Performance analysis of a hydrofoil with and without leading edge slat

Tahir YAVUZ Department of Mechanical Engineering

Baskent University Ankara, TURKEY

tyavuz@baskent.edu.tr

Hur�it AKPINAR Department of Mechanical Engineering

Baskent University Ankara, TURKEY

hursit@baskent.edu.tr

Birol KILKIS Department of Mechanical Engineering

Baskent University Ankara, TURKEY

birol@baskent.edu.tr

Özgür EROL Department of Mechanical Engineering

Baskent University Ankara, TURKEY

erol@baskent.edu.tr

Abstract— Operational effectiveness of the wind and hydrokinetic turbines depend on the performance of the airfoils chosen. Standard airfoils historically used for wind and hydrokinetic turbines had and have the maximum lift coefficients of about 1.3 at the stall angle of attack, which is about 12o. At these conditions, the minimum flow velocities to generate electric power are about 7 m/s and 3 m/s for wind turbine and hydrokinetic turbine, respectively. Using leading edge slat, the fluid dynamics governing the flow field eliminates the separation bubble by the injection of the high momentum fluid through the slat over the main airfoil- by meaning of the flow control delays the stall up to an angle of attack of 20o, with a maximum lift coefficient of 2.2. In this study, NACA 2415 was chosen as a representative of hydrofoils while NACA 22 and NACA 97 , were chosen as slat profiles, respectively .This flow has been numerically simulated by FLUENT , employing the Realizable k-� turbulence model. In the design of the wind and hydrokinetic turbines, the performance of the airfoils presented by aerodynamics CL = f (αααα,δδδδ), CD = f (αααα,δδδδ) and CL/CD = f (αααα,δδδδ) are the basic parameters. In this paper, optimum values of the angle of attack, slat angle and clearance space between slat and main airfoil leading to maximum lift and minimum drag, and consequently to maximum CL/CD have been numerically determined. Hence, using airfoil and hydrofoil with leading edge slat in the wind and hydrokinetic turbines, minimum wind and hydrokinetic flow velocities to produce meaningful and practical mechanical power reduces to 3- 4 m /s for wind turbines and 1-1.5 m/s or less for hydrokinetic turbines. Consequently, using hydrofoil with leading edge slat may re-define the potentials of wind power and hydrokinetic power potential of the countries in the positive manner.

Keywords-component; hydrokinetic turbine, hydrofoil with leading edge slat, high performance, flow separation control, hydrodynamic model

I. INTRODUCTION (HEADING 1)

In several countries many alternative studies for increasing the potential of use of renewable energy systems are recently under way in order to generate new and sustainable solutions for energy crisis and environmental-

climatic problems. The widely known alternative energy resources are wind energy, solar energy, and biogas energy. There is another alternative energy resource that is hydro-kinetic power conversion, which is at the same time more stable, continuous, and sustainable. Although this technology may be very little known, water turbines (similar to wind turbines) that can operate in deep channels with stable (constant) flows have been designed, manufactured and operated. Some countries have great hydro-kinetic potential with large and long rivers, water channels with stable and high flow rates, and Turkey is indeed quite fortunate in this respect. The performance of the hydro or wind turbines depend on the rotor, shaft, gear box, and generator characteristics.

Hydrokinetic turbines are a class of “zero-head” hydropower which uses the kinetic energy of flowing water to drive a generator. Hydrokinetic turbines operate on many of the same principles as wind turbines and share similar design philosophies. The most notable difference is that the density of water is about 850 times greater than air density, so the energy in a given flow stream is much greater for a hydrokinetic turbine than for a wind turbine. Average flow velocities for a tidal or river flow, however, tend to be an order of magnitude less than the flow velocities at a good wind site. The net impact is that the Reynolds numbers tend to be in the same range for both wind turbines and hydrokinetic turbines, which allows for much of the same experimental airfoil/hydrofoil data to be used in the design process. Additionally, hydrokinetic turbines can be analyzed and designed using the same incompressible flow techniques used for wind turbines. Unlike wind turbines, however, hydrokinetic turbines must be designed to avoid cavitations [1].

