Velocity measurements in the field of an internal gravity wave by means of speckle photography

Speckle velocimetry with forward scattering has been applied to measure and visualize the two-dimensional velocity field in an internal gravity wave. The wave was produced by towing a cylinder in vertical direction, normal to its axis, through stratified salt water. Neutrally buoyant tracer particles whose density was matched with the density distribution of the stratification were uniformly distributed in the test fluid. The experimental results verify the results of a linear theory in the far field of the wave.Most part of this work was performed when the three authors were with Institut für Thermo- und Fluiddynamik, Ruhr-Universität Bochum.     

Experimental study on the transition between stratified and non-stratified horizontal oil-water flow

The effect of oil and water velocities, pipe diameter and oil viscosity on the transition from stratified to non-stratified patterns was studied experimentally in horizontal oil-water flow. The investigations were carried out in a horizontal acrylic test section with 25.4 and 19 mm ID with water and two oil viscosities (6.4 and 12 cP) as test fluids. A high-speed video camera was used to study the flow structures and the transition. At certain oil velocity, stratified flow was found to transform into bubbly and dual continuous flows as superficial water velocity increased for both pipe diameters using the 12 cP oil viscosity. The transition to bubbly flow was found to disappear when the 6.4 cP oil viscosity was used in the 25.4 mm pipe. This was due to the low E?tv?s number. Transition to dual continuous flow occurred at lower water velocity for oil velocity up 0.21 m/s when 6.4 cP oil was used in the 25.4 mm ID pipe, while for U_so > 0.21 m/s, the transition appeared at lower water velocity with the 12 cP oil.The effect of pipe diameter was also found to influence the transition between stratified and non-stratified flows. At certain superficial oil velocity, the water velocity required to form bubbly flow increased as the pipe diameter increased while the water velocity required for drop formation decreased as the pipe diameter increased. The maximum wave amplitude was found to grow exponentially with respect to the mixture velocity. The experimental maximum amplitudes at the transition to non-stratified flow agreed reasonably well with the critical amplitude model. Finally, it was found that none of the available models were able to predict the present experimental data at the transition from stratified to non-stratified flow.     

Kinematic Parameters of a Two-Phase Flow in a Rectangular Channel

A kinematic two-phase flow pattern formed in a rectangular channel due to the interaction of a gas flow with an initially stationary or moving water layer is investigated. Using laser diagnostics and hot-wire methods, the velocity distributions in the water and the air are found for a stratified flow regime.     

Hamiltonian Formulation for Wave-Current Interactions in Stratified Rotational Flows

We show that the Hamiltonian framework permits an elegant formulation of the nonlinear governing equations for the coupling between internal and surface waves in stratified water flows with piecewise constant vorticity.     

Impingement of buoyancy-driven flows at a stratified interface

Laser-induced fluorescence (LIF) and particle-image velocimetry (PIV) are used to study both thermals and plumes impinging on a stratified interface. Data are obtained for a central slice of the flow near the stratified interface. Both the thermal and plume are generated by releasing fresh water at the bottom of a tank filled with two layers of salt water of different densities. Thermals and plumes are studied at Reynolds numbers ranging from 3,000 to 8,000, above the value for the mixing transition, a Schmidt number of about 600, and Richardson numbers from 1 to 22. The Richardson and Reynolds numbers are based on the thermal or plume characteristics (size and vertical velocity) before impingement and the initial density difference across the interface. Laser-induced fluorescence (LIF) is used to determine the maximum penetration height, rebound distance and lateral spreading velocity. The vorticity results obtained from the PIV data reveal the vortical structure near impingement. When the thermal impinges upon the stratified interface, a baroclinic eddy generated at the interface appears to merge with eddies comprising the thermal itself to form a vortex ring. This ring remains near the interface, moving mainly along the lateral or horizontal direction away from the region of impingement. These results suggest that lateral transport is significant for thermals impinging on stratified interfaces, and that ignoring such transport may greatly underestimate overall transport and mixing in such flows.     

