Ellison Kawakami

Creation and Maintenance of Cavities Under Horizontal Surfaces in Steady and Gust Flows

An experimental study of air supply to bottom cavities stabilized within a recess under a horizontal surface has been carried out in a specially designed water tunnel. The air supply necessary for creating and maintaining an air cavity in steady and gust flows has been determined over a wide range of speed. Flux-free ventilated cavitation at low flow speeds has been observed. Stable multiwave cavity forms at subcritical values of Froude number were also observed. It was found that the cross-sectional area of the air supply ducting has a substantial effect on the air demand. Air supply scaling laws were deduced and verified with the experimental data obtained.

On the stability of supercavity with angle of attack

SUMMARY Research on the supercavity stability is not only the focus question as a physical phenomenon in the field of the supercavity flows, is but also the basis of studying the control of the supercavitating flows and their vehicles. Angle of attack (AOA) is one of the most important control variables that can maintain and control the supercavity dimensions and its vehicle by changing the drag coefficient and the separation point of supercavitating flows to produce an effect on the force of supercavitating vehicle. However, the disturbances caused by angle of attack or other factors follow and influence the supercavity stability, and in this case, the stability mechanism of supercavity with AOA is different from one without AOA. Hence, the gravity and AOA models of supercavity are respectively established, and the supercavity stability is analyzed combining with Logvinovich model by researching the supercavity after disturbance in this paper. FFT is used to analyze the numerical results to get the frequency characteristics of the pulsation supercavity with and without AOA. It demonstrates that the dominant pulsation frequency of supercavity is irrelevant to the external disturbance excitation. Considering this point, the gas dynamics parameterE , the ratio of the vapor cavitation number v V to the ventilation cavitation number c V , is taken as the stability criterion parameter for the supercavity with the given AOA. The corresponding critical valuesE are obtained numerically for the different AOAs. On this basis, comprehensive consideration of the elastic parameter E and AOA D , the new stability criterion is formulated using the nonlinear least square method for supercavity with AOA.

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

A ventilated supercavity consists of a large, gas-filled bubble enveloped around an under-water vehicle that allows for significant drag reduction and increases in vehicle speed.Previous studies at the Saint Anthony Falls Laboratory (SAFL) focused on the behaviorof ventilated supercavities in steady horizontal flows. In open waters, vehicles canencounter unsteady flows, especially when traveling near the surface, under waves. Insupercavitation technology, it is critical that the vehicle remains within the cavity whiletraveling through water to avoid unwanted planing forces. A study has been carried outin the high-speed water tunnel to investigate the effects of unsteady flow on axisymmetricsupercavities. An attempt is made to duplicate sea states seen in open waters. In an effortto track cavity dimensions throughout a wave cycle, an automated cavity-tracking scripthas been developed. Using a high-speed camera and the proper software, it is possible tosynchronize cavity dimensions with pressure measurements taken inside the cavity.Results regarding supercavity appearance, cavitation parameters, and their relation tosea state conditions are presented. It was found that flow unsteadiness caused a decreasein the overall length of the supercavity while having only a minimal effect on the maxi-mum diameter. The supercavity volume varied with cavitation number, and a possiblerelationship between the two was explored. [DOI: 10.1115/1.4024382]

A comparative study of behaviors of ventilated supercavities between experimental models with different mounting configurations

Small-scale water tunnel experiments of the phenomenon of supercavitation can be carried out broadly using two different kinds of experimental models–in the first model (forward facing model, or FFM), the incoming flow first interacts with the cavitator at front, which is connected to the strut through a ventilation pipe. The second model could have the strut and the ventilation pipe preceding the cavitator (backward facing model, or BFM). This is the continuation of a water tunnel study of the effects of unsteady flows on axisymmetric supercavities. In this study, the unwanted effect of test model configuration on supercavity shape in periodic flows was explored through a comparison of FFM and BFM models. In our experiments, it was found that periodic gust flows have only a minimal effect on the maximum diameter and the cavity length can be shortened above a certain vertical velocity of periodic flows. These findings appear to be robust regardless of the model configuration.

Measurements in the wake of a ventilated hydrofoil: A step towards improved turbine aeration techniques

The purpose of this study is to develop the necessary algorithms to determine the bubble size distribution and velocity in the wake of a ventilated or cavitating hydrofoil utilizing background illumination. A simplified experiment was carried out to validate the automatic bubble detection algorithm at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the SAFL high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. Bubbles were identified in the images using an edge detection method based on the Canny algorithm. The utilized algorithm was designed to detect partly overlapping bubbles and reconstruct missing parts. After all images were analyzed, the bubble velocity was determined by applying a tracking algorithm. This study has shown that the algorithm enables reliable analysis of irregularly shaped bubbles even when bubbles are highly overlapped in the wake of the ventilated hydrofoil. It is expected that this technique can be used to determine the bubble velocity field as well as the bubble size distributions.

Measurements in the Wake of a Ventilated Hydrofoil

The purpose of this study is to develop the necessary algorithms to determine the bubble size distribution and velocity in the wake of a ventilated or cavitating hydrofoil utilizing background illumination. A simplified experiment was carried out to validate the automatic bubble detection algorithm at Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. All parts of the image that are above the global threshold are segmented by an edge detection method based on the Canny algorithm. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct missing parts. After all images have been analyzed, the bubble velocity can be determined by applying a tracking algorithm. This study has shown that the algorithm enables reliable analysis of irregularly shaped bubbles even when bubbles are highly overlapped in the wake of the ventilated hydrofoil. It is expected that this technique can be used to determine the bubble velocity field as well as the bubble size distributions.Copyright

Characteristics of Ventilated Supercavities in a Periodic Gust Flow

A ventilated supercavity consists of a large gas-filled bubble enveloped around an underwater vehicle that allows for significant drag reduction and an increase in maximum vehicle speed. Previous studies at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota 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, ventilated 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.Copyright

RECOGNITION OF HIGHLY OVERLAPPING BUBBLES IN THE WAKE OF A VENTILATED HYDROFOIL

SUMMARY A study is being carried out at Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota to develop the necessary algorithms to determine the velocity deficit and bubble size distribution in the bubbly wake behind a ventilated or cavitating hydrofoil. This is done by utilizing background illumination of the bubbly wake. A simplified experiment, to validate the proposed bubble detection algorithm, was carried out in a fish tank. Compressed air was injected into a porous media, located in the bottom of the tank, which broke up the air into bubbles that rose to the surface. Shadows of the bubbles, cast on a bright background, were collected by a high-speed camera. For processing, a reference image, taken when no bubbles were present in the flow, was subtracted from the images to reduce background noise. Once this was done, the image was segmented using an edge detection technique. The Canny algorithm was determined to be the best suited. Applying the proper thresholds ensured that only bubbles in the plane of interest were considered during analysis. Once bubble edges were detected, a curvature profile method was employed to distinguish individual bubbles within a cluster of highly overlapping bubbles. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct the missing parts. The movement of recognized individual bubbles was tracked on a two dimensional plane within a flow volume. In order to obtain quantitative results, the wake of a ventilated NACA0015 hydrofoil was investigated by applying a shadowgraphy technique and the described bubble detection algorithm. There experiments were conducted in the SAFL high speed cavitation tunnel.