CFD simulations using ANSYS FLUENT 6.3.26 have been performed on a generic SUV design and the settings are validated using the experimental results investigated by Khalighi. Moreover, an addon inspired by the concept presented by Englar at GTRI for drag reduction has been designed and added to the generic SUV design. CFD results of addon model and the basic SUV model have been compared for a number of aerodynamic parameters. Also drag coefficient, drag force, mean surface pressure, mean velocities, and Cp values at different locations in the wake have been compared for both models. The main objective of the study is to present a new addon device which may be used on SUVs for increasing the fuel efficiency of the vehicle. Mean pressure results show an increase in the total base pressure on the SUV after using the device. An overall reduction of 8% in the aerodynamic drag coefficient on the addon SUV has been investigated analytically in this study.
Sports utility vehicles are known worldwide for their ruggedness and high passenger capacity. Their ability to run on both offroad and onroad conditions makes them very peculiar for usage. Due to their high utility, a need for making them more fuel efficient has arisen in the past few years. The following data shows the average miles per gallon statistics for cars and SUVs. The increase in the fuel efficiency is due to improvements in the aerodynamics of the automobiles. For the SUVs the average miles per gallon has been fairly increasing in the past few years as shown in Figure
Average miles per gallon for automobiles in US from 1970 to 2006.
Aerodynamic effects on a running SUV play a substantial role in their fuel efficiency. Efforts have been made for making the SUVs aerodynamically and fuel efficient by using a variety of addon devices and other methods. Shape cannot be much compromised for the basic purpose of the SUV. Hence methods to alter the air flow over the surface of the SUV by using devices or by blowing air in the rear region of the SUV are widely used for drag reduction. Form drag or the pressure drag contributes to the 90% of the total drag [
A great amount of research has been done in the field of addon devices on SUVs and pickup trucks for reduction in drag forces by altering the air flow around and behind the vehicle. Some of the work has been reported here. Englar Robert [
Boat and tail addons have been used on a large scale over the years. Addons with different shapes and sizes have been experimented and substantial results have been worked out. In this study, a boat tail addon using the concept of Coanda effect [
The main features of the flow being observed using the wake pathlines are a strong downwash from the cab surface of the SUV and a strong upwash from the underbody surface. The same was reported by AlGarni et al. [
The basic model of the generic SUV design has been numerically solved using the RANS kw(sst) model at a Reynolds number of
conservation of mass:
conservation of momentum:
kinematic eddy viscosity:
turbulence kinetic energy:
specific dissipation rate:
Figures
Rear view of the basic SUV.
Side view of the basic SUV.
Computational domain for the SUV.
ANSYS FLUENT 6.3.26 is used to simulate the design and finally perform the postprocessing. Mesh included an inflation layer with 5 layers with a growth rate of 20% surrounding the SUV model to capture the turbulence.
Figure
Mesh of the basic SUV.
Postprocessing of the basic model case produced results which were similar to the experimental results by AlGarni et al. [
The mean Cp plots for the basic model for both the cab and the underbody surface were similar to the experimental ones by AlGarni et al. [
Cp plots for basic SUV.
As stated in Figure
Similarly, for the underbody surface, two local minima can be observed at
Figure
Streamlines in the center horizontal plane for basic SUV.
Figure of streamlines by AlGarni et al. [
Table
Comparison of Cp values.
S. number 

Cp (experimental values)  Cp (CFD values) 

1  5  1.0  1.0 
2  10  −0.5  −0.8 
3  110  0.5  0.65 
4  165  −0.9  −1.0 
5  350  −0.1  −0.1 
Figures
Mean velocity profiles for basic SUV,
Mean velocity profiles for basic SUV,
The addon device used on the generic SUV is inspired from the GETS (generic European transport system) as mentioned by van Leeuwen [
The addon works on the principle of Coanda effect patented by Coanda [
Addon device geometry used on the SUV.
Since the width of the SUV model
SUV with addon on the rear.
The addon SUV model is numerically solved using the ANSYS FLUENT 6.3.26 for a Reynolds number of
Mesh settings used are identical to the basic SUV case. Mesh has been optimized to have 1.2 million elements. Reference area used for the calculation of the coefficient of drag is 0.01080399 m^{2}. Convergence criteria are set to have a residual error of 10^{−3} and the solution was converged in 387 iterations. Similar to the basic SUV case, an inflation layer with 5 layers and a growth rate of 20% was used on the cab and the underbody boundary. Rear and side views of the model are shown in Figures
Addon SUV rear view.
Addon SUV side view.
The average coefficient of pressure plots have been calculated on the cab and the underbody surface and are compared in Figures
Cp plots comparison for cab surface.
Cp plots comparison for underbody surface.
For the cab surface case, most of the portion of the curve overlaps with the basic case. However, only noticeable difference is at the base of the SUV. Due to the suction created at the base, the air rushes past the addon leading to movement of air over a convex portion downwards which causes the velocity to increase and pressure to reduce. However, due to the addon effect the wake region is repressurized and the average value of Cp is high as compared to the basic model as shown in the figure.
Similar phenomenon occurs for the underbody surface and the average value of Cp is on the higher side as compared to the basic case leading to reduction in the drag force on the SUV.
Velocity pathlines and pressure contours have been plotted in the horizontal plane (
Figures
Velocity vectors for addon model SUV.
Velocity vectors for basic model SUV.
Velocity pathlines in the horizontal plane are shown in Figures
Velocity pathlines for addon model SUV.
Velocity pathlines for basic model SUV.
Velocity contours for the wake region are shown in Figures
Velocity contours in horizontal plane for addon SUV model.
Velocity contours in horizontal plane for basic SUV model.
Total pressure in horizontal plane for addon SUV model.
Total pressure in horizontal plane for basic SUV model.
At
for addon model: 114 Pa;
for basic model: 139 Pa.
Hence, total pressure in the base region is higher for the addon model as compared to the basic model.
Figure
Mean velocity profile comparison in horizontal plane,
The vertical component of velocity in the wake region in the center horizontal plane for both the models is shown in Figure
Mean velocity profile comparison in horizontal plane,
Symmetry plane divides the vehicle into two symmetrical parts and lies along the
Velocity vectors for addon model in symmtry plane.
Velocity vectors for basic model in symmtry plane.
Velocity contours in the symmetry plane for both cases are shown in Figures
Comparison of total pressure values in symmetry plane.

