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SCIENTIFIC JOURNAL OF THE TECHNICAL UNIVERSITYOF CIVIL ENGINEERING

Mathematical Modellingin Civil Engineering

BUCHAREST

Special Issue

November - 2013


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Disclaimer

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CONTENTS

PREFACE .................................................................................................................................................................... 5 CONSOLIDATION PROCEDURE FOR UNSATURATED SOILS USING A MODIFIED SETUP OF THETRIAXIAL COMPRESSION APPARATUS ........................................................................................................... 7

Adrian Liviu BUGEA

SLOPE STABILITY ANALYSIS OF UNSATURATED SOILS ......................................................................... 13

Andreea CARASTOIAN

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF INDOOR COMFORT AND ENERGYCONSUMPTION IN A TYPICAL ROMANIAN CLASSROOM FOR DIFFERENT GLAZING AREAS .... 23

Tiberiu CATALINA, Razvan POPESCU, Nicolae BAJENARU, Andrei ENE

INFLUENCE OF LONGITUDINAL VORTICES ON HEAT TRANSFER FOR AIRFLOW PASSINGTHROUGH AN INNOVATIVE SOLAR FACADE .............................................................................................. 29

Cristiana Verona CROITORU, Florin BODE, Ilinca NASTASE

DESIGN PROCEDURE FOR SIDE WALLS OF SOCKET FOUNDATIONS .................................................. 38

Ionu DAMIAN

CONCEPTION OF AN ADVANCED THERMAL MANIKIN FOR THERMAL COMFORT ASSESSMENTIN BUILDINGS AND VEHICLES .......................................................................................................................... 58

Angel DOGEANU, Ciprian CALIANU, George CHITARU, Matei GEORGESCU, Andrei TUDORACHE

ANALYSIS OF CREST CUTOFF WALL AT OSTROVUL MIC LEFT SIDE EMBANKMENT DAM ....... 69

Daniel Andrei GAFTOI, Ctlin POPESCU, Drago FR ILESCU

ENERGY SAVING ANALYSIS INSIDE A DOUBLE SKIN FACADE ............................................................. 78

Sebastian HUDITEANU, Claudia-Florentina POENARI, Bogdan-Iulian BALINT, Monica CHERECHE,

Nelu-Cristian CHERECHE

OPEN SOURCE 3D MODELING FROM RASTER IMAGES ........................................................................... 84

Gabriel Adrian KEREKES

INFLUENCES OF OXIDATION STEP AND INITIAL METAL CONCENTRATIONS ON IRON ANDMANGANESE REMOVAL EFFICIENCY .......................................................................................................... 90

Alexandru JERCAN

SEISMIC RESPONSE OF TALL BUILDINGS WITH ROCKING WALLS SYSTEM ................................... 99

Lidia MARIN, Mircea VADUVA

NUMERICAL MODELLING OF RIGID VERTICAL INCLUSIONS AS REINFORCEMENTS FORCOMPRESSIBLE SOILS..106

Iulia-Victoria TALPOS (NEAGOE)


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MATHEMATICAL MODELLING OF SHOCK WAVES GENERATED BY BLAST EVENTS AND THEIREFFECT ON CONCRETE STRUCTURES ........................................................................................................ 113

George-Bogdan NICA

MATHEMATICAL MODELING FOR THE SEDIMENTATION PROCESS IN THE RESERVOIRS ...... 122

Catalin POPESCU, Daniel Andrei GAFTOI, Drago FR ILESCU

INTEGRATED SOLUTIONS FOR IMPROVING THE ENERGETIC PERFORMANCE OF BUILDINGS .................................................................................................................................................................................. 131

Ana-Maria PASRE, Paul ANGHEL, Nelu-Cristian CHERECHE, Andrei BURLACU

R2D NUMERICAL MODELING OF FLOW THROUGH GEOMEMBRANE DEFECTS IN A LANDFILLLINING SYSTEM .................................................................................................................................................. 137

Gheorghe PANTEL

NUMERICAL MODELLING OF PILED RAFT FOUNDATIONS ................................................................. 150

rpd SZERZ

INFLUENCE ASPECTS OF EXTERNAL PARAMETERS UPON EFFICIENCY OF A NATURAL SMOKEEXHAUST SYSTEM.............................................................................................................................................. 159

Andrei-Mihai STOICA

DYNAMICS OF FREE SURFACE FLOW AROUND A CYLINDER VISUALIZATIONS AND PIVMEASUREMENTS ................................................................................................................................................ 170

Nicoleta Octavia T NASE, Florin BODE

DYNAMIC BEHAVIOR OF BUCKLING RESTRAINED BRACES AND THE INFLUENCE OFCOMPRESSION STRENGTH ADJUSTMENT FACTOR .............................................................................. 176

Mircea VADUVA, Lidia MARIN


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6 Mathematical Modelling in Civil Engineering -Special Issue - 2013

we are a privileged community that works on passionate subjects since we all chosen to stay andserve the Research, and that we can improve our condition by exchanging in a large scientificnetwork that will start to develop.This special issue presents a selection of the best papers presented at the conference. All thesubmitted papers in this special issue were fully peer reviewed by the reviewers drawn from thescientific committee, external reviewers and editorial board depending on the subject matter ofthe paper. Reviewing and initial selection were undertaken electronically. After the rigorous peerreview process, the submitted papers were selected on the basis of originality, significance, andclarity for the purpose of the workshop. All the selected papers for the present special issue aswell as all the other contributions at our conference resulted in an exciting technical program forwhich we want to warmly thank all the contributors.The members of the scientific committee were:Anton ANTON, UTCB, Bucharest, ROMANIAGiorgio SERINO, Universita Degli Studi Di Napoli Federico II, Napoli, ITALYTiziana ROSSETTO, University College London, UKRadu VACAREANU, UTCB, Bucharest, ROMANIAGabriel RACOVITEANU, UTCB, Bucharest, ROMANIAViorel BADESCU, UPB, Bucharest, ROMANIALoretta BATALI, UTCB, Bucharest, ROMANIAGabriel OPRISAN, UT Gh.Asachi, Jassy, ROMANIAAlexandru ALDEA, UTCB, Bucharest, ROMANIA Nelu - Cristian CHERECHES, UT Gh.Asachi, Jassy, ROMANIAIlinca NASTASE, UTCB, Bucharest, ROMANIAIonut - Ovidiu TOMA, UT Gh.Asachi, Jassy, ROMANIAAlexandru DIMACHE, UTCB, Bucharest, ROMANIAVasilica CIOCAN, UT Gh.Asachi, Jassy, ROMANIA

Horatiu POPA, UTCB, Bucharest, ROMANIAVictoria COTOROBAI , UT Gh. Asachi, Jassy, ROMANIAAurel SARACIN, UTCB, Bucharest, ROMANIAIonut RACANEL, UTCB, Bucharest, ROMANIAIon MIERLUS-MAZILU, UTCB, Bucharest, ROMANIAMihail Ioan SAVANIU, UTCB, Bucharest, ROMANIATiberiu CATALINA, UTCB, Bucharest, ROMANIA

We would like to thank the scientific committee, the program chairs, the organizing committeefor their work.We are grateful to all the participants and to all the persons that have contributed to a successfulfirst edition of our conference. We hope that our first goal which was making a first step indeveloping a large scientific network of young engineering researchers was reached. We alsohope that all contributors and other interested parts beneficiated scientifically from this specialissue.

The young researchers from Technical University of Civil Engineering of Bucharest.

November 2013,

Bucharest, Romania.


