Thermal Boundary Layer Response to Periodic Fluctuations

Unsteady effects impact the aerothermal performance of the turbine blade rows, originating noise, mechanical and thermal fatigue. Blade row interactions are due to the relative motion between nearby rows of airfoils, the periodic occurrence of flow distortions generated by the airfoil rows or combustors. The detailed characterization of the thermal boundary layer under periodic fluctuations is vital to improve the performance of cooled turbine airfoils. In the present contribution, we performed series of Unsteady Reynolds Averaged Navier-Stokes simulations to investigate the wall heat flux response to periodic flow velocity fluctuations, on a flat plate of 0.5 m. We investigated the boundary layer response to sudden flow acceleration and periodic flow perturbations, caused by inlet total pressure variations. Because of the flow acceleration the boundary layer is first stretched, resulting in an increase of the wall shear stress. Later on, due to the viscous diffusion, the low momentum flow adjusts to the new free stream conditions. The behavior of the boundary layer at low frequency is similar to the response to an individual deceleration followed by one acceleration. However, at higher frequencies the mean flow topology is completely altered. One would expect that higher acceleration rates would cause a further stretching of the boundary layer that should cause even greater wall shear stresses and heat fluxes. However, we observed the opposite; instead, the amplitude of the skin friction coefficient is abated, while the peak level is one order of magnitude smaller than at low frequency. Two counteracting effects influence the response of both the momentum and the thermal boundary layer. In one hand, the stagnant flow quantities propagate at characteristic velocities guiding the establishment of the mean flow conditions. On the other hand, the diffusion across the boundary layer leads the final response of the near wall region. However, the dynamic pressure gradients imposed in the mean flow modulate the viscous properties of the boundary layer through local flow acceleration, transforming the expected pattern.