Ventilated Supercavity Physics

Author:

Hansford, Samuel E

Graduate Program:

Mechanical Engineering

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

February 19, 2018

Committee Members:

Robert Francis Kunz, Dissertation Advisor/Co-Advisor
Rhett William Jefferies, Committee Chair/Co-Chair
Timothy A Brungart, Committee Member
Robert Francis Kunz, Committee Member
Jules Washington Lindau V, Outside Member

Keywords:

Supercavitation

Abstract:

This work discusses the physics of ventilated supercavities. By varying the ventilation gas, it is shown experimentally that cavity size and closure are better predicted by the mass ventilation rate than by the volumetric ventilation rate that is commonly used throughout the literature. A model of the shear entrainment mechanism is also used to show that lower density gases, such as helium, are entrained faster, on a volumetric basis, than denser gases, such as air and sulfur hexafluoride, consistent with the experimental findings. Waves on the cavity interface are shown to play an important role in cavity behavior leading to a proposed non-dimensional mass ventilation rate parameter based on the cavity surface area. Cavity dynamics and statics are affected by both the amplitude and length of the interface waves. Long wavelength disturbances appear to cause contractions in cavity size, which result in increased cavity pressure. Conversely, shorter wavelength disturbances alter the skin friction relation. The waves on the cavity interface are also shown to be the cause of tonal and broadband pressure oscillations inside the cavity that are radiated from the interface as sound. As such, cavities are not only tonal monopole noise sources, but broadband monopole noise sources as well. Well defined, gross wave motion is related to tonal noise whereas random cavity interface waves or disturbances are the source of broadband noise. There are two observed tonal noise generation mechanisms, pulsation and a feedback loop, and the controlling physics of these tones are discussed. Additionally, pulsation appears to be a cavity state rather than a stand-alone closure regime and the conditions necessary for pulsation to occur are determined. Broadband noise generation when the cavity is near a free surface is different compared to the case without a free surface, when the tunnel is filled and pressurized. When the cavity is near a free surface, the noise appears to scale with the ventilation speed but, when the free surface is removed the noise scaling is different and much more complex. Finally, a model based on the pathlines of the liquid stream around the cavity is proposed. The Froude number and cavitation number are shown to contribute to the pathlines of the liquid stream. Predicted cavity shapes can be inserted into the shear entrainment model to predict the mass flow rate required to maintain a smooth cavity.