scholarly journals Coefficient of heterogeneity in turbulent mixing zone

2000 ◽  
Vol 18 (2) ◽  
pp. 207-212 ◽  
Author(s):  
A.S. KOZLOVSKIH ◽  
D.V. NEUVAZHAYEV
Keyword(s):  
Turbulent Mixing ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Mixing Zone ◽  
Turbulent Diffusion Coefficient ◽  
Dissipative Term ◽  
Developed Turbulence ◽  
Self Similar Solution ◽  
Self Similar ◽  
Good Agreement

The paper considers the equation for heterogeneity coefficient within the turbulent mixing area in the approximation of big Reynolds numbers and small Mach numbers. A mechanism is studied of the heterogeneity coefficient dissipation due to molecular diffusion. The Kolmogorov's hypothesis on developed turbulence is used to calculate a dissipative term. The model presented allows us to take into account the heterogeneity degree in LV- and KE-models of turbulent mixing. A system of equations allowing us to calculate directly the heterogeneity degree is derived for the case of the LV-model with the turbulent diffusion coefficient which is constant over the turbulent mixing area. A self-similar solution is derived for the heterogeneity coefficient which is in good agreement with the results of experiments and direct numerical simulations. The heterogeneity coefficient averaged over the mixing area is shown to depend weakly on the density drop between the mixing materials. Thus, it is kH = 0.25 at the drop n = 1–3, and at the drop n = 20 − kH = 0.23.

LDV and PIV Velocity Results in a Thermosiphon

2003 ◽  
Author(s):  
J. Kulman ◽  
D. Gray ◽  
S. Sivanagere ◽  
S. Guffey
Keyword(s):  
Large Scale ◽  
Single Phase ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Flow Patterns ◽  
Laser Doppler Velocimeter ◽  
Mean Velocity ◽  
Developed Turbulence ◽  
The Mean ◽  
Good Agreement

Heat transfer and flow characteristics have been determined for a single-phase rectangular loop thermosiphon. The plane of the loop was vertical, and tests were performed with in-plane tilt angles ranging from 3.6° CW to 4.2° CCW. Velocity profiles were measured in one vertical leg of the loop using both a single-component Laser Doppler Velocimeter (LDV), and a commercial Particle Image Velocimeter (PIV) system. The LDV data and PIV data were found to be in good agreement. The measured average velocities were approximately 2–2.5 cm/s at an average heating rate of 70 W, and were independent of tilt angle. Significant RMS fluctuations of 10–20% of the mean velocity were observed in the test section, in spite of the laminar or transitional Reynolds numbers (order of 700, based on the hydraulic diameter). These fluctuations have been attributed to vortex shedding from the upstream temperature probes and mitre bends, rather than to fully developed turbulence. Animations of the PIV data clearly show these large scale unsteady flow patterns. Multiple steady state flow patterns were not observed.

scholarly journals Bulk turbulent transport and structure in Rayleigh–Taylor, Richtmyer–Meshkov, and variable acceleration instabilities

2003 ◽  
Vol 21 (3) ◽  
pp. 305-310 ◽  
Author(s):  
ANTOINE LLOR
Keyword(s):  
Turbulent Mixing ◽  
Fluid Model ◽  
Turbulent Transport ◽  
Model Testing ◽  
Turbulence Structure ◽  
Mixing Zone ◽  
Self Similar ◽  
Two Fluid Model ◽  
Directed Energy ◽  
Two Fluid

Directed energy and turbulence structure are shown to be crucial in understanding the growth of self-similar Rayleigh–Taylor and incompressible Richtmyer–Meshkov turbulent mixing zones. Averaging over the mixing zone is used to analyze the response of a modifiedk–ε model and a turbulent two-fluid model. Three different transport regimes are then identified by considering self-similar variable acceleration RT flows (SSVARTs), which appear as promising reference flows for model testing.

scholarly journals Experimental study of the influence of the stabilizing properties of transitional layers on the turbulent mixing evolution

