Characteristics of flow past elongated bluff bodies with underbody gaps due to varying inflow turbulence

Abstract

An experimental study was performed on two elongated bluff bodies with underbody gaps, a square-back Ahmed body and a cuboid, to investigate the effects of geometry and the approach flow conditions on the time-averaged and temporal characteristics. The flow fields produced from two approach turbulent boundary layers with moderate (∼4%) and high (∼7%) turbulence levels were studied using time-resolved and double-frame particle image velocimetry systems. With the moderate turbulence, the wake topology and the loci of the centroid of vortices exhibit the well-known toroidal structure behind the Ahmed body, though it is skewed away from the wall. Also, the regions of elevated Reynolds stresses are considerably larger in the upper shear layer compared to the lower shear layer due to the reduced underbody velocity. The dominant frequencies obtained from the velocity fluctuations, reverse flow area, and the coefficient of the first proper orthogonal decomposition (POD) mode are identical. The flow structures are more complex behind the cuboid and the Ahmed body mounted in the high turbulent flow due to the enhanced interaction between the lower and upper shear layers, which is also evident from the shape of the turbulent structures in these shear layers. Consequently, the dominant vortex shedding frequencies varied as the streamwise distance from the bodies increased. The probability density function of the reverse flow area and the POD analysis performed in the spanwise plane revealed that the bi-stability phenomenon is absent in the present study due to the significant modification of the wake topology.