Search

It is well known that ultrasound enhances drug delivery to tissues, although there is not a general consensus about the responsible mechanisms. However, it is known that the most important factor associated with ultrasonically-enhanced drug permeance through tissues is cavitation. Here we report results from research conducted using a experimental approach adapted from single bubble sonoluminescence experiments which generates very well defined acoustic fields and allows controlled activation and location of cavitation. The experimental design requires that a biological tissue be immersed inside a highly degassed liquid media to avoid random bubble nucleation. Therefore, live frog bladders were used as the living tissue due to their high resistance to hypoxia. Tissue membrane permeance was measured using radiolabeled urea. The results show that an increase in tissue permeance only occurs when cavitation is present near the tissue membrane. Moreover, confocal microscopy shows a direct correlation between permeance increases and physical damage to the tissue.

The accurate prediction of air entrainment is critical in simulating various important multiphase (air/water) flows. In this paper, we present a sub-grid air entrainment model that quantitatively predicts the rate of air entrainment and subsequent disperse bubbly flow for a plunging jet. The derivation of this model is based on the two-stage (i.e., low and high liquid jet velocity) air entrainment mechanisms suggested by Sene [Sene KJ. Air entrainment by plunging jets. Chem Eng Sci 1988;43(10):2615–23]. This model was validated against extensive experimental data for water jets in air over a wide range of liquid velocities (from around 1 to 10 m/s) for the total rate of air entrainment. It was then implemented into an Eulerian/Eulerian two-fluid computational multiphase fluid dynamics (CMFD) model, wherein the liquid and the bubbles are modeled as two distinct continua. This multiphase model, supplemented by the new sub-grid air entrainment model, was used to predict the void fraction distribution underneath plunging water jets at different depths and water jet velocities. It was found that this approach yields results that match the experimental observations very well.

Direct numerical simulations (DNS) of air entrainment by the breaking bow waves of naval surface ships are outside of the computational reach of the most powerful computers in the foreseeable future. This creates a need for physically-based models of air entrainment for applications in numerical simulations for ship design. Due to the non-linear dependence of the terminal bubble velocity with diameter most air entrainment flows have a high void fraction region immediately below the free surface. We present a model that locates this region employing the local liquid velocity and the distance to the interface. Using this model and the bubble size distributions measured by Deane and Stokes [Deane BD, Stokes MD. Scale dependence of bubble creation mechanisms in breaking waves. Nature 2002;418:839–44] we have simulated the air entrainment in the breaking wave experiments of Wanieski et al. [Waniewski TA, Brennen CE, Raichlen F. Measurement of air entrainment by bow waves. J Fluids Eng 2001;123:57–63]. Comparison against these experimental data is good. We then apply this model to simulate the flow around naval combatant DTMB 5415 and the research vessel Athena. The model predicts air entrainment in regions where it was actually observed at sea, namely the breaking bow wave, along the water/air/hull contact line and in the near-wake. To the best of our knowledge this is the first model of air entrainment that compares favorably with data at laboratory scale and also presents the right trends at full-scale.

We demonstrate how a micro-rotor self-aligns on top of a microbubble and rotates at a frequency up to 700 rpm because of the steady fluid flow produced by acoustic oscillations of the micro-bubble. The rotation frequency is controlled over a wide range by modifying the frequency of the acoustic excitation. We also show that a wide range of geometries can be used for the rotor. Finally, the possibility of using this novel concept to build a very simple micromotor is discussed.

In a bubbly flow, the presence or absence of the dispersed phase (gas) at a given instant and a given point in space depends on two factors, namely: (a) the forces acting on the bubble (or, more generally the dispersed particle) as a whole and responsible for its movement, and (b) the shape (geometry) of the bubble, which contributes to define if the spatial point in question lays inside or outside the bubble at that instant. Based on that point of view, we propose herein a two-fluid model that involves a particle-center-averaging procedure for the disperse phase, and a re-interpretation and post-processing of the results obtained. This center-averaged approach averages the disperse phase (bubbles) based on a particle center indicator function, while retaining standard phase indicator averaging for the continuous phase (liquid). The solution fields obtained are then post-processed to introduce the geometry of the bubbles in order to recover the values that are representative of the measured fields. The key idea here is to separate the geometric aspect from the dynamic aspect of the problem into two independent, successive steps. Tracking the bubble centers makes it possible to model the wall-induced forces more accurately. We take advantage of this fact to propose models for the wall normal force for very small Weber numbers and for Weber numbers of order unity. The additional tangential drag induced by the wall is also examined in light of recent experimental evidence. The resultant two-fluid model may be easily incorporated into existing two-fluid model codes. Results obtained with the new model showing agreement with experimental data are also presented.

