Aerodynamic Design

Vehicle Design

Drag Force - Aerodynamic Design

The vehicle’s global body shape governs the average aerodynamic performance, so it is the most important design feature for reducing C_D.  The shape should maintain attached flow over most of the surface and can be achieved by having a streamlined shape. 

The most efficient shapes are the simple ‘teardrop’ or the airfoil based one.  Unfortunately, these shapes aren’t necessarily optimal for overall vehicle design because they either may not gain acceptance by consumers, or can degrade other functional aspects of a vehicle such as trunk storage and dynamic stability. 

The entire car should have smooth shapes and sharp edges should be avoided at all costs since they create adverse pressure gradients that produce turbulent flow and drag.  A “smooth” design can be accomplished by creating a shape with the smallest possible curvature line, and, for instance, having the highest possible angle between the hood and the front windshield (rake angle).  The rear of the car must be carefully designed in concerns with drag since undesired separated flow in this area will create a wake, thus, increase drag.  An optimal design has the separation occur as near to the end of the vehicle as possible by incorporating a small taper as in a cutback.  Also, a reduction in the frontal area of the vehicle will greatly reduce drag.  However, reducing a vehicle’s frontal area is a very limiting solution, since that frontal area is typically predetermined for a vehicle program according to its size category (ie. small car, SUV, full-size sedan, etc).  Reducing it thus means reducing the vehicle’s overall size.

Another source of drag is the complex flow in the wheelhouses.  The airflow in the wheelhouse and around the wheels can stand for 30 percent of the total drag.  Covering the wheelhouses may stabilize the flow in those areas and reduce drag very significantly.  However, styling and functional issues (front turning wheels) often make it hard to implement this solution.  A compromise in design and functionality includes integrating the shape of the wheel well with the shape of the body.

Pressure Distribution with and without Wheel Spoilers

Rearview mirrors also contribute to a significant amount of a vehicle’s drag as shown in the pressure distribution results in the flow simulation figure to the right.  Mirrors are a protrusion on the body of the car that produces a wake behind it.  By careful design, the drag of a rearview mirror can be improved by 50% through integrating mirror in the general shape of the car.  However, the best solution for drag reduction is without doubt, the simple replacement of the rearview mirrors with rear cameras and onboard screens.  This will eliminate the entire drag produced by the rearview mirrors and will provide a great improvement for the C_D of the car.  However, this is not a practical solution due to high cost and cameras and screens would add numerous processes to the manufacturing process, not to mention that the electronics are highly environmentally intensive to produce.  Also, any fuel efficiency gain in drag reduction would be offset by the reduction in fuel economy from the increased electrical load.

The underbody of a car provides a large area of improvement for the aerodynamic drag.  An optimized underbody can provide a 25% decrease in the total drag.  An exposed underbody contains uneven and cleft structure that triggers separation and turbulence.  The flow is slowed down and the pressure gradient becomes unfavorable which increases the pressure drag.  Covering open areas with panels reduces the turbulence and straightens the flow, thus leading to a lower drag.  Also, fitting an air dam to the front spoiler can improve drag by reducing the airflow under the vehicle. 

However, adding underbody panels does present significant design issues that must be considered.  The added material increases overall vehicle weight (detrimental to fuel economy) and adds manufacturing processes requiring energy which has a negative impact on the environment.  In addition, underbody panels could pose thermodynamic conflicts (hot underbody parts) and repair obstacles as the suspension and driveline could become unaccessible.