Experimental Investigation of Unstably Stratified Buoyant Wakes

2005 ◽  
Vol 128 (3) ◽  
pp. 488-493 ◽  
Author(s):  
Wayne N. Kraft ◽  
Malcolm J. Andrews
Keyword(s):  
Vertical Velocity ◽  
Cold Water ◽  
Water Channel ◽  
Mixing Layer ◽  
Splitter Plate ◽  
Wake Region ◽  
Return To Equilibrium ◽  
Velocity Fluctuations ◽  
Unstable Stratification ◽  
Plane Wake

A water channel has been used as a statistically steady experiment to investigate the development of a buoyant plane wake. Parallel streams of hot and cold water are initially separated by a splitter plate and are oriented to create an unstable stratification. At the end of the splitter plate, the two streams are allowed to mix and a buoyancy-driven mixing layer develops. The continuous, unstable stratification inside the developing mixing layer provides the necessary environment to study the buoyant wake. Downstream a cylinder was placed at the center of the mixing layer. As a result the dynamic flows of the plane wake and buoyancy-driven mixing layer interact. Particle image velocimetry and a high-resolution thermocouple system have been used to measure the response of the plane wake to buoyancy driven turbulence. Velocity and density measurements are used as a basis from which we describe the transition, and return to equilibrium, of the buoyancy-driven mixing layer. Visual observation of the wake does not show the usual vortex street associated with a cylinder wake, but the effect of the wake is apparent in the measured vertical velocity fluctuations. An expected peak in velocity fluctuations in the wake is found, however the decay of vertical velocity fluctuations occurs at a reduced rate due to vertical momentum transport into the wake region from buoyancy-driven turbulence. Therefore for wakes where buoyancy is driving the motion, a remarkably fast recovery of a buoyancy-driven Rayleigh-Taylor mixing in the wake region is found.

Experimental Investigation of Stratified, Buoyant Wakes

Volume 1 ◽  
2004 ◽  
Author(s):  
Wayne N. Kraft ◽  
Malcolm J. Andrews
Keyword(s):  
Cold Water ◽  
Water Channel ◽  
Mixing Layer ◽  
Splitter Plate ◽  
Return To Equilibrium ◽  
Plane Wake ◽  
Image Velocimetry ◽  
Parallel Streams ◽  
Turbulence Velocity ◽  
Dynamic Flows

The development of a buoyant plane wake has been investigated experimentally. A water channel has been used as a statistically steady experiment to investigate the plane wakes. Parallel streams of hot and cold water are initially separated by a splitter plate. The streams are oriented such that the cold fluid is above the hot fluid, resulting in an unstable stratification. At the end of the splitter plate, the two streams are allowed to mix and a buoyancy driven mixing layer develops. Downstream of the splitter plate, growth of the turbulent buoyancy-driven mix is disrupted by a cylinder. The cylinder is located at the centerline of the mixing layer and associated wake. As a result the dynamic flows of the plane wake and buoyancy driven mixing layer interact. Particle image velocimetry (PIV), and a high-resolution thermocouple system are used to measure the response of the plane wake to buoyancy driven turbulence. Velocity and density measurements are used as a basis from which we describe the transition, and return to equilibrium, of the buoyancy driven mixing layer. We found for wakes where buoyancy is driving the motion, a remarkably fast recovery of a Rayleigh-Taylor mix in the wake region.

scholarly journals Detailed measurements of a statistically steady Rayleigh–Taylor mixing layer from small to high Atwood numbers

2010 ◽  
Vol 659 ◽  
pp. 127-190 ◽  
Author(s):  
ARINDAM BANERJEE ◽  
WAYNE N. KRAFT ◽  
MALCOLM J. ANDREWS
Keyword(s):  
Vertical Velocity ◽  
Mass Flux ◽  
Mixing Layer ◽  
Splitter Plate ◽  
Hot Wire ◽  
Operating Characteristics ◽  
Velocity Fluctuations ◽  
Gas Channel ◽  
Wide Range ◽  
Turbulent Mass Flux

The self-similar evolution to turbulence of a multi-mode miscible Rayleigh–Taylor (RT) mixing layer has been investigated for Atwood numbers 0.03–0.6, using an air–helium gas channel experiment. Two co-flowing gas streams, one containing air (on top) and the other a helium–air mixture (at the bottom), initially flowed parallel to each other at the same velocity separated by a thin splitter plate. The streams met at the end of the splitter plate, with the downstream formation of a buoyancy unstable interface, and thereafter buoyancy-driven mixing. This buoyancy-driven mixing layer experiment permitted long data collection times, short transients and was statistically steady. Several significant designs and operating characteristics of the gas channel experiment are described that enabled the facility to be successfully run for At ~ 0.6. We report, and discuss, statistically converged measurements using digital image analysis and hot-wire anemometry. In particular, two hot-wire techniques were developed for measuring the various turbulence and mixing statistics in this air–helium RT experiment. Data collected and discussed include: mean density profiles, growth rate parameters, various turbulence and mixing statistics, and spectra of velocity, density and mass flux over a wide range of Atwood numbers (0.03 ≤ At ≤ 0.6). In particular, the measured data at the small Atwood number (0.03–0.04) were used to evaluate several turbulence-model constants. Measurements of the root mean square (r.m.s.) velocity and density fluctuations at the mixing layer centreline for the large At case showed a strong similarity to lower At behaviours when properly normalized. A novel conditional averaging technique provided new statistics for RT mixing layers by separating the bubble (light fluid) and spike (heavy fluid) dynamics. The conditional sampling highlighted differences in the vertical turbulent mass flux, and vertical velocity fluctuations, for the bubbles and spikes, which were not otherwise observable. Larger values of the vertical turbulent mass flux and vertical velocity fluctuations were found in the downward-falling spikes, consistent with larger growth rates and momentum of spikes compared with the bubbles.