Fluid Permeability in Stratified Unconsolidated Particulate Bed

Fluid permeability of polydisperse particulate bed with finite thickness has been examined. On the assumption of creeping flow, the permeability of monodisperse particles with arbitrary arrangement is calculated by means of Stokesian dynamics approach in which the interaction between individual particles and interstitial fluid is described by multipole expansion of the Oseen tensor. We have extended such calculation method to polydisperse particulate systems which have not so dense structures (up to particle volume fraction ${\phi \sim}$ 0.2). The particles are located infinitely in space and their interaction has been taken into account by Ewald summation technique. For the spatial distribution of polydisperse particles, we consider locally stratified particulate beds and define stratification degree as a parameter which apparently and mathematically represents the thickness of the mixing region of different-sized particles. The permeability profiles in the particulate beds with different stratification degree show the dependence of local permeability on the spatial and size distribution of particles. Consequently, the calculation results indicate that the permeability of non-uniform polydisperse particulate bed can be predicted by integrating the local permeation resistance which is determined by the local specific surface area.     

Determination of flow sub-regimes in stratified air–water channel flow using LDV spectra

In gas–liquid stratified flows, pressure drop and transport across the interface are strongly influenced by the interfacial wave structure, making the determination of interfacial topography in this kind of flows very important. An objective way of characterizing the wave pattern present in the interface is proposed here. The method consists in analysing the spectra of the signal obtained from Laser Doppler Velocimetry (LDV) measurements of fluctuations occurring close to the air-sheared interface. Transitions are defined by the appearance and disappearance of peaks in the frequency spectra. The method was applied to study the transition regimes of a stratified air–water flow in a square-cross section channel. A flow pattern map for air–water channel flow is presented and compared with the maps available from the literature.     

Experiments to study baroclinic waves penetrating into a stratified layer by a quasi-geostrophic potential vorticity equation

The upward penetration of the mid-latitude baroclinic waves has been well known to influence the air motion in the stratosphere. To study this in the laboratory, we present a new type of rotating fluid annulus experiment where a radial temperature difference is imposed on water near the flat bottom to produce a baroclinic wave in a lower layer and the water surface is heated to create a stratified upper layer. To test this model, we carried out experiments to study baroclinic waves penetrating into the stratified layer by a quasi-geostrophic potential vorticity equation in the atmospheric dynamics. The baroclinic waves produced in the lower fluid layer were observed to penetrate into the stratified upper layer. As expected from the equation, the vorticities of the flows for wavenumbers 3, 4 and 5 decayed exponentially with height and showed a weaker decay for lower wavenumbers.     

Turbulence in the Stratified and Rotating World Ocean

This is an overview of knowledge, derived mainly from observations, of turbulence in the stratified and rotating World Ocean from the 1960s, when mesoscale motions with scales of 30–150 km and 100 days were discovered by neutrally buoyant floats, to the present decade and the use of SF₆“purposeful tracer” release study in the North Atlantic. Most of the ocean is stably stratified, but it contains a rotational turbulent “continua” and isolated rotating eddies, as well as Rossby waves, and many features similar to those of, say, planetary atmospheres. It differs however because (a) the presence of lateral boundaries, the continental land masses, islands, and seamounts, provides constraints to the circulation and to the propagation of eddies, and possibly substantial sources and sinks of eddy motion; (b) channels connecting the oceans to land-locked seas (e.g., the Mediterranean; the formation of water with anomalous properties, “natural tracers”, e.g., temperature or salinity, allows “interthermocline eddies” to be readily detected); and (c) convection and differential seasonal latitudinal forcing introduce upper-ocean variability and intrathermocline eddies. The focus of research interest has moved from “turbulence”per se towards study of its consequences, for example towards dispersion of material particles and dissolved solutes, and the meridional and inter-basin transfers of heat. Received 2 December 1996 and accepted 18 September 1997     

Formation of a tripolar vortex in a stratified fluid

This paper reports on an experimental study of vortices in a stratified fluid. The vortices were generated by two different stirring devices, viz. a rotating sphere and a rotating bent rod. It was found that the vortices created with the rotating sphere are mostly axisymmetric and stable, whereas the vortices produced with the bent rod generally show instabilities, under certain conditions leading to the formation of a tripolar vortex. This report concentrates on this tripolar structure and presents quantitative information about the flow obtained through streak photography of tracer particles.     

