Optical studies and simulations of hypervelocity flow fields around blunt bodies

Abstract

This dissertation focuses on the development and application of optical techniques to visualize and characterize hypersonic wake flows in shock tunnels. Simulations of these flows are conducted and used to compare with the experimental results. The work presented covers three topics. Firstly, a time-resolved diode-laser based resonantly enhanced differential interferometry visualization is introduced. This new technique is capable of visualizing the transient behavior of wake flows under close-to-rarefied conditions, which is not possible with standard schlieren or interferometric methods. Secondly, measurements of velocity and temperature using planar laser-induced fluorescence (PLIF) around a generic planetary entry capsule model are presented. The model is mounted without potentially flow-disturbing mounting structures, creating a wake flow field similar to that of a space vehicle. The first two-component velocity map of such an entry-type hypersonic flow is presented, showing the velocity vector field in the inner wake, which has not previously been measured accurately. The theoretical understanding of the PLIF Doppler velocimetry technique is extended by describing the phenomenology behind the systematic error due to laser attenuation, one of the most important sources of bias error associated with this technique. With little additional effort, the rotational temperature in the same flow field is measured as well. The third topic focuses on the comparison between the measured velocities and temperatures with simulations using the direct simulation Monte Carlo method. Good agreement is found, except in the inner wake downstream of the model close to the centerline. These discrepancies are attributed to effects of localized transition from laminar to turbulent flow, causing higher temperatures and different velocity profiles in the far wake compared to the laminar simulations. The availability of non-intrusively measured global velocity and temperature data allows effect of transition on these parameters to be quantified with unprecedented resolution.