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Cavitation Erosion Mechanism: Numerical Sumulations of the Interaction Between Pressure Waves and Solid Boundaries
No full textTo evaluate the aggressiveness power of cavitating flows and to improve prediction methods for cavitation erosion, the pressure waves emitted during bubble collapses were studied and simulated by means of the Keller's and Fujikawa and Akamatsu's physical models. The profile and the energy of the pressure waves emitted during cavity collapse were evaluated by numerical simulation. The dynamic response and the surface deformation (i.e., pit profile and pit volume) of various materials exposed to pressure wave impacts was simulated making use of a 2D axisymmetric numerical code simulating the interaction between pressure wave and an elastoplastic solid. Making use of numerical results, a new parameter b (defined as the ratio between the pressure wave energy and the generated pit volume) was introduced and evaluated for three materials (aluminum, copper and stainless steel). By associating numerical simulations and experimental results concerning pitted samples exposed to cavitating flows (volume damage rate), the pressure wave power density was introduced. This physical property of the flow characterizes the cavitation aggressiveness and can be related to the flow hydrodynamic conditions. Associated to b parameter, the pressure wave power density appeared to be a useful tool to predict the cavitation erosion power
Cavitation Characteristics on Flow with Multi Cavitation Sources
No full textThe cavitation characteristics were analyzed in this paper for the flood relief mid-outlets of Baise Multipurpose dam project, on the basis of depressurized model test of 1:50 in a specially designed vacuum chamber. The results of test demonstrate that there exists relatively distinct cavitation cloud at the gate slots and outlet diffusion zones of spillways. Mutual disturbance of cavitation noise from two sources leads to the relationship curve becoming concave downward and the frequency spectrum sound pressure level decreasing correspondingly when h is close to hm (h/hm = 1). Its pattern is not in conformity with the general law. The discovery may have certain supplementation for the standard of judging the cavitation emergence
Mechanism of Cloud Cavitation Generation on a 2-D Hydrofoil
No full textWhen a sheet cavity on a hydrofoil section attains a certain size, it starts violent periodical oscillation shedding a harmful cloud cavity downstream at each oscillation cycle.
This phenomenon is due to the occurrence of the re-entrant jet. In this paper, the behavior of the re-entrant jet was observed in detail using a transparent foil section model and a high-speed video camera. Time variation of pressure distribution on the foil was measured simultaneously.
It was found that the re-entrant jet can start at any point in sheet cavity elongating stage. Even two re-entrant jets can appear in one cycle.
When a re-entrant jet is generated upstream, the jet velocity is lower compared to the case when a re-entrant jet is generated downstream. The jet velocity is almost constant at the value determined by the location of the generation.
As a result, the cavity oscillation cycle becomes constant when it is normalized by the sheet cavity surface velocity and the maximum sheet cavity length.
The jet velocity is calculated from the pressure gradient at the sheet cavity T.E., using a simple theoretical model. The calculated jet velocity agrees with the measurement, showing that the jet velocity increases as its generation point shifts downstream. It is possible that pressure gradient at the sheet cavity T.E. is the driving force of re-entrant jet
Some Problems of Supersonic Cavitation Flows
No full textThe basic results obtained by the author in the theory of supercavities in compressible fluid have been presented. The modern numerical methods have been applied in this investigation and the basic correlations for shock in water have been considered. The characteristic features of supersonic cavitation flows and their difference from the analogous flows in the air have been noted. The buckling stability of an elastic body moving at a high speed in water has been investigated
Analysis of the Cavitating Flow Around the Horn-type Rudder in the Race of a Propeller
No full textThis paper describes numerical methods by a mixed formulation of the boundary value problem (BVP) for
the prediction of the cavitating flow around the horn-type rudder working behind a propeller. The blade BVP
is treated by the classical vortex lattice method, whereas the rudder BVP is solved by the surface
panel method. The three-dimensional flow around the rudder and the propeller is computed
simultaneously, considering the interactions between them. A modified kinematic boundary condition is
applied to predict the mean velocity and flow volume through gap flow region of the horn-type rudder.
