239 research outputs found
Sort by
Mutual Interactions of Flow Structure and Bubble Cavitation
No full textTo quantify the interaction between the flow structures and the cavitating phase, the present experimental study focuses on the velocity measurements and spectral analysis by LDV of the liquid flow properties in a cavitating Couette Taylor flow. In this experiment, where the turbulent scales take place progressively, the bubbly phase is introduced in the gap by pressure drop and a particular attention is devoted to the determination of the effects of this bubbly phase on the properties of the flow by comparison between the single phase patterns and those observed in cavitating flow. This experimental work has highlighted the particular arrangement of the cavitating phase generated by the vortex pattern and the modification of the regimes existing in the single phase flow
Application of Preconditioned, Multiple-Species, Navier-Stokes Models to Cavitating Flows
No full textA preconditioned, homogenous, multiphase, Reynolds Averaged Navier-Stokes model with mass transfer is presented. Liquid, vapor, and noncondensable gas phases are included. The model is preconditioned in order to obtain good convergence and accuracy regardless of phasic density ratio or flow velocity. Both incompressible and finite-acoustic-speed models are presented. Engineering relevant validative and demonstrative unsteady and transient two and three-dimensional results are given. Transients due to unsteady cavitating flow including shock waves are captured. In modeling axisymmetric cavitators at zero angle-of-attack with 3-D unsteady RANS, significant asymmetric flow features are obtained. In comparison with axisymmetric unsteady RANS, capture of these features leads to improved agreement with experimental data. Conditions when such modeling is not necessary are also demonstrated and identified
Cavitation Tunnel Tests For Propeller Noise of a FRV and Comparisons with Full-Scale Measurements
No full textThis study presents the results of cavitation tunnel tests carried out with model propeller of a Fisheries Research Vessel (FRV) and those of noise measurements with its full-scale propeller to validate the low-noise performance of this propeller. The tests involve the simulation of a target wake using a wake screen and the determination of the nature and extent of the observed cavitation behind the simulated wake. The measurements for the noise levels of the model propeller and their analyses are also part of the study. The net noise levels of the model propeller are extrapolated to full-scale using the scaling law recommended by the 18th ITTC Cavitation Committee. The extrapolated results are compared with the criteria recommended by the International Council for the Extrapolation of the Sea (ICES) as well as against the full-scale measurements carried out with this vessel in Japan
A New Parameter to Predict Cavitation Erosion
No full textIn order to avoid or predict cavitation erosion, it is necessary to know resistance of materials against to cavitation impacts. The cavitation impacts, which are larger than certain threshold level, only affect cavitation erosion of materials. In the present paper, existence of the threshold level was revealed experimentally. The threshold level of material is a new parameter to predict the cavitation erosion. Erosion rates for several materials were clarified by using a cavitating jet erosion test, which was new ASTM standard ASTM G134. The cavitation impacts induced by the cavitating jet were measured by means of a special made PVDF transducer, and energy of cavitation impacts was calculated. The threshold levels for several materials were revealed from the relation between the erosion rate and the energy of the cavitation impacts. A method to predict the cavitation erosion quantitatively from the threshold level of material and the measurement of cavitation impacts was proposed
Erosive Intensity Measurements of Cavitating Jet
No full textCavitating jets are widely used for cleaning, cutting, improving material strength and so forth. This paper describes the erosive intensity measurement of a cavitating jet with various nozzle configurations. Two cross-shaped nozzles, one circular nozzle with two cross wires, and two nozzles with swirl vanes, were tested. The authors expected an increase of erosive intensity to come about by the use of these nozzles, as a result of the deformation of the vortex ring in the cavitating jet. However, the experimental results reveal a decrease of erosive intensity contrary to expectations
Cavitation Bubble Behavior Near Solid Boundaries
No full textIn the present study the bubble behavior in the narrow space are experimentally and numerically examined as the gap between two parallel walls and the position of bubble induction were changed. The main results are as follows. (1) The effects of two parallel walls can be classified by the ratio of the gap between the walls to the maximum bubble radius. If the ratio>5.0 the bubble shape is almost sphere. The wall effect remarkably appears for the ratio<3.0 and the bubble deforms to be dumbbell- or cone-like shape. (2) When the gap between the walls is small, the single bubble is finally divided into two bubbles owing to the large lateral pressure. The rebound of each bubble causes impulsive pressure and damages the upper and lower wall surface. Especially, if the bubble is not created at the center between the walls, the collapse phase shift among the divided bubbles brings the further damage on the wall surface. (3) The computed motion of the bubble without non-condensable gases well explains the dumbbell- or cone-shaped bubble deformation
Cavitation in Injection Nozzles - Effect of Model Parameters and Boundary Conditions
No full textThis work deals with a numerical simulation of the effect
of injection pressure fluctuations on the cavitation
processes in injection nozzles. The numerical approach
is based on predicting the growth and collapse of bubbles
in combination with a modified Volume-of-Fluid technique
(VOF). A k-omega model is applied for turbulence
modeling. Calculations confirm that there exists phase
shift among the time history of the injection pressure,
the transient velocity, and the cavitation process
Calculation of Three-dimensional Unsteady Sheet Cavitation by a Simple Surface Panel Method "SQCM"
No full textThis paper presents a calculation method for the 3-D unsteady cavitating hydrofoil problem. The method is based on a simple surface panel method "SQCM" which satisfies easily the Kutta condition even in the unsteady problem. This method is applied to the Wagner problem, heaving hydrofoil and the hydrofoil in sinusoidal gust. We show some calculated results for partially cavitating and supercavitating hydrofoil and compare them with other calculated results
Numerical Simulation of Turbulent Flows with Sheet Cavitation
No full textA pressure-based algorithm is developed and applied to compute turbulent sheet cavitating flows. Single-fluid Navier-Stokes equations, cast in their conservative form, along with a volume fraction transport equation are employed. The flow is computed in both phases with the vapor pressure recovered inside the cavity via a mass transfer model. A pressure-velocity-density coupling scheme along with an upwinded density interpolation is developed to handle the large density ratio associated with cavitation. The method is assessed through simulations of cavitating flows over a cylindrical object and an airfoil. The results show satisfactory agreement with experimental data in pressure distribution. In addition, information such as wall shear stress distributions and related velocity and turbulence fields is highlighted for both axisymmetric projectile and NACA airfoil
Behavior of Unsteady Cavity on a Wing Section
No full textIn order to simulate the cloud cavitation on hydraulic machinery, the unsteady cavitation on a rectangular wing
section was observed in the cavitation channel of the Miyakonojo National College of Technology. The attack
angle was forced to change from 10deg. to –10deg. by a strike of hammer. As the attack angle decreased, the
sheet cavitation left the leading edge of wing and changed into cloud cavities on the mid-chord of wing