Fabian J. Bonetto

A polydisperse model for bubbly two-phase flow around a surface ship

Abstract A three dimensional polydisperse model for bubbly two-phase flow around a surface ship is presented. The Boltzmann equation for the bubble mass probability density function is evaluated using a multigroup approach with groups of constant bubble mass. The intergroup transfer mechanisms are bubble breakup, coalescence and the dissolution of air into the ocean, and their effects on the two-phase flow field are analyzed. A three dimensional two-fluid model is used for each bubble mass group to calculate the group’s average gas velocity, resulting in four scalar equations per group. The air entrainment process is modeled using simulated breaking bow waves and the steady-state evolution of the gas bubbles for zero Froude number is obtained. It was found that intergroup transfer is very important in these flows. Some of the research areas that need further improvement for the numerical prediction of polydisperse two-phase flow around a ship have been identified and are discussed.

Lateral forces on spheres in turbulent uniform shear flow

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.

An experimental study on air carryunder due to a plunging liquid jet

Abstract This paper presents experimental data on the air entrainment process for a plunging liquid jet. Local data, including the size distribution of the bubble diameter, void fraction, bubble velocities and turbulent liquid velocities were obtained using a fiber optics phase Doppler anemometer system. A dual element conductivity probe was also used to measure the spatial distribution of the local void fraction. For the larger bubbles, the turbulence of the plunging jet was parametrically controlled using a specially designed nozzle and the turbulence intensity was measured using a laser Doppler anemometer system.

Phase distribution and turbulence structure for solid/fluid upflow in a pipe

Abstract The phase distribution and turbulence structure for solid/fluid upflow in a vertical pipe were investigated. Spherical particles, approximately 2 mm in diameter, were used and runs were made with particles having two different specific gravities. In particular, ceramic particles, which were heavier than water, and expanded polystyrene particles, which were lighter than water, were used. A new method is presented for the measurement of the volume fraction in solid/fluid two-phase flows using a laser-Doppler anemometer (LDA). The measured local time fractions obtained with the LDA must be corrected, because bias is produced by the presence of natural seeding, the finite size of the measurement volume and interruptions of the laser beams by the dispersed particles. An analytical method has been developed which accounts for these effects. A single-beam traversing γ-ray densitometer was used as a reference against which to assess the volume fraction correction method. Good agreement between the the corrected LDA and γ-ray densitometer results was obtained. The volume fraction profiles show that at low flow rates the ceramic particles have an almost uniform distribution, while increasing the flow rate causes coring. In contrast, the phase distribution of the light polystyrene particles had wall peaking for both the low and high flow rates. However, wall peaking was flattened as liquid flow rate was increased.

High-Frequency Digital Lock-In Amplifier Using Random Sampling

A high-frequency digital lock-in amplifier (LIA) that uses a random-sampling scheme is proposed and tested experimentally in this paper. By using this sampling strategy, it is possible to process, without aliasing effects, periodic signals of frequencies that are several times higher than the Nyquist frequency. Analytical and numerical analyses that show the advantages and limitations of the proposed scheme are presented. A high-frequency digital LIA implementation is also described. The prototype maximum sampling frequency is 150 kHz, and its maximum signal frequency without aliasing is 2.5 MHz, limited only by the random-sampling period quantization. Experimental results that validate the proposal are presented.

The interaction of background ocean air bubbles with a surface ship

A two-fluid model suitable for the calculation of the two-phase flow field around a naval surface ship is presented. This model couples the Reynolds-averaged Navier–Stokes (RANS) equations with equations for the evolution of the gas-phase momentum, volume fraction and bubble number density, thereby allowing the multidimensional calculation of the two-phase flow for monodisperse variable size bubbles. The bubble field modifies the liquid solution through changes in the liquid mass and momentum conservation equations. The model is applied to the case of the scavenging of wind-induced sea-background bubbles by an unpropelled US Navy frigate under non-zero Froude number boundary conditions at the free surface. This is an important test case, because it can be simulated experimentally with a model-scale ship in a towing tank. A significant modification of the background bubble field is predicted in the wake of the ship, where bubble depletion occurs along with a reduction in the bubble size due to dissolution. This effect is due to lateral phase distribution phenomena and the generation of an upwelling plume in the near wake that brings smaller bubbles up to the surface.