2003 ◽  
Vol 21 (3) ◽  
pp. 369-373 ◽  
Author(s):  
Yu.A. KUCHERENKO ◽  
S.I. BALABIN ◽  
R.I. ARDASHOVA ◽  
O.E. KOZELKOV ◽  
A.V. DULOV ◽  
...  
Keyword(s):  
Diffusion Layer ◽  
Turbulent Mixing ◽  
Molecular Diffusion ◽  
Mixing Layer ◽  
Mixing Zone ◽  
Transitional Layer ◽  
Diffusion Layer Thickness ◽  
X Ray ◽  
Rayleigh Taylor Instability ◽  
Miscible Liquids

Experiments conducted on the EKAP facility at the Russian Federal Nuclear Center–VNIITF concerning the stabilization of Rayleigh–Taylor instability-induced mixing in miscible liquids by the formation of a molecular diffusion (or transitional) layer between the liquids initially were described. The experiments had an Atwood number of 1/3. The acceleration was 3500 times that of Earth's gravity, and several values of diffusion layer thickness were considered. The experiments showed that the growth of the turbulent mixing zone could be delayed by adjusting the amplitude of the initial perturbations and the characteristic thickness of the diffusion layer. This has been observed in experiments conducted with water and mercury. The mixing layer evolution was imaged using X-ray radiography.

Mixing efficiency in controlled exchange flows

2008 ◽  
Vol 600 ◽  
pp. 235-244 ◽  
Author(s):  
TJIPTO PRASTOWO ◽  
ROSS W. GRIFFITHS ◽  
GRAHAM O. HUGHES ◽  
ANDREW McC. HOGG
Keyword(s):  
Turbulent Mixing ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Scaling Analysis ◽  
Stratified Turbulence ◽  
Exchange Flows ◽  
Maximum Value ◽  
Direct Measurements ◽  
Overall Efficiency ◽  
Good Agreement

Turbulence and mixing are generated by the shear between two counter-flowing layers in hydraulically controlled buoyancy-driven exchange flows through a constriction. From direct measurements of the density distribution and the amount of turbulent mixing in steady laboratory exchange flows we determine the overall efficiency of the mixing. For sufficiently large Reynolds numbers the mixing efficiency is 0.11(±0.01), independent of the aspect ratio and other details of constriction geometry, in good agreement with a scaling analysis. We conclude that the mixing in shear flows of this type has an overall efficiency significantly less than the maximum value widely proposed for stratified turbulence.

A model of constant-flow ventilation in a dog lung

1988 ◽  
Vol 64 (5) ◽  
pp. 2150-2159 ◽  
Author(s):  
E. Ingenito ◽  
R. D. Kamm ◽  
J. W. Watson ◽  
A. S. Slutsky
Keyword(s):  
Gas Exchange ◽  
Turbulent Mixing ◽  
Molecular Diffusion ◽  
Peripheral Zone ◽  
Semiempirical Model ◽  
Mixing Zone ◽  
Constant Flow ◽  
Specific Impedance ◽  
Cardiogenic Oscillations ◽  
Predicted Values

A semiempirical model of constant-flow ventilation (CFV) is developed to test the hypothesis that a three-zone serial model with the following characteristics can explain the adequate CO2 transport observed during CFV: 1) a zone of jet recirculation immediately downstream of the catheter in which convection dominates; 2) a zone influenced by turbulence but with little or no bulk flow; and 3) a peripheral zone, free of turbulence, in which transport is governed by molecular and augmented diffusion. Interactions between turbulent eddies and cardiogenic oscillations are included using a modification of Taylor dispersion theory according to the formulation of Kamm et al. Predicted values for arterial PCO2 are reasonably similar to experimental results for He-O2, air, and SF6-O2 mixtures for catheter flow rates from 0.2 to 1.6 l/s. Specific impedance to gas exchange was found to be largest immediately proximal to the end of turbulent mixing zone, where transport is governed by low-level eddy mixing and molecular diffusion. Simulations suggest that, during CFV, cardiogenic oscillations augment gas exchange primarily by promoting turbulent eddy dispersion in the distal airways and by extending the length of the turbulent mixing zone. Even small displacements of the catheter are shown to have a dramatic effect on gas exchange.

