Effects of adverse and favorable pressure gradients on entropy generation in a transitional boundary layer region under the influence of freestream turbulence

Fundamental understanding of the entropy generation process in characteristic wall shear flows is developed. For entropy generated by fluid friction, the rates are predictable for developed turbulent flows and pure laminar flows. The main concern lies in prediction for flows undergoing so-called “bypass” transition from laminar to turbulent state. In this study, the entropy generation process in the bypass transition scenario is investigated for a flat plate boundary layer under effects of adverse and favorable pressure gradients. For the case studied, transition occurs prematurely due to the presence of strong levels of freestream turbulence. Reynolds-Averaged Navier-Stokes (RANS) models and direct numerical simulations (DNS) are implemented to study the local entropy generation in pre-transitional and transitional regions. Three of these RANS models are transitional models such as, traditional SST k-ω (2eq), SST k-ω (4eq) and k-kl-ω. They are used for prediction of the onset of transition and the results are compared with DNS. All the RANS models predicted transition onset prematurely and, consequently, over predict the integral entropy generation rate and the skin friction coefficient in the transition region. Four simulations have been performed for (1) zero, (2) favorable, (3) adverse and (4) strong-adverse pressure gradient cases. The numerical results show that the pressure gradient has a significant effect on the onset of transition and entropy generation. Transition is early and abrupt for the strong adverse pressure gradient but occurs further downstream and is longer with the increasingly favorable flows. Entropy generation rate at the wall increases as the pressure gradient grows.