F.J. Moraga

Assessment of turbulent dispersion models for bubbly flows in the low Stokes number limit

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<10−2. Most of the models successfully predicted these mixing layer data. Moreover, for Stokes numbers much smaller than unity several of the models were virtually identical. No adjustable coefficients were used in the mixing layer data comparisons.

The Modeling of Lift and Dispersion Forces in Two-Fluid Model Simulations of a Bubbly Jet

Two-fluid model simulations of a bubbly vertical jet are presented. The purpose of these simulations is to assess the modeling of lift and turbulent dispersion forces in a free shear flow. The turbulent dispersion models used herein are based on the application of a kinetic transport equation, similar to Boltzmanns equation, to obtain the turbulent diffusion force for the dispersed phase. They have already been constituted and validated for the case of particles in homogeneous turbulence and jets and for microscopic bubbles in grid generated turbulence and mixing layers

A micro-rotor driven by an acoustic bubble

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.

The effect of acoustically-induced cavitation on the permeance of a bullfrog urinary bladder

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 modeling of bubbly flows around naval surface ships at high Reynolds numbers

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.

The Modeling of Lift and Dispersion Forces in Two-Fluid Model Simulations: Part II — Boundary Layer Flows

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).© 2003 ASME

The design of acoustic resonant chambers by numerical simulation

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.

The Modeling of Lift and Dispersion Forces in Two-Fluid Model Simulations: Part I — Jet Flows

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.Copyright

Numerical Simulation of a Small Bubble Impinging Onto an Inclined Wall

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