OPTIMIZATION OF AERODYNAMICALLYMODIFIED HATCHBACK CAR USING CFD

ANALYSIS-A REVIEW

Daivik J JoshiM.E. IC AUTO, PIET, VADODARA

daivikjoshi@gmail.com

ABSTRACT

Currently automobile sectors are focusing on efficient, high speed and powerful vehicles. External car aerodynamicsstudy has great importance in overall car efficiency and ride stability, being a key element in successful automotivedesign. The present numerical work is intended to focus on the aerodynamic characterization of a hatchback vehicle byvarying and analyzing the parameters which have a considerable effect on aerodynamics of car. The parameters likecoefficient of drag and lift, pressure distribution and resistance on surface of car, wake formation and airflow separationare taken into account. After that various modifications are done on car outer surface by using different body shapesand various parts are installed which can reduce the drag and improve vehicle’s aerodynamics. For this, computationalmethod like CFD is used to analyze the effects of the varying parameters and then optimized by installing various parts,giving better ideas and relevant and accurate results from simulations to understand these effects for furtherassessment.

KEY WORDS: . External aerodynamics;hatchback vehicle; parameters; modifications; drag;improve performance; CFD.

INTRODUCTION

Study of a solid body moving throughthe atmosphere and its interactiontaking place between the bodysurfaces and the surrounding air withvarying relative speeds and winddirection is referred to as SolidBody Aerodynamics. Vehicleaerodynamics is same as solid bodyaerodynamics as vehicle is consideredas solid body. When vehicle is movingthrough atmosphere several forces areacting over it. Those forces areoffering resistance to vehiclemotion. The forces are namely Dragforce and Lift force. Aerodynamicdrag is usually insignificant at low

vehicle speed but the magnitude ofair resistance becomes considerablewith rising speed. This can be seenin Fig. 1 which compares theaerodynamic drag forces of a poorlystreamlined and a very highlystreamlined medium sized car againstits constant rolling resistance overa typical speed range. A vehicle with

a

high drag resistance tends onlymarginally to hinder its acceleration

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but it does inhibit its maximum speedand increases the fuel consumptionwith increasing speed. Body stylinghas to accommodate passengers andluggage space (Boot space), thefunctional power train, steering,suspension, brakes, wheels etc.

Fig. 1 Comparison of low and highaerodynamic drag forces with rollingresistance. [7]

Boundary layer:

Air has viscosity, that is, there isinternal friction between adjacentlayers of air, whenever there isrelative air movement, consequentlywhen there is sliding betweenadjacent layers of air, energy isdissipated. When air flows over asolid surface a thin boundary layeris formed between the main airstreamand the surface. Any relativemovement between the main airstreamflow and the surface of the body thentakes place within this boundarylayer via the process of shearing ofadjacent layers of air.

Fig. 2 Boundary layer velocity gradient[11]

The air layer velocity from inner toouter boundary layer shows aparabolic rise When air flows overany surface, air particles inintimate contact with the surfaceloosely attach themselves so that

relative air velocity at the surfacebecomes zero as shown in above Fig.2.

Skin friction:

The skin friction is the viscousresistance generated within theboundary layer when air flows over asolid surface. Skin friction isdependent on the surface area overwhich the air flows, the degree ofsurface roughness or smoothness andthe air speed.

Air streamlines:

A moving car displaces the air aheadso that the air is forced to flowaround and towards the rear. Thepattern of air movement around thecar can be visualized by airstreamlines which are imaginary linesacross which there is no flow, seeFig. 3. These streamlines broadlyfollow the contour of the body butany sudden change in the car's shapecompels the streamlines to deviate,leaving zones of stagnant air.

