IRJET- Study of Various Passive Drag Reduction Techniques on External Vehicle Aerodynamics Performance: CFD Based Approach

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 06 Issue: 05 | May 2019 www.irjet.net p-ISSN: 2395-0072
© 2019, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 1851
Study of Various Passive Drag Reduction Techniques on External
Vehicle Aerodynamics Performance: CFD Based Approach
Basudev Datta1,2,3,4, Vaibhav Goel5, Shivam Garg5 and Inderpreet Singh5
1Plant Operation Control Intern, Maruti Suzuki India Limited-Gurgaon Plant
2MBA (Operations) Student, Symbiosis Institute of Management Studies-Pune
3Former Assistant Professor, C V Raman College of Engineering-Bhubaneswar
4Former Trainee Scientist (Scientist Gr.IV(1)), Council of Scientific and Industrial Research (Dhanbad Campus)
5B.Tech. Student, Dept. of Mechanical Engineering, Chitkara University- Rajpura
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Abstract - Recent trend of fluctuation in fuel prices and associated concern regarding global warming due to greenhouse gas
emission has attracted the engineers to think over the prevailing techniques of the improving fueleconomy. Automobileengineers
have already tried various techniques to improve fuel efficiency by engine and chassis weight modifications. However, the area of
vehicle aerodynamics for improving fuel efficiency has caught the eyes of automobile researchers which involve reducing the
magnitude of drag coefficient using various drag reduction techniques. Inthepresentpaper, anattempthasbeenmade toimprove
external vehicle aerodynamics performance by exploitingvariouspassive dragreduction techniquesbasedonComputationalFluid
Dynamics (CFD) using ANSYS Fluent v14 (Non Commercial Version) to reduce fuel consumption and in turn pollution.
Key Words: CFD, body of influence, realizable K-epsilon model, lift & drag coefficients
1. INTRODUCTION
Global trend of fluctuation (most of the time increase) in fuel prices, heavyconsumptionoffuel byinefficientandoldervehicles
(Majumder and Saha, 2014), immense pressure of environmental regulatorstocontrol thegreenhousegasesemission (Sudinet
al., 2014) have increased the demand for design and development of fuel efficient vehicles. Fuel efficiency of vehicle can be
improved by various techniques such as i) fuel efficient engine technology, ii) chassis weight reduction techniques and iii)
external aerodynamic drag reduction techniques (Rohatgi, 2012;Koikeet al., 2004;Carr, 1969). Asa result,tremendousamount
of research has been carried out in the field of design of efficient engines and chassis weight reduction techniques by
automobile industry (Demmler, 1998; Buchholz, 1998). Fuel efficientenginescoupled withchassismadeofcompositematerials
(e.g., carbon fibre) have improved the fuel efficiency to some extent (Small et al., 2006) but could not provide the satisfactory
solution. It is observed that fuel consumption increases upto 50% due to aerodynamic drag (Sudin et al., 2014).
Implementation of active and passive drag reduction techniques (Mayer and Wickern, 2011, Hsu and Davis, 2010) provide a
cost-effective solution to improve fuel economy (Hucho and Sovran, 1993). Incaseofactivedragreductiontechniques,network
of actuators coupled with sensor based controllers areusedwhichmakethesystemverycomplexaswell asaddextra weightto
the vehicle effecting overall performance. However, traditional passive drag reduction techniques involve modification in
external geometry of vehicles or attaching additional static devices to reduce the drag. As a result, passive drag reduction
techniques are mostly preferred by automobile manufacturers due to its cost effectivenessandsimplicityindesign. Therefore,
in the present paper, various passive drag reduction techniqueshave beenexploitedto improve external vehicleaerodynamics
performance using Computational Fluid Dynamics (CFD) based approach to reduce fuel consumption and in turn pollution.
Author has used noncommercial version of both ANSYS Fluent v14 & Dassualt Systeme CATIA v5r19 for simulation purpose.
The second and third sections of this paper illustrate the classification of different passive drag reduction techniques and
simulation set up including solver settings respectively followed by results and discussions.
2. CLASSIFICATION OF DIFFERENT PASSIVE DRAG REDUCTION TECHNIQUES
The drag can be dealt with two different perceptions in case of vehicles: i) due to vehicle and ii) due to the fluid through which
vehicle travels. In accordance to Newton’s Third law of motion, every action has equal and opposite reaction.Similarly,incase
of fluid dynamics, due to force exerted by vehicle motion on fluid, equal andoppositeresistiveforcesi.e. dragforcesareexerted

