Suppression of vertical flow separation over steep slopes in open channels by horizontal flow contraction

Y. B. Broekema,W. S. J. Uijttewaal,R. J. Labeur

doi:10.1017/jfm.2019.972

Abstract

Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist to control flow separation, and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free-surface flows may be suppressed by contracting the flow horizontally upstream of the slope. We found that, unexpectedly, introducing lateral non-uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2. This study reveals the possibility of two different flow states over steep slopes; (i) a vertically attached flow combined with horizontal convergence, and (ii) a vertically detached flow combined with horizontal divergence. A detailed analysis of the dynamics of the two different flow states is presented. Although a predictive relation determining the transition point between the two flow states was not found in the current study, the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope. The results demonstrate a significant influence of the vertical flow state - separated or attached - on the shear stress at the confining boundaries of the flow.

Highlights

Flow separation is one of the most intensively investigated problems in classical fluid mechanics (Goldstein 1969)
In this paper we present observations that demonstrate – for the first time – that vertical flow separation over steep slopes in open channels may be suppressed by a lateral gradient in the streamwise velocity field upstream of the slope
The analogy between the vertical stretching and horizontal compression of the conveyance cross-section with the vertical stretching of a single vortex is still recognized in the observed flow field (§ 4.1), it was shown in § 4.2 that for the selected cases there are no significant differences in the development of the velocity difference over the mixing layer ( U) and the mixing layer width (δ)

Summary

Introduction

Flow separation is one of the most intensively investigated problems in classical fluid mechanics (Goldstein 1969). It occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction (Simpson 1989). Uijttewaal including streamlining of a flow body (Schlichting 1951; Schubauer & Spangenberg 1960), boundary layer suction (Prandtl 1935; Truckenbrodt 1956; Kametani et al 2015; Kornilov 2015), injection of momentum into the boundary layer (Wallis & Stuart 1958) and controlling separation by provoking it (Hurley 1961; Francis et al 1979). In this paper we present observations that demonstrate – for the first time – that vertical flow separation over steep slopes in open channels may be suppressed by a lateral gradient in the streamwise velocity field upstream of the slope

Methods

Findings

Discussion

Conclusion