Thermal dispersion from a line source in the shearless turbulence mixing layer

1990 ◽  
Vol 216 ◽  
pp. 35-70 ◽  
Author(s):  
S. Veeravalli ◽  
Z. Warhaft
Keyword(s):  
Heat Flux ◽  
Line Source ◽  
Large Scale ◽  
Mixing Layer ◽  
Distinct Region ◽  
Grid Turbulence ◽  
Fine Scale ◽  
Temperature Variance ◽  
Turbulence Mixing ◽  
The Mean

We experimentally investigate dispersion from a heated line source placed in the central region of a turbulence mixing layer. Recently described by Veeravalli & Warhaft (1989) the mixing layer has no mean shear and consists of gradients in the velocity variance and scale; it is formed from a composite grid of constant solidity from which two distinct velocity scales are formed, one on either side of the stream. Mixing is effected by intermittent turbulent penetration and diffusion. The dispersion measurements were carried out in the convective regime where both plume flapping and fine-scale turbulent mixing play a role, the latter becoming more dominant as the plume evolves. The mean and variance temperature profiles are strongly skewed (with larger tails on the low turbulence side of the flow) in the earlier stages of the plume development. Here, in the convective range, the median and peak of the mean plume are deflected toward the large-scale region. As the flow evolves the profiles become more symmetrical but as the plume enters the turbulent diffusive stage there is evidence that the profiles again became asymmetric but now with longer tails in the high turbulence side of the flow (owing to the higher diffusivity). The temperature variance and heat flux budgets are highly asymmetric but tend to exhibit many of the characteristics of the budget of a line source in decaying homogeneous grid turbulence which is also presented here. However, a distinct region of negative production (counter-gradient heat flux) is found in the temperature variance budget and this is shown to be a consequence of the asymmetry of the transverse velocity probability density function in the mixing layer. Temperature spectra, both of the time series and of the intermittency function, across the plume are described. They are shown to peak at high wavenumbers in the centre and edge of the plume and at lower wavenumbers in the intermediate region. Their form is shown to change as the plume develops fine-scale structure and flapping becomes less important.

Large-scale clustering of coherent fine-scale eddies in a turbulent mixing layer

2018 ◽  
Vol 72 ◽  
pp. 100-108
Author(s):  
Toshitaka Itoh ◽  
Yoshitsugu Naka ◽  
Yuki Minamoto ◽  
Masayasu Shimura ◽  
Mamoru Tanahashi
Keyword(s):  
Turbulent Mixing ◽  
Large Scale ◽  
Mixing Layer ◽  
Fine Scale ◽  
Turbulent Mixing Layer

The shearless turbulence mixing layer

1989 ◽  
Vol 207 ◽  
pp. 191-229 ◽  
Author(s):  
S. Veeravalli ◽  
Z. Warhaft
Keyword(s):  
Transverse Velocity ◽  
Mixing Layer ◽  
Perforated Plate ◽  
Energy Budgets ◽  
Grid Turbulence ◽  
Self Similarity ◽  
Mean Velocity ◽  
Spacing Ratio ◽  
Turbulence Mixing ◽  
Mesh Spacing

The interaction of two energy-containing turbulence scales is studied in the absence of mean shear. The flow, a turbulence mixing layer, is formed in decaying grid turbulence in which there are two distinct scales, one on either side of the stream. This is achieved using a composite grid with a larger mesh spacing on one side of the grid than the other. The solidity of the grid, and thus the mean velocity, is kept constant across the entire flow. Since there is no mean shear there is no turbulence production and thus spreading is caused solely by the fluctuating pressure and velocity fields. Two different types of grids were used: a parallel bar grid and a perforated plate. The mesh spacing ratio was varied from 3.3:1 to 8.9:1 for the bar grid, producing a turbulence lengthscale ratio of 2.4:1 and 4.3:1 for two different experiments. For the perforated plate the mesh ratio was 3:1 producing a turbulence lengthscale ratio of 2.2:1. Cross-stream profiles of the velocity variance and spectra indicate that for the large lengthscale ratio (4.3:1) experiment, a single scale dominates the flow while for the smaller lengthscale ratio experiments, the energetics are controlled by both lengthscales on either side of the flow. In all cases the mixing layer is strongly intermittent and the transverse velocity fluctuations have large skewness. The downstream data of the second, third and fourth moments for all experiments collapse well using a single composite lengthscale. The component turbulent energy budgets show the importance of the triple moment transport and pressure terms within the layer and the dominance of advection and dissipation on the outer edge. It is also shown that the bar grids tend toward self-similarity with downstream distance. The perforated plate could not be measured to the same downstream extent and did not reach self-similarity within its measurement range. In other respects the two types of grids yielded qualitatively similar results. Finally, we emphasize the distinction between intermittent turbulent penetration and turbulent diffusion and show that both play an important role in the spreading of the mixing layer.

scholarly journals Effects of multiple-nozzle distribution on large-scale spray cooling via numerical investigation

