Dynamics of manipulated bluff body wakes

Abstract

An experimental investigation of drag reduction devices for bluff bodies in ground proximity has been conducted. The main goal of the research is to gain a better understanding of the drag reduction mechanisms of several drag reduction devices. We report unsteady base pressure measurements and Particle Image Velocimetry (PIV) measurements of the velocity field in the near wake of the models. Averages of more than 200 PIV velocity vector fields were used to compute the mean velocity and turbulent stresses at several cross section planes. The mean pressure results show a significant increase in the base pressure of models with drag reduction devices, consistent with the drag reduction reported by Lanser, Ross and Kaufman for similar geometries. The drag reduction devices strongly influence the base pressure spectral peak at a Strouhal number of 0.12 suggesting that, for the present geometries, the drag reduction devices suppress large-scale turbulent motions in the wake. The PIV results show a reduction of the turbulence intensity as well as a rapid upward deflection of the underbody flow. The effect of the drag reduction devices on the length of the recirculation region in the near wake is small. INTRODUCTION The aerodynamic drag of bluff bodies has been the subject of many scientific studies. Of particular interest is the drag of bluff bodies with a blunt base because of its relevance to road vehicle aerodynamics. Research in the last few decades motivated in part by increased fuel prices has led to significant advances in understanding of drag reduction mechanisms for blunt-based bluff bodies. Mair provides an overview of techniques proposed for drag reduction of bluff bodies and points out important differences between plane and axisymmetric geometries. Most of these techniques including splitter plates, base bleed, ventilated and closed cavities, and trailing edge modifications reduce aerodynamic drag by stabilizing the large-scale vortical motions in the near wake. It follows that effectiveness of these techniques depends on the nature of the largescale motions in the wake as well as the receptivity and amplification of disturbances in the separated shear layers. Another technique frequently used to reduce the drag of bluff bodies is boat-tailing. In this case the base drag is reduced using a properly shaped afterbody that avoids boundary layer separation as the body cross section area is reduced. The external flow remains attached over a longer distance, which enhances pressure recovery and reduces the recirculation region in the near wake. Both of these effects tend to increase the base pressure and reduce drag even for relatively short boat-tailed afterbodies. Other drag reduction techniques based on the same principle include boundary layer control by blowing and/or suction, and moving walls, as well as Coanda jet blowing. An overview of drag reduction techniques for road * Graduate Research Assistant, Department of Aerospace Engineering, Member AIAA. † Associate Professor, Department of Aerospace Engineering, Senior Member AIAA. ‡ Staff Research Engineer, Vehicle Analysis and Dynamics Lab. § Director of Advanced Engineering, Saab Automobile AB. Copyright © 2000 Luis P Bernal, Published by the American Institute of Aeronautics and Astronautics, Inc. with permission AIAA-2000-2556