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Tip Leakage Vortex Cavitation from the Tip Clearance of a Single Hydrofoil
No full textAbstract
Focusing on the tip leakage vortex cavitation, experimental and numerical studies were carried out as the first step of the investigation of cavitations in tip leakage flow. For a single hydrofoil with a tip clearance, tip leakage vortex cavitations were observed for various cavitation numbers and angles of attack. To simulate the tip leakage vortex cavitation, a simple calculation of 2-D unsteady flow based on the slender body approximation with taking into account the effects of cavity growth (Watanabe et al., 2001) was made. The results of calculations show qualitative agreement with the experimental results with respect to the location and size of the cavity. The influences of the cavitation number, angle of attack, blade loading, and the size of tip clearance were simulated reasonably well
Physical Investigation of a Cavitation Vortex Collapse
No full textMost of severe cavitation erosion of hydraulic machines are found to be associated with the collapse of transient cavitation vortices downstream of a leading edge cavity. The dynamics of such a type of cavitation is studied in a Cavitation Vortex Generator (CVG). By producing the cyclic growth and collapse of a single cavitation vortex, this device provides a way to investigate the mechanisms involved in the final stage of the vapour cavity collapse.
The observations of vapour structures and emitted shock waves are based on high-speed visualisations (up to frames/s) which are obtained by using a shadowgraph video system.
Luminescence sources which are emitted during the collapse are visualised by using an intensified camera. By adjusting a short exposure time of 500 before shock waves are captured, this allows the time of minimum cavity volume and position to be estimated.
This paper presents the complex physical mechanisms of cavitation vortex collapse which have been pointed out with these simultaneous visualisations
Validation of Bubble Distribution Measurements of the ABS Acoustic Bubble Spectrometer with High Speed Video Photography
No full textMeasurement of the bubble size distribution in a liquid is very important for cavitation inception studies. In this paper we describe an acoustics based device, the ABS Acoustic Bubble Spectrometer® that measures bubble size distributions and void fractions in liquids based on the measurement of sound propagation through the tested liquid. Short monochromatic bursts of sound at different frequencies are generated by a transmitting hydrophone and received by a second hydrophone after passage through the liquid. These signals are processed and analyzed to obtain the frequency dependent attenuation and phase velocities of the acoustic waves. From these, the bubble size distribution (number of bubbles versus size) is obtained following solution of an inverse problem. In order to validate a new implementation of the instrument software, a fundamental experiment is conducted. Bubbles are generated in a controlled fashion, and then carefully mixed into a uniform distribution in a flowing system. A high-speed micro-video system is used to take videos of the bubbles at the same time and within the test volume interrogated by the ABS system. Both the acoustic data and the video frames are then analyzed using many datasets under the same conditions, and the results are compared. The two methods are seen to provide very close results within their limits of resolution and within the bubble distribution variations in the liquid. The ABS provides results very close to the time-consuming micro video photography in near real-time in a much more cost-effective fashion
Some Results Concerned with Cavitation Studies in Unsteady Hydrodynamics Laboratory of Moscow State University
No full textThe results of theoretical and experimental cavitation studies
obtained in the unsteady hydrodynamics Laboratory of MSU (founded on
academitian Sedov's initiative in 1960) are given.
The theoretical and experimental results presented are based on the
dimension and similitude theory. A number of self-similar solutions
describing unsteady jet flows (including a new solution involving a llength
parameter) are outlined. The new solution describes, e.g., the high
velocity inertial body motion in fluid. The important feature of this
self-similar solution is that the cavity form is concerved during the
process of body deceleration. Other self-similar solutions describe
different stages of fluid - blunt body impact, in particular, when the
atmosphere influence on the jet formation is taken into account.
An experimental method to measure nondimensional equation of state
for water (using a medium which includes small gas bubbles) is suggested.
The influence of gas washing down (in the form of a bubble wake) on the
drag and thrust is also considered. For high velocity flows it is shown
that one can regulate the pressure distributions on a cavity surface.
The experimental facilities which make it possible to model
practically all parameters of cavitation flows are described.
