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Research Papers: Multiphase Flows

Investigation of the Behavior of Ventilated Supercavities

[+] Author and Article Information
E. Kawakami

 University of Minnesota, Saint Anthony Falls Laboratory, 2 Third Ave. SE Minneapolis, MN 55414kawa0054@umn.edu

R. E. A. Arndt

 University of Minnesota, Saint Anthony Falls Laboratory, 2 Third Ave. SE Minneapolis, MN 55414arndt001@umn.edu

According to Franc and Michel [4], pulsating cavities depend on the ratio σc /σv , the inverse of the ratio used by Paryshev [6].

J. Fluids Eng 133(9), 091305 (Sep 15, 2011) (11 pages) doi:10.1115/1.4004911 History: Received May 20, 2011; Revised August 16, 2011; Published September 15, 2011; Online September 15, 2011

A study has been carried out to determine various aspects of the flow physics of a supercavitating vehicle at the Saint Anthony Falls Laboratory (SAFL). For the experimental work presented here, artificial supercavitation behind a sharp-edged disk was investigated for various model configurations. Results regarding supercavity shape, closure, and ventilation requirements versus Froude number are presented. Conducting experiments in water tunnels introduces blockage effects that are not present in nature. As a result, effects related to flow choking must also be considered. Two methods for computing ventilated cavitation number were compared, the first based on direct measurement of pressure and velocity, and the second technique based on measured cavity geometry and the use of previous numerical results. The results obtained are similar in character to previously reported data, but differ in measured numerical values. An attempt is made to correlate results from water tunnel experiments, where blockage has a significant effect, to an unbounded open flow. Supercavitation parameters, especially the minimum obtainable cavitation number are strongly affected by tunnel blockage.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Example of supercavitation observed in the Saint Anthony Falls Laboratory water tunnel, Wosnik [1]

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Figure 2

Air entrainment data for both disks and struts. Cavitation number computations were refined using data of Brennen [8]. From Wosnik [5].

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Figure 3

Typical entrainment curve, obtained experimentally, for a cavity formed behind a given disc at a given speed (Campbell and Hilborne [5], originally reported by Swanson and O’Neill [9])

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Figure 4

SAFL water tunnel schematic

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Figure 5

Forward facing model schematic

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Figure 6

Top: Typical cavity observed with forward facing model. Bottom: Scheme of deformation of cavity, which is ended by two vortex cords [10].

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Figure 7

Backward facing model schematic

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Figure 8

Cavities generated by the backward facing model. Top: cavity generated by 10 mm cavitator exhibiting significant disturbances from the upstream hydrofoil’s wake. Bottom: cavity generated by 20 mm cavitator. Disturbances seen on the cavity surface inside the hydrofoil wake only.

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Figure 9

Top: Short, opaque cavity highlighting first type of re-entrant jet observed. Bottom: Re-entrant jet at the closure of a cavity transiting to twin-vortex mode of closure exhibiting second type of re-entrant jet.

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Figure 10

Cavity development for the forward facing model. Note that transducer data are only available when a clear cavity is present. Fr = 23, dc  = 20 mm. Red arrows shows steps of air entrainment increase then decrease, illustrating hysteresis effects. For purposes of clarity, only a few data points are shown for Brennen’s [8] predictions once a clear cavity is established.

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Figure 11

Cavity development for the backward facing model. Note that transducer data are only available when a clear cavity is present. Fr = 23, dc  = 20 mm. Red arrows shows steps of air entrainment increase then decrease, illustrating hysteresis effects. For purposes of clarity, only a few data points are shown for Brennen’s [8] predictions once a clear cavity is established.

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Figure 12

Experimental data compared to Brennen’s [8] numerical predictions for both models. Results from the forward facing model are shown in black, while results from the backward facing model are shown in red. Calculations from Brennen [8] are shown by lines while experimental data is shown as points. Labels indicate disk diameters associated with blockage ratios from curves generated by Brennen’s [8] simulations given tunnel equivalent diameter (slightly different than diameters tested). Once again, Brennen [8] made the assumption of infinite Froude number for his calculations. As Froude number increases, data approach the minimum value as dictated by blockage.

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Figure 13

Ratio of cavitation numbers, σc /σblockage , plotted against Froude number for the forward facing model. Values correspond to clear supercavities only.

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Figure 14

Entrainment Coefficient as a function of cavitation number for constant Froude numbers. Comparison between the forward and backward facing models with results from Campbell-Hilborne [5] as presented by May [13].

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Figure 15

Equivalent freestream cavitation number plotted against Froude number. Forward facing model for all the cavitator sizes tested.

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Figure 16

Examples of closure for both models. Top: forward facing model. Bottom: backward facing model.

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Figure 17

Example of quad-vortex. Top: forward facing model. Bottom: backward facing model. Note that the images are side views. Each vortex seen in the images has a matching vortex behind it.

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