Direct Numerical Simulations of K-type transition in a flat-plate boundary layer with supercritical fluids

TL;DR

This study uses direct numerical simulations to explore the K-type transition in a flat-plate boundary layer with supercritical fluids, revealing delayed transition and complex vortex structures compared to ideal gases.

Contribution

It provides new insights into boundary layer transition with supercritical fluids, including effects of temperature profiles and non-ideal fluid behavior on vortex formation and transition delay.

Findings

Delayed vortex formation in supercritical fluids compared to ideal gases

Pseudo-boiling temperature influences secondary instabilities and vortex structures

Transition to turbulence is less violent and significantly delayed

Abstract

We investigate the controlled K-type breakdown of a flat-plate boundary-layer with highly non-ideal supercritical fluid at a reduced pressure of $p_{r,\infty}=1.10$. Direct numerical simulations are performed at a Mach number of $M_\infty=0.2$ for one subcritical (liquid-like regime) temperature profile and one strongly-stratified transcritical (pseudo-boiling) temperature profile with slightly heated wall. In the subcritical case, the formation of aligned $\Lambda$-vortices is delayed compared to the reference ideal-gas case of Sayadi et al. (J. Fluid Mech., vol. 724, 2013, pp. 480-509), with steady longitudinal modes dominating the late-transitional stage. When the wall temperature exceeds the pseudo-boiling temperature, streak secondary instabilities lead to the simultaneous development of additional hairpin vortices and near-wall streaky structures near the legs of the primary aligned $\Lambda$-vortices. Nonetheless, transition to turbulence is not violent and is significantly delayed compared to the subcritical regime.