The blade or rotor which converts kinetic energy of the wind or water current into mechanical energy is the most important component of the turbine systems. Traditionally, wind turbines use the classical standard airfoils which have the maximum lift coefficient of about 1.3. According to the fluid dynamics governing the flow field; the separation bubble by the injection of the high momentum fluid through

2011 10th International Conference on Machine Learning and Applications

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DOI 10.1109/ICMLA.2011.113

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the gap between main hydrofoil and slat by means of the flow control delays the stall up to about an angle of attack of 20o, with a maximum lift coefficient of 2.4. In this study a new computer-aided optimum design-analysis tool and a model was developed, which is capable of carrying out all the calculations for the design and analysis of adaptable, winged water turbines and to support design decision-making processes for both high-capacity hydro-kinetic energy conversion systems for cities and smaller-scale systems for individual power demands that may efficiently operate at low water flow rates. Using this model a suitable laboratory-scale pilot turbine can be manufactured and performance tested to gather and compile sufficient data to build a robust engineering knowledge base for a prototype design in the future. In this study, the hydrofoil with two leading edge slat profiles, NASA 22 and NASA 97, was chosen and the optimum geometric dimensions and flow conditions for the high hydrodynamic performance of the system were determined, which are presented in this paper. Results are also applicable for the wind energy and wind turbines.

II. HYDRODYNAMIC MODEL

In the hydrodynamic model, the blade – element momentum theory which is a combination of two different theories, namely the conservation of momentum theory and the blade element theory is used. Conservation of momentum theory refers to a control volume analysis of the forces at the rotor plane based on the conservation of linear and angular momentum. Conservation of momentum states that the loss of pressure or momentum through the rotor plane, which occurs as the fluid passes through the rotor plane, is caused by work done on the turbine blades by the moving fluid. The conservation of momentum theory then allows calculation of the induced velocities in the axial and tangential directions from the momentum lost by the moving fluid. A flow field, characterized by the axial and angular induced velocities, is used to define the local flow conditions at the rotor hydrofoils. Blade-element theory is an analysis of forces which assumes that the blades can be divided into many smaller elements which act independently of surrounding elements. Given the local flow conditions and the blade geometry, the hydrodynamic forces on these blade elements can be calculated. Blade-element theory then sums these elemental forces along the span of the blade to calculate the total forces and moments exerted on the turbine.

High-lift systems essentially modify the fluid dynamics of wings so as to avoid aircraft stalling and methods have been devised to predict boundary layer separation that causes stalling [2, 3]. Stall at high angles of attack can be retarded by either optimizing the shape of the airfoil or by adding high-lift devices such as leading-edge slats and trailing-edge flaps. The optimization of airfoils for high lift is governed by the maximum lift coefficient available from a mono-element airfoil with un-separated flow and the shape of the airfoil can be improved for high-lift situations [4–6]. Leading-edge slats are known to help avoid leading edge separation at low speeds by injection of high-momentum fluid through the gap between the slat and the main airfoil. This injected fluid adds

kinetic energy to the boundary layer [7] and hence delays leading-edge separation. Various forms of leading-edge devices have been tested, notably the fixed slat [8, 9, 10], the retractable slat [11, 12], and the Krüger flap [13]. The effectiveness of the leading edge auxiliary airfoil is dependent on its positioning. Same kind of flow behavior can be observed in the flow around double blade airfoils. In the case of using double-blade airfoil, the fluid dynamics governing the flow field eliminates the separation bubble by the injection of the high momentum fluid through the gap between two blades by meaning of the flow control delays the stall up to an angle of attack.

The main purpose of this study is to present the optimum geometric parameters, the orientation of the slat with respect to the main hydrofoil and flow characteristics for hydrofoil with leading edge slat, defined as high performance hydrofoil, that is useful for hydrokinetic turbines to generate electric power in relatively slow hydrokinetic currents. Geometric definitions of the blade hydrofoil are shown in Figure 1. The variations of the geometric parameters in the analysis were chosen to be as 0.125 < h/c1< 0.185, 0.56 < c1/c2 < 0.71, angle of attack, 0o <α <38o, slat angle, 18o<δ<37o (Plus and minus) and the Reynolds numbers (Re=VC1/ν), 1.26 x105 <Re<2.53x105.

