Influence of afterbody rounding on the pressure distribution over a fastback vehicle

Abstract Experimental analyzes were performed to understand the drag evolution and the flow field modifications resulting from afterbody rounding on the Ahmed body, a simplified vehicle model with 25 degrees rear slant. Curvature effects were investigated using balance measurements, flow visualizations, wall pressure, and particle image velocimetry measurements. The rear end of the original well-known Ahmed body has sharp connections between the roof and the rear window as well as squared rear pillars. Similarly to previous studies, rounding the roof/backlight intersection was shown to reduce drag up to 16 %. Surprisingly, additional rear curvature associated with side pillar rounding did not further modify the drag. However, the zero net effect was found to result from opposite drag effects on the slanted and vertical surfaces and to hide strong local modifications on the flow field. The tridimensional organization and vorticity transport in the near wake were analyzed and connected to the observed local increase in the pressure drag on the base. Finally, these results were shown to be generic for a realistic rounded-end car shape.

[1]  Sandrine Aubrun,et al.  Effects of suppressing the 3D separation on the rear slant on the flow structures around an Ahmed body , 2012 .

[2]  Jean-François Beaudoin,et al.  Drag and lift reduction of a 3D bluff body using flaps , 2008 .

[3]  L. Graftieaux,et al.  Combining PIV, POD and vortex identification algorithms for the study of unsteady turbulent swirling flows , 2001 .

[4]  Gunther Ramm,et al.  Some salient features of the time - averaged ground vehicle wake , 1984 .

[5]  Sinisa Krajnovic,et al.  Flow Around a Simplified Car, Part 2: Understanding the Flow , 2005 .

[6]  John P. Davis.  Wind tunnel investigation of road vehicle wakes , 1983 .

[7]  J. Lumley,et al.  A First Course in Turbulence , 1972 .

[8]  Philippe Planquart,et al.  Experimental aerodynamic study of a car-type bluff body , 2011 .

[9]  Martin A. Passmore,et al.  The importance of rear pillar geometry on fastback wake structures , 2014 .

[10]  Jeff Howell,et al.  The Effect of Backlight Aspect Ratio on Vortex and Base Drag for a Simple Car-Like Shape , 2008 .

[11]  Joseph Katz,et al.  Race Car Aerodynamics: Designing for Speed , 1996 .

[12]  Fabien Harambat,et al.  WAKE STRUCTURE AND DRAG OF VEHICLES WITH ROUNDED REAR EDGES , 2015 .

[13]  James M. Luckring,et al.  Initial Experiments and Analysis of Blunt-Edge Vortex Flows , 2008 .

[14]  P. Joseph.  Application du contrôle par jets pulsés à un corps non profilé , 2012 .

[15]  M. Thompson,et al.  The effect of aspect ratio on the wake of the Ahmed body , 2015 .

[16]  Jeff Howell,et al.  The Influence of Ground Simulation on the Aerodynamics of a Simple Car Model , 1997 .

[17]  Jean-Luc Aider,et al.  Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets , 2012 .