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

An experimental investigation was conducted for a better understanding of the turbulence behavior and the evolution of coherent structure in the heated turbulent boundary layer. 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. In the near-wall region, the decrease of fluid viscosity caused by wall heating has stabilization effects on the turbulent fluctuation. With the increase of 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 disconnection of separated turbulent structures from the turbulent production source in the near-wall region, the TKE in the outer region for the wall-heating case is less than that of the unheated case.