ABSTRACT
Aerodynamics is one of the most important factors contributing to the resistive forces acting on a vehicle when it moves through a fluid medium. Several aerodynamic forces such as lift, side force, and drag are responsible for this resistance. Reducing aerodynamic drag not only enables higher top speeds but also decreases overall fuel consumption and improves ride comfort. These factors are particularly critical for passenger cars, as they influence vehicle performance, market popularity, and marketing strategies. Consequently, researchers continuously attempt to optimize vehicle design features to reduce aerodynamic losses. This review paper presents various research works that have been carried out to reduce aerodynamic drag across different vehicle segments.
Key Words: Aerodynamic drag, Computational Fluid Dynamics (CFD) analysis, Fuel economy, Wind tunnel
1. INTRODUCTION
When a vehicle is in motion, several forces act upon it, such as drag force and downforce. Among these, aerodynamic drag is the most dominant force contributing to increased fuel consumption and reduced top speed. There are different types of drag forces acting on a vehicle, namely parasitic drag, lift-induced drag, induced drag, and wave drag. Parasitic drag is further subdivided into form drag, skin-friction drag, and interference drag.
These individual drag components are difficult to calculate separately; therefore, most studies focus on determining the overall drag coefficient of the vehicle. This coefficient can be obtained experimentally using wind tunnels, where scaled models of vehicles are tested under controlled airflow conditions. The basic equation governing aerodynamic drag is given by:D=21ρCdAV2
where,
Cd = coefficient of drag
A = frontal area
V = relative velocity of the object with respect to the fluid medium
ρ = density of air
From the equation, it can be observed that air density remains nearly constant under normal conditions. One of the primary parameters that can be effectively modified is the frontal area. Therefore, optimizing the frontal area by modifying vehicle design can significantly reduce aerodynamic drag. Vehicle design optimization is commonly achieved using aerodynamic aids such as air dams, spoilers, vortex generators, and other flow-control devices.
2. LITERATURE REVIEW
R. H. Heald (1933) [1] investigated four automobile models to determine their drag coefficients and compared them with a model developed ten years earlier. The study revealed that eliminating fenders and other protruding elements, along with improved body fairing, significantly reduced the drag coefficient. A further reduction was achieved by removing the windshield and streamlining the entire vehicle body to resemble a thick aircraft wing section.
Shobit Sengar et al. (2014) [2] analyzed the aerodynamic forces acting on three different vehicles: the Hindustan Ambassador, Lamborghini Aventador LP 700-4, and a Formula One car. Scaled models of these vehicles were tested in a wind tunnel under varying wind conditions. A comparative analysis revealed that the Formula One car exhibited the best aerodynamic performance, followed by the Aventador and then the Ambassador. The superior performance of the first two vehicles was attributed to their low ground clearance and streamlined body profiles. Additionally, the coupe-style contours help channel airflow efficiently toward the rear, where spoilers provide enhanced stability at high speeds.
Abdellah Ait Moussa Et al. (2015)[3] worked on reduction of Aerodynamic Drag in generic trucks using
geometrically optimized rear cabin bumps. They used a 1/10th scaled half model of a generic truck and added three equally spaced bumps on the top of the cabin surface. Thereafter they used Taguchi or Orthogonal array optimization method to study the effect of these bumps on drag. Next, they tested Solid works model with and without bumps in ANSYS workbench and plotted a curve for the pressure distribution over the cabin. The authors concluded that the optimal geometrical parameters of the bumps for maximum drag reduction (i.e., 9.83%) should be as follows:(W/H0)=0.088,(L1/H0)=0.334,(L2/H0)=0.078,(h/H0)=0.062
where W, H₀, H₁, and h represent the characteristic dimensions of the bump geometry.
Figure 1. Generic truck test model
Taherkhani A. R. et al. (2015) [4] carried out experimental and computational investigations into the aerodynamics of emergency response vehicles, focusing on reducing the additional drag caused by the common practice of installing light bars on vehicle roofs. Their study revealed that reducing the fuel consumption of the YAST ambulance fleet by 5% could result in annual savings of approximately £350,000 and reduce carbon dioxide emissions by 250 tons of CO₂. These benefits could be extended across the UK’s NHS national fleet.
