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Mixing in stratified gravity currents: Prandtl mixing length  [PDF]
Philippe Odier,Jun Chen,Michael K. Rivera,Robert E. Ecke
Physics , 2009, DOI: 10.1103/PhysRevLett.102.134504
Abstract: Shear-induced vertical mixing in a stratified flow is a key ingredient of thermohaline circulation. We experimentally determine the vertical flux of momentum and density of a forced gravity current using high-resolution velocity and density measurements. A constant eddy viscosity model provides a poor description of the physics of mixing, but a Prandtl mixing length model relating momentum and density fluxes to mean velocity and density gradients works well. For $ \approx 0.08$ and $Re_\lambda \approx 100$, the mixing lengths are fairly constant, about the same magnitude, comparable to the turbulent shear length.
Mixing in thermally stratified nonlinear spin-up with sources and sinks  [PDF]
Meline Baghdasarian,Arturo Pacheco-Vega,J. Rafael Pacheco,Roberto Verzicco
Physics , 2013, DOI: 10.1063/1.4895435
Abstract: Stratified spin-up experiments in enclosed cylinders have reported the presence of small pockets of well-mixed fluids but quantitative measurements of the mixedness of the fluid has been lacking. Previous numerical simulations have not addressed these measurements. Here we present numerical simulations that address how the combined effect of spin-up and thermal boundary conditions enhances or hinders mixing of a fluid in a cylinder. Measurements of efficiency of mixing are based on the variance of temperature and explained in terms of the potential energy available. The numerical simulations of the Navier--Stokes equations for the problem with different sets of thermal boundary conditions at the horizontal walls helped shed some light on the physical mechanisms of mixing, for which a clear explanation was lacking.
On mixing times for stratified walks on the d-cube  [PDF]
Nancy L. Garcia,Jose L. Palacios
Physics , 2000,
Abstract: Using the electric and coupling approaches, we derive a series of results concerning the mixing times for the stratified random walk on the d-cube, inspired in the results of Chung and Graham (1997) Stratified random walks on the n-cube.
Resonant Triad Instability in Stratified Fluids  [PDF]
Sylvain Joubaud,James Munroe,Philippe Odier,Thierry Dauxois
Physics , 2012, DOI: 10.1063/1.4706183
Abstract: Internal gravity waves contribute to fluid mixing and energy transport, not only in oceans but also in the atmosphere and in astrophysical bodies. We provide here the first experimental measurement of the growth rate of a resonant triad instability (also called parametric subharmonic instability) transferring energy to smaller scales where it is dissipated. We make careful and quantitative comparisons with theoretical predictions for propagating vertical modes in laboratory experiments.
Penetrative convection in stratified fluids: velocity and temperature measurements
M. Moroni ,A. Cenedese
Nonlinear Processes in Geophysics (NPG) , 2006,
Abstract: The flux through the interface between a mixing layer and a stable layer plays a fundamental role in characterizing and forecasting the quality of water in stratified lakes and in the oceans, and the quality of air in the atmosphere. The evolution of the mixing layer in a stably stratified fluid body is simulated in the laboratory when "Penetrative Convection" occurs. The laboratory model consists of a tank filled with water and subjected to heating from below. The methods employed to detect the mixing layer growth were thermocouples for temperature data and two image analysis techniques, namely Laser Induced Fluorescence (LIF) and Feature Tracking (FT). LIF allows the mixing layer evolution to be visualized. Feature Tracking is used to detect tracer particle trajectories moving within the measurement volume. Pollutant dispersion phenomena are naturally described in the Lagrangian approach as the pollutant acts as a tag of the fluid particles. The transilient matrix represents one of the possible tools available for quantifying particle dispersion during the evolution of the phenomenon.