on vehicle by fluid which tries to resist its motion. However, the drag which a vehicle exerts due to a fluid stream is the
summation of pressure drag and skin friction drag (Hucho and Sovran, 1993). Pressure drag depends upon geometrical
configuration of system due to phenomenon of boundary layer separation from the surface of rear window and formation of
wake region behind the vehicle which accounts for more than 80% of the total drag (Majumder and Saha, 2014). In such
conditions, large pressure difference is developed across front and rear section of a car. As a result, vehicle experiences a drag
force in the direction of air movement. In addition to that, speedofvehicleandwakeregionproducedduetothemovingvehicle
also affects the magnitude of drag (Singh, 2004, Sudin et al., 2014). Itisgeneral tendency that40%ofoverall induceddragforces
are concentrated at rear section of a car geometry (Chainani and Perera, 2008; Sudin et al., 2014). Though drag forcesaremore
concerning factor, lift forces also play considerable role in improving the dynamicstabilityofa vehicle.Largeristhevalue oflift
coefficient, higher is the instability of vehicle. The generation of negative lift forces is an added advantage while driving at
higher speeds. For such purposes some special devices like spoilers are required to be installed. The negative lift forces
increase the grip of the car tires on the road (Ahmed and Chacko, 2012). On the other hand, the heavy and large sized vehicles
(trucks, buses etc.), design becomes aerodynamically inefficient due to large frontal area and non-streamlined body shape
which consumes upto 65% additional fuel to overcome the induced drag resistance (Altaf et al., 2014, Wahbaet. al., 2012) even
if it has better tyre grip with surface. It means aerodynamic drag largely dependentongeometrical configurationof thebodyof
a vehicle in addition to fluid flow conditions i.e., laminar or turbulent. In case of streamlined bodies (e.g. sports cars), one can
reduce the drag to some extent by changing the flow conditions i.e. varying Reynolds number. But situation becomes complex
in case of bluff bodies with sharp corners (buses, trucks) where Reynolds numbervariationhasnoeffectonaerodynamicdrag.
Hence, structural modification of a vehicle is required to reduce the drag to control the flow separation (Altaf et al., 2014). The
various structural modifications can be donebyusingthe followingtechniqueseitherindividuallyorincombinationforpassive
drag reduction.
2.1 Rear Tail Flaps
It is most common type of passive drag reduction techniques. Tail flaps are always installed in the rear section of the vehicle.
The flaps are usually installed horizontally at the top and bottom sections of utility vehiclesandverticallyalongsidesincaseof
bluff body vehicles as shown in Fig. 1(a) and Fig. 1(b) respectively. Numerical investigationofvertical tail flapsonsportsutility
vehicles conducted by Wahba et al. (2012) clearly indicated that the diversion ofairtakesplaceintolowpressure wakezoneto
reduce drag upto 18% by improving the wake pressure recovery process. Thoughsomeparameterssuchascrossectional area,
chord length and angle of attack must be varied in such a way that vehicle aerodynamic performance is optimized(Sudinetal.,
2014). Sudden upward deflection of air flow underneath the car causes reduction in drag coefficient by 30% due to reduced
turbulent intensities in near wake zones, shortening and weakening of recirculation zones and reduction in base pressure
(Khalighi et al., 2013). Lots of research has been carried to determine the influence of angle of attack ondragandlift coefficient
to determine optimal angle of attack (Sharma and Bansal, 2013).Windtunnel analysisbyFourriéetal. (2011)clearlyshowsthat
widening of separated zone which obstructs the process of development of counter rotating longitudinal vortices near lateral
edges of rear window causing significant reduction in magnitude of drag.
(a) Horizontal tail flaps at 20° angle of attack installed in
rear section of passenger car
(b) Vertical tail flaps installed in rear section of SUV (Wahba
et al., 2012).
Fig. 1: Different installation techniques of rear tail flaps for utility and bluff body vehicles