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Mathematical Modelling in Civil Engineering - Special Issue - 2013 7

CONSOLIDATION PROCEDURE FOR UNSATURATED SOILS USING AMODIFIED SETUP OF THE TRIAXIAL COMPRESSION APPARATUS

Adrian Liviu BUGEA PhD Student, Technical University of Civil Engineering, Faculty of HydrotechnicalEngineering, e-mail: [email protected]

Abstract: This paper presents the proposed diffusion procedure for obtaining controlled suctionsamples, in order to determine the soil water characteristic curve (SWCC). The method wasmentioned by D.G. Fredlund [1] and it was adapted to the triaxial compression equipment in thegeotechnical laboratory of the Technical University of Civil Engineering Bucharest. Based uponthe translation axis method, initially under a theoretical form, an optimal modified setup has beendesigned. Moreover, a consolidation procedure permitting the determination of the volumetricdeformations is being analyzed, in order to obtain a volume variation law as a function of themoisture content under an imposed value of suction.

Keywords: unsaturated soils, matric suction, volume variation law, SWCC

1. IntroductionLast decades trend in Soil Mechanics is represented by the understanding and modeling of theunsaturated soil behavior, state in which over 80% of the cases of soil samples are found.Moreover, this is of great interest as the direct foundation systems are founded on this type ofsoil and certain particularities of their behavior are not covered by classical saturated soilsmechanics, among which we can specify collapsible or swelling-contractile properties of eoliandeposits or clayey layers.Based on the works of researchers like D.G. Fredlund, H. Rahardjo [1], A. Gens or A. Lloret [2],the knowledge accumulated in the field of agriculture has been transferred in the geotechnicalengineering, while from theoretical background new methods and apparatuses have beendeveloped ( [3], [4], [5], [6]). Initially using mercury or water burettes, in order to measure eitherwater pressures or pore air pressures, the nowadays technology implied in these tests evolved tosuch a precision that almost infinitesimal fluid volumes may be measured or applied to thesamples.This paper aims at presenting an evolved methodology to determine the constitutive surface ofsoils, in terms of net stresses, void ratio and matric suction, using a classic triaxial compressiontest apparatus, modified in order to be used in unsaturated soil tests. The sensors for monitoringthe sample during the test and the conditions applied to it in terms of cell pressure, back pressureand pore air pressure are not new, but the entire setup and methodology have been designed insuch ways that it may offer a clear image on the behavior of the soil.

2. Variation of indices and their determination

Following the molding of the sample to a ratio between the diameter and the height of 1:2, forwhich it is recommended to have a 50x100mm sample due to the stiffness reason, the sample ismeasured and the initial mass and physical parameters are being identified. Based on the relationof the general density with respect to the solid skeleton density, porosity and moisture content,one obtains all the indices to define soils initial state.During the actual consolidation test, following the setup of the sample in the modified triaxialapparatus, a cell pressure, back pressure and air pore pressure combination is applied to the

sample until all the displacement and internal stresses are stabilized. At the end of the stage, thevoid ratio, the matric suction and the net stresses that describe the new physic and mechanicalstate of the sample are saved. This combination belongs to a surface that connects these three


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components, describing the stresses that act on an unsaturated soil sample and its deformationstate. This surface is obtained using either odometer test monoaxial infinitely confinedcompression test, or triaxial compression test. The Ct compression index is related to the at or avcompression index obtained using the oedometer test, determined using the saturated soilmechanics relation:

(1)where e is the variation of the void ratio and p the change in pressure. If the unloading case isconsidered, the ats coefficient is obtained. The Cm index is determined considering volumetricvariation during a drying test.Following this stage, new stress combinations may be applied, in order to obtain more points onthe net-e-(ua-uw) surface.

Fig. 1 - Constitutive surface of an unsaturated soil [1]

Although Fredlund and Rahardjo [1] implied that this surface may be determined by the aid ofodometer compression test and suction test using the translation of axes technique pressure plate drying test, a new method that involves both wetting and drying paths can be developed. Ifthe air pore pressure is lowered, the newly obtained matric suction difference between air andwater pore pressures, is lowered, therefore, the soil sample will arrive to an equilibrium state byadding water to its content the wetting path. If on the contrary, the air pressure is increased, thewater content in the sample will reach lower values.

3. Proposed setup of apparatus

Based on the aforementioned techniques, a modified triaxial test setup is being proposed, inorder to simplify the installation methodology of the sample and increase the accuracy of themeasured deformations.Most of todays unsaturated triaxial compression apparatuses are using the double wall method,in order to reduce the error regarding the deformations of the inner cell, affecting the volumecalculations of the fluid that creates the hydrostatical pressure on the sample, and, finally, thecalculations of the volume variations of the sample itself. This indirect method of determiningthe volumetric variations of the soil is both subjected to errors and difficult to use due to itscomplexity degree of setting up the sample inside the two celled apparatus.Therefore, a more simple and direct setup has been developed, involving a dynamic triaxialcompression apparatus, designed initially for saturated soil mechanics. The main difference between the saturated and unsaturated apparatuses are the presence of a high air entry value


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ceramic stone, which can maintain a pressure difference between water and air and the possibility to control another inflow fluid the air. Therefore, using the top cap drain, as shownin Fig. 2, an air controller is linked to the upper part of the sample and through a porous stone theair pore pressure is varied. In the pedestal, a ceramic stone is placed, in order to keep theequilibrium between the pore air pressure (denoted with ua) and water back pressure (keptconstant and denoted with uw), without getting air into the water system. Moreover, the ceramic

disk is always held in a saturated state, due to its high capillarity and the water pressure applied bottom-up.

Fig. 2 The proposed unsaturated single cell triaxial setup

In order to overcome the problem of varying cell volume, the direct method of usingdisplacement transducers to measure the radial and vertical deformations of the sample is being proposed. As Fig. 2 shows, in the case of vertical displacement transducer, a LVDT (linearvariable differential transformer) is attached to the sample and its vertical deformation ismeasured. Considering the case of isotropic consolidation, the loading ram is not touching the


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top cap the undocked status, the vertical deformations cannot be determined but attaching thelocal vertical displacement sensor. In the case of anisotropic K 0 (coefficient of active earth pressure at rest), consolidation step, although the distance between the porous stone outersurfaces is known, as the initial value was declared to be equal to the initial height of the sample,a more accurate displacement monitoring can be done using the attached sensor.Although it is possible to back-calculate the radial deformation of the sample, knowing the cellvolume and the sample height variation, a more accurate method is to measure directly on thesample the lateral surface variation - its radial deformation. In order to measure this type ofvariation, another LVDT is placed on a ring attached to the sample (Fig. 2) and whose initial position is set to a null value. All the measured linear deformations are treated as the perimeterdeformations of the cross section, taking into account that the expected displacement value is small.As the tests are undergoing, diffused air may accumulate under the ceramic porous stone asFredlund [1] presented. Although several devices have been designed in order to determine thevolume of diffused air, such as the bubble pump [7], or even a simple system to flush thediffused air beneath the ceramic disks [8], a newer technique developed by Lawrence [9] is to beused. It consists of rapidly changing the pressure in the pore water line, in order to determine theair volume using the ideal gas law. It still must be taken into consideration that these tests,especially on clayey soils are running for a longer period several days to weeks, and thediffused air volume may exceed the total volume of water of the tested sample. Therefore, aflushing apparatus must be connected to the aforementioned setup, in order to relieve and keep atlow values the volume of the diffused air (Fig. 3).

Fig. 3 - The flushing device according to Fredlund [1]

The flushing of the diffused air is performed by opening the ball valve and moving to the tank asufficient amount of water which will be carrying along the air bubbles. The water level is brought back to the initial level by the pressure and volume water controller. The difference between the initial position and the new one offers the volume of diffused air in terms of


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Adding to these results of the consolidation tests the radial and vertical displacements also may be plotted in different systems, with respect to the matric suction, applied stresses and moisturecontent or void ratio, in the end, offering a more complex view on the behavior of theunsaturated soils, especially used for the case of predicting the swelling and contraction properties of soils ([13], [14]).If the case of determining the shear resistance and the dynamic parameters with respect to thesoil matric suction, the considered setup may be used, but the results and the methodology is stillsubmitted to discussions.

