Effect of Rear Spoiler and Diffuser Angle on Aerodynamic ...indusedu.org/pdfs/IJREISS/IJREISS_1994_94345.pdf · Md. Saifur Rahman and Khushbu Yadav, International Journal of Research

Md. Saifur Rahman and Khushbu Yadav, International Journal of Research in Engineering, IT and Social Sciences,

ISSN 2250-0588, Impact Factor: 6.452, Volume 08, Special Issue, June 2018, Page 105-117

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Effect of Rear Spoiler and Diffuser Angle on Aerodynamic

Characteristics of a Sedan

Md. Saifur Rahman

M.Tech Scholar

Amity University, Noida, Uttar Pradesh, India

Khushbu Yadav

Assistant Professor

Amity University, Noida, Uttar Pradesh, India

Abstract

Aerodynamic characteristics play an important role on stability and fuel economics of a

vehicle. The rear spoiler and underbody are two passive drag reducing devices that are integral

in reducing the aerodynamic drag of the vehicle. The aim of the research is to alter the diffuser

angle and spoiler angle at various speeds for studying the drag and lift characteristics. The

method of Computational Fluid Dynamics is used to analyze the aerodynamic properties

pertaining to the variation in diffuser angle and spoiler angle. Rise in fuel prices has driven

design engineers to enhance aerodynamics through minimal design changes. Another reason

that has grabbed attention is the fact that automotive vehicles have become so much faster

experiencing uplift force which creates unexpected accidents. The presented work is an

extension of previous studies that involved changing the angle of a diffuser or spoiler to study

the effects on a passenger vehicle. The vehicle model used for analysis is a very generic one

with the diffuser and spoiler mounted at the rear of the vehicle. Vehicle model has been

designed on SolidWorks 2015 and CFD analysis done on ANSYS Fluent. The drag and lift

coefficients are recorded for different cases of angle change, namely at 2, 5, 7, 10 and 13

degrees. The variation in angle is kept same for both spoiler and diffuser and observations

made accordingly.

Keywords: CFD, Drag, Diffuser, Lift, Spoiler





INTRODUCTION

The study of aerodynamics has yielded fruitful results over time. Be it any means of transport,

aerodynamics plays an important role in improving the design of vehicle to make it more

efficient and effective. Among road vehicles, from race cars to trailers, there has always been a

special emphasis on reducing the aerodynamic drag. Both active as well as passive drag

reducing components are crucial and reduce vehicle fuel consumption to up to about 50% at

highway speeds [1].

There are two key components that influence aerodynamic, namely skin friction drag and

pressure drag. For most of the part, pressure drag is the dominant force and depends greatly on

vehicle geometry because of boundary layer separation phenomenon. This results in the building

up of wake region at the rear end of the vehicle. The point where separation occurs defines the

wake region size and also evidently, the value of aerodynamic drag. An inefficient aerodynamic

geometry causes uncontrolled drag which in turn increases consumption of fuel.

A rear spoiler is an automotive part whose purpose is to disrupt undesirable air movement

around the body of a moving vehicle. Studies show that for maintaining tyre and road contact of

vehicle, rear spoilers are designed so as to decrease the lift of the vehicle. A diffuser is an

external component of an automotive vehicle whose main function is to accelerate and smoothen

airflow transition under the car. This causes the pressure beneath the vehicle to get affected

leading to creation of downforce on the vehicle.

The lift and drag coefficients give immense information in aiding to redefine the aerodynamics

of the vehicle. While altering the shape of the vehicle for different drive conditions is

impractical, the accessories of a vehicle can be easily adjusted to control the aerodynamic wake.

In the presented work, the aim of the study is to get the drag and lift coefficient values by

simulation of air flow around the vehicle surface. The vehicle body is modified for different

inclination angles and then the corresponding values recorded, which could improve the

aerodynamics of the vehicle.

LITERATURE REVIEW

Existing research is mostly focused on one component and analyzing its characteristics to achieve

some results. The different studies pertaining to the subject area have been reviewed for insight

into the subject matter.





Sudin et al. [2] reviewed the research performances of active and passive flow control of

vehicles. The study showed that failure to recover pressure in the wake region was a major factor

that compliments aerodynamic drag. Kang et al. [3] developed and studied an actively translating

rear diffuser device for passenger vehicles. A movable arc-shaped semi-diffuser device is

designed to maintain the streamlined configuration of the vehicle. Ahmed et al. [4] analyzed the

effects of a diffuser in a car for generation of down force. The various drag and lift forces acting

on the vehicle at different speeds for different diffuser angles were listed out and compared by

plotting them. Das et al. [5] examined the effects of a rear end spoiler at different angles on a

passenger vehicle. Cakir [6] studied the effects of a rear spoiler on a passenger vehicle and

presented a numerical simulation of the flow around the vehicle with spoiler mounted at the rear

end.

