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2006-01-2905

Design of Chassis and Aerodynamic Improvements of a Solar Energy Powered Vehicle

Nelson Garcia Assistant Professor. Simon Bolivar University

Javier Palencia Assistant Professor. Simon Bolivar University

Jorge Clarembaux Undergraduate Student. Simon Bolivar University

Yoana Milazzo Undergraduate Student. Simon Bolivar University

Copyright © 2006 Society of Automotive Engineers, Inc

ABSTRACT

The main objective of this research is the aerodynamic study of a high efficiency chassis of a solar propulsion vehicle. Experimental and numerical methodologies were used. The experimental methodology was based on profile studies in a subsonic wind tunnel using a 1:12 scale model and wind velocities untill 35 m/s. The numeric study used a 1:1scale prototype. A computational tool that simulates CFD ambient was used. The validated mesh, contains 577969 elements and 135212 nodes. Visualization techniques were used to study the current lines and detachment areas, corresponding these with the numeric study. Also the aerodynamic drag contribution of the driver cabin and the drag variation due to floor effect was studied.

SCOPES

The goals to achieve in this investigation work are:

• Determine the behaviour of the aerodynamics coefficients of a model in order to calculate the forces on it.

• Analyze scale effects like: limitations of the tunnel area, use of a ground plane fixed and the variation of Reynolds number.

• The aerodynamic drag contribution of the driver cabin.

• Study of the pressure drag through the (momentum method) wake and visualization of the airflow.

LIST OF SYMBOL AFm = Model frontal area AFp = Prototype frontal area b = Prototype width ; 1,8m c = length of the prototype; 5m

CD = Drag coefficient

CL = Lift coeffcient CM = Moment coeficient D = Drag force

h = Gap model-ground plane L = Lift force M = Momentum ρ = air density

Re = Reynolds number V = air velocity

INTRODUCTION

The increasing interest in the use of clean energies, has motivated to a group of the Simón Bolivar University, to be interested in the use of the solar energy like alternative source of vehicles propulsion. On this way, the participation in the competition "Formulates Sun Grand Prix 2006-Texas" is proposed. In it a group of conditions are settle down to propose the design of an driven automobile impelled by solar energy. The first estimations made according to the desired necessities to cover during the competition, presented a tendency to construct to vehicles with an 5 HP electrical motor and a 350 kg weight, at a 22 m/s cruise speed. The obtained results were directed to consider the aerodynamic behavior of the vehicle USB-Solar 2006, specifically focused in areas like velocity profile in different positions; zones of the flow stagnation; flow deattachment and vorticities generation; characterization of the prototype wake at different experimental speeds; pressures profile produced under the prototype; verify the similarity of the flow lines between the wind tunnel model and the prototype used in CFX-5.6TM. The importance of this project is to promote ideas oriented to the take advantage of alternative energies and design oriented to environment conservation, using the solar energy for the propulsion of the vehicle. Taking into

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account that this one is a resource that has taken more and more importance in investigation subjects in our country and the world.

This way the participation in the competition “Formulates Sun Grand Prix 2006-Texas” is proposed. In this a series of conditions are settles down to design an driven automobile impelled by solar energy [1]. The first estimations made according to the necessities to cover during the competition, presented a tendency to construct to vehicles with an electrical 5 HP motor and a 350 kg weight, at a 22 m/s cruise speed. This way a study was made previously to determine the size of the chassis of the vehicle, so that it adapted to the established regulation.

The analysis of the characteristics of several aerofoil profiles was made to adapt them to the adjustable body of the chassis, looking for the lowest drag coefficient [3]. The dimensions of the vehicle are 5.0x1.8x0.96 m of length, wide and high respectively. Therefore a model on scale 1:12 was used to be adapted to the wind tunnel (opened cycle, constructed by the company "Plint & Partners" in Berkshire, England) at the Fluid Mechanics Laboratory in Simón Bolívar University. Based on a study of similitud [4]; that allow to validate a computatinal model of the prototype on 1:1scale by the use of Computacional Fluid Dynamics.