Jeff Howell et al. (2013) [5] investigated the drag and lift coefficients of a bluff body using wind tunnel testing. The model closely represented a quarter-scale hatchback vehicle. The researchers varied the taper length from 0.075 m to 0.225 m and the taper angle from 5° to 25° to examine their effects on aerodynamic drag under different conditions. The results were presented graphically, as shown below.
Chart 1. Effect of taper length and taper angle on drag coefficient
Yingchao Zhang et al. (2009) [6] described the implementation of a virtual wind tunnel test simulation in their study. Using computational methods, they obtained aerodynamic drag coefficients, velocity contours, and pressure distributions. The authors proposed several recommendations to reduce the aerodynamic drag of the vehicle design. They concluded that CFD-based numerical simulation is a simple, effective, convenient, and rapid approach for aerodynamic analysis during the car styling process.
Abdulkareem S. H. Mahdi-Obaidi et al. (2014) [7] performed experimental and numerical investigations on an open-wheel race car designed by students of Taylor’s University. Wind tunnel testing and ANSYS Fluent simulations were conducted to study the effect of radiator air-channel geometry on drag optimization. The results showed that increasing the radiator channel tilt angle from 36° to 72.5° reduced the drag coefficient from 0.619 to 0.563. The experimental and numerical results were in close agreement, with a deviation of only 7.7%.
J. Abhinesh et al. (2014) [8] conducted a CFD analysis on two Volvo intercity bus models—one existing model and one modified version—to reduce aerodynamic drag and fuel consumption. The analysis revealed a drag reduction of approximately 10%. The drag coefficient of the original model was 0.8, while that of the modified model was reduced to 0.7.
Francesco Mariani et al. (2012) [9] numerically investigated a race car model developed by students at the University of Perugia. Their study focused on optimizing aerodynamic drag by modifying the vehicle’s nose design. The original configuration was referred to as Model A, while the modified configuration was designated Model B. In Model B, a front wing was added, the headrest was modified, an air extractor was incorporated, and a wing was added to the front tire. The resulting improvements in aerodynamic forces are illustrated below.
Figure 2. Percentage improvement of aerodynamic forces for Model B relative to Model A
Ashfaque et al. (2014) [10] discussed the analysis of drag force on a car using a low-speed wind tunnel. In their experimental setup, a Pitot tube, manometer, and a solid object (airfoil) were used. The velocity of the airflow, along with drag and lift forces, was calculated. They observed that the design of a low-speed open-circuit wind tunnel differs somewhat from conventional wind tunnels, particularly due to its flexible diffuser. The construction of the wind tunnel was low-cost, simple in design, and utilized materials readily available in the market. The setup was found to be useful for educational and research purposes.
Keisuke Nisugi et al. (2004) [11] investigated aerodynamic drag reduction in vehicles using a feedback flow control system. In their study, a sensor in the form of a control-flow nozzle was mounted on the vehicle to provide velocity and pressure information to the controller. Based on these inputs, the controller activated an actuator that operated the control port, enabling blowing and suction of air. The nozzle was positioned on a section of the front windshield. With proper systematic calculations and control, a drag reduction of approximately 20% was achieved compared to a vehicle without feedback flow control.
L. Anantha Raman et al. (2016) [12] conducted a comparative study of different aerodynamic drag reduction techniques aimed at reducing fuel consumption in vehicles. Passive tests were carried out on an SUV model by extending the rear end (rear fairing), adding a rear plate (rear screen), and installing vortex generators of delta-wing and bump shapes. Drag reductions of 6.5% and 26% were achieved using rear screens and rear fairings, respectively. Among the vortex generators, the delta-wing type was found to be the most effective in reducing drag.
