Direct Numerical Simulation of K-type and H-type transitions in a flat-plate boundary layer with supercritical fluids

Supercritical fluids are often used in industrial processes. Yet, their transition to turbulence has not been elucidated. Therefore, we investigate a transitional flat-plate boundary layer with fluids at supercritical pressures. K-type and H-type breakdowns are simulated by means of Direct Numerical Simulations at a Mach number of 0.2. For each scenario, two different non-ideal regimes with respect to the pseudoboiling temperature are considered at a reduced pressure of p_r=1.1: subcritical (liquid-like) and transcritical (pseudoboiling). The latter is characterised by sharp gradients in thermophysical properties close to the Widom line. To model the non-ideal gas effects, the Peng-Robinson cubic equation of state is used. In the subcritical regime, the formation of aligned (K-type) and staggered (H-type) Λ-vortices, which are followed by hairpin-shaped eddies in the late-transitional stage, is delayed, as the wall temperature approaches the Widom line. Conversely, in the transcritical regime (where the wall temperature is higher than the pseudoboiling temperature), the classical breakdown patterns are highly altered by the non-ideal gas effects. A combination of strong vortical structures, resembling Λ-vortices, and high-low-speed streaks with high-low-density fluid is found. Hence, transition is further delayed.