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2013 年 02 月 20 日
Seminar - Applications of high-speed imaging for compressible flows by Dr. Stuart Laurence

Date:   20 February 2013 (Wednesday)

Time:   11:00 am – 12:00 pm

Venue:  EF305, The Hong Kong Polytechnic University

Abstract:

In this presentation, the author will discuss several areas of his research in which high-speed imaging has allowed critical insights into flow phenomena and/or provided a means for convenient non-intrusive measurement. The first part of the talk will focus on an investigation of transient unstart phenomena in a scramjet (supersonic combustion ramjet) combustion chamber. Unstart is a particularly important problem in scramjets, being responsible, for example, for the recent failure of the second X-51 flight experiment. In this investigation, the unsteady phenomena signalling the onset of unstart are investigated in the HyShot II scramjet configuration. Experiments are carried out in the HEG reflected-shock wind tunnel: high-speed Schlieren and OH* chemiluminescence visualizations, combined with surface pressure measurements, allow links to be drawn between observed flow and combustion features. Incipient unstart is found to take the form of an unsteady shock train that travels up the combustion chamber. While boundary-layer separation is seen to develop during the shock-train propagation, an analysis of the visualization data shows the primary mechanism responsible for the transient behaviour to be thermal choking; this is confirmed by unsteady numerical simulations of the configuration. This finding contradicts previous studies and gives a lower choking limit than predicted by an integral analysis, showing the localized nature of thermal choking to be crucial. Finally, a simple theoretical model is developed to elucidate features of the shock-propagation behaviour. The second part of the talk will touch briefly on three further problems in which high-speed imaging has proved invaluable: laminar-turbulent transition in hypersonic boundary layers; force and moment measurement on capsule geometries in high-enthalpy flows; and aerodynamic interactions between free-flying bodies.

Biosketch:

Dr. Stuart Laurence carried out his undergraduate studies at the University of Auckland, New Zealand, earning a B.Sc.(Hons)/B.A. in Applied Mathematics, Physics and Philosophy in 2001. He was then accepted to the graduate program in the Graduate Aeronautical Laboratories, California Institute of Technology (GALCIT) where he received his MS in 2002 and Ph.D. in 2006. His thesis work, carried out under the supervision of Hans Hornung, was an experimental, numerical, and theoretical study of the aerodynamic interactions between bodies in high-speed flow. Following a post-doc at GALCIT, investigating the aerodynamics and aerothermodynamics of the NASA Orion capsule, Dr. Laurence moved to the German Aerospace Center, Goettingen, where he has been since in the capacity of a research scientist. His research is primarily experimental in nature, focusing on such areas as reacting and combusting flows, hypersonic aerodynamics, boundary-layer transition, and the development of non-intrusive measurement techniques.