Qualitative assessment of the bi-stable states in the wake of a finite-width double backward facing step

Abstract

A B S T R A C TThe flow past a simplified frigate shape model which is analogous to a finite-width double backward facing step is investigated numerically using well-resolved large eddy simulations at Re ' 8 Â 10 4 . The geometric configuration of this model permits the occurrence of bi-stable flow states, with an asymmetrical flow topology being observed in the lateral midplane behind each step. In each flow state, the flow at the top step is anti-symmetrical to that observed on the bottom step, and the two flow states are anti-symmetrical to each other. Furthermore, incorporating a base cavity on the top step leads to the suppression of the asymmetrical flow topology on both the steps. The recirculation bubble formed by the downwash at each step is elongated when the base cavity is used, as compared to that observed in either of the two flow states, resulting in a decrease of the drag coefficient. In each of the two flow states, the unequal strength of the streamwise vortices close to the lateral edges on the top step has been identified as the likely cause of the asymmetrical flow on the bottom step.

The double backward-facing step: interaction of multiple separated flow regions

“…Herry et al (2011) observed a bi-stability downstream of the first step. This was investigated further in the studies of Rao et al (2019) and Zhang et al (2021), who employed geometries more representative of ships upstream of the first step. However, the authors are not aware of any studies that have systematically varied the streamwise separation between the two steps to study the interaction of the separated flow regions.…”

Heat Transfer Improvement in a Double Backward-Facing Expanding Channel Using Different Working Fluids

“…Over the last several years, many studies have focused on using nanofluids with separation flow in BFSs and FFSs, which have been considered by various researchers [18][19][20][21][22][23][24][25][26][27][28][29][30] with similar conclusions. Recently, the modeling of turbulent heat transfer over a microscale BFS was presented by Sadeq et al [31].…”

Drag reduction of ship airflow using steady Coanda effect

“…This suggests that Case 4 has less control effectiveness with lower pressure recovery, and its drag reduction will require higher energy consumption. Note that the drag reduction of 5.34% and 6.22% may sound small, but is much more significant compared to the drag reduction of less than 2% achieved by the base cavity flow control applied on the SFS2 ship model in Rao et al (2019). Fig.…”

“…For the present case, 𝛥𝑠 + is less than 55 and the maximum 𝛥𝑙 + is 21. Based on various numerical studies on the ship model with the grid size ranging from 6 to 21 million cells (Zhang et al, 2018;Rao et al, 2019;Zhang et al, 2021), the grid resolution of more than 37 million cells for the present study is sufficient. Therefore, no further grid independent study is conducted.…”