Mathematical modeling of two problems of wave dynamics in heterogeneous media

Two problems of heterogeneous media mechanics are investigated in the paper. The first one, concerned with the shock wave/dust layer interaction, is solved within the framework of the equilibrium model of heterogeneous media mechanics. The second problem deals with the simulation of a Riemann problem for a stratified layer of solid particles.This paper is based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31–August 5, 2005     

Evolution of the diffusion-induced flow over a sphere submerged in a continuously stratified fluid

The problem of the stabilization of the diffusion-induced flow over a sphere submerged in a continuously stratified fluid is solved using both asymptotic and numerical methods. The analytical solution describes the structure of the main convective cells, including thin meridional jets flowing along the surface and plumes spreading from the flow convergence regions above the upper and lower poles of the sphere which gradually return the fluid particles to the neutral buoyancy horizon. The total width of the flows adjacent to the surface exceeds the thickness of the salinity deficit layer or the density boundary layer. The numerical solution of the complete problem in the nonlinear formulation describes the main convective cells and two systems of unsteady integral waves formed in the vicinity of the sphere poles. At large times, out of the entire system of internal waves only those nearest to the neighborhood of their horizon of formation remain clearly defined. The calculated flow patterns are in agreement with each other and the data of shadow visualization of the stratified fluid structure near a submerged obstacle at rest.     

Insight into the interaction between water and surfactant aerosol particles based on molecular dynamics simulations

The ubiquitous surfactant significantly influences the hygroscopic growth of atmospheric aerosol particles. However, knowledge on the morphology of surfactant particles after the adsorption of water is insufficient. In this study, the interaction between water and particles composed of surface active malonic acid (MA) or adipic acid (AA) are simulated based on the molecular dynamics method. The key point is the combined effect of temperature and water content on the structural properties of particles and the surface propensity of surfactants at the equilibrium state. Results show that demixed structure 1 with the adsorption of water clusters on acid grain, mixed structure and demixed structure 2 with acids coating on water droplet can be observed. With temperature increasing from 160 K to 330 K the surface propensity of MA and AA increases first and then weakens. Near the standard atmospheric temperature (280–330 K), the surface propensity of MA and AA increases with increasing water content and alkyl group, and its sensitivity to temperature and water content varies regularly. Moreover, all surfactants at the particle surface orient their hydrophobic groups toward the gas. These findings improve our insight into the surfactant partitioning and further assist in more accurate prediction of the particle hygroscopic growth.     

PTFE@SiO2核壳型添加剂在不同实验条件下的水润滑性能

采用原位聚合法制备了聚四氟乙烯@二氧化硅(PTFE@SiO₂)复合粒子. 利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、粒径分布仪以及傅立叶红外光谱仪(FTIR)对复合粒子的形貌、粒径分布和组成结构进行了分析. 结果表明:所制备的复合粒子呈核壳型复合结构,粒子尺寸处于亚微米级别. 分散性试验表明:该粒子在水中具有良好的分散性和稳定性. 利用LSR-2M往复式摩擦磨损试验机测试了不同试验条件下复合粒子在水环境中的摩擦学性能,结果表明:在常温、0.019 m/s滑动速率以及0.2 μm的钢盘粗糙度下,PTFE@SiO₂润滑剂具有最佳的摩擦学性能;相比于纯水和添加了PTFE/SiO₂的润滑剂,其摩擦系数降低了近80%,磨损体积减小了1~2个数量级. 分析表明:PTFE@SiO₂复合粒子优异的水润滑性能主要与核壳结构的存在以及在摩擦过程中形成的高质量转移膜密切相关.      

Rheological studies in the bulk and at the interface of Pickering oil/water emulsions

The rheological behavior and interfacial properties of olive oil–water emulsions stabilized by surfactant and clay particles (smectite) were studied to evaluate the effect of particles and surfactant distribution both in the bulk phase and at the oil–water interface. The temperature sweep of surfactant solutions and emulsions with and without clay particles showed the critical effect of the solid particles on the viscosity change. The mechanism of adsorption of surfactant molecules onto clay particles has a direct impact on the micellization and gelling temperatures. Indeed, the presence of clay particles caused a slight decrease in the micellization temperature and a total cancellation of the gelling phenomenon. Dynamic interfacial tension values demonstrated that clay particles would not compete with the surfactant for adsorption at the interface. However, the significant increase in the elastic properties of the interface that was observed accounts for their accumulation in the vicinity of the interface, probably at the level of surfactant polar head groups. Thus, the clay particles would form a mechanical barrier, preventing coalescence of emulsion droplets.     