A model for the wallinduced forces on bubbles for application in two-fluid model simulations of bubbly flows has been derived. The wall-induced force is obtained as the integral of an excess pressure calculated from the solution of the lubrication equation. Our numerical results are compared with experiments and it is shown that the wall-induced force is a function of the buoyancy and added mass forces, modulated by a smoothed Heaviside function that accounts for the proximity to the wall. This model explicitly accounts for the Reynolds, Eotvos and Weber numbers of the bubble and is valid in a larger range of these numbers than previous models.

We present analytical results for stable stratified wavy two-phase flow and functional forms for the various interfacial force densities in a two-fluid model. In particular, we have derived analytically the components of the non-drag interfacial force density [Drew, D.A., Passman, S.L., 1998. Theory of Multicomponent Fluids. Springer-Verlag, New York; Nigmatulin, T.R., Drew, D.A., Lahey, R.T., Jr., 2000. An analysis of wavy annular flow. In: International Conference on Multiphase Systems, ICMS’2000, Ufa, Russia, June 15–17], Reynolds stress tensor, and the term, (p˜cli−p¯cl)∇αcl, where p˜cli is interfacial average pressure, p¯cl the average pressure, and αcl is the volume fraction of the continuous liquid phase. These functional forms should be useful for assessing two-fluid closure relations and Computational Multiphase Fluid Dynamics (CMFD) numerical models for stratified wavy flows. Moreover, it appears that this approach can be generalized to other flow regimes (e.g., annular flows).

Two-fluid turbulent dispersion models have been compared with direct numerical simulations (DNS) of a decaying turbulence bubbly flow in the low Stokes number limit, St≈10−3. Because of the absence of empiricism, DNS results represent an excellent means of assessing turbulent dispersion models. Sufficiently far away from the inlet of the channel, where the turbulence was fully developed, these turbulent dispersion models were able to predict the DNS results when a Schmidt number, Scb=0.83, was used. This result highlights the fact that even bubbles of diameter, Db=42 μm, considerably smaller than the Kolmogorov length scale, η=75 μm, do not behave as passive scalars for which Scb=1. In addition, these models were also assessed against a bubbly mixing layer flow having a low Stokes number, St

The lateral force on a tethered rigid sphere submerged in a turbulent, uniform shear flow of water was measured. Periodic and non-periodic motions of the sphere were observed depending on flowrate, shear and sphere density. The direction of the observed lateral force was opposite to that predicted by inviscid theory and increased in magnitude as the sphere’s Reynolds numbers based on relative velocity, Re ,a nd average shear, Rer, increased. The lateral force was found to correlate with the product Re Rer. The data suggests that a sign reversal occurs at relatively small values of the product Re Rer, where the lateral force is dominated by inviscid eAects. The results are explained assuming that the lateral forces on rigid spheres are a consequence of two competing factors: namely, inviscid lift forces and the vortex shedding-induced lateral forces which are dominant for higher Reynolds numbers. An estimate of the kinetic energy in the wake was used to show that the vortex shedding-induced lateral forces correlate with the product Re Rer and are in a direction opposite to the inviscid lift force. Combining the experimental data of this study with similar data a correlation for the lift coeAcient of spheres in turbulent shear flows was developed. This correlation is applicable to turbulent multiphase flows having a spherical dispersed phase. # 1999 Elsevier Science Ltd. All rights reserved.