5. Conclusions

This paper presented a setup to modify a classic dynamic triaxial compression apparatus in orderto determine unsaturated soil consolidation surface, in terms of applied net stresses, matricsuction and void ratio.The aim of this new setup is to overcome some of the difficulties presented by other authors andobtaining optimum installation, testing procedures and conditions in terms of device complexityand sample monitoring, as well as accuracy of results.Although at first it has been designed bearing in mind only the monitoring of the swelling-contractive behavior of clayey soils under constant stress state (consolidation step), furtherresearch in terms of determining the shear resistance and even dynamic parameters ofunsaturated soils is developed, aiming at offering a clearer and more precise view on the entireresponse of the soil material under different types of stresses that may lead to failure.

References

[1] Fredlund D., Rahardjo H., 1993, Soil mechanics for unsaturated soils, New York: John Wiley & Sons Ltd..[2] Romero E., Gens A., Lloret A., 2001, Temperature effects on the hydraulic behaviour of an unsaturated clay,

Geotechnical and Geological Engineering, nr. 19, pp. 311-322.[3] Padilla J., Houston W., Lawrence C., Fredlund D., Houston S., Perez N., 2006, An automated triaxial testing

device for unsaturated for unsaturated soils, 4th International Conference on Unsaturated Soils, Arizona,.[4] Nishimura T., Fredlund D., 2003 A new triaxial apparatus for high total suction using relative humidity, 12th

Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, Singapore.[5] Lauer C., Engel J., A triaxial device for unsaturated sand - New developments, Dresden.[6] Ho D. , Fredlund D., March-June 1982, A multistage triaxial test for unsaturated soils, Geotechnical Testing

Journal , vol. 5, pp. 18-25,.[7] Bishop A., Donald I.,1961, The experimental study of partially saturated soils in the triaxial apparatus,

Proceedings of the 5th International Conference on Soil Mechanics and Foundation Engineering, Paris,.[8] Padilla J., Perera Y., Houston W., Fredlund D., Perez P., 2006 Quantification of air diffusion through high-air

entry value ceramic disks, Proceedings of the 4th International Conference on Unsaturated Soils, Carefree,.[9] C. Lawrence, 2005, Pressure pulse technique for measuring diffused air volume, Proceedings of the

International Symposium on Advanced Experimental Unsaturated Soil Mechanics, Trento,.[10] Rahardjo, H., Fredlund D., December 1996 Consolidation apparatus for testing unsaturated soils,

Geotechnical Testing Journal, vol. 19, nr. 4, pp. 341-353,.[11] Wulfsohn, D., 1994, Triaxial testing of unsaturated agricultural soils, n International Summer Meeting,

Kansas City,.[12] Alonso E., Gens A., Josa, A., 1990 A constitutive model for partially saturated soils, Geotechnique, vol. 40,

nr. 3, pp. 405-430,.[13] Hung Q., Fredlund D., 2004, The prediction of one-, two- and three dimensional heave in expansive soils,

Canadian Geotechnical Journal, vol. 41, nr. 4, pp. 713-737,.[14] Fredlund D., 1975 Prediction of heave in unsaturated soils, n 5th Regional Conference, Banglore,.


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SLOPE STABILITY ANALYSIS OF UNSATURATED SOILS

Andreea CARASTOIAN PhD student Technical University of Civil Engineering, Bucharest,e-mail: [email protected] ;

Abstract: The paper presents the slope stability analysis of unsaturated soils consideringgeotechnical parameters and groundwater level variations due to saturation degree. Infiltrations areone of the main factors causing slope failures. The main parameters associated with slope stabilityanalysis are the characteristics of water flow, change of pore-water pressure and shear strength ofsoils. The saturation degree of soils highly influences the geotechnical parameters. The finiteelement method was used to evaluate the stability of the slope.

Keywords: slope analysis, unsaturated soil, groundwater, finite element method, geotechnical parameters

1. Introduction

The stability factor of a slope can be computed using the finite element method by reducing thesoil strength until the slope fails. The resulting stability factor is the ratio of the actual shearstrength of the to the reduced shear strength at failure. In total stress analysis of soil slopes, totalstress shear strength parameters ( and ) are often used. Pore pressures are not considered.These total stress analyses are appropriate in the short term only and not in the long term whereslope stability is a minimum (Simons et al., 2001). [1]It is important to formulate a slope stability analysis method, which can track the failure processfrom the initial deformation to the ultimate failure

2. Slope Stability Analysis

The slope stability analysis is an analytical tool for assessing the stability of a slope by using asimple failure model in the analysis.The required safety factor depends on the consequences of losses in terms of property, lives andcost of repair in the event of slope failure. The safety factor is also dependent on the reliability ofdesign parameters.A factor of safety is placed on shear strength parameters;

- The strength parameters are independent of stress-strain behavior;- Some or all of the equations of equilibrium are used to the determine the safety factor;- Forces involved in the equilibrium methods are statically indeterminate.

2.1. Stress Analysis Method (SAM)

The failure of soil slopes came in a wide variety of conceivable manners. The qualitativedefinition is given in a book by Terzaghi, Peck and Mesri (1996): The failure of a mass of soillocated beneath a slope is called a slide. It involves a downward and outward movement of theentire mass of soil participating in the failure. [2]Slope stability analysis is globally performed by means of two methods: the simplified methodand the numerical analysis method. The limit equilibrium which belongs to the simplifiedmethods is one way to approach slope stability problems and is most widely used in practice.

However, this method cannot consider the stress hysteresis effects during the formation of slopeand the variation of stress for foundations due to the groundwater. Although the finite elementtechniques considers the formation process of slope and the property of foundation, the method


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requires high cost and long analysis time, and the slope stability data for evaluation purposes isrelatively insufficient.In this method, stress analysis is first performed on the slope using the finite element method.Based on the stress analysis results, the factor of safety for potential sliding surface is calculated,and the critical section is determined using the minimum safety factor. [3]

The safety factor in the finite element method is generally defined as:(1)

Where, is the shear stress, and is the shear strength according to Mohr-Coulomb failurecriteria.

Fig. 1 - Stress components of sliding surface [4]

The initial failures for most of the unsaturated soil slopes have small depth-to-length ratios andform the failure planes parallel to the slope surface; hence, the use of infinite slope analysis forstability evaluation is thus justified (Collins and Znidarcic, 2004). The factor of safety of theslope is calculated by using a modified Mohr-Coulomb failure criterion (Fredlund et al., 1978;Fredlund and Rahardjo, 1993):

c u tan u u tan (2)

FOS

(3)

Where, c is the effective cohesion, is the effective frictional angle, is the netnormal stress, is the pore-air pressure, is the pore-water pressure, is the matricsuction, is the internal friction angle due to matric suction.The unsaturated friction angle ( ) depicts the increment rate of shear strength due to an increasein suction and it can be obtained by performing a series of triaxial compression tests undervarious matric suction conditions. In these tests the pore air pressure ( ) control and transducerare installed to measure the matric suction . For finite slope analysis, the factor ofsafety (FOS) of an unsaturated slope is expressed as:where W is the weight of slice, which is the product of (the total unit weight) and (vertical depth ofthe assumed slip surface) and is the slope angle.

The limit equilibrium methods calculate a factor of safety which, by definition, is assumed to bethe same at all points along the potential slip surface. This is reasonable only at failure, when allthe slices are on verge of failure; that is, when the factor of safety equals unity for each slice. Inreality, the local factors of safety will vary somewhat along the slip surface, for some slices itmight be one and higher value for others. In the case of brittle materials, even small hydrostaticloads can reduce the local factors of safety to less than unity and the progressive failuremechanism may be triggered. This happens, for instance, in over-consolidated clay that canexhibit residual shear strength under drained loading and in loose, saturated sands under


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undrained loading. Therefore, the methods cannot explicitly model the mechanism of progressivefailure (Pyke, 1991).[4]Modeling of slope failure by the finite element method must address the following issues:

- Occurrence of large deformations;- The effect of three dimensional conditions;- Accurate following of the equilibrium path under hardening, softening and snap-back

behavior;- Occurrence of narrow, continuous localized zone on which the slope slides.

