increasing granular ow rate with obstructionsquantumfi.net/papers/murray2016increasing.pdf ·...

Papers in Physics, vol. 8, art. 080003 (2016)

Received: 22 November 2015, Accepted: 19 February 2016Edited by: L. A. PugnaloniReviewed by: C. M. Carlevaro, Instituto de Fısica de Lıquidos y Sistemas Biologicos,

La Plata, Argentina.Licence: Creative Commons Attribution 3.0DOI: http://dx.doi.org/10.4279/PIP.080003

www.papersinphysics.org

ISSN 1852-4249

Increasing granular flow rate with obstructions

Alan Murray,1 Fernando Alonso-Marroquin2∗

We describe a simple experiment involving spheres rolling down an inclined plane towardsa bottleneck and through a gap. Results of the experiment indicate that flow rate can beincreased by placing an obstruction at optimal positions near the bottleneck. We use theexperiment to develop a computer simulation using the PhysX physics engine. Simulationsconfirm the experimental results and we state several considerations necessary to obtain amodel that agrees well with experiment. We demonstrate that the model exhibits clogging,intermittent and continuous flow, and that it can be used as a tool for further investigationsin granular flow.

I. Introduction

When does an obstruction placed near a bottleneckincrease the flow rate of discrete objects movingthrough the bottleneck? Answering this questionis of great utility on many scales. For example, thesafe evacuation of pedestrians moving through con-fined environments (train stations, stadiums, con-cert halls, etc) in emergency situations can be ofvital importance. And grain falling through a silo,where efficient flow is necessary for production andclogging is undesirable, is another example.

As stated by Magalhaes et al. in Ref. [1], ‘Gran-ular materials are ubiquitous either in nature orin industrial processes’ and so a fundamental un-derstanding of their motions is of intrinsic inter-est, both from physics and engineering perspec-tives. The quantitative study of the flow of granu-lar materials has been performed for many decades[2], yet there is enormous scope for many interest-ing and creative techniques to be developed: exper-

∗E-mail: [email protected]

1 SAE Creative Media Institute, Sydney, Australia.

2 School of Civil Engineering, The University of Sydney,NSW 2006, Australia.

imental, theoretical and computational. Granularflow around an obstruction placed near a bottleneckis of particular interest as it has many potential ap-plications and this has been investigated by manyauthors [1, 3–7].

Increases in flow rate achieved through place-ment of an obstruction near a bottleneck have beenreported by several investigators [8–10]. Anothercommonly reported phenomenon is that of ‘clog-ging’, also described as the formation of arches [4].There are also many novel investigations of gran-ular flow. For example, Wilson et al. [3] considergranular flow of particles that are completely sub-merged under water and Lumay et al. [7] investi-gate the flow of charged particles that repel eachother. Clearly, granular flow is a very wide andactive area of research.

Several authors have investigated the use ofphysics engines used in games, an example of whichcan be found in the work by Carlevaro and Pug-naloni [15]. In this paper we present a real time,3D computer simulation using the PhysX physicsengine. The model can be used in the study ofgranular flow as it exhibits continuous flow, inter-mittent flow, clogging, and has several parametersthat an investigator can control as the simulation

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Papers in Physics, vol. 8, art. 080003 (2016) / A. Murray et al.

is occurring. The computer model is based on anexperiment that uses spheres for the particles andthree different obstruction shapes. We use exper-imental data to calibrate and validate our model.The model can then be used to determine how thehighest flow rates may be achieved both with re-gard to obstruction position and to find optimalobstruction shapes.

The paper is organized as follows: we present themodel in section II. Then we describe the experi-ments in two parts. Section III deals with flow ratewithout obstructions, and section IV investigatesflow rate with obstructions. We discuss the limita-tions in section V, and conclude in section VI.

