|Cross-sectional view of the new spoiler attached to the back of a modeled minivan. Click to enlarge. Source: Inchul Kim|
A team of researchers has used the principles of fluid dynamics and numerical simulation to design a new rear spoiler for bluff-backed vehicles (such as minivans and SUVs) that can reduce drag and lift significantly.
In a paper published in the International Journal of Vehicle Design, Inchul Kim of Metacomp Technologies, in Agoura Hills, California, working with Xin Geng and Hualei Chen of the University of Michigan-Dearborn report that the aerodynamic drag and lift on a mini-van moving at 108 kph (67 mph) are reduced by 5% and more than 100%, respectively, when the new spoiler is attached to it.
With as much as 65% of the power required for ground vehicles to travel on a highway at 70 miles per hour is being consumed due to aerodynamic drag, the reductions from the spoiler could increase fuel economy by up to several miles per gallon, the researchers say.
A traveling vehicle experiences two main aerodynamic forces: drag and lift. Drag (air resistance) is in the opposite direction to the movement. Lift is perpendicular to the drag and to the road. The greater the drag, the more energy is needed to propel the vehicle and so the more fuel that it uses to cover the same ground at a given speed. Lift reduces the amount of friction between the tires and the road and so lowers traction. This also reduces efficiency as well as deleteriously affecting handling.
Total aerodynamic drag acting on a moving body is the sum of friction drag (the drag force component on the body due to the shear stresses) and pressure drag (the drag force component on the body due to the pressure).
The total drag on a streamlined body such as an airfoil is contributed mainly by the friction drag and is very low at high Reynolds numbers(CD ≈ 0.01 at Re ≈ 106). In contrast, the drag on a body with a bluff back is contributed mainly by the pressure drag and is very high (CD ≈ 1.2 at Re ≈ 106). This, the researchers say, is because the boundary layer flow separates from the rear top of the body so that the pressure magnitude at the rear of the body is much smaller than that at the front.
Conventional spoilers resemble an inverted plane wing and generally work by increasing the downward force on the back of the vehicle as well as improving the flow of air across the bluff rear. The new rear spoiler resembles a wave in profile rather than a wing.
In developing the spoiler for bluff-backed vehicles, the researchers focused on three conditions:
The rear-spoiler should act as a diffuser when it is attached to the roof end of a vehicle so that the pressure on the back of a vehicle with the spoiler should be higher than that on the back of a vehicle without the spoiler.
Normally any additional attachment to a vehicle causes an additional drag due to the presence of it that is attached to a moving body. The rear-spoiler should be designed in such a way that the drag on the spoiler itself should be much smaller than the total drag or be negative (i.e., in the direction of the vehicle motion).
The lift on the spoiler itself should be minimal if it is positive or be negative (i.e., in the direction to the ground).
The scientific principle and geometry of the new rear-spoiler are completely different from those of conventional spoilers that have been used so far. The new rear spoiler acts like a diffuser when it is attached to the back of a vehicle, and thus, the pressure on the back of a vehicle with the new spoiler is higher than that on the back of a vehicle without it.
The new rear spoiler can be applied to other minivans, vans, sports-utility-vehicles and buses. With the new spoiler attached, the vehicles having a bluff back would have not only a higher mileage but also better stability.—Kim et al. (2008)
Kim, K., Geng, X. and Chen, H. (2008) Development of a rear spoiler of a new type for mini-vans, Int. J. Vehicle Design, Vol. 48, Nos. 1/2, pp.114–131 doi: 10.1504/IJVD.2008.021155