The effects of unstable stratification and mean shear on the chemical reaction in grid turbulence

2000 ◽  
Vol 408 ◽  
pp. 39-52 ◽  
Author(s):  
KOUJI NAGATA ◽  
SATORU KOMORI
Keyword(s):  
Chemical Reaction ◽  
Turbulent Mixing ◽  
Mixing Layer ◽  
Stratified Flow ◽  
Laser Doppler Velocimeter ◽  
Grid Turbulence ◽  
Unstable Stratification ◽  
Sheared Flow ◽  
Small Scales ◽  
Neutrally Stratified

The effects of unstable thermal stratification and mean shear on chemical reaction and turbulent mixing were experimentally investigated in reacting and non-reacting liquid mixing-layer flows downstream of a turbulence-generating grid. Experiments were carried out under three conditions: unsheared neutrally stratified, unsheared unstably stratified and sheared neutrally stratified. Instantaneous velocity and concentration were simultaneously measured using the combination of a laser-Doppler velocimeter and a laser-induced fluorescence technique. The results show that the turbulent mixing is enhanced at both large and small scales by buoyancy under unstably stratified conditions and therefore the chemical reaction is strongly promoted. The mean shear acts to enhance the turbulent mixing mainly at large scales. However, the chemical reaction rate in the sheared flow is not as large as in the unstably stratified case with the same turbulence level, since the mixing at small scales in the sheared neutrally stratified flow is weaker than that in the unsheared unstably stratified flow. The unstable stratification is regarded as a better tool to attain unsheared mixing since the shearing stress acting on the fluid is much weaker in the unstably stratified flow than in the sheared flow.

scholarly journals Modulation of the velocity gradient tensor by concurrent large-scale velocity fluctuations in a turbulent mixing layer

2015 ◽  
Vol 777 ◽  
Author(s):  
O. R. H. Buxton
Keyword(s):  
Velocity Gradient ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Mixing Layer ◽  
Small Scale ◽  
Velocity Fluctuations ◽  
Turbulent Mixing Layer ◽  
Local Maxima ◽  
Leibler Divergence ◽  
Small Scales

The modulation of small-scale velocity and velocity gradient quantities by concurrent large-scale velocity fluctuations is observed by consideration of the Kullback–Leibler divergence. This is a measure that quantifies the loss of information in modelling a statistical distribution of small-scale quantities conditioned on concurrent positive large-scale fluctuations by that conditioned on negative large-scale fluctuations. It is observed that the small-scale turbulence is appreciably ‘rougher’ when the concurrent large-scale fluctuation is positive in the low-speed side of a fully developed turbulent mixing layer, which gives further evidence to the convective scale modulation argument of Buxton & Ganapathisubramani (Phys. Fluids, vol. 26, 2014, 125106, 1–19). The definition of the small scales is varied, and regardless of whether the small-scale fluctuations are dominated by dissipation or have the characteristic features of inertial range turbulence they are shown to be modulated by the concurrent large-scale fluctuations. The modulation is observed to persist even when there is a large gap in wavenumber space between the small and large scales, although local maxima are observed at intermediate length scales that are significantly larger than the predefined small scales. Finally, it is observed that the modulation of small-scale dissipation is greater than that for enstrophy with the modulation of the vortex stretching term, indicative of the interaction between strain rate and rotation, being intermediate between the two.

Wavelet analysis of shearless turbulent mixing layer

2021 ◽  
Vol 33 (2) ◽  
pp. 025109
Author(s):  
T. Matsushima ◽  
K. Nagata ◽  
T. Watanabe
Keyword(s):  
Wavelet Analysis ◽  
Turbulent Mixing ◽  
Mixing Layer ◽  
Turbulent Mixing Layer

Effect of Particle Residence Time on Particle Dispersion in a Plane Mixing Layer

1993 ◽  
Vol 115 (4) ◽  
pp. 751-759 ◽  
Author(s):  
Tsuneaki Ishima ◽  
Koichi Hishida ◽  
Masanobu Maeda
Keyword(s):  
Gas Phase ◽  
Residence Time ◽  
Time Scale ◽  
Characteristic Time ◽  
Mixing Layer ◽  
Particle Dispersion ◽  
Laser Doppler Velocimeter ◽  
Characteristic Time Scale ◽  
Two Phase ◽  
Particle Residence Time

A particle dispersion has been experimentally investigated in a two-dimensional mixing layer with a large relative velocity between particle and gas-phase in order to clarify the effect of particle residence time on particle dispersion. Spherical glass particles 42, 72, and 135 μm in diameter were loaded directly into the origin of the shear layer. Particle number density and the velocities of both particle and gas phase were measured by a laser Doppler velocimeter with modified signal processing for two-phase flow. The results confirmed that the characteristic time scale of the coherent eddy apparently became equivalent to a shorter characteristic time scale due to a less residence time. The particle dispersion coefficients were well correlated to the extended Stokes number defined as the ratio of the particle relaxation time to the substantial eddy characteristic time scale which was evaluated by taking account of the particle residence time.

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