Mashud et al. [7] researched on the aerodynamic behavior of an airfoil for specific spoiler

position and presented their observations. The spoiler used for the research was such that it

extended at an angle of 7 degree with the horizontal. Aulakh [8] studied the effect of underbody

diffuser of vehicles in a convoy to understand the effect of inter-vehicular spacing and upper

body geometry of vehicles and the resulting aerodynamics. Hamut et al. [9] investigated the

effects of rear spoiler geometry on a sports car and conducted a numerical analysis to understand

air flow around the exterior of the vehicle. Hu et al. [10] researched the influence of diffuser

angle on a sedan and studied the resulting aerodynamic characteristics, without the separator and

end plate. Marklund [11] in thesis studied the impact of underbody and diffuser flow for

passenger vehicles. The study was initially conducted on bluff bodies, the results analyzed and

then the findings applied to full-size vehicles.

From literature survey, it has been observed that the maximum work regarding reduction of

aerodynamic drag has been focused on independent components.

GEOMETRIC MODEL

At high speeds, the shape of the vehicle becomes an important factor for the drag force acting on

the vehicle, which makes modelling important for analysis. Instead of modelling a separate part

for the diffuser and then integrating it with the main assembly of sedan and spoiler, the rear

underside of the sedan has been simply cut to function as a rear diffuser. The analysis of the

outcome would be quite similar to when a diffuser is designed as a different part of its own. The





diffuser is such arranged at the rear underside that it extends along the length of the rear bumper

of the sedan. There are, however, no vanes in the diffuser.

A. Geometric Model of Vehicle

The vehicle that has been used is a newly developed

passenger sedan. To avoid complexities, the

geometry of the sedan was kept as simple as

possible making it resemble more like a coupe. The

sedan was designed using SolidWorks 2015, where

the side profile was first drafted after which it was

extruded to the width of the vehicle. Only the

significant outline was modelled with the omission

of fenders, grilles, door panels, wheel details, etc. to

name a few. The front and rear windshields

integrate with the roof of vehicle to make a

hemisphere shaped upper body. Wheel arches are

not included and the underbody is kept flat.The

length, width and height of the sedan are 3m, 1.2m

and 0.5m respectively with the ground clearance

being 0.29m. Fig. 1 shows the relevant dimensions

of the generic model of the vehicle.

B. Geometric Model of Vehicle

The spoiler has been modeled from the base by keeping

the dimensions suited to the sedan. The CAD model of

the spoiler has also been designed on SolidWorks 2015,

to be assembled with the sedan CAD model for further

analysis. The length, width and height of the spoiler

body are 1m, 0.16m and 0.05m respectively. No vertical

supports are modeled for the spoiler as they would have

a negligible effect due to their sleek geometry. Fig. 2

Fig. 1: Side and Top view of the Vehicle

with dimensions

Fig. 2: Side and Top view of the Spoiler

with dimensions





shows the relevant dimensions of the generic model of the spoiler.

C. Final CAD Assembly

The spoiler has been integrated into the sedan and the

diffuser cut at the rear underside of the sedan. The

spoiler has been appended to the sedan using the

SolidWorks Assembly feature. The diffuser has been

simply cut into the rear underbody. Fig. 3 shows the

final CAD assembly containing the sedan, spoiler and

diffuser acting as one unit. Six different cases

including original model are designed. The spoiler and

diffuser angle was set to 2, 5, 7, 10 and 13 degrees

respectively for each case.

NUMERICAL SIMULATION

The numerical simulation has been conducted on ANSYS Fluent. All the cases have been

analyzed with the same configuration for mesh generation, boundary condition and solver.

A. Mesh Generation

A triangular type surface mesh is generated on ANSYS Fluent, as can be seen in Fig. 4 and 5.

This type of mesh has been used due to its proximity to changing curves and bends. The mesh

was such generated that the solid body to be studied was meshed with fine elements. The small

sized elements transitions as it grows in size to the areas where accuracy is not required.

Computational time is greatly reduced as flow separation occurring at rear end of vehicle can be

accurately predicted.The bounding box dimensions are 3.8m, 2.02m and 1.7634m for length,

breadth and height respectively. To resolve boundary layer smoothly, since boundary layer

separation is important, five layers of inflation are added around the vehicle surface and the road.

