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Toward Efficient Computation of Heat and Mass Transfer Effects in the Continuum Model for Bubbly Cavitating Flows
No full textThe Rayleigh-Plesset equation is used extensively to model spherical bubble dynamics, yet it has been shown that it cannot correctly capture damping effects due to mass and thermal diffusion. Full single bubble models have been successfully used to study these diffusion effects, but these are too computationally expensive to implement into
the continuum model for bubbly cavitating flows since the diffusion equations must be solved in the radial direction at each position in the flow. The focus of the present research is the development of simpler and more efficient bubble dynamic models that capture the important aspects of the diffusion processes. We present some preliminary results from a full bubble model that has been developed to provide insight into possible simplifications. This in turn can be
used to develop and validate simpler models. The full model is contrasted to the Rayleigh-Plesset equation, and a suggestion for possible improvement to the Rayleigh-Plesset equation is made
Surge Instability on A Cavitating Propeller
No full textThis study details experiments investigating a previously unrecognized surge instability on a cavitating propeller
in a water tunnel. The surge instability is first explored through visual observation of the cavitation
on the propeller blades and in the tip vortices. Similarities between the instability and previously documented
cavitation phenomena are noted. Measurements of the radiated pressure are then obtained, and the
acoustic signature of the instability is identified. The magnitudes of the fluctuating pressures are very large,
presumably capable of producing severe hull vibration on a ship.
The origins of the instability are explored through separate investigation of the cavitation dynamics
and the response of the water tunnel to volumetric displacement in the working section. Experiments
are conducted to quantify the dynamics of the propeller cavitation. Finally, a model is developed for the
complete system, incorporating both the cavitation and facility dynamics. The model predicts active system
dynamics (linked to the mass flow gain factor familiar in the context of pump dynamics) and therefore
potentially unstable behavior for two distinct frequency ranges, one of which appears to be responsible for
the instability
Physically Realistic Models of Catastrophic Bubble Collapses
No full textWhen gas micro-bubbles are forced with an acoustic field they typically undergo a slow expansion followed by a violent collapse. During the expansion phase a significant amount of vapor enters the bubble. While much of this vapor is expelled as the bubble collapses, significant excess vapor is trapped in the interior during this violent collapse. As the bubble collapses this vapor is significantly heated and undergoes dissociative chemical reactions. Applications which take advantage of these acoustically driven chemical reactions are generally known as sonochemistry. Using the results of direct numerical simulations as a base, reduced models of the transport and gas dynamics of this sonochemical process are carefully developed. The models are compared to experimental data, showing that the reduced formulations can be used to predict real phenomena
Fission of Collapsing Cavitation Bubbles
No full textHigh-speed observations (for example, Lauterborn and Bolle 1975, Tomita and Shima 1990, Frost and Sturtevant 1986) clearly show that though a collapsing cavitation bubble
approaches its minimum size as a coherent single volume, it usually reappears in the first rebounding frame as a cloud of much smaller bubbles or as a highly distorted single volume (see, for example, figure \ref{F2}). This paper explores two mechanisms that may be responsible for that bubble fission process, one invoking a Rayleigh-Taylor stability analysis and the other utilizing the so-called
microjet mechanism. Both approaches are shown to lead to qualitatively similar values for the number of fission fragments and the paper investigates the flow parameters that effect that number. Finally, we explore the effective damping of the Rayleigh-Plesset single bubble calculation which that fission process implies and show that it is consistent with the number of collapses and rebounds which are observed to occur in experiments
Measurement of Cavitation Erosive Aggressiveness by Means of Structure Born Noise
No full textA measuring system was developed to estimate the cavitation erosive aggressiveness in a centrifugal pump during operation. The system is based on acoustical means. Acoustic pressure waves in water and vibrations of machine casings underlie several disturbances especially by changes of the transmission behaviour of the fluid during operation. Thus, an approach based on the structure born noise measured directly on the mechanical structure exposed to the cavitation was chosen to minimise this problems. The sensor, the amplifier and the signal processing and transmission devices have then to be placed within the rotating part of the machine. The measurements of the system have been compared with results of measurements using material specimen and coatings of soft copper. The physical relation between the acoustical event amplitudes detected by the system and the sizes of pits found on the specimen is discussed