Figure 1. Geometry of hydrofoil with leading edge slat

Figure 2. Structured grid of hydrofoil-slat arrangement for CFD Analysis

The FLUENT code was used as the advanced tool in this study. Detailed analysis was made using the FLUENT code including Spalart-Allmaras’, k-ε turbulent model. The results are all obtained for two dimensional computations although three dimensional effects are present within the separated region. No-slip boundary conditions are used at solid surfaces. The grid used for the double blade hydrofoil is generated by the GAMBIT [14] program, and is shown in Fig. 2.

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The grid extends from 7 chords upstream to 15 chords downstream. The upper and lower boundary extends 12 chords from the profile. Different size grids are used to ensure grid independence of the calculated results. This is achieved by obtaining solutions with increasing number of grid nodes until a stage is reached where the solution exhibits negligible change with further increase in the number of nodes. Consequently, the grid size giving the grid-independent results are selected and the total number of cells is adopted as 55000 nodes.

The FLUENT code solves the RANS equations using finite volume discretization. Second-order upwind discretization in space is used, and the resulting system of equations is then solved using the SIMPLE coupled solution procedure until certain convergence criteria are satisfied. The convergence rate is monitored during the iteration process by means of the residuals of the dependent variables of the governing differential equations. Convergence is also checked using the relative differences between two successive iterations for each of the integrated force and moment coefficients.

The results shown in this study were obtained for two-dimensional computations although three dimensional effects are present and relevant within the separation bubble. Free stream boundary conditions were used in the upstream, downstream, and outer boundaries. No-slip boundary conditions were used at solid surfaces. In order to resolve the boundary layer, 10 layers were introduced and first layer is 0.1mm from the wall. Hence the first grid point off the wall in the normal direction was placed at a distance less than y+ =10 in wall coordinates

III. RESULTS AND DISCUSSIONS

The design parameters, so called optimum parameters, for wind or hydrokinetic turbines are defined at the maximum value of the ratio of the lift coefficient to the drag coefficient, CL/CD with respect to the geometric and flow parameters. Results show that, the optimum geometric parameters of the hydrofoil-slat arrangement were obtained to be as h/c1= 0.165 and slat angle of 24.5o. Results obtained for three cases, two leading edge slats and without slat, at δ=24.5o, h/c1=0.165 and Re =2.53x105 are presented in Table 1. As seen in the Table, the maximum values of the CL/CD for the three cases were obtained at α=12o. Thus, the optimum angle of attack is 12o. Hence, the design parameters of the different hydrofoil-slat arrangements are CL/CD =24.9 and 26.24 for slat Naca 97 and Naca 22 respectively. Without slat, hydrofoil gives the maximum value of CL/CD is about 13.32. Thus, using leading edge slats, NACA 97 or NACA22 increase the design values of CL/CD from 13.32 to 24.9 and 26.24 which represent about % 95 increases. Comparing results, it can be said that hydrofoil with leading edge slat arrangement, NASA 2415-NASA22, gives highest performance. Maximum lift coefficient for this arrangement, CLmax=2.2 was obtained at the angle of attack, α=20o and then hydrofoil became to the stall condition.

. The hydrodynamic performance of the hydrofoil-slat arrangement, NASA2415-NASA97 at the optimum angle of attack, α=12o are presented in Table 2. Design parameters for this arrangement are, angle of attack αopt = 12o, slat angle δopt=24.5o, CLopt=1.49, CDopt=0.059 and Cl/CDopt=24.9 (or CD/CLopt=0.040).

TABLE I. HYDRODYNAMIC PERFORMANCE OF HYDROFOILS WITH AND WITHOUT SLAT ARRANGEMENTS AT δ = 24.5O, H/C1 = 0.165 AND RE =

2.53X105.

Angle of attack, αααα (o).

