Self-similar behavior of turbulent impinging jet based upon outer scaling and dynamics of secondary peak in heat transfer

摘要

The flow field of an impinging wall jet created by the impingement of a turbulent axisymmetric jet normal to a flat surface was characterized by the particle image velocimetry technique. Experimental data is analyzed to explore two basic features of the impinging jet: first, to bring out unexplored aspects which are responsible for secondary peak in heat transfer distribution and to understand the reason for discrepancies in the existing observations about the peaks in heat transfer. Second, to analyze the self-similarity of radial wall jet based upon outer scaling. Measurements of the cross-wise mean velocity and turbulence statistics were initially used to explain the dynamics of secondary peak. Our results show that flow separation/reattachment occurs along the surface. At the reattachment location an intense increase in crosswise velocity, normal stresses and higher mixing are evident, which would lead to a peak in heat transfer. The separation/reattachment location is further found to depend upon the specific stage of vortical structure and is a function of surface spacing. The location of maximum value of mean cross-wise velocity and normal stress is located at the intersection of inner and outer shear layers. While the maximum Reynolds shear stress location is shifted to the outer shear layer and is located between the location of maximum velocity and jet half-width. The impinging jet exhibits a self-similar behavior as evident by the collapse of mean velocity and turbulent stress profiles when scaled with appropriate parameters. The outer scaling is able to bring out the self-similar profile of mean velocity and normal stresses. However, the shear stress profile does not show the self-similar behavior by the use of outer scaling. Data in the inner shear layer show small scatter compared to the outer shear layer especially close to the surface. The results show that the outer scales are not suitable to scale the data in the inner layer. It is also observed that the presence of vortical structure in wall jet delays attainment of self-similarity and the location beyond which self-similarity is observed is a function of surface spacing. These results aid in interpretation of heat transfer behavior from a flat surface and provide comprehensive benchmark data for theoretical modeling of the flow.