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Roger E.A. Arndt, Principal Investigator

Unsteady Sheet/Cloud Cavitation

Experimental research in this project has been able to identify and quantify several features of the complex physics associated with the unsteady cavitation phenomena. As part of the research, these researchers have developed the tools necessary to validate the computational codes being developed. By developing a numerical model in concert with experimentation, reliable predictive tools are ensured.

This research indicates that the entire experimental system must be modeled in order to effect a realistic evaluation of computational models. Therefore, it is necessary to model most of the water tunnel used in these experiments. These researchers are continuing to use fluent, which played a major role in previous work. Further efforts are also made in exploiting capabilities in calculating two phase flow. The ability to use the added features of fluent have important ramifications in developing new experimental techniques for the measurement of cavitating flows.

Can a commercial code calculate a complex flow? FLUENT 5 was used to calculate the cavitating flow over a NACA 0015 hydrofoil. The graph on the left compares the calculated cavity length with experiment, where F is the cavitation number and " is the angle of attack. Although the cavity length appears to correlate well with the parameter F/2", the vertical extent of the cavitation varies substantially with " as shown in the figure on the right.

Previously, comparisons were made between measured experimental data and simulations for the wake of a hydrofoil. These measurements were made in the St. Anthony Falls Laboratory (SAFL) high-speed water tunnel. The basic instrumentation used in this study consisted of hydrophones, accelerometers, flush mounted piezoelectric transducers, and video recording. Two hydrophones were mounted in the water tank above the test section. The hydrofoil designed for this project is approximately two-dimensional, spanning the test section in the vertical direction with an NACA-0015 cross section.

Velocity measurements were made using a TSI Color Burst LDA/LDV system with a beam splitter and an IFA Processor in the backscatter mode. The data were collected at various distances behind the trailing edge of a hydrofoil. The data are plotted in the form [Umax - u(y)] / Umax x (x / c)0.5 versus y / (x x c)0.5, where Umax is the free stream velocity, c is chord length, u(y) is the axial component of velocity in the wake, y is normal to the flow with negative values on the suction side of the foil, and x is the distance downstream of the trailing edge of the foil. Comparisons are made with the large eddy simulation (LES) simulations of Professor Charles C.S. Song of the Civil Engineering Department and the St. Anthony Falls Laboratory at the University of Minnesota using an RSM model.

Very recently, these researchers have identified a total of three mechanisms for cavitation induced instabilities that result in lift oscillations with totally different spectral characteristics in the three regimes. This would not have been possible without the benefit of a coordinated experimental/numerical approach. The numerical simulations were based on a sophisticated LES approach by Professor Song and his associates. The next phase of numerical work is determining the suitability of less complex simulations using a modification of a commercial code. This is allowing wide applicability of the research in the design process for a wide range of liquid handling devices. The first application is being done in the hydropower field.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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