Comparisons between one- and two-dimensional experiments and non-equilibrium wet steam CFD simulations are conducted paying attention not only to pressure profiles and wetness but also to the more difficult to match droplet diameter. To achieve the objective of optimizing the match of data the heat transfer model between the droplet and its surrounding vapor proposed by Young and the corrections to classical nucleation theory proposed by Moore and popularized by Gerber are adopted. It is found that the proposed models produce a better agreement with droplet diameter, without affecting the overall quality of the predictions for other quantities.Copyright © 2013 by ASME

In this paper we derive a model based on lubrication theory to describe the interaction of a bubble with an inclined wall. The model is an extension of the model derived by Klaseboer, Chevailier, Mate, Masbernat, Gourdon [2001. Model and experiments of a drop impinging on an immersed wall. Physics of Fluids 13(1), 45–57.] and Moraga, Drew, Larreteguy, Lahey [2005. Modeling wall-induced forces on bubbles for inclined walls. Multiphase Science and Technology 17(4), 483–505.] in the case of a horizontal wall. We consider bubbles of diameter 1–2 mm, which corresponds to high Reynolds numbers Re ∼ O( 100 ), and moderate deformation effects (with a Weber number of O(1)). Predictions of the model are compared with experimental visualizations of air bubbles rising in water toward an inclined wall. The dynamical behavior of bubbles is observed to depend on the wall inclination. We find that the model reproduces the bubble trajectories for wall inclinations smaller than 55 ◦ –60 ◦ . This critical value for the wall inclination corresponds to an experimentally observed transition in the bubble bouncing behavior, which agrees with the observations of Tsao and Koch [1997. Observations of high Reynolds number bubbles interacting with a rigid wall. Physics of Fluids 468, 271.]. We show that the main features of our lubrication-based model for rebound with an inclined wall can be expressed with a simple force model proposed by Moraga et al., suitable for use in direct numerical simulations of multiphase flow. 2007 Elsevier Ltd. All rights reserved.

This paper presents a numerical modeling of the collision between a small bubble -of a few hundred microns, initially moving at terminal velocity, and an inclined wall, with relevance to drag reduction schemes. The theoretical model uses the lubrication theory to describe the film drainage as the bubble approaches the wall, and compute the force exerted by the wall as the integral of the excess pressure due to the bubble deformation. The model is solved using finite differences. The trajectory of the bubble is then determined using equations of classical mechanics. This study is an extension of previous work by Moraga, Cancelos and Lahey, [Multiphase Science and Technology, 18,(2),2006] where the simulation and comparison with experiments was carried out for a horizontal wall. In the present study where the wall is inclined, the bubble trajectory is no longer onedimensional and axisymmetry around the vertical axis is lost, allowing for more complex behavior. The influence of various parameters (Reynolds number, Weber number) is examined. Numerical results are compared with the experimental data from Tsao and Koch [Physics Fluids 9, 44, 1997]Copyright © 2006 by ASME

Two-fluid model simulations of a bubbly vertical jet are presented. The purpose of these simulations is to assess the modeling of turbulence dispersion and lift forces in a free shear flow. Although turbulence dispersion forces have previously been validated using simpler canonical flows and microscopic particles or bubbles, there was a need to asses the model performance for larger bubbles in more turbulent flows. This method, of validating two-fluid models in flows of increasing complexity has the advantage of excluding, or at least minimizing, the possibility of cancellation of errors when modeling several forces. In a companion paper (see Part-II), the present two-fluid model is extended to a boundary layer in which forces induced by the presence of a wall are important. The turbulent dispersion models used herein are based on the application of a kinetic transport equation, similar to Boltzmann’s equation, to obtain the turbulent diffusion force for the dispersed phase [1, 2]. They have already been constituted and validated for the case of particles in homogeneous turbulence and jets [3] and for microscopic bubbles in grid generated turbulence and mixing layers [4]. It was found that it is possible to simulate the experimental data in Ref. [5] (See Figures-1 to 4) for a bubbly jet with 1 mm diameter bubbles. Good agreement is obtained using the model of Brucato et al. [7] for the modulation of the drag force by the liquid phase turbulence and a constant lift coefficient, CL. However, little sensitivity is observed to the value of the lift coefficient in the range 0 < CL < 0.29.