In total stress analysis, the pore water pressure within the soils is ignored, only the free water body is considered. Free water can be modeled as a material with unit weight, but has nostrength (Figure 2a) or can also be modeled as an equivalent loading on the slope (Figure 2b).Since the free water body acts as counter weight, a submerged slope is always more stable than adry one in total stress analysis.

Fig. 2 - Total stress analysis of the slope [4]

2.2. Strength Reduction Method (SRM)

The finite element method is a precise numerical analysis method which satisfies the forceequilibrium, compatibility condition, constitutive equation and boundary condition at each pointof a slope. It simulates the actual slope failure mechanism and determines both the minimumfactor of safety and the failure behavior. It can also reflect real in-situ conditions better than mostmethods. Moreover, it can determine the failure process without assuming any failure planes inadvance. (Griffith 1999; Matsui, 1990).[5]There are two types of methods used in the finite element method to analyze slope stability

Strength Reduction method and Indirect method. The Strength Reduction method is a directmethod, which gradually reduces the shear strength of sloped ground materials until failuretakes place. Failure is assumed to occur when the analysis does not converge. Under thiscondition, the maximum strength reduction factor where the analysis fails to converge becomesthe factor of safety. The Indirect method determines the factor of safety by applying thecalculated stresses combined with the conventional Limit theory method. The StrengthReduction method uses a finite element technique first proposed by Zienkiewicz (1975). We nowfocus on a Gauss point, A, of an element in a sloped ground structure to calculate the factor ofsafety of a slope as shown in Figure 3. The stress state at this point is represented in a Mohrcircle. In order to represent the sliding surface, the shear stress at the point is divided by a factorof safety, F, so that the Mohr circle for the stress state of the fictitious sliding surface becomestangent to the failure criterion.


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Fig. 3 - Strength Reduction Method[6]

That is, the stress state of the point is corrected to the failure state. An increase in the number of points results in a global slope failure. As soon as a finite element solution diverges, the analysisstops and the limit value, F, becomes the minimum factor of safety for the slope. This methodrequires stability in the numerical analysis, but returns consistent results and evaluates the actualfailure behavior. [6]To determine the minimum stability factor of the slopes, the elasticity modulus and Poissonsratio are assumed to be constant. The cohesion and friction angle are simultaneouslyreduced, and the factor of safety, is determined at the diverging point. The factor of safety forslope failure is determined on the basis of shear failure as follows:

(4)

Where, is the shear strength of the sloped material.This is computed according to the Mohr-Coulomb criterion.

(5)Where,

- Coefficient of shear strength;

- Coefficient of shear strength;

SRF Strength reduction failure.In order to determine the SRF accurately, it is necessary to trace the resulting values of causing theslope to fail. The incremental parameter is increased in very small steps even though it may extend theanalysis time duration. Otherwise, calculating the minimum factor of safety may face difficulties.

3. Example studies

3.1. Studied cases

The finite element stability analyses were performed on homogeneous slopes. Two sets of runswere performed on each slope. The first case was a homogeneous slope consisting of the two soiltypes considering the influence of a horizontal water table at various depths, such as 3m, 5m, 7mand 10m. In the second set, on the same geometry, I analyzed the influence of the variation of theshear strength parameters. For both sets I performed the slope stability analysis using stressanalysis method (SAM) in 2D and shear reduction method in 3D (SRM). The analyses were performed using the finite element software Midas GTS.Table 1 indicates the parameters for the finiteelement slope stability analysis for the first case,while Table 2 shows the corresponding parameters for the second case.


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- 2D results using SAM methodwith water table at 3 m depth. FOS = 1,62.

Fig. 7 - a) Displacements contour of the slope; b) Shear strength of the slope.

- 2D results using SRM methodwith water table at 3 m depth. FOS = 0,975.

Fig. 8- a) Displacements contour of the slope; b) Shear strength of the slope.- 2D results using SAM method with water table at 5 m depth. FOS = 1,53.

Fig. 9 - a) Displacements contour of the slope; b) Shear strength of the slope.- 2D results using SRM methodwith water table at 5 m depth. FOS = 1,075.

Fig. 10- a) Displacements contour of the slope; b) Shear strength of the slope.- 2D results using SAM methodwith water table at 7 m depth. FOS = 1,54.



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- 2D results using SRM method with water table at 7 m depth. FOS = 1,15.


- 2D results using SAM method with water table at 10 m depth. FOS = 1,65.

Fig. 13 - a) Displacements contour of the slope; b) Shear strength of the slope.- 2D results using SRM method with water table at 10 m depth. FOS = 1,3.

Fig. 14- a) Displacements contour of the slope; b) Shear strength of the slope.

We can observe the difference of the value of the safety factor using those finite element methods. Usingthe stress analysis method, the value of the safety factor is bigger than 1.2, which means that it is stable.In the other method, by reducing the shear strength parameters, the program finds the most unfavorablesituation, reducing the value of the safety factor to slope instability.The next figures show the comparison of FOS with the shear strength parameters, influenced by thefluctuation of water table.

Fig. 15 16 - FOS values versus shear strength parameters, influenced by fluctuation of water table.


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Numerical result for slope analysis soil 2Following the same steps the results for soil 2 are presented in the figure 16.We observe the influence of the shear strength parameter modification. The initial value of safetyfactor (FOS = 1.706) is lesser than the corresponding value (FOS = 1.38) for soil 1. Applying thestrength reduction method, the safety factor remains in the unstable zone.

3.3. Numerical Results for the Second Case

In the second study I present a comparison between 2D and 3D analyses. The hypothesis is toobserve the evolution of the safety factor by modifying the shear strength parametersconsecutively, like in table 2.

- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,706. FOS (SRM) = 1.2.and

Fig. 17 - a) 2D - Displacements contour of the slope; b) 2D - Shear strength of the slope - SAM

Fig. 18 - a) 3D - Displacements contour of the slope; b) 3D - Shear strength of the slope SRM

- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,64. FOS (SRM) = 1.2.[1]. and

Fig. 19 - a) 2D - Displacements contour of the slope; b) 2D - Shear strength of the slope SAM



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- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,59. FOS (SRM) =1.05.

and



- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,53. FOS (SRM) =0.95.

and



The results are shown in the next figure, comparing the safety factor with the shear strength

parameters.


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Fig. 25 - Comparison of FOS on SAM and SRM methods

In this case, we can observe the difference between those two types of analyses. In 2D, applying the SAMmethod we can observe that the value of the stability factor is in the stable zone, modifying in each stepsthe shear strength parameters. With the same condition, applying the strength reduction method, in 3D,the value of stability factor is calculated using the reduced strength, being in the unstable zone.

4. Conclusions

The paper presents a comparison of two methods the stress analysis method and the strength reductionmethod using the finite element method for analyzing the slope stability for an unsaturated case.The slope stability analysis of unsaturated ground requires to simultaneously computing deformation andgroundwater flow with time dependent boundary conditions.According to the results we can observe the advantages of the second method. SRM slope analysis can produce insights into the failure mechanisms, and their formation, in ways that may not be as evident inlimit-equilibrium analysis.

The factor of safety is calculated using reduced strength, and the critical cross section is the area wherethe maximum shear strain occurs.Slope stability analysis of unsaturated ground requires to simultaneously computing deformation andgroundwater flow with time dependent boundary conditions.

