PIV-LIF determination of mean velocity field and Reynolds stress tensor in a cavitating mixing layer

Abstract

The purpose of this experimental study was to analyze a 2D cavitating shear layer. The global aim of this work was a better understanding and modeling of cavitation phenomena from a 2D turbulent shear flow to rocket engine turbopomp inducers. This 2D mixing layer flow provided us a well documented test case to be used for comparison between the behavior with and without cavitation. Similarities and differences led to characterize effects of the cavitation on the flow dynamic. The run fluid was liquid water. The experimental facility allowed us to set two distinct configurations with different cavitation levels: - CDM: a mixing layer flow (U1 = 15.8 m/s for the high speed side and U2= 3.5 m/s for the low speed side) - MD: a downward facing step flow (U1 = 13.5 m/s and U2 = 0 m/s). The development of Kelvin-Helmholtz instabilities was observed at the interface. Vaporizations and implosions of cavitating structures inside the vortices were also observed. PIV-LIF(Particle Image Velocimetry Laser Induced Fluorescence) system was used to measure the velocity of the liquid phase. Instantaneous velocity fields were measured in the whole flow. The self similarity of the flow was characterized by the dimensionless analysis of the mean and fluctuating velocity fields. Parameters that characterized the flow dynamic were studied and quantified: Vorticity thickness, growth rate and Reynolds tensor components. Turbulent kinetic energy and the anisotropy tensor components were also analyzed and estimated. General behaviors of the two configurations have been observed: - In the CDM case the mixing area developed along the x-axis a turbulent shear area, growing linearly, showing a constant growth rate over the studied cavitation parameter range. - The MD case was more complex, presenting a flow separation with a large recirculating area and a quite large positive pressure gradient. The reattachment point moved depending on the cavitation level. The recirculating area seemed to have an unsteady behavior and was certainly pulsing and shedding vortices downstream. Successive vaporizations and condensations of the fluid particles inside the turbulent area have generated additional velocity fluctuations due to the strong density changes associated with the vaporization and condensation processes. However, the mean spatial development of the mixing area was only barely affected over the studied cavitation number range. The main results of this study clearly showed that the turbulence-cavitation relationship inside a mixing layer is not only driven by a simple change of compressibility properties of the fluid in the turbulent field due to the presence of a twophase flow.