Topology, Braids, and Mixing in Fluids  [PDF]
Jean-Luc Thiffeault,Matthew D. Finn
Physics , 2006, DOI: 10.1098/rsta.2006.1899
Abstract: Stirring of fluid with moving rods is necessary in many practical applications to achieve homogeneity. These rods are topological obstacles that force stretching of fluid elements. The resulting stretching and folding is commonly observed as filaments and striations, and is a precursor to mixing. In a space-time diagram, the trajectories of the rods form a braid, and the properties of this braid impose a minimal complexity in the flow. We review the topological viewpoint of fluid mixing, and discuss how braids can be used to diagnose mixing and construct efficient mixing devices. We introduce a new, realisable design for a mixing device, the silver mixer, based on these principles.
Solubility and Mixing in Fluids  [PDF]
Robert Holyst
Physics , 1996,
Abstract: Traditionally, solubility and mixing belong to the realm of chemistry and material sciences and many standard textbooks on physical chemistry treat this problem, usually within the scope of thermodynamics. Here apart from thermodynamics we shall discuss other issues such as: the statistical mechanics theories of mixtures, relation between intermolecular interactions and demixing, coupling between ordering and demixing, and kinetics of demixing which includes spinodal decomposition in binary liquid mixtures. The following special examples will be discussed: polymer blends,diblock copolymers, liquid crystals, and ternary mixtures including surfactants (amphiphiles). Within the scope of physics of solubility and mixing one can study such diverse phenomena as: mixing of two simple liquids, collapse of the polymer chain in the solvent, flocculation of colloidal particle upon the addition of the polymer chains, mixing of two polymer components in a liquid state, ordering of copolymers and ternary mixtures of oil, water and surfactant or formation of micelles in aqueous solutions. In reality, very rarely we have completely pure substances, thus the properties of mixtures and the phenomenon of mixing and demixing are of prime importance for science and technology.
Mixing in shear thinning fluids
Ameur, H.;Bouzit, M.;
Brazilian Journal of Chemical Engineering , 2012, DOI: 10.1590/S0104-66322012000200015
Abstract: in the present study, a cfd characterization of the flow generated by curved-blade impellers in a cylindrical unbaffled vessel was carried out. the tank diameter was 300 mm, with a flat bottom. the liquid height was equal to the vessel diameter. the fluids simulated have a shear thinning behavior. analyses concern the effect of the impeller speed, the fluid rheology and the number of impeller blades on the induced flow patterns and the power consumption. the predictions were compared with literature data and a satisfactory agreement was found.
Turbulence and turbulent mixing in natural fluids  [PDF]
Carl H. Gibson
Physics , 2010, DOI: 10.1088/0031-8949/2010/T142/014030
Abstract: Turbulence and turbulent mixing in natural fluids begins with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretions on a spinning pair releases 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscous stresses and negative turbulence stresses work against gravity, extracting mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until strong-force viscous stresses freeze out turbulent mixing patterns as the first fossil turbulence. Cosmic microwave background temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered as plasma photon-viscous forces permit gravitational fragmentation on supercluster to galaxy mass scales. Turbulent morphologies and viscous-turbulent lengths appear as linear gas-proto-galaxy-clusters in the Hubble ultra-deep-field at z~7. Proto-galaxies fragment into Jeans-mass-clumps of primordial-gas-planets at decoupling: the dark matter of galaxies. Shortly after the plasma to gas transition, planet-mergers produce stars that explode on overfeeding to fertilize and distribute the first life.
Oscillation and levitation of balls in continuously stratified fluids  [PDF]
I. Biro,B. Gyure,I. M. Janosi,K. G. Szabo,T. Tel
Physics , 2007, DOI: 10.1063/1.2927461
Abstract: The free motion of balls is investigated experimentally in continously stratified fluid in a finite container. The oscillation frequency is found to be very close to the local Brunt-Vaisala frequency. The effect of added mass proves to be practically negligible. The evolution of rear jets is demonstrated, and a kind of long term levitation is found. We show that the classical viscous drag would lead to a much stronger damping than observed in the experiment. This is interpreted as a consequence of the feedback from the previously excited internal waves following their reflection from the boundaries. A phenomenological equation with a modified drag term is proposed to obtain a qualitative agreement with the observations. We point out that the inclusion of a history term would lead further away from the observed data.
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