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.

Experimental Investigation of Statistically Steady Rayleigh-Taylor Mixing at High Atwood Numbers

2005 ◽  
Author(s):  
Arindam Banerjee ◽  
Malcolm J. Andrews
Keyword(s):  
Mixing Layer ◽  
Splitter Plate ◽  
Hot Wire ◽  
Single Wire ◽  
Mixture Flow ◽  
Gas Channel ◽  
Gas Streams ◽  
Velocity Statistics ◽  
Self Similar ◽  
Atwood Number

In the present work, a novel gas channel experiment was used to study the non-equilibrium development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing a Helium-Air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of the splitter plate leading to the formation of an unstable interface and initiation of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, short transients and was statistically steady. The facility was capable of large Atwood number studies (At ∼ 0.75). Here, we describe recent work to measure the self similar evolution of mixing at large density differences (At ∼ 0.1). Diagnostics include a constant temperature Hot Wire anemometer, and high resolution thermocouple measurements. The Hot Wire probe gives velocity statistics of the mixing layer. A multi-position single-wire technique was used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the structure of mixing as it develops to a self-similar regime in this flow.

Large-scale structure and entrainment in the supersonic mixing layer

1995 ◽  
Vol 284 ◽  
pp. 171-216 ◽  
Author(s):  
N. T. Clemens ◽  
M. G. Mungal
Keyword(s):  
Mach Number ◽  
High Speed ◽  
Large Scale ◽  
Mixture Fraction ◽  
Mixing Layer ◽  
Mie Scattering ◽  
Planar Laser ◽  
The Cross ◽  
Convective Mach Number ◽  
Stream Direction

Experiments were conducted in a two-stream planar mixing layer at convective Mach numbers,Mc, of 0.28, 0.42, 0.50, 0.62 and 0.79. Planar laser Mie scattering (PLMS) from a condensed alcohol fog and planar laser-induced fluorescence (PLIF) of nitric oxide were used for flow visualization in the side, plan and end views. The PLIF signals were also used to characterize the turbulent mixture fraction fluctuations.Visualizations using PLMS indicate a transition in the turbulent structure from quasi-two-dimensionality at low convective Mach number, to more random three-dimensionality for$M_c\geqslant 0.62$. A transition is also observed in the core and braid regions of the spanwise rollers as the convective Mach number increases from 0.28 to 0.62. A change in the entrainment mechanism with increasing compressibility is also indicated by signal intensity profiles and perspective views of the PLMS and PLIF images. These show that atMc= 0.28 the instantaneous mixture fraction field typically exhibits a gradient in the streamwise direction, but is more uniform in the cross-stream direction. AtMc= 0.62 and 0.79, however, the mixture fraction field is more streamwise uniform and with a gradient in the cross-stream direction. This change in the composition of the structures is indicative of different entrainment motions at the different compressibility conditions. The statistical results are consistent with the qualitative observations and suggest that compressibility acts to reduce the magnitude of the mixture fraction fluctuations, particularly on the high-speed edge of the layer.

Experimental and computational investigations of flow dynamics in LPP combustor

2017 ◽  
Vol 121 (1240) ◽  
pp. 790-802 ◽  
Author(s):  
Y. W. YAN ◽  
Y. P. Liu ◽  
Y. C. Liu ◽  
J. H. Li
Keyword(s):  
Recirculation Zone ◽  
Swirling Flow ◽  
Mixing Layer ◽  
Experimental Tests ◽  
Flow Dynamics ◽  
Reynolds Stresses ◽  
Lean Combustion ◽  
Low Emission ◽  
Image Velocimetry ◽  
Combustion Technology

