Dynamics of coherent structures in turbulent flow over a backwards-facing step

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University of New Brunswick


A long run-time (t = 157sec) Large Eddy Simulation is performed to elucidate the role of several key features of the turbulent flow over a Backwards-Facing Step. Preservation of the instantaneous fluctuating pressure field was central to the simulations and allowed the calculation of both field and wall-pressure statistics. Multi-point spectral results showed that the majority of the field pressure fluctuations were well represented by wall-pressure measurements at all but the highest frequencies examined. The peak spectral frequency was found to increase with streamwise distance from the step. It was shown that the increase in peak frequency was much lower than the increase in convective velocity of the structures, which indicates that mean flow acceleration is not solely responsible for this behavior. Low-pass filtering of the instantaneous velocity fields was employed to highlight the mechanisms responsible for the peak frequencies associated with so-called shear-layer flapping near St ≈ 0:01, which has been heavily debated in the literature. The low-frequency behavior was found to determine the instantaneous streamwise reattachment point as well as the area of the primary recirculation zone. The results indicate that flow impingement on the vertical step face results in pairs of highly three-dimensional counter-rotating vortices, with an approximate spanwise wavelength of λ[subscript z] ≈ 2h, that wander back and forth along the step face. This results in a side-to-side sweeping of the flow downstream of these features, particularly near the reattachment location. The dynamics of large-scale vortex-shedding were then isolated from the low-frequency behaviors using band-pass filtering of both the pressure and velocity fields. Examining the evolution of the structures showed that the regions of band-pass filtered fluctuating pressure increased in size as they convected downstream, and their wall pressure signature increased accordingly. The combination of this elongation and the increasing mean velocity causes the discovered increase in peak spectral frequency. The instantaneous fluctuating wall-pressures were found to be well-representative of the fluctuations above the wall, particularly for the largest regions of organized pressure fluctuations, owing to the fact that smaller-scale fluctuations that occurred farther from the wall were obscured by the larger fluctuations closer to the wall.