ESPI of a transient shock wave flow using an ultra-fast digital camera

The application of electronic speckle pattern interferometry (ESPI) to the visualization of a typical high-speed compressible flow is investigated. ESPI is an interferometric technique that has established itself as a reliable alternative to holographic interferometry in the measurement of small displacements and of vibrations, and is increasingly being used in flow visualization. It can instantly and in real time produce interferometric images in digital form on a video screen, with no photographic processing being required. In this paper two flows are examined, the one a low speed flow of a thermal plume arising from a hot soldering iron, for which real-time visualization is achievable; and the other single frame imaging of a shock wave emerging from a small round open-ended shock tube. ESPI is shown to be a valuable tool in the visualization of compressible flows, and a good alternative to holographic interferometry in obtaining quantitative density data about a flow field. A method for obtaining interferograms with finite fringe-width is presented. The main benefit of using ESPI for flow visualization is that the interferometric image is immediately accessible for viewing on a monitor, so avoiding the tedious photographic holographic reconstruction process. Advances in camera technology are fast overcoming its disadvantage, low image resolution.