Fig. 3 Streamline air flow around car[7]

Aerodynamic drag. [7, 5]

Pressure drag: When viscous air flowsover and past a solid form, vortices

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are created at the rear causing theflow to deviate from the smoothstreamline flow, see Fig. 4 (a).Under these conditions the air flowpressure in front of the solid objectwill be higher than atmosphericpressure while the pressure behindwill be lower than that of theatmosphere, consequently the solidbody will be dragged (sucked) in thedirection of air movement. Thiseffect is created in addition to theskin friction drag.

Pressure drag can be reduced bystreamlining any solid form exposedto the air flow, for instance a roundtube (see Fig. 4(b)) encourages theair to flow smoothly around the fronthalf and part of the rear before flowseparation occurs thereby reducingthe resistance by about half that ofthe flat plate. The resistance of a

tube can be further reduced to about15% of the flat plate by extendingthe rear of the circulating tube inthe form of a curved tapering lobe,(see Fig. 5(c)). Even biggerreductions in resistance can beachieved by proportioning the tubesection (see Fig. 5(d)) with afineness ratio a/b of between 2 and 4

with the maximum thickness b setabout one-third back from the nose,(see Fig. 4). This gives a flowresistance of roughly one-tenth of around tube or 5% of a flat plate.

Fig. 4 (a & b) Air flow over variousshaped sections [7]

Fig. 5 (c & D) Air flow over variousshaped sections[7]

Aerodynamic lift: [7, 5]

Lift coefficients:

The aerodynamic lift coefficient Cl

is a measure of the difference inpressure created above and below avehicle's body as it moves throughthe surrounding viscous air. Anuplift known as positive lift is

undesirable asit reduces thetyre to groundgrip whereas adown forcereferred to asnegative liftenhances thetyre's roadholding.

Vehicle lift:

The direct slower moving under-side

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and the indirect faster moving topside air-stream produces a higherpressure underneath the car than overit, consequently the resultantvertical pressures generated betweenthe upper and under surfaces producea net up thrust or lift. Themagnitude of the lift depends mainlyupon the styling profile of both overand under body surfaces, the distanceof the underfloor above the ground,and the vehicle speed.

Fig. 6 Aerodynamic lift [7]

Classification of Various AerodynamicModifications on Car body: [7]

Now a days to reduce the fuelconsumption is the primary goal inthe automotive industry due tolimited fuel resources by whichprotecting the global environment andconservation of energy resources.Reduction of drag and lift force, yawmoment, flow separation, etc. byimproving the aerodynamic shapes areessential for reduce the fuelconsumption as well as to improvedriving performance.Variousaerodynamic shapes and parts are asdepicted below:

Profile edge rounding or chamfering:

There is a general tendency foraerodynamic lift and dragcoefficients to decrease withincreased edge radius or chamfer:experiments carried out showed for aparticular car shape (see Fig. 7 (a))how the drag coefficient was reducedfrom 0.43 to 0.40 with an edgeradius/chamfer increasing from zeroto 40 mm (see Fig. 7 (b)), and therewas a slightly greater reduction withchamfering than rounding the edges;

however, beyond 40 mm radius therewas no further advantage inincreasing the edge radius orchamfer.

Fig. 7 [a, b] Influence of fore bodybonnet (hood) edge shape on dragcoefficient [7]

Bonnet slope and windscreen rake:

Increasing the bonnet (hood) slopeangle α from zero to roughly 10reduces the drag coefficient, butbeyond 10 the drag reduction isinsignificant, (see Fig. 1.15(b)).Likewise, increasing the rake angle γreduces the drag coefficient (seeFig. 8 (c)) particularly when therake angle becomes large; however,very large rake angles may conflictwith the body styling.

Fig. 8 [a, b, c] Bonnet slope andwindscreen rake angle versus dragcoefficient. [7]

Spoiler:

A spoiler helps to reduce both drag

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and lift. Also helps in maintainingcontrol over the car during highspeed cornering. Spoiler can beinstalled in front and rear of carbut generally rear spoiler iscommonly used. A rear spoiler asshown in below Fig. 9, helps inincreasing down force and maintainscars stability.

