Jin-Keun Choi

Modeling of surface cleaning by cavitation bubble dynamics and collapse

Surface cleaning using cavitation bubble dynamics is investigated numerically through modeling of bubble dynamics, dirt particle motion, and fluid material interaction. Three fluid dynamics models; a potential flow model, a viscous model, and a compressible model, are used to describe the flow field generated by the bubble all showing the strong effects bubble explosive growth and collapse have on a dirt particle and on a layer of material to remove. Bubble deformation and reentrant jet formation are seen to be responsible for generating concentrated pressures, shear, and lift forces on the dirt particle and high impulsive loads on a layer of material to remove. Bubble explosive growth is also an important mechanism for removal of dirt particles, since strong suction forces in addition to shear are generated around the explosively growing bubble and can exert strong forces lifting the particles from the surface to clean and sucking them toward the bubble. To model material failure and removal, a finite element structure code is used and enables simulation of full fluid-structure interaction and investigation of the effects of various parameters. High impulsive pressures are generated during bubble collapse due to the impact of the bubble reentrant jet on the material surface and the subsequent collapse of the resulting toroidal bubble. Pits and material removal develop on the material surface when the impulsive pressure is large enough to result in high equivalent stresses exceeding the material yield stress or its ultimate strain. Cleaning depends on parameters such as the relative size between the bubble at its maximum volume and the particle size, the bubble standoff distance from the particle and from the material wall, and the excitation pressure field driving the bubble dynamics. These effects are discussed in this contribution.

NOISE DUE TO EXTREME BUBBLE DEFORMATION NEAR INCEPTION OF TIP VORTEX CAVITATION

A study on the tip vortex cavitation inception based on extreme bubble deformation and jet noise is presented. First, two preliminary experiments involving bubble splitting between two plates in the absence of swirl are performed to provide a correlation between the numerically computed splitting/jet noise and the measured noise. The bubble behavior and pressure signal predicted by the axisymmetric method are compared with those recorded simultaneously by using a high-speed video camera and a hydrophone. Then, numerical studies on the bubble behavior in the tip vortex flow field are conducted. The tip vortex flow near a hydrofoil is provided by a viscous flow computation, and the bubble behavior is simulated by an axisymmetric boundary element method which accounts for the provided vortex flow field. The characteristics of the bubble behavior and jet noise over a range of cavitation numbers are investigated. The effect of initial bubble nucleus size and the Reynolds number effect of the tip vortex flow on...

Disinfection of gram-negative and gram-positive bacteria using DynaJets® hydrodynamic cavitating jets.

Cavitating jet technologies (DynaJets®) were investigated as a means of disinfection of gram-negative Escherichia coli, Klebsiellapneumoniae, Pseudomonas syringae, and Pseudomonas aeruginosa, and gram-positive Bacillus subtilis. The hydrodynamic cavitating jets were found to be very effective in reducing the concentrations of all of these species. In general, the observed rates of disinfection of gram-negative species were higher than for gram-positive species. However, different gram-negative species also showed significant differences (P. syringae 6-log(10) reduction, P. aeruginosa 2-log(10) reduction) under the same conditions. Disinfection of E. coli repeatedly showed five orders of magnitude reduction in concentration within 45-60-min at low nozzle pressure (2.1 bar). Optimization of nozzle design and operating pressures increased disinfection rates per input energy by several orders of magnitude. The power efficiencies of the hydrodynamic cavitating jets were found to be 10-100 times greater than comparable ultrasonic systems.

Modelling single- and tandem-bubble dynamics between two parallel plates for biomedical applications

Carefully timed tandem microbubbles have been shown to produce directional and targeted membrane poration of individual cells in microfluidic systems, which could be of use in ultrasound-mediated drug and gene delivery. This study aims at contributing to the understanding of the mechanisms at play in such an interaction. The dynamics of single and tandem microbubbles between two parallel plates is studied numerically and analytically. Comparisons are then made between the numerical results and the available experimental results. Numerically, assuming a potential flow, a three-dimensional boundary element method (BEM) is used to describe complex bubble deformations, jet formation, and bubble splitting. Analytically, compressibility and viscous boundary layer effects along the channel walls, neglected in the BEM model, are considered while shape of the bubble is not considered. Comparisons show that energy losses modify the bubble dynamics when the two approaches use identical initial conditions. The initial conditions in the boundary element method can be adjusted to recover the bubble period and maximum bubble volume when in an infinite medium. Using the same conditions enables the method to recover the full dynamics of single and tandem bubbles, including large deformations and fast re-entering jet formation. This method can be used as a design tool for future tandem-bubble sonoporation experiments.