References

[1] Simons, N.E, Menzies, B.K. and Matthews, M.C., 2001 - A Short Course in Soil and Rock Slope Engineering,Thomas Telford, London.

[2] Terzaghi, K., Peck, R., B., and Mesri, G., 1996 - Soil mechanics in engineering practice, third ed. John Wiley &Sons, Inc.

[3] Charles, W., W., Ng., Bruce, M., 2007- Advanced Unsaturated Soil Mechanics and Engineering, Taylor &Francis Group, New York;

[4] Pyke, R., M., 1991 - TSLOPE Users Guide, TAGA Engineering Systems & Software Lafayette, California, ()[5] Griffiths, D.V., and P.A. Lane, 1999- Slope Stability Analysis by Finite Elements, Geotechnique, vol. 49, no.

3, pp. 387-403.[6] Shukra, R. and Baker, R., 2003 - Mesh Geometry Effects on Slope Stability Calculation by FLAC Strength

Reduction Method Linear and Non-Linear Failure Criteria. In Proceedings of the 3rd International FLACSymposium, Sudbury, Ontario, Canada, eds. R. Brummer et al, pp. 109-116.

[7] *** MIDAS GTS SOFTWARE, 2012, User Guide.


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EXPERIMENTAL AND NUMERICAL INVESTIGATION OF INDOORCOMFORT AND ENERGY CONSUMPTION IN A TYPICAL ROMANIAN

CLASSROOM FOR DIFFERENT GLAZING AREAS

Tiberiu CATALINA Lecturer, PhD, Technical University of Civil Engineering - Romania, Faculty of Building

Services, e-mail: [email protected] POPESCU - Lecturer, PhD, Technical University of Civil Engineering - Romania, Faculty of BuildingServices, e-mail: [email protected] BAJENARU Eng., Technical University of Civil Engineering - Romania, Faculty of Building Services,e-mail: [email protected] ENE- Student, Technical University of Civil Engineering - Romania, Faculty of Building Services,e-mail: [email protected]

Abstract: This article is divided in two parts: in the first part the indoor conditions in a typicalclassroom using experimental measurements are illustrated and in the second part, using numericalsimulations, the impact of glazing area on the energy consumption for heating/lighting and indoorcomfort is analyzed. It was found that in this typical classroom the indoor levels of CO2 do notfulfill the requirements for a good air quality. However, thermal comfort is achieved as the air

temperature is around 20oC. Moreover, the illuminance level is also achieved as the window area isaround 25% from the floor area. During the numerical simulations we have studied a similar

classroom with the one from the experimental campaign. Four windows-to-floor-area-ratios have been studied in terms of energy consumption, thermal and visual comfort. It has been found that alarger area of window affects greatly the cooling demand, but has a slight impact on the operativetemperature. Moreover, the energy needed by the artificial lighting is reduced by 51% if wecompare a window-to-floor-ratio (WFR) of 15% to WFR 30%. It is recommended not to overpass aWFR of 25% as this will affect too much the energy consumption.

Keywords: experimental measurements, indoor environmental quality, numerical simulations,energy consumption, design optimization

1. IntroductionThe reduction of energy consumption is an important point on the agenda of objectives to beattained by EU countries by year 2020. At the same time, we cannot neglect the true purpose ofa building: to provide the occupants a comfortable and healthy indoor environment. Indoorenvironmental quality (IEQ) is a concept that deals not only with thermal conditions but it alsogoes much further, because it involves air quality, lighting and acoustics [1]. Given the manyinteractions between building energy performance and IEQ, these two issues must beaddressed and researched in a connected manner. IEQ and energy are closely linked and onlyan integrated research project can ensure that improvements in energy efficiency do not reduceIEQ, and that improvements in IEQ do not decrease energy efficiency, either. This issue isclearly expressed even by the European Energy Performance of Buildings Directive2002/92/EC which underlines that the expression of a judgment about the energy consumptionof a building should be always combined with an analysis of the IEQ. The problem is evenmore complicated as these are conflicting criteria [2] and finding an optimal solution can bequite difficult for the current knowledge frontier.Considering this panorama, it is highly important to search for solutions of building design that provide the highest benefit for both energy saving and IEQ at the lowest cost. Educationalfacilities are among the most important fixtures in our community [3], where children spendaround 25% of their time inside school classrooms, this area being like their second home [4].As schools present a much higher occupancy than any other building, it is vital to have anindoor climate that will not affect the comfort, health or intellectual performance of thestudents [5]. In all the existing educational facilities in Romania many complaints were madeabout discomfort, poor air quality (health problems) and expensive bills for facility operation.This research project proposal emphasizes research on schools because these types of buildings


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Fig.2 - Indoor comfort and air quality variation inside the classroom

From Figure 2 it can be noticed that the CO2 level before the pupils arrive to school was situatedat 600 ppm and then increased up to 2800 ppm. The indoor air quality norms are not fulfilled asthe maximum allowed value should be around 800 ppm, which is a translation of a poor airquality. The explanation for this high values is that the classroom does not have a ventilationsystem to supply the fresh air and that the windows have a good air tightness. This lack of freshair is translated in a low energy consumption zone, but at the same time in a low air quality. Dueto heavy rains the day before the measurement we have recorded a high value for the relativehumidity, while the indoor levels are in the range of 55% to 60%, more than acceptable. Theoutside temperature during that period was low with values around 11oC which triggered theheating system to start but only for 2 hours.Due to building thermal inertia and heat gains from the pupils the indoor air temperature waskept in a comfortable range of 20oC to 22oC. The indoor illuminance was also measured and the

mean value for the entire period overpasses the visual comfort standards [12]. It must bementioned that during the measurements the sky was covered by thick clouds and the artificiallighting was turned on for the school hours. It can be concluded from this experimentalinvestigation that the building performs well in terms of thermal and visual comfort but has aserious problem with the indoor air quality. Moreover, it was observed that the artificial lightingcontrol is not optimized and there is a waste of electric energy.As noise problems are concerned, it was found that the global sound pressure level was higher(45 dB(A)) than 35 dB(A) which is the limit value for a comfortable environment. This is due tothe sound that is propagated from the outdoor to the indoor space mainly through the windows.The exterior environment was found to be noisy as the traffic is intense and multiple trucks are passing by the school faade.

3. Numerical simulations

After the experimental analysis we have done a theoretical study of a classroom that has the sizeof 8m x 6m x 3.2 m (L x l x h) using numerical simulations. We found these dimensions relevantas most of our studied classrooms had these sizes. One of the most trustworthy solutions toevaluate the indoor conditions and energy consumption of a building are the dynamicsimulations. It is clear that during the design stage of a new building it is important to verify theimpact of a certain parameter on the building performance. As the building faade is a keyelement for the indoor conditions we wanted to study the impact of the window area on the

thermal comfort and energy consumption. Like for the experimental studied classroom thissimulated virtual classroom has a good air tightness of 0.2 ach/h and the installed windows aredouble pane glazing windows. The walls are made of brick and are insulated with 7.5 cm of


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polystyrene. The structure of the walls, the floor and the ceiling corresponds to real schools. Thesimulated classroom is oriented WEST and is situated at the last floor of a building in order tohave the most disadvantageous case from the thermal point of view. The ceiling is adjacent withan attic insulated with 10 cm of mineral wool.