II. The Model

We use PhysX [12], a freely available real time, 3Dphysics engine for computations. PhysX is widelyused in the games industry as, when necessary, itfavors speed and stability of computation over ac-curacy. Integration of the equations of motion isdone using an unconditionally stable, semi implicitEuler scheme yielding algebraic equations that aresolved using a progressive Gauss-Seidel algorithm,whilst enforcing the Signorini conditions [16]. Thetime step can be adjusted for greater accuracy atthe cost of speed and hence, real time performanceof the engine. For this study, we left the time stepat its default value of 0.02 seconds.

PhysX uses the Coulomb model for friction andrestitution is a measure of the loss of kinetic energybetween colliding particles. Details and further ref-erences about the rigid body system and frictionmodel used in PhysX can be found in the work byTonge [16].

The model incorporates the following parame-ters which are freely specifiable and chosen to agreewith experimental results: friction, restitution, andsphere diameter. In addition, our model allows usto vary the angle of inclination of the plane, the gapwidth and the distance from the obstruction to thegap. We incorporate imperfections in the spherediameters by allowing them to vary by up to 5%.The model uses 1551 particles whilst ≈ 1000 parti-cles were used in the experiment. Each simulationis performed 20 times.

Unity [13], a freely available game engine, is usedas our programming interface to PhysX. We also

use Blender [14], a freely available 3D modelingpackage for modeling the obstructions.

III. Flow rate without obstructions

The basic set up of the experiment with obstruc-tions is shown in Fig. 1(a), where particles arereleased from above the obstruction. The parti-cles themselves are spheres with diameters: 6.15mm, 7.75 mm and 11.9 mm, referred to as ‘small’,‘medium’, and ‘large’ respectively, and are shownin Fig. 1 (b). Note that without obstructions, theparticles are released at the gap as shown in Fig.1(c).

Figure 1: (a) Basic set up of the experiment withan obstruction. (b) Particles are spheres of threedifferent diameters. (c) Experiment of flow ratewithout obstructions, the particles are released atthe gap.

We use an inclined plane that makes an angleof 8o with the horizontal. The plane has retain-ing walls, walls that form a bottleneck and releasebarriers as shown in Fig. 1(a). We pack a knownnumber of spheres of a specified size, shown with-out texture in Fig. 1(b), onto the plane and releasethem from rest so that they collide with the bottle-neck walls, and flow through the gap. The walls ofthe bottleneck make an angle of 30o to the retainingwalls.

Under gravity, the spheres move down the in-clined plane, come into contact with the bottleneckwalls and flow through the gap. The number of par-ticles is known and the time taken for all particlesto flow through the gap is measured, from which the

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flow rate, J , is determined. The flow rate is thusdefined as the number of particles passing throughthe gap per second.

The experiment is repeated for various gapwidths, with the corresponding flow rates mea-sured. The entire experiment is performed individ-ually for small, medium, and large spheres. All di-mensions (particles, plane, bottleneck angles, etc)are known, as are the coefficients of friction andrestitution of the particles. As per the experiment,in our model we release the particles at rest fromthe gap as shown in Fig. 1 (c).

It was found that there is a relatively sharp rise inflow rate as the gap is increased. It is precisely thisrange of distances where the flow rate transitionsfrom clogging to intermittent to continuous flowrates. The model also exhibits clogging as shownin Fig. 2 (a), and intermittent flow. Therefore, inorder to get reliable and reproducible flow rates, wedecided to perturb the particles just sufficiently toprevent extended periods of clogging which therebyaffect flow rates. As described by Garcimartin etal. [11], the problem of clogging can effectively beeliminated by applying external vibrations.

Figure 2: The model exhibits clogging (a) at thegap and between the obstruction and the bottle-neck walls (b), consistent with the experiment.

In a similar way, we use the technique of ‘shak-ing’. In the case of flow rate without obstructions,the bottleneck walls were shaken as shown in Fig.3(a) from a maximum of 0.5 mm diminishing tozero over one second, every five seconds. The pa-rameters that define a shake are: shake amplitude,direction of shake, shake duration and time betweenshakes. These parameters can be set arbitrarily inthe model and, whilst other times and distanceswere tested, the above were found to be just suffi-

Figure 3: (a) directions of shaking of the bottle-neck walls in the experiment of flow rate withoutobstructions. (b) directions of shaking in the ex-periment of flow rate with obstructions. Shaking isnecessary to avoid clogging between the obstructionand the bottleneck walls. (c) obstructions used inexperiments. The V obstruction is the A obstruc-tion rotated through 180o.

cient to yield reproducible results.