2.2 Ground Effect
Air flow across the car body with abrupt change in geometrical shape causes development of abnormal high pressure zones.
Due to development of high pressure zone in the car roof,significantamountofdownwardforceis exertedonundercarriagei.e.
in low pressure zone (Ahmed and Chacko, 2012). Since the ground effect techniquesisbasedonBernoulli’sprinciple, therefore,
it is suggested to reduce frontal ground clearance area as depicted in Fig 2(b). It causes the air velocity to increase gradually
producing low pressure zones underneath the car i.e. undercarriage section of a car. The entire process not only reduces the
drag coefficient but also improves the vehicle dynamic stability. Dynamic stability of a car is improved due to addition of
negative forces to tyre gripping force which counters the lift forces developed.
2.3 Diffuser
It is well known fact that high exit velocity of air underneath the rear section of a vehicle causes widening of flow separation.
Recirculation of air due to difference in pressure across upstream and downstream at rear section ofa carcausesincrementin
induced drag. In order to avoid the same, diffuser technique is used (Ahmed and Chacko, 2012).Itinvolvescertainmodification
of underbody/undercarriage tray of the car especially, in the rear section as illustrated in Fig 2(c) (Katz, 2006)soastoprevent
the re-circulation of air at rear section by restoring air velocity at exit point.
Various researchers have done different modifications to utilize diffuser technique. Mazyan (2013) modified front head with
rear wings which reduced the coefficient of drag by 21%. Marklund et al. (2013) studied the influence of ground proximity of
underbody tray and diffuser angle on drag coefficient based on flow physics and wake analysis for Sedan passenger car with
low ground clearance. They found that coefficient of drag is reduced by 2-3%. Modification of underbody tray shape wasdone
similar to venturi nozzle without endplates to reduce drag (Huminicetal., 2012). CFDsimulationbySudinetal. (2014)indicates
that drag coefficient reduces by 4% when movable arc-shaped semi-diffuser devices in underbody tray is installed in order to
facilitate streamlined flow of air. Therefore, it can be concluded that the area of crossection exposed to the incoming air flow
may be increased gradually at particular slant angle to minimize the recirculation.
2.4 Vortex Generator
It is a small vane shaped aerodynamic surface used in aerospace sector that produces vortices in the air flow. Such devicesare
mostly installed at the top end of rear section of a car just before declination of rear wind screen of the car as shown in Fig.
2(d). It is an inherent property of the vortex generator to induce drag initially. However, at the same timeit reducesmoredrag
in comparison to the initially induced drag by delaying flow separation at the downstream side (Dubey et al., 2013). The
purposes of vortex generator are: i) to control the boundary layer transition and ii)todelaytheflowseparationtogeneratethe
strong negative lift forces thereby improving dynamic stability of a car (Ahmed and Chacko, 2012). It works on the principle of
exchange of momentum between upstream and downstream flows. Downstream air pressure increases with respect to
upstream at rear end due to tapered shape of vortex generators. As a result, a resisting forceisgeneratedinoppositedirection.
The phenomenon of delay in flow separation at the roof of the rear section of Sedan and Hatchback istestedusingGAMBIT and
FLUENT after installation of bump shaped vortex generators. CFD analysis data indicates that both liftanddragcoefficientcan
be reduced by installing vortex generator at the rear section of the car (Koike et al. 2004).
2.5 Front Bonnet Duct
In order to minimize the coefficient of drag, designers need to focus how to eliminate or minimizethehighpressure zones.The
high pressure zones are associated with speed of a car. It is obvious that higher is the air speed, lower is the pressure. Usually,
due to sudden increase in angle encountered by fluid flow withfrontsectionofa car,airspeeddrasticallyreduces resultinginto
generation of high pressure zones. Therefore, a front bonnet duct is generally used for this purpose as shown in Fig 2(e). The
purposes are mainly: i) to reduce the high pressure zones in the front section of a car to reduce drag coefficient (Ahmed and
Chacko, 2012) and ii) additional air cooling of engine.

2.6 Rear Spoiler
The basic design of rear spoiler is taken from the wings used in the aircrafts (Ahmed and Chacko, 2012).Intheaircraftindustry,
wings are used to generate lift forces, however, in case of automobile industry, it is used to minimize unfavorable air
movement. Rear spoilers are usually inverted wings installed over the trunk lid in rear section of a car as illustratedinFig2(f).
It has tendency to diffuse the air flowing around the vehicle, by minimizing the turbulent kinetic energy at the rear section of
the car. It results into generation of large amount of negative lift forces due to development of downwards pressure (Zakem,
2008; Daryakenari et al., 2013). This device not only reduces the drag but also improves dynamic stability of a car. In earlier
phase of development of spoilers, these were installed just for decorationpurpose.But researchshowsthattodayistheneedof
these spoilers for reducing the drag significantly and improving the dynamic stability (Hu et al., 2011).
2.7 Rear Fairing
It is one of the rarely used passive drag reducing techniques. Since,itrequirescompletegeometrical overhaulingof rearsection
of a car influencing the aesthetics of the same. It generally involves conversion of rear section of the car into truncated cone
design with rectangular or square sectional base similar to that of the tail section of an aircraft as shown in Fig. 2(g). A careful
attention must be given to design the slant angle of rear fairing in order to minimize the flow separationphenomenon.Sudden
change in slant angle of rear fairing must be avoided. It is reported that installation of rear fairing has reduced the drag
coefficient by 26% (Rohatgi, 2012).
(a) (b) (c) (d)
Fig 2: Various Passive drag reducing techniques installed in base model of car
(a) Rear Tail Plate, (b) Ground Effect, (c) Diffuser, (d) Vortex Generator, (e) Front Bonnet Duct, (f) Rear spoiler and (g)
Rear fairing
3. MODEL DESCRIPTION
The generic model of a car was designed using Dassualt Systeme CATIA v5r19 software for the present study. It was imported
to ANSYS Fluent v14 for CFD simulation purpose as shown in Fig. 3 below. Though the car was initiallydesignedwithitsactual
dimension as shown in Fig. 3 (base length=4m, base width=2m and height=1.8m) but the design was reduced to 1:10 scale to
facilitate faster computational process.
(e) (f) (g)

