REYNOLDS STRESS MEASUREMENTS IN THE WAKE OF A STACK PARTIALLY IMMERSED IN A TURBULENT BOUNDARY LAYER

Some characteristics of the Reynolds stress field in the wake of a stack were experimentally studied in a low-speed wind tunnel using thermal anemometry. The cross-flow Reynolds number was ReD = 2.3×10, and the jet-to-cross-flow velocity ratio was varied from R = 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-stack-height ratio of /H = 0.5 at the location of the stack. The spanwise characteristics of the time-averaged streamwise and wallnormal velocities varied along the stack height (z/H) and with the value of R. Likewise, the streamwise and wallnormal turbulence intensities as well as the Reynolds shear stress varied with the stack height z/H and were strongly influenced by the value of R. The behaviour of the shear correlation coefficient indicated that there is more turbulent mixing within the vicinity of the free end of the stack than at mid-height. INTRODUCTION The complex flow field associated with a stack is encountered in many engineering applications, especially in the area of pollution dispersion. The mixing and dispersion of the jet issuing from a stack depend on the ambient conditions, the jet exit characteristics, and the geometry of the stack. The simplest stack geometry can be represented by a uniform finite circular cylinder mounted normal to a ground plane. Due to the influence of the free end and the boundary layer on the ground plane, there are marked changes in the near-wake flow structure along the cylinder height. These changes are strongly influenced by the cylinder’s aspect ratio, AR (= H/D, where H and D are the height and diameter of the cylinder, respectively). Two pairs of counter-rotating streamwise vortex structures have been identified within the wake of the finite circular cylinder (Sumner et al. 2004). The first pair, called the tip vortex structures, is formed very close to the free end and is associated with a downward directed velocity along the wake centreline known as “downwash”, while the second pair, called the base vortex structures, is found close to the ground plane. Kármán vortex shedding from the finite circular cylinder is influenced by the tip vortices (Sumner et al. 2004). Compared with a finite circular cylinder, the presence of the jet flow issuing from the stack gives rise to an even more complicated flow structure, both around the stack and in its wake. For a non-buoyant jet, the extent of this complexity depends on the jet-to-cross-flow velocity ratio, R (= Ue/U, where Ue and U are the mean jet exit velocity and the freestream velocity, respectively). The flow regimes in the stack wake have been classified differently by various researchers. For example, three distinct flow regimes have been identified for R varying between 0 and 3: the downwash flow regime for R < 0.7, the crosswinddominated flow regime for 0.7  R < 1.5, and the jetdominated flow regime for R  1.5 (Adaramola et al. 2007). Each flow regime has distinct flow structures evident within the turbulence and time-averaged velocity fields (Adaramola et al. 2007 and Adaramola et al. 2007b). Various zones can also be identified within the jet itself, depending on the value of R and the corresponding flow regime ( et al. 2005). Recently, Adaramola et al. (2007a) identified up to three pairs of streamwise vortex structures within the stack wake (Fig. 1), depending on the value of R. These vortex structures influence the turbulence characteristics within the wake of the stack. In the downwash flow regime (Fig. 1(a)), Sixth International Symposium on Turbulence and Shear Flow Phenomena Seoul, Korea, 22-24 June 2009