Détermination du Champ de Vitesse du Sillage d'une Sphère Hypersonique à l'Aide de Peignes de Sondes Ioniques

Abstract

The technique of measuring convection velocity in the hypersonic wake with a pair of in-line probes has been applied on a large scale to the mapping of the velocity field in the hypersonic wake through the use of a transverse survey array containing up to 8 ion-probe pairs. Measurements are reported of the velocity field in the wakes of 2.7 in. diameter spheres flown at 14 500 f/s in ballistic range atmospheres at 7.6 Torr and at 20 Torr of nitrogen. The array technique leads to convection velocity results which are in excellent agreement with the mean wake velocity data obtained by the sequential spark technique in an air atmosphere. In addition, the technique has permitted an extension of the mapping of the velocity field of 15 000 f/s spheres to considerably smaller axial distances than was possible with the spark method because of the difficulty of forming distinct sparks at the higher levels of ambient ionization encountered in the near wake.A comparison of the data obtained at 7.6 Torr and at 20 Torr shows that the amplitude of the velocity distribution in the wake, as defined by the velocity on the wake axis, is higher at higher pressure. Considering the wake width, however, it is found that the width of the velocity distribution is larger at the lower pressure of 7.6 Torr than is the case at 20 Torr, at least in the near wake. However, at axial distances greater than 300 or 400 diameters, the data for the two pressures tend to overlap. These observations appear to be related to other information indicating that for 2.7 in. diameter spheres launched at 7.6 Torr, the wake is not fully turbulent until about 300 or 400 diameters.