AN EXPERIMENTAL STUDY OF DISPERSED LIQUID/LIQUID TWO-PHASE UPFLOW IN A PIPE

This paper presents experimental data for dispersed liquid/liquid upflows. Water was the continuous phase and mineral oil was the dispersed droplet phase. For this flow regime reduced gravity bubbly flow phenomena was simulated because the mineral oil and water had almost the same density. The mean velocity and turbulence fields, the size distributions of the oil droplets, the volume fraction, and interfacial area density distribution were measured using fiber optic Laser Doppler Anemometer (LDA) and phase Doppler Anemometer (PDA) systems. Significantly, the results presented in this paper are similar to those for bubbly air/water flows in microgravity conditions (Kamp el at., 1995).

AN EXPERIMENTAL STUDY OF PHASE DISTRIBUTION AND TURBULENCE STRUCTURE FOR SOLID/LIQUID FLOW IN A HORIZONTAL PIPE

Abstract Local measurements of the velocity and phase distribution were made for solid/liquid two-phase flows in a horizontal pipe using a multidimensional laser Doppler anemometer (LDA) system. The pipe used had a 30.6 mm inner diameter and was made of a special optically clear material, Fluorinated Ethylene Propylene (FEP), which has the same index of refraction as water. The test section and laser probes were submerged in water so that no optical corrections were necessary. Simultaneous measurements of the liquid and particle phases were obtained and discriminated based on differences in the residence time that each particle had when crossing the LDAs measurement volume. Positive buoyant and negative buoyant solid spherical particles, about 2 mm in diameter, were used in this study. A complete data set of the phasic mean velocities, turbulence and volume fractions were taken for each particle type. The results presented herein are complete and self-consistent, and can be used to assess the predictive capabilities of multidimensional two-fluid computational fluid dynamic (CFD) models.

Analytical study of the acoustic field in a spherical resonator for single bubble sonoluminescence

The acoustic field in the liquid within a spherical solid shell is calculated. The proposed model takes into account Stokes wave equation in the viscous fluid, the membrane theory to describe the solid shell motion and the energy loss through the external couplings of the system. A point source at the resonator center is included to reproduce the acoustic emission of a sonoluminescence bubble. Particular calculations of the resulting acoustic field are performed for viscous liquids of interest in single bubble sonoluminescence. The model reveals that in case of radially symmetric modes of low frequency, the quality factor is mainly determined by the acoustic energy flowing through the mechanical coupling of the resonator. Alternatively, for high frequency modes the quality factor is mainly determined by the viscous dissipation in the liquid. Furthermore, the interaction between the bubble acoustic emission and the resonator modes is analyzed. It was found that the bubble acoustic emission produces local maxima in the resonator response. The calculated amplitudes and relative phases of the harmonics constituting the bubble acoustic environment can be used to improve multi-frequency driving in sonoluminescence.

Real-time particle image velocimetry based on FPGA technology

Particle image velocimetry (PIV) allows measuring distributed flow velocity fields. It is well established as an experimental tool in modern fluid dynamics research, being applied to liquid, gases and multiphase flows. Images of tracer particles are processed by means of a statistical strategy, which makes its real-time implementation difficult to achieve. In this paper, we describe the design and implementation of an embedded architecture for real-time PIV based on FPGA technology. The proposed scheme has allowed us to exploit the low-level parallelization in both, the direct cross-correlation computation and interrogation windows handling. We propose a bus architecture to manage multiple interfaces among the processing modules and external devices. By using this scheme, we achieved design flexibility and improved processing speed. Major benefits of the speed improvement are enhanced experimental capabilities like feedback control, on-line flow regimen visualization, and a significant speed up in off-line processing. We show experimental results of a physical field of velocities calculated in real-time.