2019 ◽  
Vol 23 (5 Part B) ◽  
pp. 3015-3024
Author(s):  
Qiang Xie ◽  
Zuobing Chen ◽  
Gong Chen ◽  
Yongjie Yu ◽  
Zheyu Zhao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Large Scale ◽  
Heat Transfer Performance ◽  
Spray Cooling ◽  
Transfer Performance ◽  
Enhance Heat Transfer ◽  
Cooling Performance ◽  
The Mean ◽  
Real Practice

Spray cooling has been widely employed in many applications due to its high flux removal ability. A previous study has been conducted to reveal the large-scale spray cooling performance of an industrial used single nozzle. Continuously, influence of multiple-nozzle distribution has also been numerically investigated in present work. The mean heat flux and its standard deviation and uniformity are used to qualify the cooling performance. A flat wall with 1.6 m in length and 1.0 m in width has been taken as the research object. Effects of nozzle number, distance and offset have been parametrically compared. It is found that increasing nozzle number could promote mean heat flux, improve the uniformity of cooling patterns and enhance heat transfer performance. A best nozzle number of 10 could be obtained by an equation fitting. Decreasing nozzle distance turns out to be detrimental to heat transfer. The reason comes from the collisions and interactions of two too adjacent nozzles. Based on choices in real practice, two types of arrays i. e. perpendicular and skew array have been discussed and compared. It is concluded that the skew array could obtain higher heat flux with more uniform distribution.

Transport equations for the normalized nth-order moments of velocity derivatives in grid turbulence

2021 ◽  
Vol 930 ◽  
Author(s):  
S.L. Tang ◽  
R.A. Antonia ◽  
L. Djenidi
Keyword(s):  
Large Scale ◽  
Transverse Velocity ◽  
Longitudinal Velocity ◽  
Transport Equations ◽  
Small Scale ◽  
Grid Turbulence ◽  
Mean Velocity ◽  
Local Isotropy ◽  
Advection Term ◽  
The Mean

Transport equations for the normalized moments of the longitudinal velocity derivative ${F_{n + 1}}$ (here, $n$ is $1, 2, 3\ldots$ ) are derived from the Navier–Stokes (N–S) equations for shearless grid turbulence. The effect of the (large-scale) streamwise advection of ${F_{n + 1}}$ by the mean velocity on the normalized moments of the velocity derivatives can be expressed as $C_1 {F_{n + 1}}/Re_\lambda$ , where $C_1$ is a constant and $Re_\lambda$ is the Taylor microscale Reynolds number. Transport equations for the normalized odd moments of the transverse velocity derivatives ${F_{y,n + 1}}$ (here, $n$ is 2, 4, 6), which should be zero if local isotropy is satisfied, are also derived and discussed in sheared and shearless grid turbulence. The effect of the (large-scale) streamwise advection term on the normalized moments of the velocity derivatives can also be expressed in the form $C_2 {F_{y,n + 1}}/Re_\lambda$ , where $C_2$ is a constant. Finally, the contribution of the mean shear in the transport equation for ${F_{n + 1}}$ can be modelled as $15 B/Re_\lambda$ , where $B$ ( $=S^*{S_{s,n + 1}}$ ) is the product of the non-dimensional shear parameter $S^*$ and the normalized mixed longitudinal-transverse velocity derivatives ${{S_{s,n + 1}}}$ ; if local isotropy is satisfied, $S_{s,n + 1}$ should be zero. These results indicate that if ${F_{n + 1}}$ , ${F_{y,n + 1}}$ and $B$ do not increase as rapidly as $Re_\lambda$ , then the effect of the large-scale structures on small-scale turbulence will disappear when $Re_\lambda$ becomes sufficiently large.

Organized structures in a reattaching separated flow field

1984 ◽  
Vol 143 ◽  
pp. 413-427 ◽  
Author(s):  
T. R. Troutt ◽  
B. Scheelke ◽  
T. R. Norman
Keyword(s):  
Vortex Dynamics ◽  
Large Scale ◽  
Mixing Layer ◽  
Separated Flow ◽  
Turbulence Energy ◽  
Hot Wire ◽  
Mean Flow ◽  
Large Scale Vortex ◽  
The Mean ◽  
Important Time

Spanwise structures in a two-dimensional reattaching separated flow were studied using multisensor hot-wire anemometry techniques. The results of these measurements strongly support the existence and importance of large-scale vortices in both the separated and reattached regions of this flow. Upstream of reattachment, vortex pairings are indicated and the spanwise structures attain correlation scales closely comparable to previously measured mixing-layer vortices. These large-scale vortices retain their organization far downstream of the reattachment region. However, pairing interactions appear to be strongly inhibited in this region. It is suggested that large-scale vortex dynamics are primarily responsible for some of the important time-averaged features of this flow. Notably, the reduction of turbulence energy in the reattachment region and the slow transition of the mean flow downstream of reattachment are attributed to effects associated with these vortices.

scholarly journals Thermodynamic Coupled Modes in the Tropical Atmosphere–Ocean: An Analytical Solution

2010 ◽  
Vol 67 (5) ◽  
pp. 1667-1677 ◽  
Author(s):  
Faming Wang
Keyword(s):  
Heat Flux ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Large Scale ◽  
Decadal Climate Variability ◽  
Coupled Modes ◽  
Meridional Winds ◽  
Wes Feedback ◽  
The Mean ◽  
Decadal Climate

