University of Twente Student Theses


Experimental research on cavitation in a water tunnel at the University of Michigan

Deijlen, Lisa (2017) Experimental research on cavitation in a water tunnel at the University of Michigan.

Abstract:Previous research studied the effect of cavitation on the wake behind a symmetric wedge and discovered that the shedding frequency from the far wake, the von Karman vortex street, nondimensionalized as the Strouhal number(St) is constant at high cavitation numbers (σ), but increases, peaks and then decreases again when the cavitation number decreases. To understand this behavior several experiments are conducted in a water tunnel at the University of Michigan. Here, the cavitation number is varied by reducing the pressure in the tunnel and the flow in the wake is visualized using high-speed videos and X-ray densitometry. From these images the vortex spacing, velocity of the vortices and the Strouhal number are obtained. For high cavitation numbers (σ > 2.5) this has been done using a MATLAB script to detect the vortices, finding the centers and calculating the values. To better understand the properties of a cavitating wake, from both Strouhal number and vortex spacing perspective, the properties of the von Karman vortex street were examined by Saffman and Schatzman (1981) using an inviscid model for the vortex street which modeled the dependence of the vortex spacing with the drag coefficient for a given vorticity ratio between the far wake and shear layer. The insight from their analysis is used to understand the effect of cavitation in altering the vortex spacing and Strouhal number. Since the drag on the object was not measured, the cavitation numbers from the experimental results are used to calculate the corresponding drag coefficients. The drag coefficients are calculated using the Riabouchinsky model including a correction for the wall effects. With the help of estimated drag and measured vortex spacing, the experimental results are compared with the theoretical analysis of Saffman and Schatzman (1981). With the assumption of a constant vorticity and velocity in the shear layer for different cavitation numbers it is possible to draw conclusions about the vorticity. Until the Strouhal peak (σ =2.05) the vorticity of the vortices decreases and after the peak the vorticity will stay approximately constant. This indicates that the presence of cavitation will influence the wake dynamics behind a wedge. The amount of influence needs to be investigated in further research.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Cavitation - Experimental research - High-speed imaging - Vortex street properties
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