Fig 3: Dimensional specification of base model car
4. CFD SIMULATION SETUP
External Aerodynamic evaluation of air flow over a vehicle can be done using either analytical method or CFD approach. It is
obvious that for solving the air flow problems, we have to compute the Navier-Stroke equation and continuity equation.
Analytical methods can be used to solve air flow problems over simple geometries like laminar flow overa flatplate.However,
in case of complex fluid flow i.e. turbulent flow over bluff bodies,itisalmostimpossible tosolveanalyticallybothNavier-Stroke
equation and continuity equation even using latest version of computers due to largecomputational time. Toreducethesame,
a time averaged Navier-Stokes equation along with turbulent models is used.
In the present study, CFD simulation is carried out using Viscous realizable K-Ɛ Turbulence Model (2 equations) with Non-
equilibrium wall function as near wall treatment. The model used in current study is very robust with reasonable
computational time and widely used by automobile designers. Steps of CFD Analysisinvolves:(i) Selectionofbasemodel ofthe
vehicle, (ii) Designing the base model of vehicle using Dassualt Systeme CATIA v5r19, (iii) Applying boundary conditions for
CFD Simulation on base model of vehicle in ANSYS Fluent v14, (iv) Generationofvirtual windtunnel environmentusingANSYS
Fluent v14, (v) Determination of lift and drag coefficient of base model of vehicle using ANSYS Fluent v14 and (vi)
Determination of lift and drag coefficient of base model of vehicle with passive drag reduction technologies installed using
ANSYS Fluent v14.
The base model of the car in this paper have symmetrical geometry. Hence, symmetrical fluid flow field has been taken into
consideration for numerical simulation purpose (Xingjun et al., 2011; Xingjunetal. 2010;MajumderandSaha, 2014). Inorderto
further minimize the computational time and resources, more refined mesh and half of the base model car is used for CFD
Simulation. Since, the computational domain is a general requirement for any external fluid flow analysiswhichisdefinedasa
box/spherical/cylindrical shaped domain surrounding the structure dependingupontypeofstructuretobeevaluated. Incase
of cars, box shaped domain is preferred which is filled with fluid and has boundariesdefiningthecharacteristicsoffluidflowas
illustrated in Fig. 4.
(a) (b)
Fig 4: Computational domain of Base model car (a) Isometric View and (b) Side View
Recent trends in CFD simulation indicates that size of computational domain is one of the important factors. It is needed to
facilitate efficient computation and capturethedetailsregardingchangeinfluidflowcharacteristics.Ingeneral,keeping inview

the Length of a car=L, 2L, 3L and 6L have been taken as size of computational domain (Majumder and Saha, 2014) on lateral
sides, upstream and downstream sides of the car,respectively. Thecompletemodel wasmeshedusingANSYSFluentv14which
is essentially composed of tetrahedral and prismatic meshes as shown in Fig. 5. Prismatic meshing was done in order to
facilitate detail capturing of surface forces acting on the vehicle with accuracy. It is implemented on vehicle body, wheels and
road surface. Program controlled inflation was inserted on the model to apply boundary layers.The best practicesformeshing
in external vehicle aerodynamics recommends that value of first aspect ratio, maximum number of layers and growth rate
should be kept as 5, 5 and 1.2 respectively as shown in Fig. 5 while meshing (Majumder andSaha, 2014).Inordertocapturethe
details regarding formation of vortices at rear section of the car, finer meshing is required. Hence, local mesh refinement was
done wherever accuracy is required. Another computational domain was built around the vehicle body to refine the mesh
closer to the car and coarsen the rest of domain space i.e. body of influence. Introduction of body of influence feature around
the vehicle body reduces the computational effort and captures the detailed change in fluid flow characteristics. In general,
dimension of body of influence is 0.5L in lateral sides, 0.5L and L in upstream and downstream directions respectively which
extended by 0.5L above the roof (Ahmad et al., 2010; Skaperdas and Kolovos, 2009).
(a) (b)
Fig 5: Meshing in ANSYS Fluent v14
(a) Grid Topology and (b) Tetrahedral mesh and Prism Layers
4.1 Solver Setting
ANSYS Fluent v14 CFD solver is used for numerical simulation of vehicleexternal aerodynamicstoevaluatetheperformanceof
different passive drag reduction techniques.Fluentsolver usesNavier-strokesequationswhicharecomposedofcontinuityand
momentum equations as discussed below in Eqns. (1) and (2).
Continuity Equation:
(1)
Momentum equations:
(2a)
(2b)
(2c)
Some assumptions are made for simulation set up which are: i) steady state inlet air velocity, ii) zero degree yaw angle, iii)
constant pressure at outlet, iv) no slip condition near vehicle surfaceandv)inviscidfluidflowconditionsnearlateral sidewalls,
roof and road or surface of wind tunnel. Table 1 and 2 illustrates the solver setting and model settings used in CFD simulation.