Measurement of Turbulent Mass Mixing Caused by a Particle Wake

2003 ◽  
Author(s):  
Toru Koso ◽  
Eiji Koyama ◽  
Takayuki Mikamoto
Keyword(s):  
Reynolds Number ◽  
Diffusion Coefficient ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Molecular Diffusion ◽  
Mixing Time ◽  
Measured Concentration ◽  
Turbulent Diffusion Coefficient ◽  
Number Range ◽  
Photochromic Dye

When a particle moves in a fluid, the fluid is disturbed by the particle and the fluid mass is mixed. This phenomenon is commonly observed in particle-laden flows and dispersed bubble flows. This mass mixing can be composed of two mechanisms. One is the mass transfer by convective flows that are induced by the reaction of the particle drag and the other is the turbulent mass mixing in the turbulent wake of the particle. The effect of the former one can be evaluated using the previous studies on the particle drag. However, the effect of the turbulent mixing is little understood. The turbulent mixing caused by a particle wake is investigated by visualization and noninvasive concentration measurement using a photochromic dye. A sphere brass particle of 5mm in diameter is dropped in kerosene filled in a vertical pipe and the mixing of dye is visualized. The photochromic dye, which is activated by an ultraviolet light, keeps its color in a few minutes after the activation. A part of the fluid is activated without disturbances and is subjected to the mixing by the particle wake. The visualized dye patterns indicate the dye is mixed isotropically by large-scale vortex motions when the particle sheds the vortices. Furthermore, the photochromic concentration measuring (PCM) technique is developed to obtain the concentration of the mixed dye. This PCM technique is based on the Lambert-Beer’s law for light adsorption and provides the average dye concentration along the light path. The measured concentration distribution shows rather isotropic mixing in longitudinal direction. The turbulent diffusion coefficient (TDC) is calculated from the temporal changes in the measured concentration distributions. The evaluated TDC shows strong time-dependency, which is attributed to the change in scale and strength of wake vortices. The TDC is about 104 times larger than the molecular diffusion coefficient at its maximum. The effect of particle Reynolds number on the turbulent mixing is also investigated for the Reynolds number range from 263 to 3290. The observed mixing patterns show a drastic change at the critical Reynolds number on the vortex shedding from the particle. The Reynolds number dependency on the non-dimensional TDC and mixing time are discussed.

Formation and propagation of waves during cavitation

2003 ◽  
Vol 3 ◽  
pp. 128-141
Author(s):  
K.R. Zakirov
Keyword(s):  
Experimental Data ◽  
Equation Of State ◽  
Compressible Fluid ◽  
Numerical Scheme ◽  
Similar Solution ◽  
Self Similar Solution ◽  
Self Similar ◽  
Laser Experiments ◽  
Propagation Of Waves ◽  
Good Agreement

The paper explores the collapse of a vapor bubble in a compressible fluid, accompanied by radiation from a divergent wave. The compressibility of a fluid is modeled using an isentropic equation of state of theta. The formation of a perturbation In the neighborhood of a bubble upon collapse, and then its propagation in surrounding the liquid upon re-expansion of the bubble. Divergent The wave is fairly accurately recorded in laser experiments breakdown of liquid, using sensors installed at some distance from the bubble. As for the formation of disturbances, in the literature there is a self-similar solution obtained for a liquid with equation Theta's state, which describes the final stage of the collapse. Based on this self-similar solution, a numerical scheme. After that, the numerical solution of the diverging wave is compared with available experimental data. Received good agreement with experiment.

Simulation of Unsteady Flow Around a Cylinder

2015 ◽  
Vol 3 (2) ◽  
pp. 28-49
Author(s):  
Ridha Alwan Ahmed
Keyword(s):  
Stokes Equations ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Mean Value ◽  
Navier Stokes ◽  
Cylinder Surface ◽  
Navier Stokes Equations ◽  
Flow Around A Cylinder ◽  
Numerical Grid ◽  
Good Agreement

       In this paper, the phenomena of vortex shedding from the circular cylinder surface has been studied at several Reynolds Numbers (40≤Re≤ 300).The 2D, unsteady, incompressible, Laminar flow, continuity and Navier Stokes equations have been solved numerically by using CFD Package FLUENT. In this package PISO algorithm is used in the pressure-velocity coupling.        The numerical grid is generated by using Gambit program. The velocity and pressure fields are obtained upstream and downstream of the cylinder at each time and it is also calculated the mean value of drag coefficient and value of lift coefficient .The results showed that the flow is strongly unsteady and unsymmetrical at Re>60. The results have been compared with the available experiments and a good agreement has been found between them

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