G. Ooms

Simultaneous flow visualization and Reynolds-stress measurement in a turbulent boundary layer

Flow visualization and Reynolds-stress measurement were combined in an investigation of a turbulent boundary layer in a water channel. Hydrogen bubbles were used to visualize the flow; a laser-Doppler anemometer capable of measuring two velocity components was applied to measure the instantaneous value of the Reynolds stress. Owing to the three-dimensional, time-dependent character of the flow it was rather difficult to identify flow structures from measured velocity signals, especially at larger distances from the wall. Despite this difficulty a method based on the instantaneous value of the Reynolds stress could be developed for detecting bursts in the wall region of the boundary layer. By this method the three-dimensional, time-dependent character of the flow is taken into account by attributing to the same burst ejections occurring successively with very short time intervals. This identification procedure is based on a comparison on a one-to-one basis between visualized flow structures and measured values of the Reynolds stress. The detected bursts were found to make a considerable contribution to the momentum transport in the boundary layer.

Particle-fluid interactions in grid-generated turbulence

The effect of small particles on decaying grid-generated turbulence is studied experimentally. Using a two-camera system, instantaneous fluid-phase and particlephase measurements can be obtained simultaneously. The data obtained with this system are used to study the decay behaviour of the turbulent flow. The role of particle size, particle density and volume load is studied in a number of different cases. These cases are chosen so that the individual role of these parameters can systematically be evaluated. Addition of particles to the flow has significant effects on the decaying turbulence: first, the onset of the turbulent decay appears to shift upstream; second, the flow becomes anisotropic as it develops downstream. The latter is observed as an increase in integral length scale in the vertical direction. The rate at which the flow becomes anisotropic can be predicted using a new parameter: the product of the non-dimensional number density and the Stokes number (referred to as the ‘Stokes load’). This parameter, combining the relevant fluid and particle characteristics, is a measure for the energy redistribution leading to anisotropy. In addition to redistributing energy, the particles also produce turbulence. However, this only becomes evident when the grid-generated turbulence has decayed sufficiently, relatively far downstream of the grid. The turbulence production by particles can also account for the observed decrease in slope of the power spectrum, which leads to a ‘cross-over’ effect. The production of turbulence by the particles can be predicted using a model for the momentum deficit of the particle wakes. The validity of this approach is confirmed using conditional sampling of the fluid velocity field around the particles.

Particle-Turbulence Interaction in a Homogeneous, Isotropic Turbulent Suspension

A review is given of numerical, analytical, and experimental research regarding the two-way coupling effect between particles and fluid turbulence in a homogeneous, isotropic turbulent suspension. The emphasis of this review is on the effect of the suspended particles on the spectrum of the carrier fluid, in order to explain the physical mechanisms that are involved. An important result of numerical simulations and analytical models (neglecting the effect of gravity) is that, for a homogeneous and isotropic suspension with particles with a response time much larger than the Kolmogorov time scale, the main effect of the particles is suppression of the energy of eddies of all sizes. However for a suspension with particles with a response time comparable to or smaller than the Kolmogorov time, the Kolmogorov length scale will decrease and the turbulence energy of (nearly) all eddy sizes increases. For a suspension with particles with a response time in between the two limiting cases mentioned above the energy of the larger eddies is suppressed, whereas the energy of the smaller ones is enhanced. Attention is paid to several physical mechanisms that were suggested in the literature to explain this influence of the particles on the turbulence. In some of the experimental studies, certain results from simulations and models have, indeed, been confirmed. However, in other experiments these results were not found. This is attributed to the role of gravity, which leads to turbulence production by the particles. Additional research effort is needed to fully understand the physical mechanisms causing the two-way coupling effect in a homogeneous, isotropic, and turbulently flowing suspension. This review contains 47 references.

Core-annular flow through a horizontal pipe: Hydrodynamic counterbalancing of buoyancy force on core

A theoretical investigation has been made of core-annular flow: the flow of a high-viscosity liquid core surrounded by a low-viscosity liquid annular layer through a horizontal pipe. Special attention is paid to the question of how the buoyancy force on the core, caused by a density difference between the core and the annular layer, is counterbalanced. From earlier studies it is known that at the interface between the annular layer and the core waves are present that move with respect to the pipe wall. In the present study the core is assumed to consist of a solid center surrounded by a high-viscosity liquid layer. Using hydrodynamic lubrication theory (taking into account the flow in the low-viscosity liquid annular layer and in the high-viscosity liquid core layer) the development of the interfacial waves is calculated. They generate pressure variations in the core layer and annular layer that can cause a net force on the core. Steady eccentric core-annular flow is found to be possible.