Table 1: Solver Settings
CFD Simulation type 3D
Solver ANSYS Fluent v14 (Non Commercial Version)
Formulation Implicit
Time Steady State
Velocity Formulation Absolute
Gradient Options Cell based
Porous Formulation Superficial Velocity
Table 2: Model Settings
4.2 K-epsilon (K-Ɛ) Model
The K-epsilon model is proposed by Launder and Spalding which is one of the most widely used turbulence models by
automobile design engineers (Launder and Spalding, 1974). The model is composed of two transport equations which
represents the turbulent fluid flow characteristics. It uses eddy viscosity approach to model the Reynolds stresses. History
effects (convection and diffusion of turbulent energy) are also taken into account (Launder and Spalding, 1974) in this model.
The turbulent kinetic energy, K and turbulent dissipation, Ɛ are the firstandsecondtransportedvariableswhichdeterminethe
turbulence energy and scale of turbulent fluid flow, respectively. The model also provides an alternativemethodformoderate
to highly complex fluid flow which earlier used to algebraically prescribetheturbulent lengthscales (Jonesetal., 1972;Launder
and Sharma, 1974). It was in fact introduced in order to improve the mixing-length model. K-Ɛ model is found to be useful in
case of i) free-shear layer flow conditions with small pressure gradients and ii) wall-bounded and internal fluid flows. It is
observed that with increase in pressure gradient, accuracy reduces drastically (Bardinaetal., 1997).Themainadvantageof K-Ɛ
model is that the implementation is relatively simple and easier to converge to the solution with reasonable predication.
Though, it too has some shortcomings: i) it is valid only for fully turbulent flows, ii) mandatory implementation of wall
functions, iii) Poor flow predication in case of swirling and rotating flow with strong separation (CD-adapco website; Karthik,
2011) and iv) unable to perform well under large and adverse pressure gradients (Wilcox, 2006). Keeping in view of the
demerits of the existing standard K-Ɛ model, in the present study realizable K-Ɛ model has been considered by author.
4.3 Realizable K-epsilon (K-Ɛ) Model
It is an improved version of existing standard K-Ɛ model (Shih et al., 1995). It consists of a new formulation for the turbulent
viscosity along with a new transport equation for the dissipation rate, Ɛ.Itisderivedfromthe exactequationfortransportation
of the mean-square vorticity fluctuation. It also satisfies certain mathematical constraints on the Reynolds stresses which are
regularly associated with physics of turbulent fluid flow. Main advantages of realizable K-Ɛ model are:i)improved predictions
of the spreading rate in case of both planar and round jets, ii) improvedperformanceincaseoffluidflowinvolving rotationand
recirculation under strong adverse pressure gradients and iii) ability to capture details of fluid flow over the complex
structures (CD-adapco website; Karthik, 2011). Based on the above mentioned discussions, the subsequent solver setting
conditions are illustrated in Table 3-8 for ease of simulation.
Turbulence Model Realizable K-epsilon (2 equations)
K-epsilon Model Realizable
Near wall treatment Non-equilibrium wall function
Operating Conditions Ambient

