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

一个不均匀的和随时间变化的密度分布是广泛的技术液体流动的特征。它们可以来自沿壁面的热传递过程，不同流体的混合过程或通过液体传播的冲击波。在所有这些情况下，流场和密度场之间都发生了相互作用。因此，为了研究和解释这种瞬态流动问题，两种光学测量技术将以一种新的方式结合起来，同时进行速度和密度场测量。测量技术主要有微分干涉法和微粒子图像测速法（Micro- piv）。将中性浮力流体示踪剂添加到流体中进行微piv测量，并通过连续粒子图像对的二维相互关联来量化流体速度。测试部分由平行光线照射，平行光线同时用于拍摄干涉图像。通过计算干涉光线的相位差，从这些干涉图像确定密度梯度。由于这些相位差源于流体的折射率变化，因此测量技术可以同样应用于确定温度、浓度、密度和压力梯度。计划中的实验装置允许用相同的激光脉冲暴露干涉和粒子图像。据我们所知，这种实验装置的实现是新的，它将为研究各种技术流动问题中遇到的瞬态流动及其密度分布之间的相互作用过程提供可能性。首先，将给出沿加热板的静止层流流动的原理证明。然后，将对差分干涉测量的空间分辨率和灵敏度进行评估。因此，将同时进行干涉和水听器测量，以比较从单个崩溃气泡传播的冲击波的压力幅值。迄今为止，人们对液体中溶解气体对空化流动的影响还只是定性地了解。因此，将进行干涉与微piv相结合的测量，研究其对激波强度的影响以及由此引起的流速变化。大量空化气泡的数值模拟通常是在有限的数值分辨率条件下进行的，只允许对压力振幅和速度分布进行定性预测。因此，测量结果将为数值模拟的验证提供重要的数据来源。

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.