An investigation of bluff body flow structures in variable velocity flows

Abstract

The present study explores three-dimensional vortex-dynamics past a wall-attached bluff body kept in a variable velocity field with numerical simulations. A trapezoidal pulse of mean velocity, consisting of acceleration phase from rest followed by constant velocity phase and deceleration phase to rest, is imposed at the inlet of the computational domain similar to the experimental study of Das et al. [“Unsteady separation and vortex shedding from a laminar separation bubble over a bluff body,” J. Fluids Struct. 40, 233–245 (2013)]. For a wide range of Reynolds numbers ([Formula: see text]), acceleration Reynolds numbers ([Formula: see text]), and deceleration Reynolds numbers ([Formula: see text]), different stages of flow evolution are systematically analyzed. The flow evolution starts with the formation of a primary vortex followed by a two-dimensional circular array of spanwise vortex tubes by inflectional shear-layer instability. At a sufficiently high Reynolds number, the shear layer vortices originated from two-dimensional fluctuations deformed by three-dimensional instabilities, giving fragmented streamwise vorticity. In addition, long-wavelength “tongue-like structures” and short-wavelength “rib-like structures” are evident near the top wall and the bluff body, respectively. The streamwise vorticity generation equation indicates that the spanwise vortex tubes initially tilt, resulting in streamwise vorticity, further amplified by the vortex stretching process. The distinct flow features, including mode shape, frequency, and growth rate associated with the shear-layer instability, are identified using the dynamic mode decomposition (DMD) algorithm. Using the maximum growth rate criteria, the DMD technique successfully separates the coherent shear layer modes associated with two-dimensional shear layer instability from the flow field.

The onset of turbulence in decelerating diverging channel flows

“…Since the inlet velocity profiles are built upon the assumption of a two-dimensional channel, using a subdomain guarantees the application of a slip boundary on the sidewalls. The work of Sarath & Manu (2022) provides additional information about the procedure to obtain the inflow velocity profile.…”

“…In contrast, the other instability develops in the braid region with a higher spanwise wavenumber. Recently, by analysing the three-dimensional coherent structures arising in the wake of a wall-attached body, Sarath & Manu (2022) showed that the vortices shed from boundary layers displayed simultaneously both elliptical and hyperbolic instability.…”