DUAL HOLOGRAM SHEARING INTERFERENCE TECHNIQUE FOR WIND-TUNNEL FLOW-FIELDS TESTING

A novel optical diagnostic technique, dual hologram shearing interfero metry, for measuring density gradients of different phase objects is p roposed and demonstrated. The lateral shearing is achieved by using a phase grating. A holographic interferometer has been developed and des igned on the base of a single pass Z type conventional schlieren devic e. The interferometer's scheme is insensitive to acoustical disturbanc es, similarly to the conventional schlieren layout, and is capable of recording holograms with a continuous wave laser during the wind tunne l run. The features of the technique make it tolerant to both the temp oral coherence of the laser light source and to the relatively low, sc hlieren quality optical windows of the wind tunnel's test section. The obtained reconstructed lateral shearing interferograms with a large r egion of overlap have high contrast and may have an arbitrary orientat ion and/or spacing of the background interference fringes. It is belie ved that the proposed approach will become a useful tool for visualiza tion and accurate mapping of the density gradients of gas dynamic flow fields, in wind and shock tunnels, where acoustic noise problems may dramatically affect reference beam holographic schemes.