Boundary Layer Transition in Mach 4 Flow with Varying Angle of Attack

Abstract

Boundary layer transition in hypersonic flow is often studied at Mach numbers above 4, where second mode instabilities are known to drive the breakdown from laminar to turbulent flow. At Mach 4, however, these second mode disturbances are weaker, and transition may instead be influenced by first mode or mixed instability mechanisms. Because of this, transition behavior in the Mach 4 regime is less clearly understood, especially under varying angle of attack conditions. To investigate this, Mach 4 flow over a sharp nose cone was studied in the University of Maryland Multi-phase Investigations Supersonic Tunnel (MIST) Ludwieg tube using high speed Schlieren imaging. Several test cases with varying angle of attack and fill pressure were analyzed using Spectral Proper Orthogonal Decomposition (SPOD) to identify the dominant coherent disturbances and examine their downstream development. Transition onset was estimated from rapid increases in modal amplitude and energy, indicating a later transition at more negative angles of attack and an earlier transition at more positive angles of attack. Transitional cases showed dominant instability frequencies between approximately 31 and 49 kHz. These results provide additional insight into boundary layer transition in Mach 4 flow.