The nature of the vortical structures in the near wake of the Ahmed body

Abstract

This study presents the results from high-spatial-resolution water-channel velocity-field measurements behind an Ahmed body with 25°rear slant angle. The Ahmed body represents a simplified generic model of a hatchback automobile that has been widely used to study near-wake flow dynamics. The results help clarify the unresolved question of whether the time-mean near-wake flow structure is topologically equivalent to a toroidal vortex or better described by a pair of horizontally aligned horseshoe vortices, with their legs pointing downstream. The velocimetry data presented allows the tracking of the vortical structures throughout the near wake through a set of orthogonal planes, as well as the measurement of their circulation. The spanwise vortices that form as the flow separates from the top and bottom rear edges are shown to tilt downstream at the sides of the body, while no evidence is found of a time-mean attached toroidal vortex, at least for the Reynolds number (based on the square root of the frontal area) of Re ffiffiffiffi ffi FA p ;30; 000 under consideration.

Adaptive control of the dynamics of a fully turbulent bimodal wake using real-time PIV

“…This simple modification leads to drastic modifications of the overall structure of the wake, together with large variations of the aerodynamic drag coefficient. As an illustration, the slanted configuration (α = 25 • ) involves large-scale streamwise longitudinal vortices (Lienhart et al, 2002;Beaudoin et al, 2004), together with an unsteady recirculation bubble over the inclined surface (Hinterberger et al, 2004;Krajnović and Davidson, 2005), a pair of a horseshoe vortices containing each a recirculation bubble (Venning et al, 2017), and spanwise Kelvin-Helmholtz vortices, together with smaller-scales turbulent structures.…”

Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations

“…However, to be able to directly compare simulations with the available high-resolution PIV measurements performed in Refs. [8] and [30], the body follows the configuration of these experiments: supported on two NACA0030 stilts with a chord length of 0.79H and height of 0.17H, respectively, where H is the height of the body and equals 72.3 mm. The length, L, and width, W, of the model is 261 mm and 97.3 mm, respectively.…”

“…It has been seen in several studies that the flow around the rear of the Ahmed body with 25 deg slant angle is insensitive to the Reynolds number [8,9,27]. To be able to directly compare the simulation results to previous high-resolution PIV measurements [8,30], the same Reynolds number as in the measurements is used for the simulations. The Reynolds number, Re L ¼ 9:34 Â 10 4 , is based on the length of the body.…”

“…To achieve similar flow conditions as in the PIV measurements [8,30] the full test-section is modeled, Fig. 1.…”

“…Utilizing the flow field insensitivity with respect to the Reynolds number, over the rear of the Ahmed body, as used in the high-resolution PIV measurements performed by Venning et al [8] and [30], makes it well suited for further comparative studies of LES predictability.…”