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Research Papers: Multiphase Flows

Investigation of the Behavior of Ventilated Supercavities in a Periodic Gust Flow

[+] Author and Article Information
Seung-Jae Lee

e-mail: sjlee@umn.edu

Ellison Kawakami

e-mail: kawa0054@umn.edu

Roger E. A. Arndt

e-mail: arndt001@umn.edu
Saint Anthony Falls Laboratory,
University of Minnesota,
Minneapolis, MN 55414

Blockage ratio is defined as the ratio of the cross sectional area of the tunnel test section to that of a cavitator.

Significant wave height (Hs) and modal wave period (Tw) are defined as the average height of the highest one-third waves in a wave spectrum and the wave period at which the peak wave energy in a wave spectrum occurs, respectively.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 17, 2012; final manuscript received May 1, 2013; published online June 3, 2013. Assoc. Editor: Olivier Coutier-Delgosha.

J. Fluids Eng 135(8), 081301 (Jun 03, 2013) (13 pages) Paper No: FE-12-1449; doi: 10.1115/1.4024382 History: Received September 17, 2012; Revised May 01, 2013

A ventilated supercavity consists of a large, gas-filled bubble enveloped around an underwater vehicle that allows for significant drag reduction and increases in vehicle speed. Previous studies at the Saint Anthony Falls Laboratory (SAFL) focused on the behavior of ventilated supercavities in steady horizontal flows. In open waters, vehicles can encounter unsteady flows, especially when traveling near the surface, under waves. In supercavitation technology, it is critical that the vehicle remains within the cavity while traveling through water to avoid unwanted planing forces. A study has been carried out in the high-speed water tunnel to investigate the effects of unsteady flow on axisymmetric supercavities. An attempt is made to duplicate sea states seen in open waters. In an effort to track cavity dimensions throughout a wave cycle, an automated cavity-tracking script has been developed. Using a high-speed camera and the proper software, it is possible to synchronize cavity dimensions with pressure measurements taken inside the cavity. Results regarding supercavity appearance, cavitation parameters, and their relation to sea state conditions are presented. It was found that flow unsteadiness caused a decrease in the overall length of the supercavity while having only a minimal effect on the maximum diameter. The supercavity volume varied with cavitation number, and a possible relationship between the two was explored.

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Figures

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Fig. 2

Schematic of optical experimental setups

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Fig. 3

Sample output from the cavity-dimension tracking process used on images collected from a high-speed camera

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Fig. 4

Characteristics of the normalized amplitude (ε/dc) of a periodic gust flow measured at 20 mm upstream from the cavitator. Note that ε/dc and fgdc/U∞ are equivalent to AM and LM in Eq. (1), respectively.

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Fig. 5

Comparison of characteristics of a periodic gust flow with sea-state condition at sea. Note that symbols are the measured values and open boxes indicate the approximated values corresponding to the sea-state condition at sea. L and A mean the frequency and amplitude of a gust flow normalized by the diameter of a cavitator, respectively.

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Fig. 6

Cavity images at different gust frequencies (fgdc/U∞) and angle of attacks (αg). Note that the images are one of side views of instantaneous supercavity images taken by high-speed camera.

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Fig. 7

Mean plots and standard deviations of maximum diameter (Dmax) of a supercavity and its location (xDmax, yDmax)

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Fig. 8

Mean plot of cavity length (Lc) normalized by wavelength of gust flows (λg) versus the normalized frequency (fgDc/U∞)

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Fig. 9

Cavity length (Lc/dc) plotted against maximum velocity of the gust generator (vg). The dotted line indicates a polynomial-fit curve.

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Fig. 10

Ratio of horizontal location of maximum cavity diameter (xDmax) to cavity length (Lc)

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Fig. 11

Trajectories of maximum cavity diameter during a cycle. Note that x- and y-axis in plots mean xDmax/dc and yDmax/dc, respectively, and the grid spacing is equal to dc.

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Fig. 12

Schematic drawings of three type trajectories for maximum cavity diameter

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Fig. 13

Time history of the horizontal position of maximum cavity diameter (xDmax/dc) and the ventilated cavitation number (σc/σcSteady) according to the angular position of the gust generator αg. Sampling rate of 1 kHz.

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Fig. 14

Ratio of the ventilated cavitation number in a periodic gust flow (σc) to the ventilated cavitation number in a uniform flow (σcSteady)

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Fig. 15

Ventilated cavitation number (σc/σcSteady) plotted against the standard deviation of xDmax/dc. Note that σc is the ventilated cavitation number in a periodic gust flow and σcSteady is the ventilated cavitation number in a uniform flow.

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Fig. 16

Cross-correlation coefficient (ρxy), time delay (τo), and phase angle (ϕ) between the ventilated cavitation number and the cavity volume

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Fig. 17

Wave spectra for different significant wave heights (Hs) and modal wave periods (Tw)

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Fig. 18

Comparison of characteristics of a periodic gust flow in experiments with each wave component in idealized wave spectra. Note that cavitator diameter of a full-scale supercavitating vehicle is assumed to be 0.25 m. L and A mean the frequency and amplitude of a gust flow normalized by the diameter of a cavitator, respectively.

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