The drag, lift and moment aerodynamic forces, as an airplane, have influence in the automotive vehicles since this faces up an airflow which changes according to its velocity. Nowdays, it is been used in the automotive field in order to improve the performance in vehicles, like the motorcycles of high performance used either in racing or private use he great automovilistic racing competitions like Formule 1, look for in theirs designs low the drag (D) and get a negative lift (downforce) that improves the traction of the wheels [1].

Figure 1. Chassis of Solar Car Prototype on selected airfoil.

EXPERIMENTAL METHODOLOGY

For the aerodynamic study, we used a subsonic open-circuit wind tunnel. Because of its dimensions, it was necessary to do a similitude analysis in order to get the dimensions of the model. The project included: the developing of the experimental assembly and the obtention of the characteristics curves. The values of drag, lift and moment coefficients were quantified, definying them as:

AV

DCD2

21 ρ

= (1)

AV

LCL2

21 ρ

= (2)

LAV

MCM

*21 2ρ

= (3)

The figure 2 shows the drag, lift and moment forces on the prototype.

Figure 2. Forces actuating on the prototype.

WIND TUNNEL. The experiments are made in a

subsonic open-circuit wind tunnel which it can reach velocities between 30 and 35 m/s and has a test section of 120 cm length and 45x45 cm transverse area. It has a balance of three components that allows to measure forces in horizontal and vertical direction (lift and drag), besides the par (moment) in the length plane symmetry of the model. A study of the variation of the velocity was done[2] with regard to the average velocity of the test section, it can be observed how the difference of velocity is not more than 0.25%. SIMILITUDE ANALYSIS. The similitude analysis included Geometric and Dynamic similitude. The geometric similitude is related with the geometric similarity that it must exist between the model and the prototype. The dynamic similitude must guarantee that the relation of forces which act on the model are the same proportionaly to the forces acting on the prototype. Due to the characteristics of the airflow (density, cinematic viscosity) and the ranges of velocity where these tests are made, the use of Reynolds number would be the adequate to do the study of dynamic similitude. With regard to this, the drag coefficient tends to have little variation, being in some cases independent of the Reynolds number from Re=2*105 to Re=6*105[3] approximately. For this study Re of the model is between 103 and 104 while is 105 in the prototype. Another factor in this type of tests is the flow blockage produced by the walls of the tunnel, causing mistakes in the results. To reduce the blockage effect of the walls, the model must be the smallest possible. However, this would bring another consequences, as getting far of the real Reynolds number and have a model with less details. So, there is a compromise between the accuracy level of the model and the features the wind tunnel has. Because of this, to get good results it is recommended that the relation between the frontal and transverse area of the test section

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does not exceed 7,5% [4]. To fulfil this requirement, a scale factor of 1:12 was taken, what gets a relation of tunnel-model areas of 7,46%. According to this scale, the model has 416 mm length, 150 mm width and 80 mm height with a frontal area 0.01518 m2 roughly. The blockage effect overestimates the aerodynamics coefficients values. Therefore, it exist a correction factor (FC) that involves the model area and the transverse section of the wind tunnel (At).

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

⎟⎠⎞

⎜⎝⎛ +

= 2

*411

1

AtAm

FC ( 5 )

The correction factor applied was 0.95. VISUALIZATION. The fluorescent tufts and mini-tufts visualization techniques permit to see the surface stream lines and the potential presence of vortices. The tufts are sticker to the model and are made highlighted by the fluorescent light [6]. In this case we used a tuft of 0.16 mm nylon thickness.

EXPERIMENTAL ASSEMBLY

MODEL. The wind tunnel used in the experiments it is designed to study airfoils and not other kind of models. So, the model needed to be attached like an airfoil did. The main goal is that the model has the major geometric accuracy possible, mainly in the frontal area since this is which faces up the airflow directly. The construction was made of several materials: wood, plaster, plastic putty, copper, and others.

