J. Fluid Mech. (2009), vol. 622, pp. 33–62. c  2009 Cambridge University Press doi:10.1017/S0022112008005090 Printed in the United Kingdom 33 Three-dimensional instantaneous structure of a shock wave/turbulent boundary layer interaction R. A. HUMBLE†, G. E. ELSINGA, F. SCARANO AND B. W. VAN OUDHEUSDEN Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Netherlands (Received 1 November 2007 and in revised form 3 September 2008) An experimental study is carried out to investigate the three-dimensional instantan- eous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid, similar to what has been reported in other supersonic boundary layers. Three-dimensional vortical structures are found to be associated with the low-speed regions, in a way that can be explained by the hairpin packet model. The instantaneous reflected shock wave pattern is observed to conform to the low- and high-speed regions as they enter the interaction, and its organization may be qualitatively decomposed into streamwise translation and spanwise rippling patterns, in agreement with what has been observed in direct numerical simulations. The results are used to construct a conceptual model of the three-dimensional unsteady flow organization of the interaction. 1. Introduction One of the most engaging yet perplexing phenomena in high-speed fluid dynamics is the interaction between a shock wave and a turbulent boundary layer (SWTBLI). Such interactions occur in a wide variety of internal and external aeronautical applications, and a detailed understanding of the flow physics remains a necessary prerequisite for accurate flow-field prediction, efficient design of high-speed aerospace vehicles, as well as advances in combustion processes (Dolling 2001). A vast body of research has therefore been accumulated over the years describing the interaction’s most salient features. Reviews of much of the early work, concerning two-dimensional interactions, may be found in Green (1970), Hankey & Holden (1975), and Adamson & Messiter (1980). More recent reviews, with emphasis on the unsteadiness properties, including three-dimensional interactions, may be found in D´ elery & Marvin (1986), Dolling (2001), and Smits & Dussauge (2006). One aspect that has attracted particular attention has been the large-scale unsteadiness of the shock wave system and pulsating of the interaction zone, particularly in view of the relatively large discrepancy between the dominant frequencies of the shock motion and the characteristic frequency of the incoming boundary layer. As noted by Wu & Martin (2008) for instance, numerous studies † Email address for correspondence: r.a.humble@tudelft.nl