On The Flow Structures And Heat Transport In The Near Wake Of A Wall-Mounted Finite-Length Square Prism

The flow structure and heat transport properties of the wake behind a wall-mounted finite-length square prism were investigated in the present paper by various experimental techniques, such as: Hot-wire, Cold-wire, PIV, LDV and flow visualization. The prism width is 20mm, and three aspect ratios of the finite-length square prism are investigated, i.e. 3, 5 and 7. All the experiments were conducted in the sub-critical, so that the results can be compared with most of the results reported in literatures. The main work is focused on the influence of aspect ratio to the near wake structure of wall-mounted prism, the effects of initial boundary layer conditions on the near wake structures and the heat transport properties in the near wake of finite-length square prism. It is found that, the near wake of o wall-mounted finite-length square prism consists of three main parts, i.e. spanwise shear flow, free-end shear flow and upwash flow from near the prism base, which forms spanwise vortex, tip vortex and base vortex. The near wake flow structure is characterized by the interaction between these three vortices.The aspect ratio of the finite-length square prism has significant influence of the near wake structure. When H/d is greater than a critical value, the near wake is dominated by anti-symmetrical Kármán vortex except near the free-end region. When H/d is smaller than the critical value, the near wake will be dominated by arch-type vortex. Both the tip vortex and the base vortex tend to change the near wake structure from anti-symmetrical to symmetrical. So, probability of the anti-symmetrical vortex is small, when the free-end downwash flow and the upwash flows are strong. The probability of anti-symmetrical vortex will increase when the influence of downwash and upwash flow retreats. The influence of the tip vortex is stronger than that of the base vortex, and it survives longer in the near wake.The initial boundary layer conditions have a profound effect on the near wake of a wall-mounted finite-length square prism. When boundary layer thickness increases, the upwash flow will be enhanced while the downwash flow will be relatively suppressed. The effects of boundary condition are significant near the free end that is well beyond the boundary layer. With increasing boundary thickness, the probability of anti-symmetrical vortex is reduced in the lower part of the wake, but increased in the upper part. Accordingly, the Reynolds stresses are weakened in the lower part of the wake but strengthened in the upper part.The heat transport in the wake of finite-length square prism was investigated at the streamwise position of x*=10. Since the near wake is highly three dimensional, the Kármán vortex is quite weak the streamwise position. The coherent fluctuation temperature and heat flux are remarkably smaller than those in the two-dimensional square cylinder wake. The incoherent heat flux in dominated in the near wake. For the prism with H/d=5 and 7, the heat transport is different at different height within the wake. Periodicity can be observed at the middle height of the wake, but the coherent heat flux is still smaller than the incoherent heat flux at that height. While approaching to the free end or prism base, the periodicity of incoherent heat flux diminishes, which is corresponding to the weaken of periodicity of the near wake at that area. For the prism with H/d=3, the behavior of heat transport is similar at z*=2 and 1.