Dissimilarity between the velocity and temperature fields in a perturbed turbulent thermal boundary layer

Abstract

Dissimilarity between the streamwise velocity and temperature fields is investigated in a perturbed turbulent thermal boundary layer using direct numerical simulation. Perturbations are provided into a turbulent boundary layer by uniform blowing or suction from a spanwise slot. The skin-friction coefficient and Stanton number are significantly changed due to blowing and suction. The streamwise evolution of the Stanton number is very different from that of the skin-friction coefficient because the pressure gradient induced by blowing or suction plays an important role in the momentum transport. Before the slot, the mean streamwise velocity changes smoothly, as compared to the mean temperature, due to the pressure gradient, and a similar behavior is also observed in the variations of the streamwise velocity and temperature fluctuations. Above the slot, the change in the mean streamwise velocity is smaller than that in the mean temperature, and after the slot the mean streamwise velocity recovers slowly to the unperturbed flow as compared to the mean temperature. By investigating each term in the mean streamwise velocity and temperature budgets, it is concluded that the mean pressure gradient is the main source of dissimilarity between the mean streamwise velocity and temperature fields. The budgets for the variances of the streamwise velocity and temperature fluctuations show that the difference between the production terms in both budget equations, which results from the existence of the mean pressure gradient, is the primary source of dissimilarity between the streamwise velocity and temperature fields above the slot, whereas downstream of the slot, the velocity pressure-gradient term (i.e., pressure gradient fluctuation) is another important source of dissimilarity. Dissimilarity between the streamwise velocity and temperature fluctuations caused by blowing persists longer in the streamwise direction than that by suction.