Program controlled surface mesher was used and the transition ratio was set to be 0.272 with a

growth rate of 1.2.

Fig. 3: CAD Model of Vehicle with Spoiler





B. Boundary Conditions

The enclosure has been specified with the inlet face for velocity flowing into the enclosure and

the outlet face for pressure formation at rear of vehicle model. A model is initialized for

atmospheric pressure as constant output pressure. The different velocities set as input for the

simulations were 19.5 m/s, 25 m/s, 30.5 m/s and 38.9 m/s. The wall surfaces and the symmetry

surfaces at the top and side are treated as “no-slip” condition by the solver for the case of a

stationary wall. The boundary conditions applied for the study are shown in Table I. All test cases

have the same boundary conditions employed to study the variation for different spoiler and

diffuser angles, pertaining to different velocities.

TABLE I BOUNDARY CONDITIONS APPLIED ON THE STUDY FOR ALL CASES

Region Boundary Conditions

At Inlet Turbulence Intensity = 5%

Inlet Velocity = 19.5m/s, 25m/s, 30.5m/s, 38.9m/s

At Outlet Outlet Pressure, Reference Pressure = 0 Pa

Top, Side and Ground Wall

Reference Area (for drag and lift coefficients) Frontal Area = 8.6 m2

Reference Temperature 300 K

C. Solver

The ANSYS Solver is capable of solving the governing equations related to flow physics

problems. For the solution setup, a steady state pressure-based solver was used. Due to its

stability and ease of convergence the standard k-epsilon model with standard wall functions was

the chosen as the turbulence model. The solution has been initialized using hybrid initialization

and SIMPLE scheme was set as the iterative algorithm. First order upwind discretization scheme

has been used for turbulent kinetic energy and turbulent dissipation rate. Also, second order

upwind discretization has been used for momentum and energy.

Fig. 4: Mesh Generation (Side view) Fig. 5: Mesh Generation (Close up view of

Rear)





SIMULATION RESULTS

The simulation has been executed for four different inlet velocity conditions as cruise speed

directly impacts the formation of aerodynamic wake behind the vehicle. The aerodynamic lift,

drag and flow characteristics of a generic sedan passenger vehicle with a spoiler and diffuser at

different angles have been numerically investigated. The test case analysis at four different

boundary inlet velocity conditions produced results, which are compiled and then presented for

understandable insight.

A. Color Contour Analysis

The velocity distribution of the air flow causes aerodynamic loads to act on the vehicle, which

amounts to the velocity existing at various parts of the vehicle surface, for some applied velocity.

The total drag and lift coefficient is affected by the amount of velocity, as velocity is squarely

proportional to pressure.

A comparison of the contours gives an insight into the behavior of the pressure region at the rear

of the vehicle. The blue region at the rear of the vehicle in the images indicate the area where the

boundary layer speed relative to vehicle lowers down to almost zero. This is where the low

pressure zone is created causing the air to become turbulent. As a result drag increases and so

does instability. The contours in Fig. 6-8 show that with the increase in inclination angle,

formation of low pressure zone decreases at the rear of the vehicle thereby affecting the drag

acting on the vehicle.

The rear spoiler and diffuser can be observed to smoothen the air flow. The transition from the

roof to spoiler and from underbody to diffuser becomes gentle causing a delay in flow separation.

Apart from this, the high pressure in front of the spoiler helps in generating negative lift by

creating downforce on the vehicle.





a. 2 degree

b. 5 degree

c. 7 degree

d. 10 degree

e. 13 degree

a. 2 degree

b. 5 degree

c. 7 degree

d. 10 degree

e. 13 degree

Fig. 6 (a-e): Velocity Contours at

70kmph Fig. 7 (a-e): Velocity Contours at

90kmph





a. 2 degree

b. 5 degree

c. 7 degree

d. 10 degree

e. 13 degree

a. 2 degree

b. 5 degree

c. 7 degree

d. 10 degree

e. 13 degree

Fig. 8 (a-e): Velocity Contours at

110kmph Fig. 9 (a-e): Velocity Contours at 140kmph





B. Graphical Representation

Once the simulation was finished, the drag and lift coefficients were plotted based on the

findings. These values of coefficient are essential for the ease of understanding and examining

the optimal inclination angle of spoiler and diffuser. The effect of inclination angle is validated

for varying vehicle velocity. From observations recorded in Table II, the maximum value of Cd is

at 2 degree inclination angle of spoiler and diffuser (V = 70 kmph), and the minimum value of Cd

is at 5 degree inclination angle of spoiler and diffuser (V = 140 kmph). There is not much

difference between the maximum and minimum value of Cd. According to Table III, the highest

value of Cl is at 2 degree inclination angle of spoiler and diffuser (V = 70 kmph), and the lowest

value of Cl is at 13 degree inclination angle of spoiler and diffuser (V = 90 kmph). It can also be

noticed that Cl value variation is constant for changing inclination angle at different vehicle

velocities.

