Delay of Boundary-Layer Transition over a Slender Cone in Supersonic Particle-Laden Flow

A multiphase Ludwieg-tube facility is used to study the influence of introducing micrometer-level (9 to [Formula: see text] diameter) liquid droplets into the freestream flow on the boundary-layer transition over a sharp, slender (5 deg half-angle) cone at Mach [Formula: see text]. Previous computational studies indicated that such droplets can induce transition in high-speed flows, leading to the expectation that the transition process, which in this experiment is dominated by first-mode instability waves, would be accelerated in the multiphase flows as compared to clean flow. Contrary to this expectation, however, schlieren visualizations of the boundary layer reveal that the transition Reynolds number can be substantially increased, by up to [Formula: see text], with droplets present. Considering that freestream acoustic disturbances generated within the turbulent nozzle boundary layers are generally the dominant mechanism leading to transition in such conventional high-speed facilities, it is hypothesized that the particles in the flow are altering the tunnel freestream noise profile. Focused laser differential interferometry is thus employed to characterize the freestream disturbance environment: a droplet-diameter-dependent attenuation of the acoustic disturbances over a wide frequency range is observed as compared to the clean flow, with a maximum reduction in the power spectral density of approximately one order of magnitude. Therefore, in the transition experiments, the attenuation of existing acoustic noise appears to dominate over particle-induced disturbances under the present conditions.