An experimental study on the effects of wall heating on coherent structures in a turbulent boundary layer

Abstract

An experimental investigation was conducted for a better understanding of the turbulence behavior and the coherent structure in a turbulent boundary layer over a heated plate, which is horizontal facing upwards. To explore the effects of wall temperature on the turbulent boundary layer behavior, the boundary layer flow was observed at four different temperatures between the plate and inlet flow (δT=0, 15, 30, and 50 K), with the momentum Reynolds number of the inlet flow set to Reθ=1196. We show that based on the comparison of turbulent kinetic energy (TKE) distribution between unheated- and heated-wall cases, the wall-heated turbulent boundary layer can be divided into three regions, namely, the near-wall region, the intermediate region, and the outer region. The relationship between the TKE distribution and the coherent structures in each region is explored by comparing the detailed flow-field measurement results for the unheated- and heated-wall cases. In the near-wall region, the decrease in fluid viscosity caused by wall heating has stabilization effects on the turbulent fluctuation. With the increase in wall temperature, the streaky structures display a continuous decrease in their spanwise meandering and an increase in their streamwise coherency. The intermediate region ranges from the logarithmic region to the thermal boundary layer edge. The buoyant force caused by wall heating has a significant effect on the turbulence behavior in this region. Under the influence of buoyant force, the large-scale coherent structures for the wall-heating case were found to contain more kinetic energy and incline away from the wall with a larger angle, which leads to the increased TKE in the intermediate region for the wall-heating case. In the outer region, the occurrence of separated turbulent structures is measurably more common for the wall-heating case. Owing to the shedding of the turbulent structures from the turbulent production source (i.e., large-scale coherent structures originating from the near-wall region), the TKE in the outer region for the wall-heating case is less than that of the unheated case.