Publisher Summary The pressure and velocity fields within a resonant chamber are calculated with a finite element and the linear fluid/structure interaction code ATILA™. This chapter presents a paper, the objective of which is to create a standing wave of pressure strong enough to produce cavitation at the bulk of the fluid and understand the effect of ultrasonic-induced cavitation pressures on the transport of drugs through biological membranes. This chapter investigates the bladder of a frog. An acoustic chamber, which resonates at a desired frequency, can be designed to get controlled cavitation. The acoustic field is produced using a piezoceramic transducer. This transducer is driven by a wave generator through an audio amplifier. The acoustic test chamber can be immersed in the bladder of a frog, which is filled with a mild salt-water solution. The chamber is designed with a system that enables flow circulation. This chapter shows that simulations exist for a complex chamber designed to explore the effects of cavitation on the drug permeability of tissues.

Publisher Summary This chapter presents the simulations of the bubbly flow around a naval combatant at a quasi-full-scale Reynolds number that incorporate all the model improvements. The numerical predictions of the steady bubbly flow around naval combatant DTMB-5415 at Reynolds number, Re = 108 and zero Froude number are presented in the chapter. A monodisperse bubble source, Db = l60 μm, at an arbitrary location upstream of the ship are used in the simulations. The objective of these simulations is to address numerical and modeling issues. In addition to the usual forces on the bubble phase, a bubble-to-bubble contact pressure model is introduced in the chapter. The contact pressure model always pushes bubbles away from the local maximum of void fraction. Because the most important maximum of void fraction is located approximately one bubble radius from the wall, this model may tend to push one half of a bubble toward the wall and the other half away from it.

Two-fluid model simulations of a bubbly vertical boundary layer with point injection are presented. A new bubble turbulence dispersion model, designed to be used with RANS type turbulence models, was formulated and compared with recent data of [1] and [2]. These data showed that bubble migration toward the wall is controlled by the coherent large scale liquid structures within the boundary layer. The model is based on the application of a kinetic transport equation, similar to Boltzmann’s equation, and the idea that by selectively removing bubbles from the liquid eddies within the boundary layer, bubble capture at the wall introduces a preferential direction of migration and/or nonhomogeneous, anisotropic dispersion. This is the first model capable of predicting all the types of void fraction profiles observed experimentally for point injection. It is shown that without this new model, two-fluid model simulations fail to predict the experimental data. In addition, a new physical interpretation of the data of [1] and [2] is presented, which strongly suggests that the quantity controlling bubble migration toward the wall and bubble dispersion, is the boundary layer drift parameter (i.e., the ratio of the bubble’s terminal velocity to the free-stream liquid velocity).

The fundamental and tenth harmonics were used to produce stable single-bubble sonoluminescence in water. By varying the phase difference between the harmonics, it was possible to enhance the sonoluminescence light emission by as much as a factor of 2.7 compared with single-frequency excitation. Absolute measurements of the bubble radius evolution were carried out using the two-detector technique. Unlike previous observations, these measurements and complementary fits of the Rayleigh-Plesset equation reveal that the maximum bubble radius does not change significantly with phase angle between the harmonics. Therefore, increased sonoluminescence intensity does not have to correlate with increases in maximum bubble radius prior to collapse. We believe that a more violent bubble collapse rate (driven by the very-high-frequency component) is responsible for the enhanced light emission under this type of mixed excitation. It was further found that the presence of the tenth-harmonic frequency component led to significant enhancements in the stability of the bubble undergoing sonoluminescence. This allowed the bubble to be driven at the fundamental frequency at 2.0 bars pressure amplitudes, which are significantly above often-reported thresholds of 1.4 bar itself, thereby leading to increased levels of light emission (by more than 250%).

The introduction of a rotating frame allows the use of a potential function to solve the problem of a rigid oblate ellipsoid undergoing rotation and translation in a rotating and straining inviscid flow. Virtual mass and lift force coefficients are calculated, including a coefficient associated with the ellipsoid rotation contribution to the lift force. Significantly, the computed coefficients can depart from sphere values. Virtual mass, Lift force, Ellipsoid, Inviscid flow.