Organized structures in a reattaching separated flow field

Abstract

Spanwise structures in a two-dimensional reattaching separated flow were studied using multisensor hot-wire anemometry techniques. The results of these measurements strongly support the existence and importance of large-scale vortices in both the separated and reattached regions of this flow. Upstream of reattachment, vortex pairings are indicated and the spanwise structures attain correlation scales closely comparable to previously measured mixing-layer vortices. These large-scale vortices retain their organization far downstream of the reattachment region. However, pairing interactions appear to be strongly inhibited in this region. It is suggested that large-scale vortex dynamics are primarily responsible for some of the important time-averaged features of this flow. Notably, the reduction of turbulence energy in the reattachment region and the slow transition of the mean flow downstream of reattachment are attributed to effects associated with these vortices.

Behavior of Separated and Reattaching Flow Formed over a Backward Facing Step.

“…In this figure, the points where the production by shear stress (−uv(∂U /∂ y + ∂ V /∂ x)) reaches its maximum (point A in the diagram) and where the transverse diffusion term (−∂vk/∂ y) changes its sign (i.e., zero diffusion points, points B and C) are shown. To indicate shear layer growth, the position where the gradient of the mean longitudinal velocity, ∂U /∂ y, reaches its maximum (point D) and the maximum slope thickness, 4) δ ω , are indicated. The maximum slope thickness is defined by δ w = U e /(∂U /∂ y) max , where U e denotes the maximum longitudinal velocity at each streamwise station.…”

“…[1][2][3][4][5][6][7][8][9] One of its features is the complexity of turbulent structures. The flow just downstream of the step has a similar flow structure to the plainmixing layer.…”

“…Because of this structure, the reattachment point moves upstream and downstream at a very low frequency, which is known as "flapping." 4) Turbulent energy balance is a methodology that can assist in understanding the turbulent flow structure of the backward-facing step. 3,5,8,9) The turbulent energy production term and the dissipation term are dominant inside the separated shear layer.…”

US Air Force 1989 Research Initiation Program. Volume 4.

“…In the step flow, since the step height is large compared with the upstream boundary layer thickness, the flow field immediately downstream of the step can be considered a boundary layer in transition to a free mixing layer (Troutt, Scheelke, and Norman, 1984). Similarly, the flow over a pitching airfoil can be initially associated with a free shear layer, and as st -is susceptible to small perturbations via the Kelvin-Helmholtz instabilities (Ho and Huerre, 1984).…”

Investigation of reattachment length for a turbulent flow over a backward facing step for different step angle

“…The presence of a separated flow, together with a reattaching flow, gives rise to increased unsteadiness, pressure fluctuations, structure vibrations and noise, as it also exhibits an unsteady structure with a large scale vortex in the separated shear layer and low-frequency motion around the reattachment region with fluctuation of an instantaneous reattachment point (Troutt et al, 1984). The flow over a single side backward facing step provides a classic example of above said flow field.…”