Leading-Edge Effects on Freestream Turbulence Induced Transition of an Organic Vapor

The freestream turbulence-induced transition of a dense-gas boundary layer past a thick leading edge representative of turbine blades is investigated with large-eddy simulations. Due to the high Reynolds number conditions, typical of Organic Rankine Cycle applications, transition occurs early on the blade. In such conditions, the freestream turbulence is characterized by relatively large scales compared to the boundary layer size, but at the same time small compared to the blade thickness. These turbulent structures wrap around the large leading-edge and strongly influence the downstream evolution of the transitional boundary layer, by modulating the appearance and evolution of turbulent spots. Combined with the favorable pressure gradient, this effect delays and smooths the transitional region over a wider chordwise extent compared to flat plate bypass transitions with comparable levels of freestream turbulence. Laminar streaks are generated inside the transitional boundary layer in the form of clusters, modulated by the intense large-scale structures that develops at the leading-edge. Despite their low population, the low-speed streaks are found to be the turbulent spots precursors through two mechanisms: streak instabilities and streak interactions. The main effect of the use of an organic vapour is the high-Reynolds-number effects in the leading-edge receptivity process. Another intriguing peculiarity of the dense-gas is the appearence of near-wall spanwise-oriented vortices below the turbulent spots. Such structures have been observed in supersonic air flows on cold walls. Despite the subsonic Mach number in the transition region for the present configuration, their presence is associated to the large heat capacity of the organic working fluid that almost suppresses friction heating.