A multi-year, multi-vehicle study was conducted to quantify the aerodynamic drag
changes associated with drag reduction technologies for light-duty vehicles.
Various technologies were evaluated through full-scale testing in a large
low-blockage closed-circuit wind tunnel equipped with a rolling road, wheel
rollers, boundary-layer suction and a system to generate road-representative
turbulent winds. The technologies investigated include active grille shutters,
production and custom underbody treatments, air dams, wheel curtains, ride
height control, side mirror removal and combinations of these.
This paper focuses on mean surface-, wake-, and underbody-pressure measurements
and their relation to aerodynamic drag. Surface pressures were measured at
strategic locations on four sedans and two crossover SUVs. Wake total pressures
were mapped using a rake of Pitot probes in two cross-flow planes at up to 0.4
vehicle lengths downstream of the same six vehicles in addition to a minivan and
a pick-up truck. A smaller rake was used to map underbody total pressures in one
cross-flow plane downstream of the rear axle for three of these vehicles.
The results link drag reduction due to various technologies with specific changes
in vehicle surface, rear underbody and wake pressures, and provide a database
for numerical studies. In particular, the results suggest that existing or
idealized prototype technologies such as active grille shutters, sealing the
external grille and ride height control reduce drag by redirecting incoming flow
from the engine bay or underbody region to smoother surfaces above and around
the vehicle. This mechanism can enhance the reduction in wheel drag due to
reduced wheel exposure at lowered ride height. Sealing the external grille was
found to redirect the flow more efficiently than closing the grille shutters,
and resulted in greater drag reduction. Underbody treatments were also found in
some cases to redistribute the flow around the vehicle to reduce pressure drag
in addition to underbody friction drag. The magnitude and spatial extent of the
measured pressure changes due to the various technologies were often consistent
with the amount of drag reduction.
