Effect of large-scale periodic unsteadiness on the heat transport in the downstream region of a junction boundary layer

Abstract

The heat transport in the boundary layer downstream of a junction between a streamlined cylinder and a wall is examined. This flow is characterized by large-scale periodic motions, attributable to vortex shedding, which affect the turbulent heat transport and may contribute to enhanced wall heat transfer. Experimental data obtained using a triple-wire turbulent heat-flux probe were analyzed using triple decomposition and a conditional sampling technique that separated the large-scale periodic motions from the background turbulence. The periodic motions lead to large-scale movements of the instantaneous thermal boundary-layer interface and contribute significantly to the overall Reynolds heat fluxes. The intermittency of the temperature signal revealed that the hot/cold interface penetrated much closer to the wall than in an undisturbed two-dimensional (2-D) boundary layer. Two different mechanisms that might be responsible for the observed transport phenomena are discussed: (1) a portion of the shed vortex filament that is skewed by the mean strain field of the wake and boundary layer; and (2) large counter-rotating stream-wise vortices identified from mean-flow measurements that oscillate in the unsteady wake of the obstacle.