Characterization of Cavitation Fields From Measured Pressure Signals of Cavitating Jets and Ultrasonic Horns

Cavitation pressure fields under a cavitating jet and an ultrasonic horn were recorded for different conditions using high frequency response pressure transducers. This was aimed at characterizing the impulsive pressures generated by cavitation at different intensities. The pressure signals were analyzed and statistics of the amplitudes and widths of the impulsive pressure peaks were extracted. Plots of number densities and cumulative numbers of peaks as functions of peak amplitude, peak width, and the power of the ultrasonic horn or the jet were generated. The analysis revealed the dominance of pulses with smaller amplitudes and larger durations at lower cavitation intensities and the increase of the amplitudes and reduction of the pulse widths at higher intensities. The ratio of the most probable peak amplitude to peak width was computed. A representative Gaussian curve was then generated for each signal using a characteristic peak amplitude and the corresponding most probable peak duration/width. This resulted in a proposed statistical representation of a cavitation field, useful to characterize cavitation fields of various intensities. [DOI: 10.1115/1.4024263] Prediction of cavitation erosion on propellers, ship structures, and in general on any structure subjected to cavitation is of great interest to many industries. However, this task is often difficult and selection of new materials or material protection coatings that are cavitation erosion resistant is instead most often based on laboratory testing using accelerated erosion methods. These aim at comparing within short time periods the resistance of a new material relative to other standard materials. Erosion in the real field occurs over long durations of exposure, while accelerated erosion tests, by definition, involve subjecting the material to an erosion field that is significantly more “intense” than the actual cavitation that the studied material will be subjected to. The validity of such an approach is however not obvious, as it has been observed that the relative resistance of two materials can be different at different “intensities” of cavitation [1,2]. However, the definition of cavitation “intensity” is not universal. One classical definition [3,4] using an integral quantity is based on a concept similar to that of the acoustic intensity. This expresses the cavitation intensity at a selected point on the material subjected to cavitation as, ð1=qcÞ P N 1 P 2

Relationship between space and time characteristics of cavitation impact pressures and resulting pits in materials

Cavitation erosion studies require a well-defined measure of the aggressiveness of the subject cavitation field. One proposed method of cavitation field strength evaluation is to use pitting tests on a selected material sample subjected to the cavitation field. These relatively short duration tests record pits or permanent deformations from individual cavitation events during the cavitation incubation period. The pitting test results are dependent on the load and the material used in the tests and a good understanding of the pit formation mechanism is required to correlate the loads with the deformations. In this study, finite element numerical simulations are conducted to examine the response of several selected materials to imposed loads representing cavitation events. The magnitude, duration, and spatial extent of the loads are varied, and the effects of these on the material deformations are studied. Next, the effects of material properties, such as yield stress, Young’s modulus, and plastic modulus on the pitting characteristics are elucidated. Material responses are found to be drastically different between metals and compliant materials and to depend significantly on load duration and spatial extent in addition to the magnitude.

Optimum Configuration of an Expanding-Contracting-Nozzle for Thrust Enhancement by Bubble Injection