Effects on Initial Development of Rayleigh-Taylor Instabilities Due to a Change in Initial Conditions

2007 ◽  
Author(s):  
Michael Peart ◽  
Robert Gore ◽  
Malcolm J. Andrews
Keyword(s):  
Growth Rate ◽  
Cold Water ◽  
Initial Conditions ◽  
Water Channel ◽  
Mixing Layer ◽  
Early Time ◽  
Hot Water ◽  
Splitter Plate ◽  
Initial Development ◽  
Parallel Streams

The effect on the initial development of Rayleigh-Taylor mixing due to a change in initial conditions has been experimentally studied. A water channel facility at Texas A&M University has been used to provide a statistically steady experiment for the investigation of buoyancy-driven turbulent mixing. Parallel streams of hot and cold water are separated initially by a splitter plate. The streams are oriented in such a way to place cold water above the hot water. Upon the termination of the splitter plate, the two streams are allowed to mix and a buoyancy-driven mixing layer develops. The growth rate of the mixing layer has been experimentally measured using image analysis techniques. Our studies have shown that introducing broadband initial disturbances can have a significant effect on the growth rate of Raleigh-Taylor instabilities, however, the mechanism that controls energy transfer at early time is not clear and requires further investigation.

On the cavity-actuated supersonic mixing layer downstream a thick splitter plate

2020 ◽  
Vol 32 (9) ◽  
pp. 096102 ◽  
Author(s):  
Jianguo Tan ◽  
Hao Li ◽  
Bernd R. Noack
Keyword(s):  
Mixing Layer ◽  
Splitter Plate ◽  
Supersonic Mixing ◽  
Supersonic Mixing Layer

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 Physical controls on orographic cirrus inhomogeneity

2007 ◽  
Vol 7 (14) ◽  
pp. 3771-3781 ◽  
Author(s):  
J. E. Kay ◽  
M. Baker ◽  
D. Hegg
Keyword(s):  
Vertical Velocity ◽  
Cloud Cover ◽  
Particle Growth ◽  
Physical Processes ◽  
Velocity Fluctuations ◽  
Ice Nuclei ◽  
Weather Model ◽  
Model Cloud ◽  
Homogeneous Freezing ◽  
The Relationship

Abstract. Optical depth distributions (P(σ)) are a useful measure of radiatively important cirrus (Ci) inhomogeneity. Yet, the relationship between P(σ) and underlying cloud physical processes remains unclear. In this study, we investigate the influence of homogeneous and heterogeneous freezing processes, ice particle growth and fallout, and mesoscale vertical velocity fluctuations on P(σ) shape during an orographic Ci event. We evaluate Lagrangian Ci evolution along kinematic trajectories from a mesoscale weather model (MM5) using an adiabatic parcel model with binned ice microphysics. Although the presence of ice nuclei increased model cloud cover, our results highlight the importance of homogeneous freezing and mesoscale vertical velocity variability in controlling Ci P(σ) shape along realistic upper tropospheric trajectories.

Dynamics of buoyancy-driven flows at moderately high Atwood numbers

2016 ◽  
Vol 795 ◽  
pp. 313-355 ◽  
Author(s):  
Bhanesh Akula ◽  
Devesh Ranjan
Keyword(s):  
Volume Fraction ◽  
Mixing Layer ◽  
Mie Scattering ◽  
Splitter Plate ◽  
Total Probability ◽  
Reynolds Stresses ◽  
Turbulence Statistics ◽  
Velocity Statistics ◽  
Anisotropy Tensor ◽  
Atwood Number

Simultaneous density and velocity turbulence statistics for Rayleigh–Taylor-driven flows at a moderately high Atwood number ($A_{t}$) of $0.73\pm 0.02$ are obtained using a new convective type or statistically steady gas tunnel facility. Air and air–helium mixture are used as working fluids to create a density difference in this facility, with a thin splitter plate separating the two streams flowing parallel to each other at the same velocity ($U=3~\text{m}~\text{s}^{-1}$). At the end of the splitter plate, the two miscible fluids are allowed to mix and the instability develops. Visualization and Mie-scattering techniques are used to obtain structure shape, volume fraction profile and mixing height growth information. Particle image velocimetry (PIV) and hot-wire techniques are used to measure planar and point-wise velocity statistics in the developing mixing layer. Asymmetry is evident in the flow field from the Mie-scattering images, with the spike side showing a more gradual decline in volume fraction than the bubble side. The spike side of the mixing layer grows 50 % faster than the bubble side. PIV is implemented for the first time in these moderately high-Atwood-number experiments ($A_{t}>0.1$) to obtain root-mean-square velocities, anisotropy tensor components and Reynolds stresses across the mixing layer. Overall, the turbulence statistics measured have shown different scaling compared to small-Atwood-number experiments. However, the total probability density functions for the velocities and turbulent mass fluxes exhibit behaviour similar to small-Atwood-number experiments. Conditional statistics reveal different values for turbulence statistics for spikes and bubbles, unlike small-Atwood-number experiments.

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