Calculation of the Added Mass and Damping Forces on Supercavitating Bodies
No full textA linearized formulation for the unsteady forces experienced by supercavitating bodies is developed in terms of their added mass and damping coefficients. The formulation is general, but is applied here using an axisymmetric base flow. Expressions for the added mass, damping and restoring tensors are derived in a form suitable for incorporation in a numerical "flight" simulation tool for supercavitating vehicles. Disk and conical cavitating bodies are investigated. It is found that the added mass in surge can actually be negative for small values of the reduced frequency. The physical interpretation of this phenomenon is provided
Numerical Modeling of Sheet and Tip Vortex Cavitation with Fluent 5
No full textThis paper presents numerical work made with the
commercial CFD code Fluent 5 by Fluent Inc. The main investigation constitutes a 2D NACA 0015 hydrofoil grid. The grid is designed to resemble a physical setup found at Saint Anthony Falls Laboratory (SAFL) in Minneapolis, MN, USA. The study is done for cavitation numbers ranging from inception to supercavitating at two different angles of attack. All numerical results are compared to corresponding experimental results from the SAFL tunnel in a quantitative manner. The general characteristics of the cavitating
flow was very well predicted. Especially, the cavity length is calculated with high accuracy. Lift variations as a function of cavitation number are also reasonably well predicted. The current model, however, seems to have problems with the dynamics of the system.
The next section presents a 3D calculation on a NACA 66_2-415 elliptical planform hydrofoil. This was done primarily to investigate the 3D ability of the solver. The results are
validated with experimental data. Special focus is given to tip vortex strength and lift variation as a function of cavitation number. Both are well predicted
Numerical Simulation of Bubble Dynamics in a Vortex Flow Using Navier-Stokes Computations and Moving Chimera Grid Scheme
No full textA numerical scheme using Navier-Stokes computations was applied to simulate bubble dynamics in a vortex flow. A moving grid generation scheme and a Chimera grid scheme were integrated with the Navier-Stokes solver to automatically generate the appropriate grids based on the moving bubble surface. A general free surface boundary condition was implemented to describe the bubble surface motion. The numerical scheme was validated by comparing results with those obtained from the Rayleigh-Plesset equation and from the potential flow solver, 3DynaFS, for bubble dynamics in an infinite volume of quiescent water with/without the gravity effect. Important numerical factors that influenced the accuracy of solution are reported. Finally, the computations were conducted for bubbles released within a Rankine vortex. Different bubble behaviors were observed for different cavitation numbers
An Improved Boundary Element Analysis of Cavitating Three-Dimensional Hydrofoils
No full textA theoretical analysis of three--dimensional cavitating flows based on a
boundary element approach which includes rotational and viscous flow
effects is presented.
Partial sheet cavitation is studied by a closed--cavity nonlinear model
which includes the prediction of the cavity detachment point.
The trailing vorticity path is described by a wake--alignment technique.
Viscosity effects are included via a viscous/inviscid technique based
on boundary--layer assumptions.
Cavitation modelling, wake alignment and viscous--flow correction
are integrated into the potential--flow solution by BEM through
a coupling procedure.
Comparisons of present numerical results with experiments and numerical
results in the literature show the capability of the proposed methodology
to take into account viscosity effects on attached cavitation, and
to analyze the flowfield in the tip--vortex region
Some Problems of Development of Cavitation Technologies for Industry Application
No full textUsage of cavitation engineering in production has shown efficiency of cavitation technologies. The development of
the given direction of researches has allowed to develop principles of designing of cavitation vehicles. To explain
effects, observed at a cavitation, which are connected to intermixing, acceleration of chemical processes in the liquid
environments, obtaining of effective thermal generators etc. In this connection in the present paper some effects are
considered and the attempt of their theoretical substantiation bound with a cavitation in viscous and anomalously
viscous liquids is made. In opinion of the writers the considered phenomena can be the basis of creation floppy and
comprehensive systems for obtaining products with the high requirements to quality in relation to homogeneity and
dispersibility