We varied the values of coefficient of friction andcoefficient of restitution in the model until we gotgood agreement with experimental results. It wasfound that varying the coefficient of restitution hadlittle to no effect on flow rate.

We found a coefficient of friction of 0.24 yieldedmodel results that gave good agreement with ex-perimental results. The experimentally measuredvalue of the spheres was found to be 0.28±0.4. Theresults for each diameter sphere are shown in Fig.4. The model results agree with the experimentalresults quite well.

IV. Flow rate with obstructions

The experiment is repeated with medium spheres(of diameter 7.75 mm). However, this time, ob-structions (shown in Fig. 3 (c)) are placed at vary-ing distances away from the gap, which is now fixedat a width of 3.3 cm.

It was found that there was a relatively sharprise in flow rate as an obstruction is moved away

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Figure 4: Flow rate without obstruction versus gap in dimensionless form. Comparison of experimentwith simulation using spheres of three different diameters. J is the flow rate, g is the acceleration dueto gravity, d is the particle diameter and W is the gap width.

from the gap. It is precisely this range of distanceswhere the flow rate transitions from clogging to in-termittent to continuous flow rates. The model alsoexhibited clogging as shown in Fig. 2(b), and in-termittent flow. Therefore, in order to get reliableand reproducible flow rates, we decided to perturbthe particles just sufficiently to prevent extendedperiods of clogging which thereby affect flow rates.

With the gap set at 3.3 cm, no clogging was ob-served for spheres of 7.75 mm at the gap. However,with the presence of an obstruction, clogging wasobserved in areas between the obstruction and thebottleneck walls as shown in Fig. 2(b) and Fig.3(b). To get consistently measurable flow rates inthe model, once again, we decided to perturb thesystem using the technique of ‘shaking’, describedabove. This time, we decided to shake the obstruc-tion also shown in Fig. 3(b). The obstructionwas shaken by a maximum of 1 mm diminishingto zero over one second, every five seconds. Thefreely specifiable parameters that define a ‘shake’are the same as those for shaking the bottleneckwalls. Figure 5 shows the maximum flow rates forthe three obstructions, where the ‘waiting room’ ef-fect, discussed in Ref. [8], occurs for the cylinderand the V obstruction but not the A obstruction.

Figure 5: Peak flow rates with obstructions, wherethe ‘waiting room’ effect is present for the cylinderand the V obstruction but not for the A obstruc-tion.

Figure 6 shows flow rates as measured experi-mentally and with the model. Both sets of resultsindicate increases in flow rates for certain ranges ofdistance of obstruction to the gap. For the model,each graph shows error bars and data that weremore than two standard deviations away from themean were not included in the statistical analysis.This was necessary as, even with the shaking tech-nique described above, the particles did occasion-ally clog for extended periods of time, thereby af-fecting the flow rate measurements.

Clearly, the model shows all obstructions im-

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Figure 6: Results of flow rate with obstructions versus distance in dimensionless form. J is the flow rate,g is the acceleration due to gravity, d is the particle diameter, and D is the distance from the obstructionto the gap. The experimental results in (a) show that each obstruction has a range of distances whereflow rate is greater than with no obstruction. The numerical results (b-d) show that the model alsopredicts that each obstruction has a range of distances from the gap where the flow rate is greater thanflow rate with no obstruction.

prove flow rate for a range of distances, and reas-suringly, the further the obstruction is moved awayfrom the gap, the closer we get to flow rates withoutobstruction.

In the case of the cylinder, due to its shape andsize relative to the bottleneck, we could only startat distances ∼ 7 cm from the gap. We see thatthere is a steady rise in flow rate as the cylinder ismoved further away from the gap. A maximum flowrate is reached and then a slightly sharper decreaseis reached until we get flow rates the same as if noobstruction is present.

