Turbulent Heat-Transfer Enhancement in Boundary Layers Exposed to Free-Stream Turbulence

Direct numerical simulations (DNS) are performed to study the effect of free-stream vortical forcing on a thermal turbulent boundary layer. In presence of external perturbations, the heat-transfer rate from the wall is increased relative to the unforced case. An explanation is provided, and starts from the free-stream forcing which enhances the Reynolds stresses inside the boundary layer, and in particular the wall-normal component. As a result, the wall-normal heat flux is also increased, which has the dual effect of distorting the base temperature profile and enhancing the production of scalar variance; both contribute to the increase in the wall heat-transfer rate. In addition, the flow sustains higher thermal fluctuations, even though the free-stream forcing is only vortical, and not thermal. These changes are accompanied by modification of the spectra of the thermal field in the outer region of the boundary layer, where large-scale thermal structures are formed in response to the large-scale velocity motions. In the near-wall region, the thermal structures are modulated by the outer hydrodynamic field and are strengthened relative to the unforced flow.