Table 3: Boundary Conditions at Velocity Inlet
Velocity Inlet
Velocity magnitude (m/s) 27.778 (steady and measured along normal
direction to the boundary)
Turbulence Specification Method Intensity and viscosity Ratio
Turbulence Intensity 1%
Turbulence viscosity Ratio 10
Table 4: Boundary Conditions at pressure-outlet
Pressure outlet
Gauge pressure magnitude (Pascal) 0
Gauge pressure direction Normal to boundary
Turbulence specification method Intensity and viscosity ratio
Backflow turbulence intensity 5%
Backflow turbulent viscosity ratio 10
Table 5: Wall zone conditions
Base model car surface No slip wall boundary conditions
Road surface Inviscid wall boundary conditions
Side walls Inviscid wall boundary conditions
Table 6: Fluid properties of air
Fluid
properties of
air
Material type Fluid
Fluent fluid material Air
Density (kg/m3) 1.225
Kinematic Viscosity (kg/m-s) 1.7894×10-5
Table 7: Solution Methods
Pressure-velocity coupling scheme Coupled
Spatial discretization Gradient Least squares cell based
Pressure Standard
Momentum Second order upwind
Turbulent kinetic energy Second order upwind
Turbulent dissipation rate Second order upwind
Convergence Criterion Continuity 10-3
X-velocity 10-3
Y-velocity 10-3
turbulent kinetic energy, K 10-3
turbulent dissipation, Ɛ 10-3

Table 8: Solution Controls
Flow courant number 50
Explicit relaxation factors Momentum 0.25
Pressure 0.25
Under relaxation factors Density 1
Body forces 1
Turbulent kinetic energy 0.8
Turbulent dissipation rate 0.8
Turbulent viscosity 0.95
Energy equation is not taken into consideration as there is no thermal activity due to fluid flow. Coupled scheme is used as
iterative algorithm in present study due to its robustness and efficient single phaseimplementationincaseofsteadystatefluid
flow.
5. RESULTS AND DISCUSSION
The results of different passive drag reduction techniques using CFD simulations have been discussed below in brief.
5.1 CFD Simulation of Base model car
The pressure coefficient plot of base model car without using any passive drag reduction techniques clearly indicates that the
value of pressure coefficient overshoots at stagnation point in front section of a car as shown in Figs. 6(a) and 6(b).
(a) (b)
Fig 6: (a) Pressure Coefficient on base model car surface and (b) Total pressure on base model car surface,
velocity inlet, pressure outlet, symmetry-side wall, symmetry-top wall and road
Figs. 7(a) and 7(b) depicts the magnitude of drag and lift coefficients (0.33846 and 0.25313) of base model car after
successfully fulfilling the convergence criteria respectively.

(a) (b)
Fig 7: (a) Drag coefficient and (b) Lift Coefficient of base model car
Figs. 8(a) and 8(b) clearly indicates that there is an increase in negative pressure and generation oflargenumberof vorticesat
rear section of base model car. Therefore, we need to concentrate on redesigning of the rear section of the car such that flow
separation phenomenon is eliminated or minimized.
(a) (b)
Fig 8: (a) Velocity contour and (b) Total pressure contour of base model car
5.2 CFD Simulation of base model car with passive drag reducing devices
It is general tendency in automobile industry that multiple types of passive drag reducing technologies are used at the same
time. Therefore, in the current study, multiple passive drag reducing techniques have been used on our base model car as
discussed in subsequent sections.
5.2.1 Rear Tail Plates
The main purpose of rear tail plates is to reduce turbulent intensities in near wake zones, shortening and weakening of
recirculation zones and reduction in base pressure by sudden upward deflection of air flowing underneath the car. Angle of
attack of rear tail plates has large influence on magnitude of drag coefficient. The base model car is fitted with rear tail plateof
25 mm length (reduced scale) at 20° angle of attack as illustrated in Fig. 9(a) and 9(b).

(a) Overall system (b) Installation details
Fig 9: Base model car fitted with rear tail plate at 20° angle of attack
5.2.2 Ground Effect with Rear Tail Plate
The base model car front and rear end ground clearance has been reduced by 50% and increased by 25% with respect to
reference design respectively along with rear tail plate of 25 mm length (reduced scale) installed on car roof at an angle of
attack of 20° as shown in Fig. 10(a) and 10(b) to facilitate large pressure differential across top and bottom sections of the car
for improving dynamic stability.
(a) Overall System (b) Geometrical configuration
Fig 10: Base model car with undercarriage unit modified for Ground effect and fitted with
rear tail plate at 20° angle of attack in rear section of car roof
5.2.3 Diffuser with Rear Tail Plate
The front ground clearance has been reduced by 75% with respect to earlier design with the inclusion of elevation angle of 5°
starting from point located at 0.5L from front section of the base model car in undercarriageunitalongwith reartail plate of25
mm length (reduced scale) installed on car roof at an angle of attack of 20° as illustrated in Fig. 11(a) and 11(b) to facilitatethe
development of high pressure zones underneath the car.