GROUND PLANE. The use of an elevated ground plane is necessary to simulate the real conditions of the study. To simulate the ground, it was used an elevated ground plane of the kind flat plate. This one is easier to build than the boundary layer suction grounds or a moving ground and has an acceptable performance if it is studied the boundary layer effect. Moreover, The gaps between the ground plane and the model (h= 1, 3 mm) are done in order to determine the effect that produces the boundary the ground plane fullfilled the next characteristics: enough length and width dimensions in order to avoid the effect of its edges on the airflow around the model, heigth adaptation, smooth finish

Figure 3. Assembly of the model in the wind tunnel. A) Model-Balance attach, B) Ground plane and C) Static

pressure lines.

(in order to get a thin boundary layer), stable behaviour facing the airflow, totally flat an easy to attach to the wind tunnel. The measures of the ground plane are: 600 mm length, 270 mm width and 152 mm heigth [7]. In the Fig. 4, it can be observed the final assembly in the test section of the wind tunnel.

EXPERIMENTAL TESTS

The study done included the next tests:

• Test where it is changed the gap between the ground plane and the wheels of the model. (Fig. 4 y 5).

• Tests done with and without cabin (Fig. 7). • Test with a wake rake behind the model to validate

with numerical prototype (Fig 8) . • Visualization of the surface streamlines.

Figura 4. Gap of ground plane-model h.

layer in the measurement of the variables and to know the error that this can lead up [8], besides taking into account that it is impossible stick the model to the ground plane since the balance could not measure the forces.

RESULTS

The errors obtained in the experimental study are quantified using two methods: error propagation and standard deviation. The error propagation gives the error produced by the precision of the instruments used during the data acquisition and it is calculated by the standard propagation error. Otherwise, the standard deviation shows the dispersion data got in each test. Like dispersion error is defined once the standard deviation that corresponds to a confidence level of 91%[9]. In this way we get a maximum error in the values of CD=±0,11 CL=±0,24 y CM=±0,10.

In order to use in the experimentation a model study with and without cabin was defined, with the purpose of perceiving the note the aerodynamic differences when the cabin is used.

Additionally, two separation distances between the artificial ground and the model of h=1 mm and h=3 mm was settled down. The distances established between the wheels and the artificial ground have great influence on the results, because the components originated by development of the boundary layer in the artificial ground and the model are not the same that the ones for the prototype of the solar car. By this reason the separation between the wheels and the ground differs in how the model is submerged in the thickness developed by the boundary layer of the ground. In this way the next graphics are presented with the pertinent analices.

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Fig 5. (Above) Experimental Model without cabin. (Down) Assembly in the wind tunnel of the experimental

model with cabin

The curves tendency of the drag coefficient referred to the Reynolds number is related to the exposed surface and the form of the car, approximately the surface of the model without cabin is 1,426 cm2 and with cabin 1,464 cm2. This difference is one of the reasons for which the CD is greater in the second configuration that in first, because it increases the drag due to the skin friction effect. Another reasons of the drag force increases in the configuration with cabin is the fact that just in front of it a high pressure zone is generated because the air flow is faced with an unexpected wall and reduces its speed, behind the cabin the flow becomes to accelerate and this differential pressure generates a drag force additional. Additionally to the previous effect, the flow disturb due to a cabin, originates different conditions at the exit of the car; now the wake that could leave before this situation has a greater effect due to the flow separation of the car surface earlier than is espected and the vortices generated due to the low pressure causes a i drag increase. For a certain body there are a variety of flow situations that depend on the Reynolds number, also note in the fig 6 that for a case of 3 mm separation with the floor, exists a point in which the drag coefficient CD is greater than for 1 mm case, this change is due to conditions of the flow. The existence of a remarkable difference between the distances of the wheels to the ground and the drag, is due to the reduction in magnitude of the distance, increasing the cut force because the flow is now through a smaller area and in addition the boundary layers created by the model as by the plate disturb the flow still more.

