Streamwise Aspect Ratio Effects on Turbulent Flow Separations Induced by Forward–Backward-Facing Steps

Abstract

Separated and reattached turbulent flows induced by two-dimensional forward-backward-facing steps with different streamwise lengths submerged in a thick turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The aspect ratios of the step range from 1 to 8, and the Reynolds number based on the free-stream velocity and step height is 13 200. The thickness of the incoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, principal stretching, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and unsteadiness of turbulent separation bubbles are studied. The results indicate that the mean flow reattaches over the step for forward-backward facing steps with aspect ratios of 2 and higher. Two local peaks of Reynolds stresses occur irrespective of AR, and for a sufficiently large AR, a third local peak of Reynolds stresses appear in the shear layer emanating from the trailing edge of the bluff bodies. The streamwise decay of Reynolds stresses is slower for smaller AR cases. Incoming coherent structure is strongly disturbed near an inclined edge where principal stretching switches orientation abruptly. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area over the bluff bodies shows remarkable correlation, which signifies the flapping motion of separation bubble.

Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

“…Similar observations were also made by Tafti & Vanka (1991) for the flow separation over a long rectangular bluff body at a lower Reynolds number (). The flapping motion at a similar frequency was also reported for flow separations induced by a backward-facing step (Eaton & Johnston 1982), a forward–backward facing step (Chalmers, Fang & Tachie 2021), and adverse pressure gradient (Mohammed-Taifour & Weiss 2016).…”

Reynolds Number Effects on Turbulent Wakes Generated by Rectangular Cylinders With Streamwise Aspect Ratios Between 1 and 4

“…Figures 4.7 and B.6 show that the topology of the Reynolds shear stress (u ′ v ′ ) is characterized by alternate regions of elevated positive and negative stresses. The positive region aligned along the mean separating streamline near the leading edge has been reported for rectangular cylinders (Sohankar, 2006;Cimarelli et al, 2019;Kumahor and Tachie, 2022) and right trapezoidal cylinders (Kang and Tachie, 2023) in uniform flow as well as separated flows over wall-mounted rectangular cylinders submerged in turbulent boundary layers (Fang and Tachie, 2020;Chalmers et al, 2021). There also exists a positive region of u ′ v ′ in the wake vortex which is formed due to the vortex shedding motions emanating from the trailing edge (Lyn et al, 1995;Kumahor and Tachie 2022).…”

“…) are observed below the mean separating streamline, indicating sweep events (u ′ > 0 and v ′ < 0) or high-speed flow turbulence transport moving downward. The switching of dominance between ejection and sweep events along the mean separating streamline has been observed for rectangular cylinders in uniform flow (Kumahor and Tachie, 2022), flows over wall-mounted rectangular cylinders (Chalmers et al, 2021) and a square cylinder with varying gap ratios (Addai et al, 2022) submerged in turbulent boundary layers. The switching between ejection and sweep events occurs close to the trailing edge for AR1 and AR2.…”