Fig 11: Base model car with undercarriage modified into diffuser and fitted with rear tail plate at
20° angle of attack in rear section of car roof
5.2.4 Front Bonnet Duct with Rear Tail Plate and Diffuser
It has been installed over bonnet of base model car at an elevation angle of 26.565° along with diffuser and rear tail plate
similar to that of discussed earlier as depicted in Fig. 12(a) and 12(b) to reduce the formation of high pressure zones in front
section of a car. Earlier, this technique has merely implemented for improving the air intake of engine.Elevationangleoffront
bonnet duct facing the wind screen has been designed in such a way that at least half of the air flow is directedawayfromfront
wind screen. It reduces the drag coefficient by preventing stagnation of air flow.
Fig 12: Base model car fitted with front bonnet duct, diffuser and rear tail plate at 20° angle
of attack in rear section of car roof
5.2.5 Rear Spoilers with Rear Tail Plate and Diffuser
It has been installed in the rear section of the base model car at an angle of attack of 26.565° with leading and trailing edge
located at the heights of 60 and 80 mm (reduced scale), respectively along with diffuser and rear tail plate similar to that of
discussed earlier as illustrated in Fig. 13(a) and (b). It is used to generate enormous amount of negative lift forces with little
increment in drag coefficient. While designing the rear spoiler, care must be taken regarding angleofattack togeneratestrong
negative lift forces. The height of installation of the rear spoiler must be equal to the vertical height of the rear wind screen

which provides the least diverted air stream to the rear spoiler thereby producing ground effect by developing high pressure
zone under the rear spoiler.
(a) Overall System (b) Installation setup of rear spoiler
Fig 13: Base model car fitted with rear spoiler, diffuser and rear tail plate at 20° angle of attack in rear
section of car roof
5.2.6 Vortex Generator
The trailing edge of the base model car roof has been fitted with 10 mm long and 25 mm height (reduced scale) vortex
generators at an angle of attack of 18° as shown in Figs. 14 and 15 to reduce the development of high pressure zones near rear
wind screen. Though, this technique not only reduces a good amount of drag coefficient but also induces small amountofdrag
simultaneously.
Fig 14: Top view of vortex generators installed on end of
base model car roof
Fig 15: Geometrical configuration of
vortex generator ( h = 25 mm, reduced
scale )
5.2.7 Rear fairing
The shape of rear fairing has been taken similar to that of truncated cone with rectangular base as shown in Fig. 16(a) and
16(b). It has minimum to maximum slant angle from 19° to 25° starting from beginning of rear wind screen to rear end and
rear wheel to rear end of the base model car. It minimizes flow separation phenomenon occurring at the rear section.

(a) Overall System Geometrical configuration
Fig. 16: Base model car with rear fairing
The computation process was done till solution converges after implementing the boundary condition as stated in Tables 3-8
while running ANSYS Fluent v-14. As a partofpost-processingcomponent,graphical plotsshowingconvergenceof magnitudes
of drag (Cd) and lift (Cl) coefficients during computation process are illustrated in Fig. 17 after installing the above mentioned
passive drag reduction devices/techniques. Similarly, the velocity and pressure contours are depicted in Fig. 18.
(a) (b)
(a) Drag Coefficient and (b) Lift Coefficient of the base model car fitted with rear tail plate at 20° angle of attack
(c)
(d)
(c) Drag Coefficient and (d) Lift Coefficient of the base model car with undercarriage unit modified for Ground
effect and fitted with rear tail plate at 20° angle of attack in rear section of car roof

(e) (f)
(e) Drag Coefficient and (f) Lift Coefficient of the base model car with undercarriage modified into diffuser and
fitted with rear tail plate at 20° angle of attack in rear section of car roof
(g) (h)
(g) Drag Coefficient and (h) Lift Coefficient of the base model car fitted with front bonnet duct, diffuser and rear
tail plate at 20° angle of attack in rear section of car roof
(i) (j)
(i) Drag Coefficient and (j) Lift Coefficient of the base model car fitted with rear spoiler, diffuser and rear tail plate
at 20° angle of attack in rear section of car roof
(k) (l)
(k) Drag Coefficient and (l) Lift Coefficient of the base model car fitted with vortex generators

(m) (n)
(m)Drag Coefficient and (n) Lift Coefficient of the base model car fitted with rear fairing
Fig. 17: Plots illustrating convergence of drag and lift coefficients as per convergence criteria for various passive
drag reducing devices which are installed in base model car
(a) (b)
(a) Velocity contour and (b) Total pressure contour of the base model car fitted with rear tail plate at 20°
angle of attack
(c) (d)
(c) Velocity contour and (d) Total pressure contour of the base model car with undercarriage unit
modified for Ground effect and fitted with rear tail plate at 20° angle of attack in rear section of
car roof