0

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WFR 15% WFR 20% WFR 25% WFR 30%

E n e r g y

[ k W h / a n ]

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Fig.3- Annual heating, cooling and electric energy consumption

The simulations were done for the entire school year (15 September to 15 June) and we havetaken into account the occupation periods with the holidays/weekends/days hours, the heat gainfrom artificial lighting and from pupils. The time step of the simulations was set to 1 hour andthey were realized using the TRNSYS 16.0 software.The same scenarios of window-to-floor-ratios were used to investigate the impact on the heatingand cooling demand during the coldest and warmest day of the school year (see Figure 4). If forthe heating we do not observe large variations for the cooling demand we have a much largerdifference of up to 1500 W between WFR 15% case and WFR 30% scenario.Like it was mentioned previously the purpose of the numerical study was to check the building performance for different glazing areas. We have considered four cases of window-to-floor-ratio(WFR): 15%, 20%, 25% and 30%. From Figure 3 it can be noticed that there is an increase of 24

% for the heating demand between the WFR 15% case and WFR 30%. At the same time there isan increase of 51.4% of the cooling demand for the same comparison. On the other hand weobserved a reduction by 43% of the electric energy if we increase the WFR from 15% to 30%.

0100020003000400050006000700080009000

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Fig.4 - Heating and cooling demand for the coldest/warmest day of the school year

Thermal comfort is defined by ASHRAE [13] as that condition of mind which expressessatisfaction with the thermal environment and is assessed by subjective evaluation. In our case weexpress the thermal comfort using the operative temperature which translates the influence of bothair temperature and radiant surrounding temperature. The set point temperature for the air was

considered of 20o

C and it can be observed that during the coldest day the operative temperature islower than 20oC, but also equal for the WFR scenarios. For the summer period the differences between the four scenarios of WFR are higher but are still low with values around 0.5oC.


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4. Conclusions

The data obtained in this article were found from experimental measurements and from thenumerical analysis of the window to floor area ratio (WFR). In this research paper the indoorcomfort is studied experimentally during one day using professional equipment that allowed themeasurement of air quality, air temperature, humidity and illuminance level. The measurements

showed that the air temperature is comfortable for the intellectual activities even if the outdoorair was around 11oC. The CO2 level was found to be much higher that the value proposed by thestandards with values of 2800 ppm. Despite the cloudy day, the indoor visual comfort isachieved by both natural and artificial lighting. The glazing area is a key element in the design ofnew classrooms, and that is the reason for studying different cases of WFR. If for the operativetemperature there is a slight difference between the WFR 15% and WFR 30% for theheating/cooling consumption we have larger differences. A larger area of window signifies lowerenergy consumption for the artificial lighting. The energy reduction can go up to 51% if using aWFR of 30% compared to 15%. It can be concluded that a low WFR is not sufficient for a gooddaylight while a higher WFR will increase the energy consumption. It is recommended that theWFR should not overpass 25% or to be lower than 20%.

Acknowledgements

This work was supported by a grant of the Romanian National Authority for Scientific Research,CNCS UEFISCDI, project number PN-II-RU-TE-2012-3-0108.

References

[1] A.C.K. Lai, K.W. Mui, L.T. Wong, L.Y. Law, An evaluation model for indoor environmental quality (IEQ)acceptance in residential buildings, Energy and Buildings, Year 2009, Volume 41, Pages 930936.

[2] Tiberiu Catalina, Vlad Iordache, IEQ assessment on schools in the design stage, Building andEnvironment, Volume 49, Pages 129-140, Year 2012

[3] Adelman H.S., Taylor L., Classroom climate, Encyclopedia of school psychology, Thousand Oaks, CA: Sage,Year 2005.

[4] Grimsrud D., Bridges B., Schulte R., Continuous measurements of air quality parameters in schools, BuildingResearch and Information, Year 2006, Volume 34, Issue 5, Pages 447458.

[5] Bartlett K.H., Martinez M. and Bert j., Modeling of occupant-generated CO2 dynamics in naturally ventilatedclassrooms, Journal of Occupational and Environmental Hygiene, Volume 1, Issue 3, Year 2004, Pages 139148.

[6] Faustman EM, Silbernagel SM, Fenske RA, Burbacher TM, Ponce RA., Mechanisms underlying children'ssusceptibility to environmental toxicants, Environmental Health Perspectives, Year 2000, Volume 108, Issue 1,Pages 13-21.

[7] World Health Organization Methods for monitoring indoor air quality in schools, Report of a meeting, Bonn,Germany, 4-5 April 2011, Regional office for Europe.

[8] Daisey JM, Angell WJ, Apte MG., Indoor Air Quality, ventilation and health symptoms in schools: an analysisof existing information, Indoor Air, Year 2003, Volume 13, Pages 53-64.

[9] J.Jalas, K.Karjalainen, P.Kimari., Indoor air and energy economy in school buildings, Proc. Of HealthyBuildings,Year 2000, Volume 4, Pages 273-278.

[10] Becker R., Goldberger I., Paciuk M., Improving energy performance of school buildings while ensuring indoor airquality ventilation, Building and Environment Volume 42, Issue 9, September 2007, Pages 32613276

[11] Mikola A., Voll. H, Koiv T., Rebane M., Indoor climate of classrooms with alternative ventilation systems,GEMESED Proceedings of the 4th WSEAS international conference on Energy and development - environment biomedicine, Year 2011.

[12] European Standard EN 15251, Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics, August 2007.

[13] ASHRAE. ANSI/ASHRAE Standard 55-2004 Thermal Environmental Conditions for Human Occupancy.Atlanta : American Society of Heating, Refrigirating and Air-Conditioning Engineers, 2004. ISSN 1041-2336.

[14] CIE Technical Comittee 4.2, The Availability of Daylight, Technical Report No. NR, ComissionInternationale de lEclairage, Paris, 1975


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INFLUENCE OF LONGITUDINAL VORTICES ON HEAT TRANSFERFOR AIRFLOW PASSING THROUGH AN INNOVATIVE SOLAR

FACADE

Cristiana Verona CROITORU-Lecturer, PhD, Technical University of Civil Engineering Bucharest, Faculty of

Building Services, Politehnica University of Bucharest;Florin BODE-Lecturer, PhD, Technical University of ClujNapoca;Ilinca NASTASE-Associate Professor, PhD, Technical University of Civil Engineering Bucharest, Faculty ofBuilding Services, e-mail: [email protected]

Abstract: Renewable energy represents an attractive solution to fulfil two requirements: indoorenvironmental quality and energy efficiency. Passive solar systems are easy to implement andeffective in areas with high solar potential. The Unglazed Transpired Solar Wall (UTSW) is madeof metal cladding with perforations, installed at several centimetres from a building wall, creating acavity. The air is forced to pass through this heated perforation, and thus a heat transfer between thefluid and the metal takes place. Several measurements and CFD simulations were performed on aninnovative perforated solar wall. This study is a preliminary analysis approach on the importance ofthe orifice shape of the perforated panel as a heat transfer parameter. The results found in literature

were compared with experimental and CFD results. A good agreement was found. Changing thegeometry of the perforations will increase on one hand the perforation perimeter and on the otherhand will generate complex fluid dynamics, resulting in a higher efficiency of heat recovery ofthese devices.

Keywords:unglazed transpired solar wall; solar energy; energy efficiency; CFD modelling

1. Introduction

The new European Directives concerning energy performance of buildings imposes significantreduction of the energy consumption. For this reason, the EU Members have adopted drasticregulation in order to achieve high building performance. On the other hand, the indoor qualityhas become an important parameter when conceiving residential or office buildings. The requestsof the occupants are more exigent and achieving the indoor comfort is one of the most importantchallenges for civil engineers. Generally, the buildings sector consumes 35.3% from the totalenergy demand. This energy demand is caused mainly by the HVAC (Heating Ventilating andAir Conditioning) Systems. During the winter season in cold countries, the heat demand of the building represents the highest percentage from the total amount of energy demand, while duringthe summer, air treatment or ventilation is a major consumer of electrical energy. In this context,the use of renewable energies is an attractive solution for fulfilling the two requirements: indoorenvironmental quality and energy efficiency. Among renewable energies, the use of solar passivesystems are easy to implement and efficient from the accessibility point of view in the zones

with solar potential.The Unglazed Transpired Solar Wall (UTSW) is made of metal cladding with perforations,installed at several tens of centimetres from a building wall, thus creating a cavity through whichair circulates.The schematic drawing of this type of solar collector is as illustrated in Fig. 1. The metalcladding is heated by the solar radiation from the Sun and ventilation fans create negative pressure in the air cavity, drawing in the solar heated air through the perforated panel. The air isgenerally taken off the top of the wall (due to air temperature gradients in the cavity) ensuringthat all of the produced solar heat is collected, and then distributed in the building via theventilation system. In the summer conditions, the system can work only during the night for free

cooling ventilation, while during the day the air layer has an insulation role.


