Studies of the luminous hypersonic wake

Abstract

Measurements of the width of luminous wakes behind hypervelocity hlunt bodies launched in ballistic ranges are presented. Experiments utilizing an image converter camera, a race track drum camera, and a photoelectric wake scanner have obtained data over 1200 body diameters of wake length on 0.22, 0.55, and 1.0-in.-spherical projectiles for freestream pressures from poo = 0.2 to 10.0 cm Hg and projectile velocities from Vm = 12,000 to 20,500 fps. The results of the luminous wake width measurements are shown to agree well with the viscous core width data obtained with shadowgraph and schlieren techniques. It is found that the luminous wake growth has a pressure dependence in the transition and laminar flow regimes, but not in the turbulent regime from 2.0 to approximately 76 cm Hg. The turbulent experimental results are compared to the wake growth theories of Lees and Hromas and Lykoudis. The simple asymptotic turbulent wake growth correlation, w/(CDA)112 = K[x/(CDA)ll2]l t is shown to give good agreement with the data with K = 0.66. Finally, the occurrence of asymmetries in the wake is documented. The polarization of these asymmetries seems to add further observational evidence of the occurrence of large scale structure in the turbulent hypersonic wake. HE structure of the wake behind bodies traveling at hypersonic speeds in air is controlled by a complex interaction of aerodynamics and chemistry, with the added complexity of flows that may vary from laminar, unsteady, and periodic, to fully turbulent. Several attempts have been made to predict some of the gross features of the hypersonic wake such as the rate of growth of the inner viscous core or boundary-layer induced wake.14 These theoretical predictions have been compared with measurements of the width of the turbulent viscous wake obtained in various ballistic range experiments.59 All of the experimental measurements on wake width reported to date have utilized the schlieren or shadowgraph technique and, therefore, measure a region in the wake defined by density gradients. The sensitivity limit of these techniques has, until now, restricted their applicability to measurements of the unsteady or turbulent wake. When plastic projectiles are fired at hypersonic velocities into a ballistic range, considerable ablation occurs and the wake becomes highly luminous.10. n The ablation products may be thought of as a dye, which defines the viscous wake in much the same manner as dyes are used to delineate lowspeed flow patterns.12 If this assumption is correct, then the luminous wake seen in hypersonic experiments should correspond to the viscous wake. In the present investigation, these ablation products are used as an observable to