(e) (f)
(e) Velocity contour and (f) Total pressure contour of the base model car with undercarriage modified
into diffuser and fitted with rear tail plate at 20° angle of attack in rear section of car roof
(g) (h)
(g) Velocity contour and (h) Total pressure contour of the base model car fitted with front bonnet duct,
diffuser and rear tail plate at 20° angle of attack in rear section of car roof
(i) (j)
(i) Velocity contour and (j) Total pressure contour of the base model car fitted with rear spoiler, diffuser
and rear tail plate at 20° angle of attack in rear section of car roof

(k) (l)
(k) Velocity contour and (l) Total pressure contour of the base model car fitted with vortex generators
(m)
(n)
(m) Velocity contour and (n) Total pressure contour of the base model car fitted rear fairing
Fig. 18 : List of velocity and pressure contours of various passive drag reducing devices which are installed
in base model car
6. CONCLUSION
The external vehicle aerodynamic characteristics of base model car attached with various passive drag reducing
devices/techniques have been studied using ANSYS Fluent v14 in this paper. The comparison of effect of installation of such
devices on the base model car has been shown in Table 9 in terms of magnitudes of drag and lift coefficients. The velocity and
total pressure contour of base model car clearly indicate that the installation of such devices has great influence on i) wake
zone formation, ii) streamlining of fluid flow over the car surfaceandiii)reducingairrecirculationwithminimizedzoneofflow
separation behind rear section of a car.
Table 9: Drag and Lift Coefficient of Base model car and Base model car fitted with passive drag reducing devices
DESIGN CONFIGURATION
DRAG
COEFFICIENT
(Cd)
LIFT COEFFICIENT
(Cl)
% CHANGE
IN Cd
%
CHANGE
IN Cl
Base Car model without any passivedragreducing
technology
0.33846 0.25313 Not
Applicable
Not
Applicable
BASE CAR MODEL WITH PASSIVE DRAG REDUCING TECHNOLOGY
Rear tail flaps (20° angle of attack) 0.27604 0.15625 . 2 .2 2
Ground Effect with rear tail plate (20° angle of
attack)
0.26569 0.16250 2 . .
Diffuser (ground clearance reduced by half) with 0.30988 0.20313 . .

rear tail plate (20° angle of attack)
Front bonnet Duct (26.565° elevation angle)
fitted with diffuser and rear tail plate (20° angle
of attack)
0.31099 0.23750 . .
Rear Spoilers fitted with diffuser and rear tail
plate (20° angle of attack)
0.40313 0.10078 +19.107 .
Vortex Generator 0.29375 0.20625 .2 . 2
Rear fairing 0.28750 0.18750 . 2 . 2
Note: and signs indicate increment and decrement in values of Cd and Cl
Hence, Installation of such passive drag reducing technologies either individually or in combination would help in improving
fuel consumption and dynamic stability ofbasemodel carbyreducingaerodynamicdragincostefficientmannerincomparison
to active drag reducing technologies.
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BIOGRAPHIES
Mr. Basudev Datta has 4+ years of experience in R&Dand TeachingSector.HehasworkedasAssistantProfessor
in Department of Mechanical Engineering, C V Raman College of Engineering-Bhubaneswar and Trainee
Scientist (Scientist Gr.IV(1)) in CSIR-Central Institute of Mining and Fuel Research, Dhanbad.Hehascompleted
Bachelor of Engineering in Mechanical Engineering Stream from Visvesvaraya Technological University-
Belgaum and Master of Technology in Mine Safety Engineering Stream from Academy of Scientific and
Innovative Research-New Delhi (An Institute of National Importance). He is a Gold Medalist and recipient of
many accolades during his UG and PG. During his tenure in CSIR, he was engaged in various CSIR Labs network
projects sanctioned under 12th Five Year Planning Commission Committee (2012-2017). He has published 5
research papers in International Conference & Journals till date. He is currently pursuing MBA in Operations
Management from Symbiosis Institute of Management Studies-Pune.
Mr. Vaibhav Goel is currently pursuing B.Tech. in Mechanical Engineering Stream from Chitkara University
Institute of Engineering&Technology-Rajpura.Hehasextensivehand-on experienceinAutomobileEngineering,
Operations Management, Production Engineering and Robotics through various mini-projects.

Mr. Shivam Garg is currently pursuing B.Tech. in Mechanical Engineering Stream from Chitkara University
Institute of Engineering &Technology-Rajpura. He has extensive hand-on experience in CAD tool such as
AutoCAD, Tribology, Automobile Engineering and Robotics through various mini-projects.
Mr. Inderpreet Singh is currently pursuing B.Tech. in Mechanical Engineering Stream from Chitkara University
Institute of Engineering &Technology-Rajpura. He has extensivehand-on experienceinFEAtool suchasANSYS,
Composite Materials and Automobile Engineering through various mini-projects.

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