Schlieren-based techniques for investigating instability development and transition in a hypersonic boundary layer

• DOI: 10.1007/s00348-014-1782-9
• Publication Date: 2014-07-22T02:08:19Z
• Authors: S. J. Laurence; A. Wagner; K. Hannemann
• Keywords: Schlieren imaging; Expansion tunnel; Frame rate

Three variants of schlieren techniques are employed to investigate the development of second-mode instability waves in the hypersonic boundary layer of a slender cone in a reflected shock tunnel. First, a previously proposed technique using high frame rate (i.e., at least as high as the dominant instability frequency) schlieren visualization with a continuous light source is shown to provide repeatable measurements of the instability propagation speed and frequency. A modified version of the technique is then introduced whereby a pulsed light source allows the use of a higher-resolution camera with a lower frame rate: this provides significant benefits in terms of spatial resolution and total recording time. A detailed picture of the surface-normal intensity distribution for individual wave packets is obtained, and the images provide comprehensive insight into the unsteady flow structures within the boundary layer. Finally, two-point schlieren deflectometry is implemented and shown to be capable of providing second-mode growth information in the challenging shock tunnel environment.