CL/CD-with slat-NACA 97

CL/CD-with slat-NACA 22

CL/CD-without slat

0 3.96 4.1 7.12 6 15.13 16.79 21.16 8 21.26 25.54 18.74

10 23.21 25.94 15.89 12 24.9 26.24 13.32 14 23.11 23.68 6.93 16 20.59 22.76 --- 20 17.98 19.22 ----

TABLE II. HYDRODYNAMIC PERFORMANCE OF NACA 2215-NACA 97 ARRANGEMENT AT α = 12O, RE = 2.43X105

h/c1 Slat angle, δδδδ (o)

CL CD CL/CD

0.135 37 1.046 0.093 11.3 0.15 37 1.0802 0.08939 12.08 0.165 22 1.4603 0.05993 24.37 0.165 24.5 1.49 0.05985 24.9 0.165 27 1.4445 0.05978 24.16 0.165 29.5 1.385 0.06153 22.51 0.165 32 1.3141 0.0656 20.03 0.165 34.5 1.2237 0.0732 16.72 0.165 37 1.1037 0.0876 12.6 0.165 39.5 0.9378 0.1139 8.234 0.18 37 1.046 0.08852 12.5

The aerodynamic performance of the hydrofoil with

leading edge slat seemed to be not changed in the range of the Reynolds number considered. Some representative results of pressure and velocity distributions around and pressure coefficient variation over the hydrofoil with leading edge slat arrangement are presented in Figure 3. As seen in Figure 3a, pressure reaches a minimum values about -3.62 kPa in the upper chamber and maximum values about 1.41kPa in the lower chamber regions. Looking the pressure coefficient distributions in Figure 3c, each blade has very important contributions to the lift forces on the hydrofoil with leading edge slat arrangement. Velocity distribution around the blade shows that maximum velocity 2.81 m/s (with respect to the current velocity of 2 m/s) appears to be over the mean hydrofoil and slat. There are not any separation region at these geometrical and flow conditions.

The power coefficient or efficiency of the hydrofoil-slat arrangement can be determined form the formula,

( )( ) ( )[ ] rrLDrrP dCotCCCosSinSinCosSinCk

λλϕϕλϕϕλϕϕλ

λ

λ

222

18

−+−= �

where λr and λ = 5.0 are the local tip and tip velocities, respectively. Using design parameters, CD/CL = 0.040, CL = 1.49 and CD = 0.059, the efficiency is found to be 0.48.

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Thus, using hydrofoil –slat arrangement in wind and hydrokinetic turbines can have certain advantages to generate electrical power at moderate wind velocities and low fluid currents.

IV. CONCLUSIONS

Flow and performance characteristics of the hydrofoil-slat arrangement has been computationally investigated in the range of the Reynolds number from 1.26 x105 <Re<2.53x105. The commercial code FLUENT with Spalart-Allmaras, k-ε turbulent model, was used in the numerical analysis.

Using the hydrofoil-slat arrangement, the maximum lift coefficient generated increases from CLmax = 1.369 to CLmax = 2.200, giving a lift coefficient increase of _ΔCLmax =0.831. The delay in stalling is illustrated as the angle of attack increase from �s =14� to �s =20�, i.e. an increase in stalling angle by Δ�s = 6�. Design parameters are CL= 1.49, CD = 0.058 and CD/CL= 0.040.

Using the hydrofoil-slat arrangement in wind and hydrokinetic turbines, minimum wind velocities and hydrokinetic current velocities to produce economical electric energy will be 3- 4 m /s for wind turbines and 1-1.5 m/s or less for hydrokinetic turbines. Consequently, the wind and hydrokinetic energy potentials of countries will be re-defined and their economic and practical potentials will increase accordingly.

ACKNOWLEDGMENT

Authors would like to thank to the Scientific and Technological Research Council of Turkey for funding this project.

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[12] M.J. Schuldenfrei, Wind-tunnel investigation of an NACA23012 airfoil with a handley page slat and two flap arrangements. NACA ARR (Wartime Report No. L-261), Washington, DC, 1942.

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Figure 3. Pressure distribution (a), velocity distribution (b) and pressure coefficient variation (c) for the hydrofoil-slat arrangement (α=100, δ=190,

h/c1=0.165, Re=2.53x105)

(a) (b)

(c)

Location (m)

CP

Lower Surface

Upper Surface

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