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Investigation of the Counterjet Developed in a Cavitation Bubble that Collapses Near a Rigid Boundary
No full textA bubble that collapses near a rigid boundary develops
a liquid jet. The jet shoots through the bubble in
direction towards the boundary. When it hits the bubble
wall a sequence of shock waves is emitted that is followed by the onset of a structure moving in opposite direction
to the jet. This structure is called counterjet. In this paper we experimentally investigate the onset of the counterjet, the evolution of its height, the duration of
its appearance and its consistence. Bubbles are induced
by means of a strong laser pulse and observed with high-speed cinematography
Numerical Modelling of Unsteady Partial Cavities Behind a Backward Facing Step
No full textThis paper presents some calculation results in the cases of steady and non-steady partial cavity. These results are obtained with the commercial Navier-Stokes codes FLUENT and STAR-CD and compared with experimental results. Under cavitation conditions the first code uses a bubbles two-phase model. The second code uses the VOF (volume of fluid) technique to calculate the interface and allows the use of either the barotropic or the bubbles two-phase model. Different arrangements of the three parameters step height, channel confinement and cavitation number are examined. Cavities observed experimentally are compared with that obtained by the calculation. We investigate the ability of the models to reproduce the non-steady behaviours (re-entrant jet thickness, shedding frequencies)
Effects of Model Size and Free Stream Nuclei on Tip Vortex Cavitation Inception Scaling
No full textBecause of the complexity of the flow field in a trailing vortex, prediction of tip vortex cavitation inception on a hydrofoil or a propeller still relies heavily on model tests and application of cavitation scaling. Experiments indicate that the major parameters influencing tip vortex cavitation inception are fluid viscosity (Reynolds number) and free stream nuclei (Weber number). A scaling method based on these two parameters has been developed in this paper. Size effects on circulation, vortex cores thickness and pressure distributions in a full-scale trailing vortex are predicted from model data based on a Reynolds scale similarity flow solution. Nuclei effect on tip vortex cavitation inception is obtained from cavitation scaling and expressed in a term called G. For prescribed pressure fields in the trailing vortex, a modified Rayleigh-Plesset bubble equation is used to evaluate effects of free stream bubbles on cavitation inception. Bubble radius history and acoustic pressure are computed for various initial bubble sizes and different release locations into the vortex core. A method based on the acoustic pressure sigma gradient is introduced to define cavitation inception. The values of G at cavitation inception are obtained. Depending on the ratio of initial bubble sizes between model and full-scale waters, the nuclei effect on tip vortex cavitation inception can be greater, equal or less than one
Supercavitation - Problems and Perspectives
No full textA brief review of some investigations, which were initiated at the IHM, and new research of the hydrodynamic
supercavitation are given in this paper. The main applications of the supercavitaion at underwater high-speed body
motion with artificial supercavity, supercavitating water jets and noise suppression are presented. The main features
of artificial supercavitation as gas leakage and supercavity closure are discussed. Further perspectives of
supercavitation method development are analyzed
Re-Entrant Jet Modelling for Partially Cavitating Two-Dimensional Hydrofoils
No full textThe computational analysis of partial sheet hydrofoil
cavitation in two dimensions is performed. Particular attention is given to the method of simulating the flow at the end of the cavity. A fixed-length partially
cavitating panel method is used to predict the height
of the re-entrant jet, using the values of the cavitation number and the drag coefficient. The jet surface is subsequently constructed and included in an updated cavity
shape. A source singularity is introduced in the fluid
domain to account for the mass flux through the part of the domain boundary represented by the re-entrant jet surface. Further iterations are performed for fixed cavitation number on the cavity with a re-entrant jet cavity
termination model. This model is seen to produce good results and displays quick convergence. A validation is accomplished by conducting a parametric analysis of the model and comparing the present calculations with other
numerical schemes. The flow around the partially cavitating hydrofoil with a re-entrant jet has also been treated with a viscous/inviscid interactive method with
favourable results
Cavitation Scale Effects - Empirically Found Relations and the Correlation of Cavitation Number and Hydrodynamic Coefficients
No full textScale effects on cavitation phenomena are departures from the classical relation due to variations in
a) water quality with regard to its cavitation susceptibility (tensile strength, concentration and size of nuclei)
b) body size and flow parameters (flow velocity, viscosity of the fluid, turbulence).
