Flows over surface-mounted bluff bodies with different spanwise widths submerged in a deep turbulent boundary layer

Abstract

The spatio-temporal dynamics of separation bubbles induced by surface-mounted bluff bodies with different spanwise widths and submerged in a thick turbulent boundary layer is experimentally investigated. The streamwise extent of the bluff bodies is fixed at 2.36 body heights and the spanwise aspect ratio ($AR$), defined as the ratio between the width and height, is increased from 1 to 20. The thickness of the upstream turbulent boundary layer is 4.8 body heights, and the dimensionless shear and turbulence intensity evaluated at the body height are 0.23 % and 15.8 %, respectively, while the Reynolds number based on the body height and upstream free-stream velocity is 12 300. For these upstream conditions and limited streamwise extent of the bluff bodies, two distinct and strongly interacting separation bubbles are formed over and behind the bluff bodies. A time-resolved particle image velocimetry is used to simultaneously measure the velocity field within these separation bubbles. Based on the dynamics of the mean separation bubbles over and behind the bluff bodies, the flow fields are categorized into three-dimensional, transitional and two-dimensional regimes. The results indicate that the low-frequency flapping motions of the separation bubble on top of the bluff body with $\mathit{AR}=1$ are primarily influenced by the vortex shedding motion, while those with larger aspect ratios are modulated by the large-scale streamwise elongated structures embedded in the oncoming turbulent boundary layer. For $\mathit{AR}=1$ and 20, the flapping motions in the wake region are strongly influenced by those on top of the bluff bodies but with a time delay that is dependent on the $AR$. Moreover, an expansion of the separation bubble on the top surface tends to lead to an expansion and contraction of separation bubbles in the wake of $\mathit{AR}=20$ and 1, respectively. As for the transitional case of $\mathit{AR}=8$, the separation bubbles over and behind the body are in phase over a wide range of time difference. The dynamics of the shear layer in the wake region of the transitional case is remarkably more complex than the limiting two-dimensional and three-dimensional configurations.

Three-dimensional structural characteristics of flow separation induced by a forward-facing step in a turbulent channel flow

“…Figure 9 also plots the in-plane linear stochastic estimation (LSE) (Adrian & Moin 1988) vectors conditioned on and at the same reference point as the contours of autocorrelations. These LSE vectors are equivalent to the vectors of two-point correlations, and represent the turbulence structure associated with the condition event (Fang & Tachie 2019 a ; Kevin & Hutchins 2019). For instance, the vectors in figure 9( a ) are .…”

“…2013; Graziani et al. 2018; Fang & Tachie 2019 b , a ), adverse pressure gradients (Mohammed-Taifour & Weiss 2016) and shock waves (Humble, Scarano & Van Oudheusden 2009). It is worth noting here that for convenience and conciseness in the subsequent discussion, we use ‘flapping motion’ to refer to a sequence of enlargement and contraction of separation bubble regardless of the contexts, although the term ‘breathing motion’ is perhaps more appropriate for separations with unsteady separating points (e.g.…”

“…For instance, Pearson et al. (2013), Fang & Tachie (2019 b ), Fang & Tachie (2019 a ) and Fang & Tachie (2020) attributed the flapping motion of separation bubbles induced by surface-mounted bluff bodies to the incoming streamwise-elongated streaky structures, while Eaton & Johnston (1982) attributed the unsteadiness of the separation bubble behind a backward-facing step to the instantaneous imbalance between the entrainment of the shear layer and reinjection near the reattachment point. It should also be noted here that all the aforementioned investigations were based on either planar PIV or hot-wire measurements.…”

“…Following Pearson et al. (2013), Fang & Tachie (2019 b ), Fang & Tachie (2019 a ) and Fang & Tachie (2020), the instantaneous reverse flow areas upstream and downstream of the FFS are used to track the instantaneous sizes of separation bubbles upstream of and over the step, which are, respectively, defined as and In the above equations, an auxiliary function is defined as if and if . In this paper the reverse flow areas upstream () and downstream () of the FFS are defined as functions of both the spanwise location () and time (), so as to reflect the 3-D spatio-temporal unsteadiness of separation bubbles observed in the supplementary movie for figure 4.…”

An assessment of Atmospheric Boundary Layer Effects on the Ship Air–wake

“…There are many published investigations on the ABL effects on flows over simplified geometries such as backward-facing steps and surface-mounted obstacles [1,4,8]. For these geometries, the boundary layer effects on the spatial velocity arrangement (such as the re-circulation region size in the wake of a step) and the turbulence behaviour are relatively well characterised.…”