An investigation of the influence of acoustic waves on the liquid flow through a porous material

An experimental and theoretical investigation has been made of the influence of high-frequency acoustic waves on the flow of a liquid through a porous material. The experiments have been performed on Berea sandstone cores. Two acoustic horns were used with frequencies of 20 and 40 kHz, and with maximum power output of 2 and 0.7 kW, respectively. Also, a temperature measurement of the flowing liquid inside the core was made. A high external pressure was applied in order to avoid cavitation. The acoustic waves were found to produce a significant effect on the pressure gradient at constant liquid flow rate through the core samples. During the application of acoustic waves the pressure gradient inside the core decreases. This effect turned out to be due to the decrease of the liquid viscosity caused by an increase in liquid temperature as a result of the acoustic energy dissipation inside the porous material. Also, a theoretical model has been developed to calculate the dissipation effect on the viscosity and on the pressure gradient. The model predictions are in reasonable agreement with the experimental data.

Influence of bubble size on the transition from low-Re bubbly flow to slug flow in a vertical pipe

An upward air–water bubbly flow in a pipe was studied experimentally, special attention being paid to the transition from bubbly flow to slug flow. The pipe diameter was 72 mm and the height 18 m. The Reynolds number based on liquid flow was low-tomoderate (Usl < 0: 2m s� 1), so that bubble break-up due to turbulence was nearly absent. Three different inlet devices were used, which had a significant influence on the initial bubble size and initial bubble concentration distribution. The transition from bubbly flow to slug flow was shown to be strongly dependent on the inlet configuration, particularly on the bubble size. A number of theoretical models for the transition from bubbly flow to slug flow were reviewed. The Taitel et al. [2] approach was combined with a bubble-size-dependent critical void-fraction expression of Song et al. [7]. This new formulation for the transition from bubbly flow to slug flow was in good agreement with the measurements. � 2002 Elsevier Science Inc. All rights reserved.

Removal of small particles from a porous material by ultrasonic irradiation

A study has been made of the removal of small particles from a porous material by means of ultrasonic irradiation. To that purpose a microscopic theoretical model has been developed to calculate the force of a traveling acoustic wave on a spherical particle attached to the wall of a smooth, cylindrical pore inside the porous material. This force was compared with the adhesion force between a small particle and a pore wall. From the comparison between the two forces the conditions were found, at which particles are detached from pore walls and removed from the porous material. The transformation of the results gained from the microscopic model to macroscopic property (permeability) of the porous material was made by means of the Kozeny relation. The aim is to be able to understand and predict qualitatively the influence of relevant parameters on the ultrasonic cleaning process. Predictions made with the theoretical model were compared with data from experiments carried out with ultrasound to remove particles from Berea sandstone. The agreement is reasonable.

Interfaces and internal layers in a turbulent boundary layer

New experimental research is presented on the characteristics of interfaces and internal shear layers that are present in a turbulent boundary layer (TBL). The turbulent/non-turbulent (T/NT) interface at the outer boundary of the TBL shows the presence of a finite jump in streamwise velocity and is characterised by a thin shear layer. It appears that similar layers of high shear occur also within the TBL which separate regions of almost uniform momentum. It turns out that they exhibit similar characteristics as the external T/NT interface. Furthermore, the spatial growth rate of the TBL, that is derived from theoretical analysis, can be correctly predicted from a momentum balance near the external T/NT interface. Similarly, the entrainment velocities for the average internal layers have been determined. Results indicate that internal layers move slower in the vicinity of the wall, whereas they move faster than the large scale boundary layer growth rate in the outer region of the TBL. It is believed that shear layers bound large scale flow regions of approximately uniform momentum. Hence, the entrainment velocities of these internal layers may be interpreted as growth rates of the large scale motions in a TBL.

On the levitation force in horizontal core-annular flow with a large viscosity ratio and small density ratio

A numerical study has been made of horizontal core-annular flow: the flow of a high-viscosity liquid core surrounded by a low-viscosity liquid annular layer through a horizontal pipe. Special attention is paid to the question how the buoyancy force on the core, caused by a density difference between the core and the annular layer, is counterbalanced. The volume-of-fluid method is used to calculate the velocities and pressures in the two liquids. At the start of the calculation the core is in a concentric position. Thereafter the core starts to rise under the influence of buoyancy until it reaches an eccentric equilibrium position where the buoyancy force is counterbalanced by hydrodynamic forces generated by the movement of a wave at the core-annular interface with respect to the pipe wall. At high Reynolds number of the flow in the annular layer core levitation is due to inertial forces, whereas at low Reynolds number viscous (lubrication) forces are responsible for levitation. We carried out two types of calculation. In the first we assume the interface to be smooth (without wave) at the start of the calculation and study how the wave develops during the rising period of the core. In the second a wave is already present at the start of the calculation.

On the influence of the particles-fluid interaction on the turbulent diffusion in a suspension

A theory for the turbulent diffusion of particles suspended in an incompressible fluid is presented. Special attention is paid to the influence on the turbulent diffusion of the two-way coupling between the carrier fluid and the dispersed particles. Also the influence of the hydrodynamic interaction between the particles and the influence of the finite-particle size are investigated. It is shown that the influence of the particles–fluid interaction and of the finite-particle size are significant. The hydrodynamic interaction between the particles appears to be negligible up to a volume fraction of 0.1. 2002 Elsevier Science Ltd. All rights reserved.