Particle-induced bridging in immiscible polymer blends

Pickering emulsions are emulsions whose drops are stabilized against coalescence by particles adsorbed at their interface. Recent research on oil/water/particle systems shows that particles can sometimes adsorb at two oil/water interfaces. Such “bridging particles” can glue together drops of oil in water or vice versa. We hypothesize that the same effect should apply in immiscible polymer blends with droplet-matrix morphologies, viz., added particles should glue together drops and give rise to particle-bridged drop clusters. We test this hypothesis in PIB-in-PDMS blends [PIB, poly(isobutylene); PDMS, poly(dimethylsiloxane)] with fumed silica particles. Direct visualization shows that the particles can indeed induce clustering of the drops, and the blends appear to show gel-like behavior. Such gel-like behavior is confirmed by dynamic oscillatory experiments. However, we are unable to conclusively attribute the gel-like behavior to droplet clustering: Association of the fumed silica particles in the bulk, which itself causes gel-like behavior, confounds the results and prevents clear analysis of the gluing effect of the particles. We conclude that PIB/PDMS/fumed silica is not a good model system, for studying particle-containing polymer blends. We instead propose that spherical monodisperse silica particles can offer a far more convenient model system, and provide direct visual evidence of gluing of PIB drops in a PDMS matrix.     

Dynamically Equilibrium Shape of Intrusive Vortex Formations in the Ocean

The problem of finding the dynamically equilibrium shape of a rotating mass of liquid with homogeneous density (lens) submerged in a stratified ocean at rest on the rotating Earth is formulated. An equation for the shape of the interface between water masses is derived. An exact solution of the problem for an anticyclonically rotating lens in a linearly stratified ocean in the neighborhood of the lens depth shows that the dynamically equilibrium shape of the interface is a triaxial ellipsoid inclined with respect to the horizon and similar to an ellipsoid of revolution for real parameters of the phenomenon. The limiting values of the latitudes at which these formations can exist are determined. Degeneration of the shape with decrease in the intrinsic lens angular rate is investigated.     

Two-phase steam-water flow through Y-junctions

An investigation has been made of steam-water flow in a vertical coplanar Y-junctions which are used, for example, in the condensate carrying pipework downstream of direct contact heat exchangers. The steam-water flow is stratified in the sloping limbs approaching the junction and this paper examines problems associated with the flooding of the junction. It is shown that at low water flowrates water falls freely through the junction, but as the water flowrate increases a condition is reached when the water starts to pile at the junction and the junction is said to be flooded. Further increase in the water flowrate causes the water level to rise within the sloping limbs approaching the junction. A simple analytical model is developed which describes reasonably well the flow phenomena observed.     

Experimental study on interfacial waves in stratified horizontal oil–water flow

Interfacial wave characteristics were studied experimentally in horizontal oil–water pipe flows during stratified flow and at the transition to dual continuous flow, where drops of one phase appear into the other (onset of entrainment). The experimental investigations were carried out in a stainless steel test section with 38 mm ID with water and oil (density 828 kg/m³and viscosity 5.5 mPas) as test fluids. Wave characteristics were obtained with a high speed video camera and a parallel wires conductivity probe that measured the instantaneous fluctuations of the interface. Experiments were conducted at 2 m and at 6 m from the inlet. Visual observations revealed that no drops are formed when interfacial waves are absent. It was also found that waves have to reach a certain amplitude before drops can detach from their crests. Wave amplitudes are increased as the superficial velocities of both phases increase. In the stratified region, the mean wave amplitude decreases by increasing the oil–water input ratio while mean wavelength increases as the slip velocity between the two-phase decreases. At the onset of entrainment, the mean amplitude and length are found to be a function of the relative velocity between the oil and water layers and of the turbulence in each layer.