Effect of a bluff body shape on a recirculation zone topology and mixing characteristics

The objective of this study is to determine how the cross-sectional shape of a bluff body controls the topology of the downstream recirculation zone and the associated mixing characteristics in turbulent non-reacting flow. Four bluff bodies with identical cross-sectional area are examined: circular (Crc-BB), square (Sqr-BB), triangular (Tri-BB), and hexagram-type star (Str-BB) geometries. A combined experimental and numerical approach is employed, with particle image velocimetry (PIV) used for velocity field measurements and high-order large-eddy simulation (LES) for detailed flow analysis. The numerical results show good agreement with the experimental data, both in terms of the mean velocity and fluctuation profiles. We demonstrate that the BB shape has a strong impact on the downstream recirculation zone in different aspects. Non-circular geometries, particularly Tri-BB and Str-BB, imprint their cross-sectional shape onto the flow, with effects persisting over several equivalent BB diameters. These shapes disrupt the formation of the Kelvin–Helmholtz vortex ring observed in Crc-BB and instead promote streamwise-aligned vortices that originate at sharp corners. This leads to significant modifications in flow statistics, including reduced velocity fluctuations along the axis, enhanced inward radial motion, and increased entrainment. All non-circular BBs improve entrainment rate compared to the circular case, with Str-BB yielding the highest increase of more than 7%. Such an increase would be highly significant in reactive configurations, potentially corresponding to a comparable rise in oxidizer transport toward the reaction zone. Additionally, Tri-BB and Str-BB promote more uniform turbulence levels within the recirculation zone, which may help prevent localized hot spots and support stable combustion.