Investigation of Drag Reduction Technologies for Light-Duty Vehicles Using Surface, Wake and Underbody Pressure Measurements to Complement Aerodynamic Drag Measurements.

Fenella De Souza,Marc Belzile,Arash Raeesi,Cheryl Caffrey,Andreas Schmitt

doi:10.4271/2019-01-0644

Abstract

Amulti-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 fullscale 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.

Highlights

Vehicle aerodynamic performance is a highly technical and complex issue
The magnitude and spatial extent of the measured pressure changes due to the various technologies were often consistent with the amount of drag reduction
Blocking the wheel curtains at baseline ride height increased the drag area by roughly half of the amount measured at reduced ride height, and caused measurable surface pressure changes only at centerline taps at the leading edge of the hood and at the highest point on the roof, where the pressure was reduced by amounts equivalent to 1.5% and 1.1%, respectively, of the freestream dynamic pressure

Summary

Introduction

Vehicle aerodynamic performance is a highly technical and complex issue. Adding a feature such as an air dam or active grille shutter to a vehicle optimized aerodynamically in its absence may not lead to the expected improvement in performance. The GESS wheel rollers do not support the full weight of the vehicle, so pre-test preparations included measuring the baseline ride height and removing the suspension springs. OEM underbody panels were removed so that their effect on drag, surface, wake and underbody pressures could be evaluated for four vehicles of this study: a crossover SUV, two compact sedans and a mid-size sedan. To investigate the maximum potential drag benefit, and the corresponding effect on surface and wake pressures, six vehicles were tested with and without their side mirrors installed: one crossover SUV, all four sedans and the pickup truck. In order to assess any possible effects of the rakes and tracks on aerodynamic drag measurements, baseline measurements were obtained with and without the underbody rake and track installed, with the wake rake at different streamwise locations and with the wake rake and track completely removed. Delta drag area measurements for the various aerodynamic treatments were calculated based on a reference condition with the same wake and underbody rake configuration

Results and Discussion

Summary

Technologies for Light-Duty Vehicles