Front Bumper:

It controls flow of air around tires,thus reducing their drag. Below Fig.9 shows car’s front bumper.

Front Skirt:

Air flow around the nose is enhancedusing front skirt without reducingradiator efficiency. Car with frontskirt is shown in Fig. 9 below.

Fig. 9 showing various parts installed in car for improving aerodynamics of car [8]

Side Skirt:

It prevents under body airflow fromdisturbing side airflow and reducesdrag. Side skirt is shown in aboveFig. 9.

Rear Skirt:

It prevents airflow at rear end ofcar and reduces wake turbulence. Carwith rear skirt is shown in above

Fig. 9.

Flat Underbody:

As shown in Fig. 9, a flat Underbodycontrols air flow under the car. Aswith flat Underbody, the generationof low pressure is advisable but ifthe Underbody is rough, than it willcreate high pressure, which in turnwill cause increment in coefficientof lift (Cl).If (Cl) is more; than carloses its traction and it adverselyaffect car’s stability.

Diffuser:

The underbody rear diffuser is one ofimportant aerodynamic add-on devices.The parameters of the diffuser,including the diffuser angle, thenumber and the shape of separators,the shape of the end plate and etc.,will affect the underbody flow andthe wake. The diffuser can work toboth reduce drag and increasedownforce for driving car, which canimprove fuel economy and operationalstability of vehicle.

Fig. 10 [a, b] Effect of rear and taper on drag coefficient [7]

Tilting upwards the underfloor rearend produces a diffuser effect whichshows a promising way to reduce thedrag coefficient, as shown in aboveFig. 10 (a, b). However, it isimportant to select the optimum ratioof length of taper to overall carlength and the angle β of upward

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inclination for best results.

LITERATURE REVIEW

Introduction [7]

Aerodynamics is a branch of dynamicsconcerned with studying the motion ofair, particularly when it interactswith a moving object. Drag and liftare two aerodynamic forces acting onanybody travelling through the airwhether it's an airplane, car ortruck. Drag is far more important forvehicles, unless it's a race car orhigh performance sports cartravelling at very high speeds. Byusing CFD it is easy to validatepressure coefficient on the centreline of the car, it is possible tofind where the low pressure exits andwhat is the drag created in themodel. Number of research papers andstudies has been conducted on variouscar models by adopting variousexperimental methods andcomputational method like CFD. Numberof reviews have been taken andanalysed below to complete thepresent study.

Literature reviews based onExperimental and ComputationalAnalysis

Chien-Hsiung Tsai a, Lung-Ming Fu b,Chang-Hsien Tai a, Yen-Loung Huang a,Jik-Chang Leong, [2007] [1] The authorin reference [1] has proposed aneffective numerical model based onthe Computational Fluid Dynamics(CFD) approach to obtain the flowstructure around a passenger car withwing type rear spoiler. The resultswere obtained by comparing both casesi.e. car with and without spoiler.The angle of attack is different fordifferent type of spoiler.Simulations are performed after

adding different types of spoilerswith different angles ofinstallation.

Fig. 11 Configuration of spoilers andCar: (a) various spoiler configurations [1]

The effect of various differentconfiguration of spoilers is onvorticity and noise level also, willhave different effect on lift anddrag coefficients and on pressuredistribution. The above Fig. 11 showsdifferent configuration of spoilers.

For all the cases considered, thedrag coefficient ranged from roughly0.4 (for case 1 in Fig 2.2) toapproximately 0.51 (for case 2 in Fig2.2) whereas the lift coefficientvaried from about 0.001 (for case 4in Fig 2.2) to approximately 0.06(for case 1 in Fig 2.2). Also, it isimportant to point out that only thespoiler for case 4produced a negativelift coefficient. Based on the casesconsidered in this work, theinstallation of a spoiler reduces thelift coefficient that leads to betterconditions for high speed driving andimproves the vertical stability ofdriving. The lift coefficient of case4, which is approximately 0.001, isthe lowest among all the casesstudied. This indicates that thespoiler design of case 4 has the bestvertical stability.