ABSTRACTA Lean Premixed Prevaporised (LPP) low-emission combustor with a staged lean combustion technology was developed. In order to study cold-flow dynamics in the LPP combustor, both experimental tests using the particle image velocimetry (PIV) to quantify the flow dynamics and numerical simulation using the commercial software (FLUENT) were conducted, respectively. Numerical results were in good agreement with the experimental data. It is shown from the observation of the results that: there is a Primary Recirculation Zone (PRZ), a Corner Recirculation Zone (CRZ) and a Lip Recirculation Zone (LRZ) in the LPP combustor, and the exchanges of mass, momentum and energy between pilot swirling flow and primary swirling flow are contributed by the velocity gradients, and the shear flow is transformed into a mixing layer exhibiting the higher Reynolds stresses, which suggests the mixing process is strictly affected by the Reynolds stresses.

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

scholarly journals Application of SLIPI-Based Techniques for Droplet Size, Concentration, and Liquid Volume Fraction Mapping in Sprays

2020 ◽  
Vol 10 (4) ◽  
pp. 1369 ◽  
Author(s):  
Yogeshwar Nath Mishra ◽  
Timo Tscharntke ◽  
Elias Kristensson ◽  
Edouard Berrocal
Keyword(s):  
Volume Fraction ◽  
Three Dimensional ◽  
Light Sheet ◽  
Calibration Procedure ◽  
Mie Scattering ◽  
Complete Characterization ◽  
Liquid Volume ◽  
Planar Imaging ◽  
Liquid Volume Fraction ◽  
Scanning Procedure

Structured laser illumination planar imaging (SLIPI)-based techniques have been employed during the past decade for addressing multiple light scattering issues in spray imaging. In this article, SLIPI droplet sizing based on the intensity ratio of laser-induced fluorescence (LIF) over Mie scattering (SLIPI-LIF/Mie) and SLIPI-Scan for extinction-coefficient (µe) mapping are applied simultaneously. In addition, phase Doppler anemometry (PDA) and numerical calculations based on the Lorenz–Mie theory are also employed in order to extract the droplets Sauter mean diameter (SMD), the droplets number density (N), and the liquid volume fraction (LVF) in a steady asymmetric hollow cone water spray. The SLIPI-LIF/Mie ratio is converted to droplets SMD by means of a calibration procedure based on PDA measurements. The droplet SMD for the investigated spray varies from 20 µm to 60 µm, the N values range from 5 to 60 droplets per mm3, and the LVF varies between 0.05 × 10−4 and 5.5 × 10−4 within the probed region of the spray. To generate a series of two-dimensional images at different planes, the spray scanning procedure is operated in a “bread slicing” manner by moving the spray perpendicularly to the light sheet axis. From the resulting series of images, the procedure described here shows the possibility of obtaining three-dimensional reconstructions of each scalar quantity, allowing a more complete characterization of droplet clouds forming the spray region.

A ‘turbulent spot’ in an axisymmetric free shear layer. Part 1

1980 ◽  
Vol 98 (1) ◽  
pp. 65-95 ◽  
Author(s):  
M. Sokolov ◽  
A. K. M. F. Hussain ◽  
S. J. Kleis ◽  
Z. D. Husain
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Large Scale ◽  
Mixing Layer ◽  
Streamwise Velocity ◽  
Reynolds Stresses ◽  
Free Shear Layer ◽  
Turbulent Spot ◽  
Air Jet ◽  
Phase Average