To validate the numerical scheme, an experiment is performed in large cavitation tunnel.
The surface pressure distributions and cavity patterns on the horn-type rudder are investigated and
compared with computational results, showing good agreement with measured results
Investigating to Cavitation Behavior of Orifice Tunnel
No full textStrong velocity scale effect on cavitation for orifice tunnels was revealed through comparing the results of model
tests in vacuum tanks and that of prototype measurements at construction site. A new formula predicting incipient
cavitation number of orifice rings is presented in this paper. The predicted data for the prototype scale are coincident
well with the prototype measured one
Cavitation Characteristics of Restriction Orifices (Experiment for Shock Pressure Distribution by Cavitation on Restriction Orifices and Occurrence of Cavitation at Multiperforated Orifices Due to Interference of Butterfly Valve)
No full textThis paper presents two experimental investigations for cavitation characteristics of restriction orifices. The first experiment is about the spatial distribution of cavitation shock pressure in a pipe at the downstream of restriction orifices.
The second experiment is the investigation of butterfly valve throttling to the cavitation in a multiperforated
orifice installed piping. From the results of the experiment for the cavitation shock pressure, it is concluded the maximum shock pressure remarkably increases with the decrease of cavitation number regardless of the orifice types. The maximum shock pressure becomes smallest on cone type orifice, and largest on single hole orifice. Multiperforated orifice is between this two.
The results of the experiment for occurrence of cavitation due to the interference of butterfly valve show that the
cavitation occurs at a relatively high cavitation number when the multiperforated orifice is placed at 1D downstream
of the butterfly valve. The butterfly valve throttling accelerates the cavitation at the multiperforated orifice because of the closed orifice installation
Spectrum of Luminescence from Laser-Induced Bubbles in Water and Cryogenic Liquids
No full textThe spectrum of the luminescence pulse from collapsing
laser-induced bubbles in water and cryogenic liquids has been measured.
For small bubbles in water the spectrum approximates that of a
black-body at about 7800 K, while for bubbles whose maximum radius is
greater than 1 mm the OH* molecular band at 310 nm becomes apparent,
a feature which is also seen in multi-bubble sonoluminescence. The
larger bubbles also show considerable instabilities in the vicinity of
the collapse minimum, where jets and fission events are observed. In
the liquid nitrogen and liquid argon the the luminescence is found to
consist of discrete lines from chromium atoms, which are compressed in the
bubble collapse to a temperature of about 4500 K. The chromium
apparently results from the vaporization by the laser of stainless
steel microparticles floating in the liquids, possibly knocked off the
cell walls by the strong shock waves generated by the laser focusing
Utilization of Cavitation for Environmental Protection- Killing Planktons and Dispersing Spilled Oil
No full textThis paper describes on the use of cavitation for environmental protection in the form of the following two subjects: killing plankton in water and dispersing spilled oil. The preliminary results showed that both uses are promising, which demonstrates the wide range of applications of cavitation utilization
Observation of Helium Two-Phase Flows in a Pipe
No full textApplications using cryogenic fluid generally encounter obstacles or complex pipe shapes such as an orifice or a converging-diverging nozzle. Therefore, a flow visualization study on two-phase cryogenic flow passing through a convergent-divergent nozzle or an orifice nozzle installed in a horizontal pipe is carried out to clarify the fundamental characteristics of the transient growth process of helium cloud cavitation. The pipe is filled with pressurized cryogenic liquid and flow immediately occurs when the on-off valve is opened. The transient growth process of the cloud cavitation induced by flow through the throat is observed from high-speed video images. Comparisons between the high-speed observations and the pressure measurements indicates that the transient growth process can be divided into two different steps by taking into account the transition point based on the observational results. Furthermore, it is confirmed that there are two types of the pressure instability in blowdown tests: one is the instability induced by density wave oscillation and the other is that induced by acoustic wave oscillation