Fully Resolving the Interaction Between Freestream and Surface Nuclei with the Surrounding Turbulent Flow Field During Cavitation Inception
Abstract
While there is ample evidence indicating that cavitation inception is dependent on theconcentration and nature of freestream and surface nuclei, very little quantitative experimental dataexits on how these nuclei interact with high Reynolds number turbulent flows as they grow frommicroscopic to visible scales. Such information is critical for modeling and predicting the onset ofcavitation, and for characterizing the so-called invisible cavitation events, which generate noise,but are hard to detect visually. Progress in this area has been hindered by our limited ability toresolve explosive processes, which occur in microseconds, and may involve four orders ofmagnitude change in the bubble size. The objective of this study is to address this challenge takingadvantage of recent advances in high speed imaging and digital holographic microscopy. Our goalis to determine how free stream and surface nuclei interact with the mean and fluctuating pressurefields in smooth and rough wall high Reynolds number turbulent boundary layers developingaround blunt nose bodies, and in vortices subjected to axial straining, which reduced the pressurein their cores. The time and spatially resolved measurements will be performed in two liquidtunnels: the JHU refractive index matched facility, and a small high-speed cavitation tunneldesigned for microscopic observations. In both cases, the cavitation tests be conducted in flowswhere the flow and pressure fields prior to cavitation inception are fully characterized and thesurface and free-stream nuclei are carefully controlled. To quantify the impact of nuclei oncavitation inception, the experiments will be performed at: (i) varying concentration and sizedistributions of freestream seed bubbles introduced upstream of the minimum pressure region, and(ii) varying surface properties and exposure history to air, including notched smooth hydrophobicand hydrophilic surfaces, as well as rough walls, which are expected to affect the spatialdistribution and durability of surface nuclei. The observation will be performed using a novel5MHz digital holographic microscopy (DHM) system that is presently being developed andintegrated. Digital holography enables us to simultaneously track in 3D and measure the size oftens of thousands of mirco-particles and bubbles of varying sizes. This new in-line DHM systemis faster than previous ones by three orders of magnitude. It features a 5 MHz camera that canrecord 180 frames, a pulsed fiber laser allowing us to reduce the exposure time to 500 ps, and acontrol system allowing us to retain data recorded prior to the slightly-delayed trigger signal. Thissystem will be used for measuring (i) the growth and collapse of freestream and/or surfacecavitation nuclei from microscopic to macroscopic scales, fully resolving the time evolution oftheir shape, (ii) the velocity and pressure distributions around the growing and collapsingcavitation bubble as it interacts with, and presumably modifies, the surrounding flow field. Thevelocity field will be characterized by seeding the water with 1 ???m neutrally buoyant particles andtracking their 3D motion using both previously-developed and recently-introduced commerciallyavailable???Shake-the-Box??? software. The potential effect of tracer particles of varying size andexposure history, which might also serve as nuclei, on cavitation inception will be determined. Theinstantaneous pressure distributions around the growing and collapsing bubbles will be determinedby spatially integrating the Largrangian acceleration of the tracer particles using in-housedeveloped procedures. The results will quantify, for the first time, the interaction of cavitationevents with the surrounding flow at high Reynold numbers, greatly enhancing our understandingthe cavitation inception process.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812635
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy