Experimental Characterization of the Pressure Field and Cavitation Inception in Tip Leakage Flows
Abstract
Leakage flows across the tip gap, from the pressure to the suction side, of ducted marine propellers roll up into a tip leakage vort ex (TLV) in a complex proces cavitation inception, making predictions of the pressure field within the TLV and its secondary components an essential element of the propeller design. Predicting the conditions for TLV cavitation inception is a major challenge owing the sensitivity of the press ure distributions in the vortex core to the tip geometry and boundary layers on the surface of the propeller blades. Understanding o f the mechanisms involved and benchmarking of numerical predictions of such flows require detailed time-resolved experimental data o n the flow structure, pressure field in the tip region of the propeller. The presently proposed study utilizes recent advancements i n 3D flow measurement and data processing techniques to fully resolve the flow and pressure fields during TLV rollup, and determine their impact on the conditions for cavitation inception. The effort consists of two parts, both involving the same propeller geometr y, but with different sizes: (i) Experiments performed at NSWC/Carderock will measure the flow downstream of a 36 diameter propelle r, focusing on the region where cavitation inception is expected to occur. JHU will support these tests with equipment, by assistanc e during tests (as needed), by manufacturing the propeller, and by providing the code and training needed for calculating the pressu re distributions from time-resolved measurements performed by collaborating groups from NASWC/Carderock, George Washington Universit y, and the University of Michigan. (ii) Experiments performed at the Johns Hopkins University (JHU) refractive index matched facilit y will utilize high speed imaging to characterize the cavitation inception process as well as stereo and time-resolved tomographic P article Image Velocimetry (PIV) for measuring the flow and pressure fields in the tip region of transparent 12 diameter propeller. The high speed imaging will determine the location, as well as the trajectory and behavior of bubbles during cavitation inception. T he impact of concentration and distribution of freestream nuclei will be determined by performing experiments with varying controlle d bubble concentrations and sizes. Taking advantage of the unobstructed views afforded by refractive index matching, stereo PIV meas urements in a series of axial, radial and meridional planes covering the entire tip region, including the tip gap, will characterize the evolution of the TLV from the rotor blade leading edge to the near field behind the rotor. Then, tomographic PIV combined with shake-the-box based particle tracking will be used for measuring the time-resolved 3D velocity distributions in selected regions, which are prone to cavitation inception. Spatial integration of the directly measured material acceleration will be used for calcula ting the 3D pressure distributions. The resulting insight will be used for assessing and testing of means to reducing the cavitation inception indices by modulating the TLV size, trajectory and strength. Based on recent experiences, the likely means to control the onset of cavitation will involve hybrid casing grooves, which entrain the entire or parts of the TLV and alter its trajectory. The impact of these casing grooves on cavitation inception will be assessed based on high-speed imaging. Geometries causing significant reductions in the inception indices without other adverse effects will be investigated further by detailed characterization of the f low mechanisms and pressure field involved. Approved for public release
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 03, 2021
- Source ID
- N000142112833
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy