International audienceCavitation erosion prediction and characterization of cavitation field strength are of interest to industries suffering from cavitation erosion detrimental effects. One means to evaluate cavitation fields and materials is to examine pitting rates during the incubation period, where the test sample undergoes localized permanent deformations shaped as individual pits. In this study, samples from three metallic materials, an Aluminum alloy (Al 7075), a Nickel Aluminum Bronze (NAB) and a Duplex Stainless Steel (SS A2205) were subjected to a vast range of cavitation intensities generated by cavitating jets at different driving pressures and by an ultrasonic horn. The resulting pitted sample surfaces were examined and characterized with a non-contact 3D optical scanner and the resulting damage computer-analyzed. A statistical analysis of the pit population and its characteristics was then carried out. It was found that the various cavitation field strengths can be correlated to the measured pit distributions and that two characteristic quantities: a characteristic number of pits per unit surface area and unit time, and a characteristic pit diameter or a characteristic pit depth can be attributed to a given "cavitation intensity level". This characterization concept can be used in the future to study the cavitation intensity of the full scale and to develop methods of full scale predictions based on model scale erosion data

The unsteady hydrodynamics of the tail flapping and head oscillation of a fish, and their phased interaction, are considered in a laboratory simulation. Two experiments are described where the motion of a pair of rigid flapping foils in the tail and the swaying of the forebody are simulated on a rigid cylinder. Two modes of tail flapping are considered: waving and clapping. Waving is similar to the motion of the caudal fin of a fish. The clapping motion of wings is a common mechanism for the production of lift and thrust in the insect world, particularly in butterflies and moths. Measurements carried out include dynamic forces and moments on the entire cylinder-control surface model, phase-matched laser Doppler velocimetry maps of vorticity-velocity vectors in the axial and cross-stream planes of the near-wake, as well as dye flow visualization. The mechanism of flapping foil propulsion and maneuvering is much richer than reported before. They can be classified as natural or forced. This work is of the latter type where discrete vortices are forced to form at the trailing edge of flapping foils via salient edge separation. The transverse wake vortices that are shed, follow a path that is wider than that given by the tangents to the flapping foils. The unsteady flap-tip axial vortex decays rapidly. Significant higher order effects appear when Strouhal number (St) of tail flapping foils is above 0.15. Efficiency, defined as the ratio of output power of the flapping foils to the power input to the actuators, reaches a peak below the St range of 0.25–0.35. Understanding of two-dimensional flapping foils and fish reaching their peak efficiency in that range is clarified. Strouhal number of tail flapping does emerge as an important parameter governing the production of net axial force and efficiency, although it is by no means the only one. The importance of another Strouhal number based on body length and its natural frequency is also indicated. The relationship between body length and tail flapping frequency is shown to be present in dolphin swimming data. The implication is that, for aquatic animals, the longitudinal structural modes of the body and the head/tail vortex shedding process are coupled. The phase variation of a simulated and minute head swaying, can modulate axial thrust produced by the tail motion, within a narrow range of ±5 percent. The general conclusion is that, the mechanism of discrete and deterministic vortex shedding from oscillating control surfaces has the property of large amplitude unsteady forcing and an exquisite phase dependence, which makes it inherently amenable to active control for precision maneuvering. [S0098-2202(00)00102-4]

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An experimental investigation was conducted in a water tunnel to explore the application of Coanda-effect circulation control to low aspect ratio wings. The facility was the Large Cavitation Channel in Memphis, TN. The intended application is to high-lift controlsurfaces (appendages) on underwater naval vehicles. Test results are interpreted in light of both theory and the extensive experience with circulation control (CC) technology at NSWCCD. The semi-span wing test model with a taper ratio of 0.76 was mounted on a load cell; a reflection plane provided for an effective aspect ratio of 2. Dual upper/lower trailing edge tangential jet slots were incorporated for bi-directional force generation. Findings include: finite-span effects on CC augmented lift are consistent with the effects on conventional lift-due-to-angle-of-attack, and cavitation in the Coanda wall jet region does not result in jet detachment or an abrupt lift stall. Wing lift augmentation ratios are up to 36 and meet expectations. Unexpected virtues of a dual-slotted configuration were found that enhance the value of CC to ship and VSTOL aircraft applications. A small flow from the second slot will significantly extend the lift capability beyond that of single slot operation by preventing what is believed to be the adverse effects of excessive turning of the wall jet at high momentum coefficients. Dual slot flow produces a merger of the two wall jets into a free planar jet that enables static thrust vectoring of the jet momentum flux over the full 0-360 deg range. This steerable-jet provides a jet-flap mode of lift development for use at very low vehicle speeds, as an extension of the high efficiency CC mode.

In order to decelerate a forward-moving submarine rapidly, often the propeller of the submarine is placed abruptly into reverse rotation, causing the propeller to generate a thrust force in the direction opposite to the submarine’s motion. This maneuver is known as the “crashback” maneuver. During crashback, the relative flow velocities in the vicinity of the propeller lead to the creation of a ring vortex around the propeller. This vortex has an unsteady asymmetry, which produces off-axis forces and moments on the propeller that are transmitted to the submarine. Tests were conducted in the William B. Morgan Large Cavitation Channel using an existing submarine model and propeller. A range of steady crashback conditions with fixed tunnel and propeller speeds was investigated. The dimensionless force and moment data were found to collapse well when plotted against the parameter η, which is defined as the ratio of the actual propeller speed to the propeller speed required for self-propulsion in forward motion. Unsteady crashback maneuvers were also investigated with two different types of simulations in which propeller and tunnel speeds were allowed to vary. It was noted during these simulations that the peak out-of-plane force and moment coefficient magnitudes in some cases exceeded those observed during the steady crashback measurements. Flow visualization and LDV studies showed that the ring vortex structure varied from an elongated vortex structure centered downstream of the propeller to a more compact structure that was located nearer the propeller as η became more negative, up to η=−0.8. For more negative values of η, the vortex core appeared to move out toward the propeller tip.