Fig 6. Drag force vs. Velocity at different hegth

Once again its verifies that the drag is proportional to the square of the speed with the tendencies obtained in the fig 12 and 13, in which the adjustments have a 99% of reliability, this is because the values of the drag coefficients obtained are closer. The greater coefficient of reported drag was of 0.0232, and corresponds to a terminal velocity condition (V=25 m/s), with smaller wheels distance to the ground (h=1 mm) and with the cabin, with a air density of 1.123 Kg/m3. At this condition with Ec. (1) in conditions of race a drag of 7.5 kg could exist on the prototype and they are equivalent to a consumed power of 2.47 HP. Studying the conditions without cabin, the force is now almost 4.7 kg, so, the cabin contribution is 2.8 kg (0.9 HP).

Fig 7.Lift vs. Velocity with cabin..

Respect to the sustenation the results were hoped. To smaller separation distance between the inferior surface of the car and ground, the positive sustenation becomes minor and is because the flow is accelerated by the reduce area, causing a pressure fall that diminishes the force, but that nevertheless is very small to avoid that the car rises. On the other hand,with referente to the differences in the sustenation of the two configurations of body proposed, is observed that when the cabin is present the sustenation becomes a little greater than without it, is due the the speed increases in the flow, and when increasing the speed

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diminishes the pressure in the superior part causing that the car experiences a vertical force in opposite direction of the gravity. This increase in the lift force is directly related to the increase in the induced drag, because it allows that the boundary layer is dettached more easily of the car surface. In order to compare the magnitude of the lift force between the model and the prototype in a race conditions, the coefficient CL correspond to the greater registered speed, the smaller separation between the wheels and the floor and a configuration with cabin. The data to consider were the following ones:

Table 1. Variables to find maximun lift

Velocidad [m/s] 25

CLmodelo 0,1436 With the previous values it is replaced in the Ec. (2), obtaining itself that the force is equivalent to 46 kg that are being diminished to him to the gross weight of the car. The importance of this one force is not if it is the relation between her and the gross weight of car is small, because even being thus, the behavior of the prototype can be seen affected before a relative wind considered, because the location of the centre of pressure is placed in front of the center of masses in almost 1.24 m/s and to 1.06 m/s of the beginning of the prototype, which does not confirm the fact that the car can raise the end, nevertheless, is due to clarify that the calculation of the previous force is related to the superficial area instead of the frontal. With the previous values it is replaced in the Ec. (2), obtaining that the force is equivalent to 46 kg, being those diminished to the gross weight of the car. The importance of this force is, not if it is the relation between it and the gross weight of car is small, even beinglioke this, the behavior of the prototype can be affected when faced a relative wind not considered, due to the location of the centre of pressure is placed in front of the center of masses in almost 1.24 m and to 1.06 m of the front of the prototype.This not confirm the fact that the car can raise, nevertheless, there are the necesity of clarify that the calculation of the previous force is related to the superficial area instead of the frontal. COMPARATIVE STUDY. The numeric study used a 1:1scale prototype. A computational tool that simulates CFD ambient was used. The validated mesh, contains 577969 elements and 135212 nodes. As the variables are dependant, the location of the lift force in front of the center of masses is related to the distribution of pressures found. As it were observed in the graphs of pressure against distance, the greater tip is positive, and it is like a pressure is being exerted on the model in opposition to the gravity. The pressure depends directly on the force but he is inversely proportional to the area, whit this last one constant parameter, then it depends solely on the force (Lift force).

Fig 8.Pressure variation under the solar car, numerical and experimental with cabin

The reason for the reduction of the speed (increase of the pressure) is due to the effect that has the artificial ground in the experimental assembly, that is to say, the air in its route through several stages, first with the aluminum lamina, the flow that crosses are not dettached the lamina because the sharping leading edge and the rugthness was diminished when polishing the plate. At the second stage the air must be divided in two parts, one will cross superior half and the other inferior half of the model, being the cross-sectional section of inferior half limited by the presence of the flat earth and the curvature of the profile, the air faces a front, reduces its speed and affect the area where a gradient of thickness with respect to the percentage of the cord is greater than the one that is located until 15% of the chord lenght, the reason: the curvature of the leading edge. Third stage characterizes by diminution of cross-sectional section and air acceleration, it causes that the suction pressure stays constant thanks to the form of the selected profile, finally the flow that crosses the inferior part of the model is obtained with an increase of the pressure by the diminution of the speed. On the other hand, the presence of the back wheel indicates the sudden increase of the pressure but, in absence of it the curve would stay in negative values until arriving at a point where the area causes the deceleration of the flow (approximately to 86% of the length overall of the model), hoping that this increase in the pressure is much smaller to the one of the maximum value and that the reduction is near zero, indicating that the pressure in the final zone of the car is the atmospheric one, being able to predict a typical wake of a configuration of aerofoil profile.

