Supersaturation Total Temperature, Pitot Pressure, and Rayleigh Scattering Measurements at Mach 10

Abstract

A fundamental research study of the supersaturation region is presented. This region is mapped in the NASA Langley 31-Inch Mach 10 Air Wind Tunnel freestream flow using physical probes (total temperature and pitot) and a nonintrusive offbody diagnostic (laser Rayleigh scattering). Data from all three methods are acquired simultaneously. Facility stagnation pressures spanned 1.0–10.0 MPa (150–1450 psi), and stagnation temperatures spanned 357–1000 K (184–1350°F). Each instrument has its own unique supersaturation region over which quantitative measurements can be obtained. The extent of each supersaturation region is a unique function of the sensitivity of the instrument selected to flowfield clustering. Laser-Rayleigh-scattering-measured density results agree to better than 15% with the freestream density computed using the GASPROPS code over a significant fraction of the supersaturation region. Evidence is presented showing that the “frozen” vibrational nonequilibrium Boltzmann population of and molecules inhibit clustering and condensation. This is the physical mechanism responsible for creating the supersaturation region. In direct conflict with the assumption in all hypersonic literature from the past 50 years, evidence is presented indicating that no hypersonic or hypervelocity freestream in which nucleation is occurring contains a frozen vibrational nonequilibrium population. This population is effectively eliminated by interacting with clusters.