The mitigation of pulsation in ventilated supercavities

Open Access
Author:
Skidmore, Grant Marston
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 04, 2016
Committee Members:
  • Jules Washington Lindau V, Dissertation Advisor
  • Jules Washington Lindau V, Committee Chair
  • Timothy A Brungart, Committee Chair
  • Robert Francis Kunz, Committee Member
  • Jonathan S Pitt, Committee Member
  • Toan Nguyen, Committee Member
Keywords:
  • Supercavitation
  • Cavitation
  • Pulsation
  • Ventilated Supercavities
  • Multiphase Flow
  • CFD
  • Noise
  • Acoustics
Abstract:
It is desirable to use ventilated supercavities to reduce the drag created by underwater bodies and obtain velocities much higher than those that are possible with fully wetted bodies. Ventilated supercavities, however, are prone to an autoresonant phenomenon known as pulsation where the supercavity radius and length oscillate with time. These oscillations in radius and length are oftentimes severe enough to cause issues with body stability and control. In this dissertation, a method to mitigate pulsation in ventilated supercavities is presented. The method, which modulates or adds a sinusoidal component to the ventilation rate, is shown to suppress pulsation numerically, experimentally, and computationally. Additionally in this dissertation, the near-field acoustic characteristics of twin vortex, re-entrant jet, and pulsating ventilated supercavities are studied experimentally. This study is then repeated computationally, with a focus on the generation and mitigation of pulsation in ventilated supercavities. The study of the near-field radiated noise from supercavities shows that pulsating supercavities generate noise that is two orders of magnitude (i.e., 40 dB) greater in level than that from comparable twin vortex and re-entrant jet supercavities. For pulsating supercavities, it is found that the interior cavity pressure and near-field radiated noise are both monotonic in frequency, with said frequency being related to the freestream velocity and the length of the dominant waves on the supercavity air/water interface. For pulsating supercavities, it is also found that, at the pulsation frequency, the cavity interior pressure spectrum level is related to the near-field and far-field noise spectrum level through spherical spreading of the sound waves from the supercavity interface. As a result, the cavity interior pressure can be used as a measure of the radiated noise. The developed method for mitigating pulsation in ventilated supercavities is shown to transition the initially pulsating supercavities to the twin vortex closure regime. A wide range of ventilation rate modulation frequencies cause the pulsating supercavity to transition into twin vortex closure, typically within 0.25 seconds of modulation initiation. Accompanying the transition from pulsation to twin vortex closure is a reduction in the radiated noise, to the continuum at the pulsation frequency, oftentimes 35 dB or more. Other modulation frequencies do not suppress pulsation but are effective at changing the supercavity pulsation frequency. The numeric exploration of the methodology focuses on solving a modified Hill's equation which captures the basics of the modulation method, and is based on Song's supercavity model. The model adequately predicts those modulation frequencies that mitigate ventilated supercavity pulsation and those that do not. The computational study utilizes 3D finite volume computational fluid dynamics (CFD) modeling, without a turbulence model, for the equations of fluid mixture motion. The gas is treated as isothermal and compressible, and the water is treated as isothermal and incompressible. The interface is captured using the High Resolution Interface Capturing (HRIC) scheme. Utilizing these models allows for the re-entrant jet, pulsating, and twin vortex closure regimes to be delineated and computationally resolved, including the expected hysteresis. To validate the use of these models, computational and experimental pulsating supercavities are generated at similar conditions. A detailed comparison of these supercavities show that the computational supercavity is able to capture the behavior of the experimental supercavity favorably. The computational exploration of the pulsation mitigation methodology was performed at conditions that not only matched those of the water tunnel experiments, but at a Froude number much larger than those explored experimentally. The computational study demonstrated that by appropriately modulating the ventilation rate, it is possible to suppress pulsation and transition the cavity to either the twin vortex or re-entrant jet closure regime at velocities much higher than those examined in the water tunnel experiments.