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Fig 9. Wake behind the model, experimental and numerically.

Fig. 10 Upper view: results of numerical simulation and visualization with black light.

Fig. 11 Side view: results of numerical simulation and visualization with black light.

CONCLUSIONS

The experimental study allowed to know the aerodynamics characteristics of the prototype through similitude analysis.

The most remarkable effect to consider in later studies, is the diminution of the positive lift force, modifing the inferior part of the body in the sense to diminish the gradient of variation of thickness with respect to the cord in the front part.

The calculated drag coefficient is small in comparison to other automobiles, is not due to forget that it has not been considered the effect that have revolving wheels on the behavior of the flow in the inferior part of the car.

The comparison of the pressure profile between the experimental datas and those of the numerical simulation show a very significant similarity, because the behavior of variation of the pressure as much on 1:12 scale was practically reflected in the wind tunnel, like a scales 1:1 of the prototype in CFX-5.6TM.

Comparing the behavior of the obtained streamlines in the wind tunnel through the black light, and the images made in the simulator, its verified that the similarity between both was highly significant. REFERENCES

[1] Potter, Merle C. y D.C. Wiggert, 1997, “Mecánica de Fluidos”.Prentice Hall, México. Segunda Edición. pp 243-244.

[2] Atramiz, E., Tesis de Grado de Ingeniería Mecánica,

2004, “Estudio experimental de la aerodinámica de un carro tipo fórmula”. Universidad Simón Bolívar.

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[3] Munson, Young. 1998, “Fundamentals of Fluid Mechanics”. Jhon Wiley, E.E.U.U., pp 610-714.

[4] Katz, Joseph, 1995, “Race Car Aerodynamics:

Designing for speed”. Robert Bentley Automotive Publishers, E.E.U.U., pp 73-74, 81-84 .

[5] Rae, William H. Jr and Pope, Alan, “Low-speed Wind

Tunnel Testing ”. Jhon Wiley, E.E.U.U., pp 213-217. [6] Atramiz E., García N., Díaz S., 2004, “Estudio

Experimental de la Aerodinámica de un Carro Tipo Fórmula”. Memorias del CONIM, pp 157-162.

[7] Melo M., Díaz S., Moreno N., García N., Bastardo R.,

2004,“Estudio del Efecto Suelo de un Vehículo Tipo Formula a través del uso de la DFC”. Memorias del CIMENICS, pp MF-77 al MF-84.

[8] Hall, Carl W. 1977, “Errors in Experimentation”.

Matrix Publishers, E.E.U.U., pp 25- 34 .

[9] Boschetti P., Cardenas E., García N., 2004,“Experimental Aerodynamic Study of UAV Wind Túnel Model for Low Reynolds Number”. Proceeding of 22nd Applied Aerodynamics Conference, AIAA2004-4969.

AUTORS

Nelson R. García P. is an Assistant Professor at Simon Bolivar University in Caracas, Venezuela. He is an Applied Aerodynamic researcher at Fluid Mechanics Laboratory and industrial consultant.

Laboratorio de Mecanica de Fluidos. Universidad Simon Bolivar. Caracas, Venezuela. Apdo 89000. Phone: +58-212-9064139. E-mail: ngarcia@usb.ve

Javier A. Palencia C. is an Assistant Professor at Simon Bolivar University in Caracas, Venezuela. He is an researcher at Mechanical Energy Convertion Laboratory

Mechanical Energy Convertion Laboratory. Universidad Simon Bolivar. Caracas, Venezuela. Apdo 89000. Phone: +58-212-9064134. E-mail: jpalenci@usb.ve