Upendra S. Rohtagi et al. (2012) [13] tested a small-scale model of a General Motors SUV in a wind tunnel under expected wind conditions and simulated road clearance. Two passive devices—a rear screen (a plate installed behind the vehicle) and a rear fairing (aerodynamic extension of the rear end)—were incorporated into the model and tested under different wind conditions. The results showed that the rear screen could reduce drag by up to 6.5%, while the rear fairing achieved a drag reduction of 26%. The study also emphasized that the effectiveness of rear screens depends on their configuration, dimensions, and placement, as well as on the geometry of the vehicle’s rear section.
S. M. Rakibul Hassan et al. (2014) [14] employed numerical methods to reduce aerodynamic drag in a racing car. Underbody modifications were carried out by slicing the underbody, allowing increased airflow suction in the low-pressure zone. A plot of drag coefficient versus slicing angle is shown below. A noticeable reduction in drag was observed. Another technique involved redirecting exhaust gases at an angle of 45° toward the low-pressure region behind the car to minimize negative pressure effects. It was observed that decreasing exhaust velocity also resulted in a reduction in the drag coefficient.
Figure 3. Coefficient of drag versus slicing angle
Mohd Nizam Sudin et al. (2014) [15] reviewed the performance of active and passive flow-control techniques for reducing vehicle aerodynamic drag. The review primarily focused on methods used to prevent or delay airflow separation at the rear end of vehicles. Passive methods such as vortex generators, spoilers, and splitters, as well as active flow-control techniques including steady blowing, suction, and air jets, were discussed. The study concluded that aerodynamic drag accounts for approximately 50% of a vehicle’s fuel consumption and that active flow-control methods are generally more effective than passive methods in reducing drag.
Rose McCallen et al. (1999) [16] conducted wind tunnel analysis on 1:14 scale Class 7 and Class 8 heavy-duty Sandia truck models to reduce aerodynamic drag and improve fuel economy. Particle Image Velocimetry (PIV) measurements were taken in the wake region of the models. Oil Film Interferometry (OFI) was used to measure skin friction, and Pressure-Sensitive Paint (PSP) techniques were applied for pressure measurements. The authors concluded that the PIV-based approach is effective in obtaining precise and accurate aerodynamic data in wind tunnel testing.
Yiping Wang et al. (2016) [17] performed numerical simulations on a generic vehicle model to optimize aerodynamic drag using a dimpled, non-smooth surface. The dimpled surface promoted turbulent airflow around the vehicle, delaying flow separation and resulting in a smaller wake and reduced form drag. A Kriging surrogate model was used to design the dimpled surface, and ANSYS Fluent was employed for CFD simulations. The results showed a reduction in drag coefficient at air speeds below 10 m/s, while a nearly constant drag coefficient was observed at speeds above 20 m/s.
Figure 3. Ahmed body with dimpled rear slant
Pikula Boran et al. (2011) [18] conducted a numerical aerodynamic analysis of the exterior of a 1:18 scale Peugeot 407 Coupe using ANSYS Fluent. Pressure distribution around the vehicle in motion was analyzed, and the numerical results were compared with experimental data. The comparison showed close agreement with only minor deviations. However, the authors concluded that for detailed analyses such as airflow around the engine compartment, wind tunnel testing is preferable to CFD simulations.
3. CONCLUSIONS
Based on the reviewed literature, it can be concluded that aerodynamic drag is one of the most significant factors influencing fuel consumption, power loss, and top speed in vehicles. It is evident that external design features play a dominant role in reducing aerodynamic drag. Most researchers have adopted similar approaches involving modifications to vehicle exterior geometry, such as the use of vortex generators, rear screens, rear fairings, fenders, and variations in rear taper and underbody angles.
Most of these aerodynamic modifications have been applied to race cars, with fewer studies focusing on passenger vehicles and heavy-load vehicles. Notably, limited research has been conducted on the combined use of front and rear spoilers specifically for passenger cars to reduce aerodynamic drag. Therefore, there exists considerable scope for further research in this area. Such modifications may be particularly beneficial for high-end passenger vehicles, where cost constraints are less critical for consumers.