These scale effects are an important consideration in the prediction of the prototype cavitation behaviour based on model tests. Test results show that, provided effects of water quality are avoided in the experiments, very clear empirical relations can be established for the scale effects on cavitation inception. The data, on which these empirical relations are based, relate to the inception of various types of cavitation as they occur on the surface of rotationally symmetric bodies, two-dimensional non lift producing bodies, or two- and three-dimensional lift producing foils. It can be shown, that the scaling relations initially found for beginning cavitation also are valid for developed cavitation. In addition lift and/or drag forces were measured on all kinds of test bodies and correlated with the cavitation number, and good correlation is obtained between the cavitation parameter and the corresponding measured drag coefficient. The new findings about the correlation of cavitation numbers with drag coefficients are believed to be the clue to why the empirically found scaling relations are so universal, and a key to the physical explanation for these scale effects
Some Models of Prediction of Supercavitation Flows based on Slender Body Approximation
No full textPaper contain results o f development of approximate methods and analysis of possibilities of flows and motion predictions for high speed movement prolate mainly axisymmetric bodies in water with supercavitation. Investigations are based on known approximation of the Slender Body Theory (SBT) and application of Matched Asymptotic Expansions Method (MAEM), integral conservation laws , heuristic models and another approaches. Alike consideration give the possibility to analyze the number of key problems as whole from united point of view. With account of two the most important ranges of speeds: moderate high speeds and very high speeds in the range of Mach Number ~0.5-2 and over it is applied accordingly two base models: of ideal incompressible fluid and izentropically compressible fluids. Every of this characteristic cases correspond different cases of applications and accordingly developed parts of the theory. The problems of development of the methods for prediction of steady and unsteady prolate supercavities for given pressure and cavities with gas injection are analyzed with account of base perturbations factors. Possibilities and state of problem of prediction supercavitation for very high speeds with account of compressibility effects are considered. Peculiarities and possibilities of predictions movement trajectory of supercavitating bodies are analyzed and influence of hydro elastic effects under motion. are investigate
Inner Structure of Cloud Cavity on a Foil Section
No full textCollapsing stage of cloud cavity and resulting violent shock pressure
have been investigated by many researchers, because the knowledge of
cavitation erosion is of practical importance and the dynamics is one
of the most challenging problems in the field of two-phase flow. When
a researche Collapsing stage of cloud cavity and resulting violent shock pressure have been investigated by many researchers, because the knowledge of
cavitation erosion is of practical importance and the dynamics is one
of the most challenging problems in the field of two-phase flow. When
a researcher intends to investigate the phenomenon numerically or
experimentally, he or she must know the structure of the cloud
cavity. The authors investigate cloud cavity, employing an off-axis
laser holography system. This paper is a brief report concerning the
inner structure of cloud cavity on a foil section
Unstable Cavitation Behavior in a Circular-Cylindrical Orifice Flow
No full textThe cavitation characteristics were experimentally investigated about long circular-cylindrical orifices of various throat lengths including the orifice with a trip wire. Especially, the shedding process of separated vortex cavity was examined in detail with the observation using high-speed photography. The cavitation impact was measured with an accelerometer. As a result, it is found that the periodic shedding of the cloud-like cavity can be observed characteristically in the transition cavitation stage and is dependent on the formation and coalescence of micro-vortex cavities on the separated shear layer as well as the reentrant motion after the shedding and collapse. It is also pointed out that it forms a feedback loop in the self-exciting mechanism. The estimation determined from two velocity ratios is shown on the Strouhal number of cloud-like cavity. The clear shedding of cloud-like cavity appears near the cavitation impact-peak region and remains almost constant, irrespective of the difference in orifice length. The transition from the throat cavitation to the jet cavitation downstream of the orifice throat can also produce the other impact-peak stage
Damage Due to Spot Cavitation on Hemisperical Cylindrical Body (Comparison Between Isolated Cavity and Parallel Cavities)
No full textWhen the Reynolds number is larger than the critical value or laminar separation is eliminated by a trip installed on a hemispherical cylindrical body, attached spot cavitation is observed occasionally. It occurs at fixed place in the vicinity of the minimum mean pressure and grows into a triangular wedge. In the present investigation, isolated and parallel spot cavitation is artificially generated on the hemispherical body adn the behavior of the cavitation is observed by instantaneous photographs. The frequency of damaging blows in the range of the flow speed of 25 to 50 m/s is obtained by counting the number of damaged pits on an aluminum specimen. When spot cavitation occurs adjacently, the cavity become rather stable. The pitting rate at the maximum damage zone by the parallel spot cavitation is much smaller than that by the isolated spot cavitation. The total pitting rates at the maximum damage zone by the isolated and parallel spot cavitation vary roughly 5th power of the flow speed for the both cases