Review of planar multiple-component velocimetry in high-speed flows

High speed flows encountered in most applications typically have very high Reynolds numbers and are often highly turbulent. Even in a laboratory scale high subsonic or supersonic (high speed) flow, velocities could vary over 3 orders of magnitude, and the turbulence spatial and temporal scales could span over 4-5 orders of magnitude. Exploring detailed physics of such flow s presents major challenges to both experimental and computational research ers. An ultimate velocimetry technique would provide detailed, accurate, vo lumetric, real-time velocity data in such flows. With that being the goal, currently there are two planar velocimetry techniques that are developing r apidly into very useful tools with the potential of providing accurate velo city information in high-speed flows. The techniques are planar Doppler vel ocimetry (PDV) and particle imaging velocimetry (PIV). Whereas PDV has been under development for a relatively short period of time and is becoming a powerful technique, more accurate in high-speed flows, PIV is an establishe d technique in low-speed flows and is now breaking ground in high-speed flo ws. The purpose of this review is to provide detailed background on these t wo techniques, to discuss the strengths and constraints of each technique, and to outline the areas in need of further improvement and development. Th e aims are to assist the novice users in their proper usage and to help tho se who are in the process of deciding which technique is more appropriate i n their specific applications.