Visualization of the shock wave/boundary layer interaction using polarization imaging
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R E G UL A R P A P E R
Lin He
•
Xiao-ge Lu
Visualization of the shock wave/boundary layer interaction using polarization imaging
Received: 8 January 2020 / Revised: 8 May 2020 / Accepted: 11 May 2020 Ó The Visualization Society of Japan 2020
Abstract The flow visualization of a shock wave/boundary layer interaction (SWBLI) has been performed using polarization imaging. This technique used the degree of linear polarization (DOLP) for flow visualization instead of the intensity information which is used in traditional imaging methods. The experimental results show that the instantaneous structures of SWBLI captured by the polarization imaging are the same as when the intensity information is used, verifying the feasibility of polarization imaging. The image fusion of the DOLP results and the intensity information will further improve the flow visualization effect. Through statistical analysis, it can be shown that polarization imaging would weaken the influence of incident light on imaging, when the intensity of the incident light is not uniform or fluctuates with time. Keywords Polarization imaging Shock wave/boundary layer Interaction Flow visualization Degree of linear polarization
1 Introduction Flow visualization techniques are of great interests in fluid mechanics, because these techniques can provide useful information about complex three-dimensional structures which have been recognized to play an important role in turbulent flows and would help further understand the physical mechanisms of turbulence. In the compressible regime, widely used techniques, such as shadowgraph and schlieren methods, are limited to investigate the three-dimensional feature of coherent structures, due to the spatial integration of such methods. The development of laser-based techniques, which avoid the spanwise integration effects of schlieren and shadowgraph, provide more powerful tools to visualize the instantaneous three-dimensional structures in compressible turbulence. For example, Smith and Smits (1995) observed the large-scale motions in a supersonic turbulent boundary layer using planar laser Rayleigh scattering. Huntley and Smits (2000) used CO2-enhanced filtered Rayleigh scattering (FRS) to visualize the boundary layer transition on a sharp-nosed elliptic cone at Mach 8, and found that the transition begins with the emergence of small-scale structures near the centerline axis of the cone. Danehy et al. (2009) used the planar laser induced fluorescence (PLIF) to acquire qualitative data about the flow structures of a Mach 5 transitional boundary layer over a discrete hemispherical roughness element. The recently developed Nano-tracer planar laser scattering (NPLS) technique has provided another useful tool to visualize the coherent structures in supersonic and hypersonic flows (Zhao et al. 2009; He et al. 2011). Though these flow visualization techniques have been applied widely in the studies of compressible flows, flow visualization results are not sufficient and improved quantitative parameter measurements
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