Entropy generation in a transitional boundary layer region under the influence of freestream turbulence using transitional RANS models and DNS

In this study, the entropy generation process in the bypass transition scenario is investigated for a flat plate boundary layer. Here 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 and energy dissipation in pre-transitional and transitional regions comprehensively. Two new transitional RANS models (SST k − ω(4eq) and k − kl − ω) were used for prediction of the onset of transition and the results are compared with DNS ones. Classical laminar theory underpredicts the observed entropy generated. In the pre-transitional boundary layer, the perturbations generated by the streaky structures modify the mean velocity profile and induce a quasi-turbulent contribution to indirect dissipation. In the transition region the pointwise entropy generation rate (S‴)+ initially increases near the wall and then decreases corresponding to the distribution predicted for a fully-turbulent boundary layer as the flow moves downstream. All the RANS models predicted transition onset prematurely and, consequently, overpredict the integral entropy generation rate and the skin friction coefficient in the transition region.