Emmanuel Guilmineau, [2007] [2] Theauthor in reference [2] has proposedthe study based on Ahmed body. The

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Ahmed body is a simplified car usedin automotive industry to investigatethe influence of the flow structureon the drag. The model of a hatchback body is simulated using all k-εmodels in CFD. Rear slant angle istaken into account. This paperinvestigates numerically the flowaround the Ahmed body for the baseslant angles 25 and 35. It is foundthat there is a 3% decrement in dragwhen different slant angles wereused. Simulations, with severalturbulence models, have been carriedout for the generic Ahmed body with25and 35 slant angles. For largerslant angle, computations with orwithout stilts were made on the bodywhich was supported in the windtunnel, gave similar results.However, drag is increased due to thepresence of the stilts. Allturbulence models predict thetopology of the flow correctly. Thereis a considerable reduction in amountof drag by using various rear slantangles.

W. Kieffera, S. Moujaesb, N. Armbyab,[2007] [3] The author in reference[3] has proposed brief literaturestudy on using the Star-CD CFD codeto perform a turbulent simulation(using a k–ε model) of the airflow onthe front and rear wings of a FormulaMazda car with different angles ofattack and the effect of the groundon the front wing. Results showingpressure and velocity distributionsand lift (Cl) and drag coefficients(Cd) for the different cases areshown graphically and the numericalmodel was set up. With the frontaerofoil close to the ground and with0 degree AOA, the velocity on theupper surface is slower than for theaerofoil in free air. On the lowersurface of the wing, the velocity ishigher for the aerofoil in ground

effect than the one in free air. For16 AOA, separation occurs at half ofthe chord length. The greater portionof lower surface is close to groundwill creates larger low pressure areaand hence greater is the downforce.

The calculated results clearly showan increase in downforce when a frontaerofoil is considered with groundeffect of about 13% to 20%. Thisincrease can be attributed to theanticipated velocity increase on theunderside of the wing, which in turndecreases the pressure on the wingfrom that side. When ground effect isconsidered, there is a slightincrease in the Cl of about 20% from0 degree to 12 degree AOA. Inaddition, there is a marked decreasein Cl by about 45%, which mayindicate that between 12 degree and16 degree AOA there is a potentialfor a “stall” condition with theaerofoil.

Mohd Akmal, [2007] [4] The author inreference [4] has proposed the effectof aerodynamics on a hatchback car.This research is focused on twoimportant aspects namely modeldevelopment and wind tunnelexperiment and the results of thisresearch focused on qualitative andquantitative values. Quantitativevalues are Co-efficient of drag andpressure distribution around themodel surface. While qualitativevalues are visualization of air flowcharacteristics over the modelsurface.

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Fig. 12 Air flow at the front edge ofcar and effect of flow separation [4]

Hugo G. Castro, Rodrigo R. Paz, MarioA. Storti, Victorio E. Sonzogni,Jorge O, [2008] [5] The author inreference [5] has proposedexperimental and numerical studies ofthe aerodynamic behaviour ofsimplified road vehicle, Experimentaland numerical analysis of the flowover the Ahmed body with a 35 degreerear slanted angle were performed.Ahmed body was chosen as a standardreference given that it wasnumerically and experimentallystudied in an exhaustive way.Comparing both, results from theexperimental tests made in this workand the other experimental referenceas well as the provided by thecomputational simulation presentssome appreciable differences. Meandrag force in the experimental tests(Cd mean = 0:40) presents a highvalue when comparing with theobtained by Ahmed et al. (1984)(Cdmean = 0:257) while the numericalobtained value had a difference lowerthan a 9% (Cd mean = 0:28) beencorrected for tare drag of stilts. Anumerical simulation of the modelwith the stilts was performed.Nevertheless, experimental valueswere still higher. it can be shownthat pressure measured in the windtunnel on the rear slanted plane arealmost in agreement, while for therear base of the model significantlyexceeds the reported values. Thisresult explains the differences indrag forces.