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A literature survey led us to some interesting conclusions: (i) a consequent part of the heattransfer between the air and the solar collector is occurring during its passage through the perforation orifices; (ii) it is preferable to have a non-uniform flow on the back of the plate.

a) b)Fig.5 -a) Schematic drawing of an unglazed transpired wall, b) Innovative perforated panel developed at ULR [1]

On the other hand, passive mixing techniques applied to HVAC air diffusion terminal units have been developed greatly during the past decade, since a collaborative research team from theUniversity of La Rochelle and UTCB dedicated numerous studies to these devices [2-11]. A newresearch direction has been started at La Rochelle University (ULR) regarding the possibility ofusing passive control for enhancing heat transfer in impinging jet flows [12]. All these studiesuse a special geometry of nozzles, ailerons or orifices, which is called lobed geometry. Anexample of such geometry is the lobed orifice. In Fig.1 b a perforated panel with lobed orifices(cross shaped or 4-lobed orifices) is presented. For the same effective area (same equivalentdiameter) the perimeter of the lobed jet is much larger than the one of the circular orifice,increasing the contact boundary between the air flow passing through the orifice and the orificethickness. Under low or moderate Reynolds numbers, such as the one characterizing the flows inthe UTSW, the analysis of the elementary lobed nozzle and orifice jets shows that the lobedshape introduces a transverse shear in the lobe troughs [7, 13-15].

2. Methods

2.1. Experimental approach

The perforated panels were placed on a rectangular box with thermally insulated walls. The boxis connected through a circular pipe to an exhausting fan, forcing the ambient air to pass throughthe perforated panel. After positioning each perforated cladding, the box is sealed with sanitarysilicone, in order not to have leaks which might perturb the tests.At a distance of 30 cm of the cladding, four Metal Halide Flood Lights were placed, eachcorresponding to a lightning level of 400 W, which are simulating the Sun radiation. Theaspiration fan creates the negative pressure necessary to force the air to pass through the perforations, so the convective heat transfer takes place. Velocity, pressure and temperature probes were placed in strategic points, either for controlling the conditions, or obtaining theresults of the measurements, as can be seen in the next figure. An acquisition data box wasemployed to record the measurement values for a certain stabilisation period. The acquisitiontime step is 60 seconds.


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a)

b) Fig.6 -a) Schematic drawing of the experimental facility b) Experimental set-up photograph: Radiation lamps,

exhaust pipe and the perforated panel

The cladding has a perforated surface of 0.49m2 (0.7m x 0.7m) and receives only a certain percentage of the total radiative intensity emitted by the lamps. Four temperature probes were placed in strategic points: t1- exit temperature; t2- ambiental temperature; t3- cladding surfacetemperature; t4- black globe temperature. The velocity and pressure probes were used to evaluatethe airflow and pressure loss of the perforated sheet. The flow rate was evaluated using theomnidirectional velocity probe from TSI, placed inside the exhaust pipe. For all themeasurements, we waited for the stabilisation time, each time three different readings of thevalues being done.The indoor temperature and relative humidity were permanently monitored. Several types of perforated panels were tested: the baseline panel with round shape perforations and theinnovative panel with lobed cross-shaped perforations. The equivalent diameter De of bothgeometries of orifices was 5mm. The porosity for each type of tested perforated panel is given bythe distance between two adjacent orifices, from centre to centre, of 13.5 mm (3C and 3R) and19 mm (4C and 4R) for each type of perforation.The perforated panels are positioned on a rectangular box with thermally insulated walls. The box is attached through a circular pipe to an exhausting fan, forcing the air to pass through the perforated panel, fact that conducted to the heating of the air.The plate collects only a certain percentage of the total radiative intensity emitted by the lamps.The radiation transmitted effectively between the source of light and the plate was considered to be around 800 W/m2 (value in agreement with the experimental conditions from [16]).

2.1. Numerical approach

The numerical simulations by the CFD approach using a RANS (Reynolds Averaged Navier

Stokes) model were performed to study the airflow and heat transfer through the two types of perforations for different values of the airflow.

t1

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Fan InsulationPerforated anel


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Given some considerations of symmetry and in order to save numerical resources for a finermesh, the numerical study was performed for a smaller perforated cladding corresponding to a

part of the experimental panel. The model comprises 25 perforations (Fig. 3), with the samespace between two adjacent centers of orifices, as in the experimental case. The metal claddinghas a size of 10 cm by 10 cm positioned at 15 cm from the exhaust surface. This approach willtest the capabilities of the CFD models to reproduce from the global point of view our previous

experimental results [17] in order to investigate in a next step which the influence of the flowdynamics is on the heat transfer enhancement.

s Fig. 7 - Section through the used grid and detail of the cross perforated cladding

The boundary conditions on the perforated cladding considered an imposed thermal flux such as inthe experimental case. The rest of the walls are considered with 0 W heat-fluxes. The air is aspiredthrough the perforations as in the experimental case. A value of turbulence intensity of 9.8%,calculated with empirical relation proposed by [18], was imposed for the inlet boundary condition.

The accuracy of a CFD simulation depends, in a high percentage, on the way of replicating thegeometry that defines the calculation domain and the heat sources, with specific boundaryconditions. The final computational domain comprises 4.5 million hybrid cells: both tetrahedraland hexahedral cells for a better characterization of the flow. Inside the orifices a first layer of0.2 mm was applied with a growth factor of 1.15. Outside the orifices, the mesh on the plate hasa first layer of 5 mm, with a growth factor of 1.15. The viscous model was chosen to be k-omegaSST in agreement to previous studies performed on the lobed perforations[19].

3. Results

The studies performed for solar walls systems showed good results in terms of energy efficiency. Inthis context the use of solar passive systems is encouraged by national regulations as they can have asignificant contribution to achieve high performances and to save energy for winter heating and for

summer cooling. Table 1 summarizes some of the case studies available in the literature. A quick survey allows us to be aware of the huge possibilities of such devices in energy recovery.For instance, the CFD study of Arulanadam et al. [20] concludes that not only metal cladding could

be used for the perforated absorber but even low conductivity materials can lead to acceptablethermal efficiency of the system, for low porosity of the transpired plate absorbers and for lowvelocity flow situations. But studies such as the ones of Van Decker et al. [21], Gunnewieck et al.[22, 23] are very interesting from our point of view, given the information related on the direct

possibilities of improvement of these devices. The early numerical study of Gunnewieck et al. [22]highlights the importance of a non-uniform flow and of a low velocity on the efficiency of unglazedtranspired solar air heaters of large area. Van Decker et al. [21] show that in no-wind conditions,about 62% of the ultimate temperature rise of the air is predicted to occur on front-of plate, 28% inthe hole and 10% on the back of the plate.


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Cordeau and Barrington [24] in their study of an UTSW, used for bringing fresh air in a broiler barn,reveal that the efficiency of the solar air pre-heaters reached 65% for wind velocities under 2 m/s, but dropped below 25% for wind velocities exceeding 7 m/s, with an annual return on investment of4.7%. Different other case studies of UTSW [16, 22-32] pointed outan energy efficiency of the system used from 52% to 68%, being an important benefit in terms of fossil energy consumptions savings.

