16 th Australasian Fluid Mechanics Conference Crown Plaza, Gold Coast, Australia 27 December 2007 Cylinder Wake – Boundary Layer Interaction in the Near Field C. J. DillonGibbons 1 , C.Y. Wong 1 , L. Chen 2 and J. Soria 1              !! "# $ %  % % "  & ’&   $!( "# Abstract The interaction between the wake generated from separation off a cylinder and turbulent structures evident in a boundary layer are of significant importance in understanding the flows for cooling towers, submerged and semisubmerged vessels. This investigation was conducted on a wall mounted circular cylinder 25.4mm in diameter in the near wake region using MCCDPIV (multigrid cross correlation digital particle image velocimetry) using a PCO4000 CCD array with full resolution of 4008 x 2672 pixels 2 per image. The investigation was conducted for (Re D ≡ DU / ν) of 3600 and 5400. It was seen in the mean after taking the ensemble average of the instantaneous results that a stagnation line was formed between x / D = 1 and 1.5 downstream of the cylinder. The region of the cavity wake was highly turbulent having the largest velocity fluctuations in this region. The shape of the stagnation line was also seen to change, where for the Re D = 3600 the stagnation line position from the cylinder changed for a given height above the flat plate surface at y / D = 0. Introduction The complex 2D flow around a cylinder has been investigated extensively in the past because important engineering applications have been identified which could potentially be advanced through understanding this flow. The application of an infinite length cylinder with a single fixed end can be seen in applications such as plume motion behind stacks, aerodynamics of cooling towers and the drag on submerged and semi submerged vessels. Many investigations have been conducted over previous decades into how the wake structure behind a cylinder behaviours, and under what conditions these structures change [2, 3, 4, 11]. Before vortex shedding takes place a cavity flow (see figure 1) can be used to describe the near field of the cylinder wake where a stagnation point occurs some distance down stream of the cylinder, where inside this region the flow is circulated back towards the body along the centre line until it bifurcates at the cylinder. The flow then circulates into vortical structures that are propagated down stream. This process is a highly 3dimenional flow where the fluid in the recirculation region must move out of the plane or continuity in the flow will not be preserved, see in figure 1. The flow that is going to be discussed in this paper is from a wallmounted cylinder, this flow represented in figure 2 has already been investigated in the past [8, 9]. This flow not only exhibits a wake formed solely from the cylinder as discussed earlier but combining this with the boundary layer flow being present on the surface of the wall, a horseshoe vortex is generated by the rotation in the boundary layer up stream of the cylinder. This horseshoe vortex separates from the boundary layer due to the adverse pressure gradient produced from the cylinder. These vortex structures then interact with the cylinder and the cavity wake discussed earlier and propagates downstream. The spanwise motion of this horseshoe vortex generates considerable out of plane motion in the centre line flow directly downstream of the cylinder, where the measurements are taken. Figure 1: Idealised example of a wake cavity flow behind a cylinder figure adapted from [4]. The aim of this investigation was to observe the mean structures and the fluctuating velocity components in the near field, where the wake structure from a cylinder and a boundary layer interact by varying the Reynolds number. Figure 2: Sketch of vortical structures in flow around a wall mounted cylinder showing coordinate system and region of interest for experiment. Experimental technique and apparatus Flow Geometry The experiments were conducted in an open topped recirculating horizontal water tunnel at the Laboratory for Turbulence Research for Aerospace and Combustion (LTRAC). The tunnel has a 5.5m long working test section, split into 5 equal viewing windows regions of 1m length, with a crosssection of 500mm x 500mm. The wake generating mechanism for this series of experiments was a circular cylinder of length, L = 600mm and diameter, D = 25.4mm. The cylinder was mounted vertically on a rigid plate 1475