In the case of the A obstruction, due to its shapeand size relative to the bottleneck, we were able toplace the obstruction very close to the gap, whichis why distances start at ∼ 0.4 cm from the gap.We see a sharp rise in flow rate as the obstructionis moved away from the gap, and then we see aleveling out of the flow rate over a range of distancesfrom 0.8 cm to 2.8 cm. There is a small peak in thisrange, but we consider this a statistical fluctuation.

Moving further away, from 2.8 cm to 3.2 cm, we

see a sharp decrease in flow rate to values that areactually below flow rates with no obstacle. This isan intriguing phenomenon, showing what we mightexpect: obstructions decrease flow rates.

The flow rate remains constant to a distance of3.6 cm and then starts rising. At a distance of 4cm, the flow rate is approximately equal to thatwith no obstructions.

In the case of the V obstruction, due to its shapeand size relative to the bottleneck, we could onlystart at distances ∼ 6 cm from the gap. We see thatthere is a steady rise in flow rate that is slower thanthat of the A obstruction, as the V obstruction ismoved further away from the gap. A maximum flowrate is reached, higher than that with no obstruc-tion, and then a decrease occurs until we get flowrates the same as if no obstruction were present.

V. Limitations of PhysX engine

We observed that the model exhibits continuousflow, intermittent flow, and clogging, all phenom-

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ena that have been identified and observed experi-mentally. The model was quite sensitive to the ab-solute sizes of the geometric structures used. Thisis true for all physics engines, as they have to be‘tuned’ to the range we would like them to workmost accurately in. For example, an engine mightbe tuned to work best with objects whose sizes areof the order of meters but it will not work for ob-jects at the scale of millimeters. To overcome thislimitation, we ran the simulations in the dimensionswhere the performance of the engine was optimal,and then used dimensional analysis to extrapolatethe result to the real experimental conditions.

We found that the particles in the model neededto be perturbed more often than in actual experi-ments, in order to get consistent flow rates. Thiscan be attributed to a real time optimization em-ployed by PhysX, known as ‘sleeping’. When a par-ticle’s angular and/or linear velocity falls below cer-tain threshold values for more than a few frames,these velocities are set to zero and the particle goesinto a ‘sleep’ state, in which no collision detectionoccurs and hence the particle’s velocity remains atzero. The particle ‘wakes up’ when it is subjected tonet forces. The reason for this optimization is thatit relieves the processor of having to perform need-less computations especially with regard to collisiondetection, thereby allowing much larger numbers ofparticles to be present and only performing the nec-essary computations as required. For this study, weleft the threshold ‘sleep velocities’ at their defaultvalues.

In spite of these limitations, we can confirm that,as reported in various experiments, it is possible toincrease flow rate of discrete particles by placing anobstruction at a suitable location near a gap.

VI. Conclusions

We have developed a model that exhibited reason-able quantitative accuracy in the case of no obstruc-tions, and good qualitative agreement when ob-structions were introduced. As in experiments, weshowed that the flow exhibits clogging, intermittentflow rates, and continuous flow rates. The modelalso confirms experimental results that placementof an obstruction near a gap can actually increaseflow rate through the gap. Better experimentalagreement with obstructions will be possible if the

roundness of the obstructions is more accuratelymodeled.

The model is quite flexible in that many param-eters can be changed via user input, and withoutmodifying the actual code. In this way, the modelcan be used as a convenient tool to suggest furtherexperiments and allows us to investigate differentobstruction shapes, both of which may help us gaina greater understanding of granular flow.

We have also described a suite of freely avail-able realtime 3D software, which is mature and hasmany online resources in the form of documenta-tion and tutorials and is being actively developed,which can be used together to create simulations ofinterest in the area of granular flow.

Acknowledgements - We thank Ivan Gojkovicand Jaeris Wu for performing the experimental partof this paper.

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Physics-for-Game.

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increasing granular ow rate with obstructionsquantumfi.net/papers/murray2016increasing.pdf ·...

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