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New Developments Around Sheet and Tip Vortex Cavitation on Ships Propellers
No full textA concept of tip vortex cavitation on propellers is described qualitatively, leading to the distinction of
trailing vortices, local tip vortices and leading edge vortices. Improvements of the inception behaviour
using this distinction are presented. Observations on developed tip vortex cavitation are given to show
that the concept of vortex bursting seems inadequate. The problem of "broadband" vibrations due to a
cavitating tip vortex is illustrated. Arguments are given for the fact that a three dimensional approach
is necessary to describe shedding of cloud cavitation at the trailing edge of a sheet cavity
Bubble Capture by Large Hydrofoil Tip Vortex
No full textThe interaction of gas bubbles with a vortex is investigated experimentally to clarify the
role of the initial bubble position on its trajectory around the tip vortex shed from a large
hydrofoil. The experiments performed with 4-6mm bubbles show that the bubbles can
undergo no capture, quick capture into the tip vortex or slow capture with a spiral
trajectory around the vortex
AZIPOD Propeller Blade Cavitation Observations During Ship Maneuvering
No full textThis paper presents results of observations of full-scale propeller blade cavitation patterns carried out on the Elation passenger cruiser, the very first ship equipped with AZIPOD electric thrusters. It offers descriptions of test conditions, cavitation photos for a number of maneuvering modes, including acceleration, steering, initial crash stop phase, some comments on cavitation patterns and conclusions about propeller blade cavitation on the pulling option of azimuthal thrusters
Thermal Damping in Cavitating Nozzle Flows
No full textRecent investigations of bubbly cavitating
nozzle flows by Wang and Brennen (1998) and by Delale et al. (2001),
where various damping effects are lumped together in an
adhoc manner, have shown flow instabilities that lead to flashing
flow solutions. Here, we investigate the stabilizing effect of
thermal damping on these instabilities. For this reason we
consider the energy equation within the bubble, assumed to be
composed of vapor and gas, in the uniform pressure approximation
(similar to that given by Nigmatulin et al., 1981 and by
Prosperetti, 1991). The partial vapor pressure is fixed by the vapor
saturation pressure corresponding to the interface temperature,
which is evaluated by the Plesset-Zwick formula assuming the
thin boundary layer approximation within the liquid. Consequently,
the partial gas pressure is evaluated by its relation to the heat
flux through the interface in the uniform pressure approximation.
The model is then coupled to the steady-state cavitating nozzle
flow equations employed by Wang and Brennen and by Delale et al.,
replacing the previously assumed polytropic law for the partial
gas pressure. The instabilities arising from the use of the
polytropic law for the gas pressure in steady
cavitating nozzle flows are seen to be stabilized by thermal
damping with or without the occurrence of bubbly shock waves
Eulerian/Lagrangian Analysis for the Prediction of Cavitation Inception
No full textAn Eulerian/Lagrangian computational procedure was developed for the prediction of cavitation inception by event rate. The event rate is governed by the number distribution of nuclei, the instantaneous pressure field in the flow, the trajectory of the nuclei, and the bubble dynamics. The development of the procedure utilized an experimental database for an axisymmetric headform known as a "Schiebe" body. The demonstration of the method in axisymmetric flows is a necessary prerequisite for application to turbomachinery flows, where the issues of grid resolution of vortices and turbulence modeling are more critical. The carrier-phase flow field was computed using an Eulerian Reynolds-Averaged Navier-Stokes solver. The Lagrangian analysis was one-way coupled to the RANS solution, since at inception, the contributions of mass, momentum, and energy of the microbubbles to the carrier flow are negligible. Probability density functions for measured nuclei populations were inverted to produce a representative population of computational bubbles, whose trajectories and growth were tracked through the flow field. The trajectories were computed using Newton's second law with models for various forces acting on the bubble. The growth was modeled using the Rayleigh-Plesset equation. The important effect of turbulence was included by adding a random velocity component to the mean flow velocity by sampling a Gaussian probability density function with variance proportional to the turbulent kinetic energy at the location of the bubble and by reducing the local static pressure by a value proportional to the turbulent kinetic energy squared. The simulation results indicate agreement with experimentally observed trends and a significant event rate at cavitation numbers above visual inception. The velocity dependence of the inception data is shown to be related to the change in the nuclei population. In this paper, the Eulerian/Lagrangian formulation is presented followed by a discussion of the simulation results of the experiment reported by Meyer, Billet, and Holl (1992)