Basic model pressure 
Addon model pressure 

500  −139  −60.5 
550  37.3  98.5 
600  96.2  98.5 
650  96.2  152 
Velocity contour for addon SUV model in symmetry plane.
Velocity contour for basic model in symmetry plane.
Total pressure for addon model SUV in symmetry plane.
Total pressure for basic model SUV in symmetry plane.
Figure
Mean velocity profile comparison in symmetry plane,
Figure
Mean velocity profile comparison in symmetry plane,
Coefficients of pressure values are compared in the base plane of the SUV and are shown in Figures
basic model: −0.288 to −0.158;
addon model: −0.192 to −0.0579.
Cp values on the base of addon model SUV.
Cp values on the base of basic model SUV.
Similarly, Figures
basic model: −21.6 to 37.3 Pa;
addon model: −7.48 to 45.5 Pa.
Total pressure values on the base of addon model SUV.
Total pressure values on the base of basic model SUV.
Table
Comparison of aerodynamic parameters for addon model and basic model SUV.
S. number  Aerodynamic parameter  Basic SUV model  Addon SUV model 

1  Form drag  1.8696 N  1.6824 N 
2  Form drag coefficient  0.3140  0.2824 
3  Total force  2.1251 N  1.9557 N 
4  Total pressure on base point  −47 Pa  −19 Pa 
5  Cp on base point  −0.096  −0.035 
6  Total coefficient of drag  0.3569  0.3283 
7  Total coefficient of lift  2.74 * 10^{−2}  4.79 * 10^{−2} 
Results clearly show that the drag forces on the addon model are lower as compared to the basic model. Also the total pressure and Cp on the base point are higher in magnitude for the addon model leading to lesser drag force.
CFD results for a generic SUV design have been presented in this paper. An addon inspired by the GETS model is added to the generic SUV and CFD analysis is performed on it. Comparison of various aerodynamic parameters is done to establish the effectiveness of the addon. The various conclusions from this study can be summarized in the following points.
Coefficient of drag for the addon SUV has reduced to 0.3283, giving a total reduction of 8.013%.
Cp plots for both cases are similar, except at the end portion of the curve due to the Coanda effect, and also the flow is attached to the addon on the upper rear region of the SUV which leads to pressurization of the wake region.
Total pressure on the base point of the SUV model has increased on addition of the addon leading to higher pressure in the wake region and hence lower overall drag, according to the steady wake drag theory. Total drag force on the SUV model has reduced from 2.1251 N to 1.9557 N.
Length of the recirculation region reduces from 1.2 to 1.12 times the width of the SUV in the symmetry plane which is a characteristic of the flow region in the wake.
The wake region for the SUV has reduced in length from 750 mm to 670 mm. The shorter is the length of the recirculation region, the lesser is the drag force on the SUV.
The width of the wake region has also reduced. Location of the recirculation vortices has changed from ±55 mm to ±35 mm in the centre horizontal plane.
Total pressure and the Cp values on the base point of the SUV increased which explains the low overall drag value on the addon model. Hence, overall reduction on the drag force on the SUV leads to increase in the fuel efficiency of the SUV vehicle.
Till now, the computational analysis of the problem has been done using the CFD and significant results showing the improvements in the aerodynamics of the SUV have been presented. So, experimental analysis of this model is recommended. Comparison of the experimental and the CFD results could be done to establish the effectiveness of the addon on the generic SUV.
Other addon devices like base bleed can be used on the SUV to reenergize the wake region and reduce the drag on the SUV.
Vortex strake devices (VSD) have been successfully tested on trucks and trailers to energize the flow behind the trailer. VSD can be used on SUVs for producing the same effect and hence reduce the drag force.
Active flow control (AFC) can be implemented on the SUV design to further reduce the drag force on the SUV and increase the fuel efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.