Development of an experimental set-up for simultaneous density and velocity field measurements in cavitating ultrasound flows by means of Differential Interferometry and Micro Particle Image Velocimetry

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An inhomogeneous and time-dependent density distribution is characteristic for a broad range of technical liquid flows. They can originate from heat transfer processes along walls, mixing processes of different fluids or from shock waves propagating through the liquid. In all these cases interaction processes take place between the flow and the density field. Hence, for the investigation and interpretation of such transient flow problems two optical measuring techniques will be combined in a novel manner to perform simultaneous velocity and density field measurements. The measuring techniques utilized for the problem are the differential interferometry and the Micro Particle Image Velocimetry (Micro-PIV). Neutrally buoyant fluid tracers are added to the flow to carry out Micro-PIV measurements and fluid velocities are quantified via a 2D cross-correlation of consecutive particle image pairs. The test section is illuminated by parallel light rays which are simultaneously used to take interference images. Density gradients are determined from these interference images by computing phase differences of interfering light rays. Since these phase differences originate from refractive index changes in the flow, the measurement technique can be applied just the same to determine temperature, concentration, density, and pressure gradients. The planned experimental set-up allows exposing interference and particle images with the same laser pulse. To our knowledge, the realization of such an experimental set-up is new and it would offer the possibility to study interaction processes between transient flows and their density distributions encountered in various technical flow problems.At first, a proof of principle will be given for a stationary, laminar fluid flow along a heated plate. Thereafter, the spatial resolution and sensitivity of the differential interference measurements will be evaluated. For that reason, simultaneous interference and hydrophone measurements will be done to compare pressure amplitudes of shock waves that propagate from single collapsing bubbles. The effect of dissolved gas in the liquid on cavitating flows is only qualitatively understood till today. Therefore combined interference and Micro-PIV measurements will be done to study its influence on shock wave strengths and resulting changes in flow velocities. Numerical simulations of large numbers of cavitation bubbles are usually done under restricted numerical resolution conditions allowing only for qualitative predictions of pressure amplitude and velocity distributions. Therefore, measurement results will provide an important data source for the validation of numerical simulations.