Debojyoti Mitra, [2010] [6] The authorin reference [6] has proposeddetailed work, experiments wereconducted with different add-onparts, like rear end spoiler & frontend spoiler in order to find out how

these add-on parts influence the dragand lift co-efficient of a basic carmodel. The results from theexperiment shows that the dragcoefficient almost remains unaffectedwith the addition of rear endspoiler, but considerably reduceslift coefficient. The addition offront spoiler increases the dragcoefficient, also reduces the liftcoefficient but not as that of rearend spoilers but, increases pressureon upper surface of vehicle. Theaddition of both the rear and frontend spoilers increases the dragcoefficient significant and bringsdown the lift coefficient to a largeextent, also increase the aerodynamicstability of a basic car model andhence they can be used in real lifewhich will be an added advantage. Asthere is an air dam in front of thevehicle a low pressure region iscreated underside, and with theincreased pressure on the uppersurface of the vehicle, gives rise toan increment in negative lift.

Conclusion

By studying various papers it isfound that, the main concern inimproving aerodynamics of road carsis achieved through modifying outerbody by installing parts like rearspoiler, front, side and rear skirt,front bumper and by changing rearslant angle, which have considerableeffect in drag reduction and thusimproving fuel consumption andoverall vehicle’s performance. Theaerodynamic drag of a road vehicle isresponsible for a large part of thevehicle’s fuel consumption andcontributes up to 50% of the totalvehicle fuel consumption at highwayspeeds. Reducing the aerodynamic dragoffers an inexpensive solution toimprove fuel efficiency. Various

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experimental and computationalmethods are used which furtherinvestigates the influence of dragand helps in getting optimumsolution. Computational method ismostly selected as it is relevant,fast and inexpensive, also providingaccurate results through simulations.

Future work

Preparing a 3D model of ahatchback car.

Perform simulation using CFD atvarying speed of car and willobserve the effect of variousparameters on performance of car.

Carry out some aerodynamicmodifications on car’s body likechanging angle between hood tofront windscreen and also byadding various add-on parts likerear spoiler and rear diffuser.

Create 3D models of rear spoilerand diffuser and by takingdifferent values of angle betweenhood and front windscreen,installing them one by one inpreviously prepared 3D model of ahatchback car and performsimulations using CFD, and willobserve change in parameters.

Again perform simulation of fullycustomized model and will observethe effect of the modification onvarious parameters and its effecton performance.

Compare the recorded observationsfrom both the 3D models i.e.customized and non-customized.

Reach on final conclusion.

REFERENCES

Papers:

Chien-Hsiung Tsai a, Lung-Ming Fu b,Chang-Hsien Tai a, Yen-Loung Huang a,Jik-Chang Leong “Computational aero-

acoustic analysis of a passenger carwith a rear spoiler” in [2007].

Emmanuel Guilmineau “Computationalstudy of flow around a simplified carbody” in [2007].

W. Kieffera, S. Moujaesb, N. Armbyab“CFD study of section characteristicsof Formula Mazda race car wings” in[2007].

Mohd Akmal, Hanif Bin Chik“Experimental study of aerodynamicsof a hatchback car” in [2007].Hugo G. Castro, Rodrigo R. Paz, MarioA. Storti, Victorio E. Sonzogni,Jorge O “Experimental and numericalstudies of the aerodynamic behaviorof simplified road vehicle” in[2008].

Debojyoti Mitra “Effect of RelativeWind on Notch Back Car with Add-OnParts’’ in International Journal ofEngineering Science and TechnologyVol. 2(4),472,476, [2010].

Heinz Heisler “Advanced VehicleTechnology” 2nd edition, WillesdenCentre, London, UK.

www.wikipedia.com/polarGAuto

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