This paper addresses the concept of thrust augmentation through bubble injection into an expanding-contracting nozzle. Two-phase models for bubbly flow in an expandingcontracting nozzle are developed, in parallel with laboratory experiments and used to ascertain the geometry configuration for the nozzle that would lead to maximum thrust enhancement upon bubble injection. For preliminary optimization of experimental setup’s design, a quasi 1-D approach is used. Averaged flow quantities (such as velocities, pressures, and void fractions) in a cross section are used for the analysis. The mixture continuity and momentum equations are numerically solved simultaneously along with equations for bubble dynamics, bubble motion, and an equation for conservation of the total bubble number. Various geometric parameters such as the exit and inlet areas, the area of the bubble injection section, the presence of a throat and its location, the length of the diffuser section and the length of the contraction section are varied, and their effects on thrust enhancement are studied. Investigation on the effect of the injected void fraction is also carried out. The key objective function of the optimization is the normalized thrust parameter, which is the thrust with bubble injection minus the thrust with liquid only divided by the inlet liquid momentum. An approximate analytical expression for the normalized thrust parameter was also derived starting from the mixture continuity and momentum equations. This analytical expression involved flow variables only at three locations; inlet section, injection section, and outlet section, and the expression is simple enough to produce a quick concept design of the diffuser-nozzle thruster. The numerical and analytical approaches are verified against each other and the limitations of the analytical approach are discussed. [DOI: 10.1115/1.4005687]

Experimental and numerical investigation of bubble augmented waterjet propulsion

This contribution presents experimental and numerical investigations of the concept jet propulsion augmentation using bubble injection. A half-3D (D-shaped cylindrical configuration to enable optimal visualizations) divergent-convergent nozzle was designed, built, and used for extensive experiments under different air injection conditions and thrust measurement schemes. The design, optimization, and analysis were conducted using numerical simulations. The more advanced model was based on a two-way coupling between an Eulerian description of the flow field and a Lagrangian tracking of the injected bubbles using our Surface Averaged Pressure (SAP) model. The numerical results compare very favorably with nozzle experiments and both experiments and simulations validation the thrust augmentation concept. For a properly designed nozzle and air injection system, air injection produces net thrust augmentation, which increases with the rate of bubble injection. Doubling of thrust was measured for a 50% air injection rate. This beneficial effect remains at 50% after account for liquid pump additional work to overcome increased pressure by air injection.

Numerical Model of Cavitating Propeller Inside of a Tunnel

The unsteady cavitating flow of a propeller subject to a nonaxisymmetric inflow inside of a tunnel is addressed. A numerical method is developed which solves for the fully unsteady propeller problem and the tunnel problem separately, with the unsteady effects of one on the other being accounted for in an iterative manner. The propeller influence on the tunnel walls is considered via potential while the tunnel walls influence on the propeller is considered via velocity. The iterative process is found to converge very fast, usually within three iterations, even for a heavily loaded propeller. The effect of the tunnel extent and the number of panels on the predicted mean propeller forces is investigated, In the case of uniform inflow the equivalent open water velocity is calculated and then compared to that predicted from Glauerts formula

Thrust Enhancement Through Bubble Injection Into an Expanding-Contracting Nozzle With a Throat

This paper addresses the concept of thrust augmentation through bubble injection into an expandingcontracting nozzle with a throat. The presence of a throat in an expanding-contracting nozzle can result in flow transition from the subsonic regime to the supersonic regime (choked conditions) for a bubbly mixture flow, which may result in a substantial increase in jet thrust. This increase would primarily arise from the fact that the injected gas bubbles expand drastically in the supersonic region of the flow. In the current work, an analytical 1-D model is developed to capture choked bubbly flow in an expanding-contracting nozzle with a throat. The study intends to provide analytical/numerical confirmation to observed phenomena and to serve as a design tool to guide practical experiments aimed at creating and studying choked bubbly flows through nozzles. Starting from the 1-D mixture continuity and momentum equations along with an equation of state for the bubbly mixture, expressions for mixture velocity and gas volume fraction were derived. Starting with a fixed geometry, an imposed upstream pressure and assuming choked flow in the nozzle, the derived expressions were iteratively solved to obtain the exit pressures and velocities for different injected gas volume fractions. The variation of thrust enhancement with the injected gas volume fraction was also studied. Additionally, the geometric parameters were varied (area of the exit, area of the throat) to understand the influence of the nozzle geometry on the thrust enhancement and on the flow conditions at the inlet. This parametric study provides a performance map that can be used to design a bubble augmented waterjet propulsor that can achieve and exploit supersonic flow. It was found that the optimum geometry for choked flows, unlike the optimum geometry under purely subsonic flows, had a dependence on the injected gas volume fraction. Furthermore, for the same injected gas volume fraction the optimum geometry for choked flows resulted in greater thrust enhancement compared to the optimum geometry for purely subsonic flows.