Table 1

Current studies on UTSW

Reference Collector area and type Airflowrate

[m3/h/m2]

Temperature rise

Efficiency Energy saving

[33] 1877 m2; vertical wall; 2% porosity; 1% canopy;

125 12.5 C 57% 917 kWh/m2/year

[33], [34] 420 m2 gross; 2%porosity; 1%canopy

72 13 C 52% 754 kWh/m2/year

[33] 27.9 m2; 2% porosity N/A N/A 63-68% N/A[35] 335 m2; corrugated dark brown

aluminum

N/A N/A N/A 195 700 kWh/m2

[35] Solar wall panel area=1.1664 m2;PV cells covered 24% of entiresurface

100 N/A Thermalefficiency 48%Combinedefficiency 51%

500-1000kWh/m2/yearFrom whichelectricity50-100 kWh/m2/year

[36] 2 m; Transpired solar collector 117 13.2CPresentstudy

1 m2 ; Transpired solar collector;0.6 % - 10% porosity; blackaluminum sheet

10-150[m3/h/m2]

9C-30C 60-70% forairflows largerthan 50[m3/h/m2]

N/A

In the present study, the heat transferred from the plate to the air (P) was quantified by the airtemperature rise, using:P=mair * c p *(T pipe-Tamb) (1)

where mairis the mass flow rate.Four cases were studied in comparison with a standard configuration of a commercial perforated panel for UTSW systems. The studied cases are: 3R - Round orifices with S= 13.5 mm, 4R - Roundorifices with S= 19 mm, 3C Cross shaped orifices with S=13.5 mm and 4C Cross shaped orificeswith S=19 mm.The efficiency of the panel was defined as:

pl T A I P

(2)where P is the heat transferred from the plate to the air, IT is the irradiation provided by the lamps tothe plate level and A is the surface of the plate of 1 m2.In Fig. 4 we represented the evolution of the thermal efficiency for the four cases investigated, incomparison with the data obtained by using a commercial UTSW and the two models proposed byBelusko et al. et Shukla et al. for UTSW without wind and with circular perforations with the sameequivalent diameter as in our case [30, 37]. For both perforation rates studied in the presentexperimental campaign we can see an advantage of the innovative perforated plate with lobed

orifices compared to the baseline round orifices panels. They present also a clear advantage for highflow rates when compared to the analytical models of Belusko et al. et Shukla et al. for panels withcircular perforations.


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a) b)Fig. 8 -UTSW efficiency for different volumetric flow rates: a) S=13.5, b) S=19 mm

If we compare the heat transfer that occurs, we can observe that the cross shape performs better, for both cases: 4C and 3C.

Fig. 9- UTSW efficiency for different volumetric flow rates: a) S=13.5, b) S=19 mm

We wanted further to test a larger domain of the airflow employed in the experimental setup for themetal cladding with perforation of 13.5 mm (3C and 3R). We evaluated the heat transfer for airflowsranging from 30m3/h up to 270 m3/h. We can observe that after a stagnant zone of heat transferred,for values of airflow of 100 m3/h we obtain a significant increase in thermal power. These findingswill be treated in a further study.

Fig. 10 -Heat transfer for a larger airflow domain: 3C and 4R comparison

Let us take a look to the CFD results compared to the experimental data. In Fig. 6 and 7 we presenttypical velocity and temperature fields in a plane passing through one of the perforation rowsadjacent to the median plane, for both geometries: cross and round shape. We can observe anincrease in temperature between inlet and outlet, intensified in the case of the cross perforation.

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3R Dymond et al. 1999

3R Belusko et al 2009

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4R Belusko et al 2008

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a) b)Fig. 11 -CFD fields for the round perforations at a flow rate of 96 m3/h: a) velocity, b) temperature

a) b)Fig. 12 - CFD fields for the cross-shaped perforations at a flow rate of 101 m3/h: a) velocity, b) temperature

In Fig. 8 we superposed experimental and numerical data for the temperature differences obtained forthe two types of perforations. While in the case of the round shape perforation the temperaturedifference between the ambient temperature and the temperature of the airflow aspirated through the perforated plate are little underestimated, in the case of the lobed perforation they are ratheroverestimated. We notice however the similarity between the experimental data of Leon et al. [16]and our numerical data.

a bFig. 13 - Temperature difference as a function of the airflow: a) circular perforations b) cross shaped perforations

In Fig. 9 thermal efficiencies for the two UTSW are given from experimental and numerical cases, incomparison with the data obtained by using a commercial UTSW and the two models proposed byBelusko et al. et Shukla et al. for UTSW without wind and with circular perforations with the sameequivalent diameter as in our case [30, 37]. For both perforation rates studied in the presentexperimental campaign we can see an advantage of the innovative perforated plate with lobed

orifices compared to the baseline round orifice panels.

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Fig. 14 -UTSW efficiency for different volumetric flow rates; Comparison with literature

They also present a clear advantage for high flow rates when compared to the analytical models ofBelusko et al. and Shukla et al. for panels with circular perforations.

4. Conclusions

The study evaluated the energy efficiency of several types of unglazed transpired solar collector(UTSW) by experimental and numerical means. The physical model used shows good results inagreement with literature. In addition, the comparison of a conventional UTSW with a newgeometry with innovative perforation leads to interesting results, with over 15% increase inthermal efficiency since the literature shows a lack of the geometry study for the perforations.These effects still need to continue the investigation. The CFD study on the unglazed transpiredsolar collector (UTSW) which is equipped with cross shape perforations shows that theexperimental conclusions can be also found by numerical means. Because such geometriesrequire very fine meshes, a scaled model of the experimental would be the answer to numerical

modelling for such case. The results showed very good agreement with the experimental study,fact that validated our model. The efficiencies calculated proved the advantage of cross-shapedmodels in comparison to classical ones. More of that, the comparison of a classical UTSW with anew one with innovative perforation geometries leads to interesting results, with more than 15%increase in thermal efficiency for volumetric flow rates higher than 100 m3/h. Further studiesregarding the best configuration still have to be conducted, including complementarymeasurement techniques.Acknowledgements: This work was supported by the grants of the Romanian NationalAuthority for Scientific Research, CNCS UEFISCDI, project number PN-II-RU-PD-2012-3-0144, PN-II-ID-PCE-2011-3-0835.

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DESIGN PROCEDURE FOR SIDE WALLS OF SOCKET FOUNDATIONS

Ionu DAMIAN - Assistant, Technical University of Civil Engineering, Faculty of Civil, Industrial and AgriculturalStructures, Department of Reinforced Concrete, e-mail: [email protected]

Abstract: Single storey structures having simple structural systems, jointed roof on cantilevercolumns are widely used nowadays for commercial buildings. To minimize the execution time andconstruction effort, a popular solution is to install precast columns on the so called socketfoundations. The column is a linear element characterized by flexural behavior, the design of whichdoes not produce difficulties. However, the foundation is made of several short elements, the behavior of which may be difficult to assess. One web element is the side wall of the socket. Thiselement behaves like a short cantilever with the load suspended at the bottom. The new version ofthe Romanian code for shallow foundations, NP 112/2012 proposes a strut and tie model for thedesign of the side wall. Anyway, the model is not fully described and the strut capacity is notverified against the results of more refined analyses.The goal of the paper is to complete the strut and tie model and to present a simple design relationfor estimating the capacity of the struts. The method is calibrated against the predicted monotonic behavior calculated using the Modified Compression Field Theory (MCFT), a formulationadequate for the analysis of squat elements. The article also includes a design example using the proposed procedure.

Keywords: socket foundation, side wall, Modified Compression Field Theory (MCFT), strut and tie

1. Introduction

Single storey structures having simple structural systems, jointed roof on top of cantilevercolumns are frequently used for commercial buildings. The current state of practice is to use precast elements for both columns and isolated footings. The design of the columns is notdifficult, because they are linear members characteri

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