A three-dimensional ‘turbulent spot’ has been induced in the axisymmetric free mixing layer of a 12.7 cm diameter air jet by a spark generated at the nozzle boundary layer upstream of the exit. The spot coherent-structure signature, buried in the large-amplitude random fluctuating signal, has been educed at three downstream stations within the apparent self-preserving region of the mixing layer (i.e. x/D = 1.5, 3.0 and 4.5) at the jet exit speed of 20 ms−1. The eduction has been performed through digital phase averaging of the spot signature from 200 realizations. In order to reduce the effect of the turbulence-induced jitter on the phase average, individual filtered signal arrays were optimally time-aligned through an iterative process of cross-correlation of each realization with the ensemble average. Further signal enhancement was achieved through rejection of realizations requiring excessive time shifts for alignment. The number of iterations required and the fraction of realizations rejected progressively increase with the downstream distance and the radial position.The mixing-layer spot is a large-scale elongated structure spanning the entire width of the layer but does not appear to exhibit a self-similar shape. The dynamics of the mixing-layer spot and its eduction are more complicated than those of the boundary-layer spot. The spot initially moves downstream essentially at a uniform speed across the mixing layer, but further downstream it accelerates on the high-speed side and decelerates on the low-speed side. This paper discusses the data acquisition and processing techniques and the results based on the streamwise velocity signals. Phase average distributions of vorticity, pseudo-streamlines, coherent and background Reynolds stresses and further dynamics of the spot are presented in part 2 (Hussain, Kleis & Sokolov 1980).

Experimental and Numerical Investigation of a Cold Turbulent Jet Impinging on a Hot Plate

2012 ◽  
Author(s):  
D. I. Maldonado ◽  
J. K. Abrantes ◽  
L. F. A. Azevedo ◽  
A. O. Nieckele
Keyword(s):  
Flow Field ◽  
Laser Doppler Anemometry ◽  
Turbulence Models ◽  
Impinging Jets ◽  
Reynolds Stresses ◽  
Hot Plate ◽  
Mean Velocity ◽  
Velocity Statistics ◽  
Reliable Assessment ◽  
Anisotropic Structures

Impinging jets are an efficient mechanism to enhance wall heat transfer, and are widely used in engineering applications. The flow field of an impinging jet is quite complex and it is a challenging case for turbulence models validation as well as measurements techniques. In the present work, a detailed investigation of a cold jet impinging on a hot plate operating in the turbulent flow regime was conducted. The flow field was characterized by both Laser Doppler Anemometry and Particle Image Velocimetry (PIV) techniques in order to collect 1st and 2nd order velocity statistics to allow a reliable assessment of the numerical simulations. Comparison was performed with two turbulence methodologies: RANS (κ–ω SST model) and LES (Dynamic Smagorinsky model). The comparison was performed to assess LES feasibility and accuracy in capturing the anisotropic structures that several tested RANS models missed. The mean velocity, instantaneous velocity, Reynolds stresses and Nusselt profiles obtained numerically are compared with experimental data. A physical insight about the general flow dynamics was obtained with the extensive amount of information available from the LES.

Density and Growth Rate Measurements in a High Atwood Number Rayleigh-Taylor Mixing

2005 ◽  
Author(s):  
Arindam Banerjee ◽  
Malcolm J. Andrews
Keyword(s):  
Growth Rate ◽  
Density Profile ◽  
Measurement Errors ◽  
Statistical Convergence ◽  
Growth Parameter ◽  
Direct Consequence ◽  
Splitter Plate ◽  
Gas Channel ◽  
Set Up ◽  
Atwood Number

A novel gas channel experiment is used to study the non-equilibrium development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air-helium mixture and the other air, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and initiation of buoyancy driven mixing. This set up is statistically steady and allows for long data collection times. Here, we describe initial measurements to determine the density profile and growth rate along the mix at low density differences (At ~ 0.05). The facility is however designed capable of large Atwood number studies (At ~ 0.75). Diagnostics include high resolution digital image analysis, which is used to determine the density profile across the mix. The growth parameter (α) is also estimated by a “moving window” calculation. The results are then verified with measurements of α made by a Constant temperature (CT) hot-wire probe and with the growth parameter obtained from small Atwood number experiments (At ~ 0.001). However, there were some inherent errors in the density profile measurements because of non-uniformity in the concentration of smoke. To verify that these errors were indeed measurement errors and not as a result of lack of statistical convergence, a detailed statistical convergence test was performed. It showed that convergence was a direct consequence of the number of different large 3D structures that were averaged over the duration of the run.

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