Comparison Of Cavitation Phenomena In Transparent Scaled-Up Single-Hole Diesel Nozzles
No full textThe structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole inclined at 90, 85, 80 and 0 degree to the nozzle axis has been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. Observations revealed that at the inception stage, cavitation bubbles were not seen at the same locations in all the four nozzles. Cavitation bubbles grew intensively and developed into cloud-like structures. Shedding of the cloud cavitation was observed. When the flow was increased further the cloud-like cavitation bubbles developed into a dense large-scale cavitation cloud extending downstream of the hole. Under this condition the cavitation started mainly as a glassy sheet at the entrance of the hole. Until this stage the spray appeared to be symmetric. When the flow was increased beyond this stage, a sheet of cavitation covered a significant part of the hole on one side, extending to the hole exit. This non-symmetric distribution of cavitation within the hole resulted in a jet, which atomized on the side where more cavitation was distributed and non-atomizing on the side with less cavitation. The distribution of cavitation in the hole was different for different nozzles
On the Collapsing Behavior of Cavitation Bubble Clusters
No full textThe present paper reports observation results of collapsing cavity bubbles on a two-dimensional foil section by a high-speed video camera, together with impulsive force measurement. Results of numerical simulations of the behavior of bubble cluster corresponding to the above condition are also shown. With these materials the authors discuss the mechanism of generation of the impulsive force due to cavitation collapse
Dynamics of Bubble Oscillation in Constrained Media and Mechanisms of Vessel Rupture in Shock Wave Lithotripsy
No full textRupture of small blood vessels is a primary feature of the tissue injury associated with shock wave lithotripsy (SWL), and cavitation has been implicated as a potential mechanism. To improve our understanding of the damage mechanism, dynamics of SWL-induced bubbles in constrained media were investigated. Silicone tubing and cellulose hollow fibers (i.d.=0.2 ~ 1.5 mm) were used to fabricate vessel phantoms, in which test fluid containing cavitation nuclei were circulated. Intraluminal bubble dynamics were examined by high-speed imaging and passive cavitation detection (PCD). Compared to the large expansion and violent inertial collapse of SWL-induced bubbles in a free field, the expansion of bubbles inside the vessel phantom is greatly constrained, leading to asymmetric elongation along the vessel axis and much weakened collapse. Conversely, the rapid, large expansion of the bubbles significantly dilates the vessel wall, leading to consistent rupture of the hollow fibers after less than 20 shocks in a XL-1 lithotripter. The rupture is dose-dependent, and varies with the spatial location of the vessel phantom inside the lithotripter field. Further, when the large intraluminal bubble expansion was suppressed by inverted lithotripter shock waves (LSW), the rupture could be avoided even after 100 shocks. Theoretical calculation confirms that the propensity of vascular injury due to LSW-induced intraluminal bubble expansion increases with the tensile pressure of the shock wave, and with the reduction of the inner diameter of the vessel, as noted in previous animal studies
Probing Luminescence from Aspherical Bubble Collapse
No full textThe luminescence from single laser produced cavitation bubbles for varying degrees of asphericity is investigated temporally, spatially, and spectrally resolved. The degree of asphericity is controlled with an adjustable rigid
boundary nearby the bubble. Temporally, single and multiple light emission events happen during a time interval of 80ns. The luminescence duration increases with increasing asphericity. Spatially, the emissions from aspherical collapsing bubbles display a pronounced halo around the central spot being several times larger than the luminescence from spherical collapse. Spectrally, the ratio of the line- to continuum emission of the sodium dublett is enhanced, whereas the total emitted energy decreases by
four orders of magnitude as compared to the spherical collapse. These findings point towards emission not only from the bubble interior but also from the liquid surrounding the bubble exited by outgoing shock waves
Cavitation Inception Witnessed by Sound Pressure Level both in Model Test and Prototype Observation
No full textThe paper concerns with the relationship between cavitation inception d1 and sound pressure level (SPL). A
case study of Xiaolangdi Hydro-project has been taken as a typical example, which demonstrates the
standard criterion of SPL indicating cavitation inception. Cavitation inception of an orifice plate used for
energy dissipation in a tunnel spillway has been witnessed and verified both in model test and prototype
observation. A design criterion has been proposed for a cavitation – free hydraulic structure. It can be served
as a reference for designers as well as preliminary findings for fundamental cavitation researchers.
Consequently, scale effects are discussed with an aim at suggesting an appropriate factor of safety in design