James Arthur, Ben Hong, Cesar Spadea

Abstract: Backward-facing step (BFS) flow phenomena is present in several engineering systems. While extensive research efforts have been invested in BFS flows, cases associated with narrow channels are relatively few. In this work, we examine the turbulent flow field of a narrow open-channel BFS to understand the effects of expansion ratio and Reynolds number variations. The physical system is modeled in an experimental facility consisting of a channel flume with a backward-facing step of height h installed in the upstream section of the flow. With an aspect ratio of 4, a narrow BFS flow configuration was achieved. By varying the flow depth for different test cases, BFS flows of expansion ratio (ER) of 1.25 and 1.50 were tested. Additionally, open-channel turbulent flow was conducted through the flume at various Reynolds number (Re) between 2900 and 11,000. A planar particle image velocimetry technique was used to capture the flow around the recirculation region. The results show that increasing Re by nearly two-fold leads to the evolution of a multi-centered primary recirculation bubble and a secondary corner bubble; while maintaining the reattachment length. Further increment of the Re and ER by 50% and 20% respectively, results in a 16% enhancement of the reattachment length. The turbulent flow, on the other hand, suggests turbulent intensities and turbulent kinetic energy are influenced primarily by Re. Overall, these results suggest important differences between closed-channel and open-channel BFS flows on the one hand, as well as narrow and wide open-channel BFS flows on the other hand.

Keywords: Turbulent flow, Backward-facing step, Particle image velocimetry.

Date Published: August 30, 2024
DOI: 10.11159/jffhmt.2024.026

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