Structure of Turbulent Wakes of Hypersonic Spheres as Inferred with Ion Probes

Abstract

Survey arrays of ion probes have been used to study the structure of the ionized turbulent wakes of 2.7 in. diameter spheres flown at 14 500 ft/s in atmospheres of nitrogen at pressures of 7.6 and 20 Torr. For various radial distances measured from the wake axis, matrices of data points have been obtained giving random values of the axial distance behind the projectile at which ionization begins, the first ionized eddy occurs, and continuous turbulence begins. A wake radius separating the significantly ionized wake from the un-ionized wake has been obtained, and found to lie intermediate between the inviscid wake radius defined by Wilson from schlieren measurements and an electron density radius corresponding to the 1/e points of the electron density distribution defined by microwave interferometric measurements. A radius defining the extent of the ionized portion of the turbulent core has been determined; this radius is of the order of 80% of the schlieren radius of the turbulent core.The continuous turbulence data can also be used to define a wake radius; this radius has been identified as the mean position of the turbulent interface for ionized eddies, because it is in agreement with estimates of the turbulent interface obtained by fitting the radial distributions of intermittency estimates prepared from independent 0.5 ms long segments of the ion probe signals. The radius corresponding to the mean position of the turbulent interface is within the schlieren radius of the wake core. In addition to determining the mean position of the turbulent front, the standard deviation of the interface has been established. These last data have been found to be in broad agreement with data derived from measurements of the wake edge using schlieren techniques.The differences in the results obtained at 7.6 and 20 Torr have been considered and found consistent with other experimentally measured data obtained at DREV; the occurrence of a transitionary type behavior in the frequency content of the probe signals from the 7.6 Torr results has been noted but not explained.