More information on the subject one can find in author's review
papers Yakimov (1982), (1991), (1992)
Experimental and Numerical Studies on a Centrifugal Pump with 2D-Curved Blades in Cavitating Condition
No full textIn the presented study a special test-pump with 2D curvature blade geometry in cavitating and non-cavitating conditions was investigated using different experimental techniques and a 3D numerical model of cavitating flows. Experimental and numerical results concerning pump characteristics and performance breakdown were compared at different flow conditions. Appearing types of cavitation and the spatial distribution of vapour structures within the runner were also analysed
Experimental Study of Ventilated Cavities on Dynamic Test Model
No full textA series of experiments were conducted to examine ventilated cavity physics. Over two hundred test runs were performed. Tow tank tests were conducted to examine the stability of ventilated cavities. The tests were performed on three different models at speeds up to 55 feet per second. One 6.25-inch diameter model allowed body motion with 3 degrees of freedom. The model also employed a cavitator that could be pitched to +/- six degrees angle of attack. The models allowed the examinationbof ventilation data at a variety of length scales and for a number of cavitator shapes and sizes. Some tests also incorporated simulated rocket exhaust. High frequency solid-state pressure transducers were used to determine the stability of the cavities. The tests confirmed that the dominant cavity frequency was correlated with the cavity length and towing speed. Dynamic model motion and the rocket exhaust both tended to enhance overall cavity stability. The impact of free shear instability on cavity stability was negligible at these speeds
Numerical Simulation of Unsteady Cavitation Flows
No full textA previously developed simple numerical code, based on the Navier-Stokes equations of compressible fluid and a virtual single phase equation of state, has been applied to further demonstrate its capability to capture highly dynamic nature of cavitating flows about a two-dimensional NACA0015 hydrofoil. Computational time steps in the order of 10 microseconds were used to capture the detailed unsteady characteristics of bubble cavitation, bubble/cloud cavitation, sheet/cloud cavitation and super cavitation. The formation and collapse of cloud cavity, and the related generation and radiation of shock waves were visualized. The collapsing of cloud cavity is observed to be a highly unpredictable turbulent phenomenon, which most frequently breaks apart into a number of smaller pieces before collapsing. Only rarely it is found to collapse spherically as a whole. If the sheet cavity is long enough so that the cloud cavity can arrive at the tail of the foil before collapsing, refraction of negative pressure wave around the tail may cause cavitation to occur on the pressure side of the foil. The unsteadiness of natural super cavity is strongly influenced by the instability of the positive vortex sheet on the pressure side of the foil
The Use of Cavitating Jets to Oxidize Organic Compounds in Water
No full textThis paper reports on the application of hydrodynamic cavitation by the use
of submerged cavitating liquid jets to trigger widespread cavitation and
induce oxidation of organic compounds in the bulk liquid solution with a two
order of magnitude increase in energy efficiency compared to the ultrasonic
means.
The results are compared to a bubble dynamics model that includes heat and
mass transport, collective bubble effects, and a first order Arrhenius
reaction rate model. Comparison of model results with experiment indicated
the reactions were limited by contaminant transport to the bubble surface
rather than by radical generation or the intensity of bubble collapse. Other
findings are the desirability of operating at atmospheric ambient pressure
and low driving pressures and of maximizing cavity surface area. These
results suggest a great potential for the use of jet cavitation in practical
scale waste treatment and remediation systems
On the Detachment of a Leading Edge Cavitation
No full textIn the present paper we present an experimental investigation of the onset and detachment of leading edge cavitation. Tests are conducted in the LMH high speed cavitation tunnel on a 2-D Naca0009 hydrofoil having 100 mm chord length. Both Particle Image Velocimetry and flow visualisation are conducted for different test conditions. At low incidence angles, in the case of non cavitating flow, the velocity field do not show any laminar separation of the boundary layer in the suction side of the hydrofoil. In the case of well developed attached cavitation, we have clearly shown that the distance between the separation and the cavity detachment points is less than our PIV spatial resolution (5.10-4 m). We have also shown through flow visualisation that an increase of the incidence angle of the hydrofoil leads to an unstable transition from bubble cavitation to attached cavitation. These observations let us believe that, in our specific experimental set-up, the flow separation is generated during the transition process from bubble to attached cavitation with a significant influence of the surface roughness and physical properties of the hydrofoil material as well as the wall pressure distribution along the suction side of hydrofoil