TABLE II DRAG COEFFICIENT VALUES FOR DIFFERENT SPOILER AND DIFFUSER ANGLES AT DIFFERENT

WIND VELOCITIES

Inclination Angle Wind Velocity

70kmph 90kmph 110kmph 140kmph

2 degree 0.12992 0.12798 0.12978 0.12965

5 degree 0.12764 0.12714 0.12681 0.12661

7 degree 0.12878 0.12853 0.12843 0.12798

10 degree 0.12743 0.12759 0.12725 0.12690

13 degree 0.12843 0.12803 0.12788 0.12768

Fig. 10: Drag Coefficient vs. Inclination Angle graph at different wind velocity





TABLE III LIFT COEFFICIENT VALUES FOR DIFFERENT SPOILER AND DIFFUSER ANGLES AT DIFFERENT

WIND VELOCITIES

Inclination Angle Wind Velocity

70kmph 90kmph 110kmph 140kmph

2 degree -0.04594 -0.04622 -0.04634 -0.04642

5 degree -0.06463 -0.06456 -0.06454 -0.06434

7 degree -0.07295 -0.07276 -0.07303 -0.07291

10 degree -0.09214 -0.09246 -0.09234 -0.09228

13 degree -0.10158 -0.10194 -0.10167 -0.10147

Fig. 11: Lift Coefficient vs. Inclination Angle graph at different wind velocity

CONCLUSION

By using numerical simulation, the aerodynamic properties of a simple sedan were studied at

different spoiler and diffuser inclination angle. The study was conducted by assuming a fixed

ground clearance of the sedan. The aim behind keeping both the spoiler and diffuser inclined at

the same angle was to observe the formation of wake when the spoiler and diffuser complement

each other. The results from simulation have shown that the angle of inclination plays a vital role

on the wake and rear underbody of the vehicle. The relation between the drag and lift coefficients

with respect to varying angles at different speeds can be clarified from the graphs plotted in

Figure 10 and 11. With increase in the inclination angle, the drag decreases and increases but





does not exceed the initial base model drag value. The negative lift on the other hand, keeps on

increasing. At higher speeds, the negative lift seems to escalate faster, reaching higher values.

The investigation revealed that among the different simulations for four wind velocities, the most

optimum inclination angle for drag is recorded at 5 degrees at 140 kmph wind velocity. The

highest negative lift induced is observed at 2 degrees inclination angle at 70 kmph wind velocity.

FUTURE SCOPE

Although the study conducted so far has borne some valuable insights, there is always scope for

improvement. Further research on this area of study would be greatly beneficial to refining the

results.

A more explicit vehicle body can be used for the study with fine meshing and increased

number of iterations to give more precise values of drag and lift coefficients.

Another scope of study would be to research different types of spoilers and diffusers and the

best possible combinations that yield the optimal outcomes.

Other passive aerodynamic drag reduction devices can be modified and the effect of their

design modification further explored upon.

The height of the spoiler can be considered as a defining parameter that can be researched

upon as the diffuser inclination angle changes.

The spoiler and diffuser can be adjusted for a combination of different angles each and their

effect studied.

REFERENCES

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Program. U.S. Department of Energy, Washington, DC.

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Aerodynamic Drag Reduction Methods. International Journal of Mechanical & Mechatronics Engineering 14.

3. Kang, S., Jun, S., Park, H., Song, K., Kee, J., Kim, K., Lee, D., 2012. Actively translating a rear diffuser device

for the aerodynamic drag reduction of a passenger car. International Journal of Automotive Technology 13.

4. A., M. A., 2016. CFD ANALYSIS OF DIFFUSER IN A CAR FOR DOWNFORCE GENERATION.

International Journal of Research in Engineering and Technology 05..

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Engineering 194.

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Theses). Santa Clara University.





7. Mashud, M., Ferdous, M., Omee, S., 2012. Effect Of Spoiler Position On Aerodynamic Characteristics Of An

Airfoil. International Journal of Mechanical & Mechatronics Engineering 12.

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Engineering 3. doi:10.1080/23311916.2016.1230310.

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