Abstract The present study provides a consistent and unified solution for the two types of thermodynamical coupled modes in the atmosphere–ocean climate system: the tropical meridional mode and the subtropical dipole mode. The solution is derived analytically from a linear model that couples a simple atmosphere to a slab ocean via the wind–evaporation–SST (WES) feedback. For a mean zonal wind, the results show that the wind (hence latent heat flux) anomaly and the SST anomaly differ in phase such that the tropical mode propagates downwind and the subtropical modes propagate upwind, with both modes being damped by the SST-driven component of latent heat flux. Despite the existence of positive WES feedback, the large-scale subtropical modes are always stable, while the tropical mode can become unstable only when the air–sea coupling is strong and the mean wind is easterly. Furthermore, the mean meridional winds break the equatorial symmetry and enable the coupled modes to intensify in the Southern (Northern) Hemisphere for a southerly (northerly) component. For realistic parameter values, these thermodynamical coupled modes have periods and damping time scales in years; hence, they may play important roles in the tropical interannual-to-decadal climate variability.

Examination of large-scale structures in a turbulent plane mixing layer. Part 1. Proper orthogonal decomposition

1999 ◽  
Vol 391 ◽  
pp. 91-122 ◽  
Author(s):  
J. DELVILLE ◽  
L. UKEILEY ◽  
L. CORDIER ◽  
J. P. BONNET ◽  
M. GLAUSER
Keyword(s):  
Proper Orthogonal Decomposition ◽  
Large Scale ◽  
Mixing Layer ◽  
Orthogonal Decomposition ◽  
Plane Mixing Layer ◽  
Large Scale Structures ◽  
The Mean ◽  
Scale Structures ◽  
Turbulent Plane ◽  
Proper Orthogonal

Large-scale structures in a plane turbulent mixing layer are studied through the use of the proper orthogonal decomposition (POD). Extensive experimental measurements are obtained in a turbulent plane mixing layer by means of two cross-wire rakes aligned normal to the direction of the mean shear and perpendicular to the mean flow direction. The measurements are acquired well into the asymptotic region. From the measured velocities the two-point spectral tensor is calculated as a function of separation in the cross-stream direction and spanwise and streamwise wavenumbers. The continuity equation is then used for the calculation of the non-measured components of the tensor. The POD is applied using the cross-spectral tensor as its kernel. This decomposition yields an optimal basis set in the mean square sense. The energy contained in the POD modes converges rapidly with the first mode being dominant (49% of the turbulent kinetic energy). Examination of these modes shows that the first mode contains evidence of both known flow organizations in the mixing layer, i.e. quasi-two-dimensional spanwise structures and streamwise aligned vortices. Using the shot-noise theory the dominant mode of the POD is transformed back into physical space. This structure is also indicative of the known flow organizations.

scholarly journals Comparison of the Global Meridional Ekman Heat Flux Estimated from Four Wind Sources

2005 ◽  
Vol 35 (1) ◽  
pp. 94-108 ◽  
Author(s):  
Olga T. Sato ◽  
Paulo S. Polito
Keyword(s):  
Heat Flux ◽  
Indian Ocean ◽  
Large Scale ◽  
North Indian Ocean ◽  
Small Scale ◽  
Tropical Regions ◽  
The Pacific ◽  
Zonal Wavenumber ◽  
The Mean ◽  
Flux Component

Abstract The variability in the meridional Ekman heat flux estimated using wind data from four different sources is examined. The wind vectors are obtained from the European Remote Sensing (ERS), Quick Scatterometer (Quikscat), and Special Sensor Microwave Imager (SSM/I) satellite instruments and from the National Centers for Environmental Prediction (NCEP) model. The datasets range over a period of 10 years except for the Quikscat, which spans the period between 1999 and 2003. The comparison of the annual mean of the zonally integrated Ekman heat flux shows some discrepancies. In comparing the four sources, the differences increase from the tropical regions toward the equator. The annual mean of the meridional Ekman heat flux is consistently smaller when estimated with the ERS data. The correlation analysis shows that ERS and the other sources have a better agreement in the tropical regions, with correlations between 0.6 and 0.8, while in the extratropical regions the correlation is 0.4. The SSM/I, NCEP, and Quikscat winds lead to better correlations, between 0.7 and 1 in the extratropical regions. The western side of the north Indian Ocean is a site where all sources are very well correlated to each other. The variability in the Ekman heat flux is determined by changes in the temperature and wind stress fields. A combination of digital filters was used to quantify the role of several regions in the frequency–zonal wavenumber spectrum of the wind in establishing the observed Ekman heat flux. The Ekman flux component that is obtained from the product of the long-term mean wind and the temperature dominates in the low latitudes of the Atlantic Ocean. Its fractional covariance reaches 0.6 in the Atlantic, in the Pacific Ocean it is at most 0.3, and it is negligible in the Indian Ocean. The temporal variability of this heat flux component is only due to the temperature variability, because the mean winds were used. Other Ekman heat flux components are obtained from the product of the filtered wind anomalies and the temperature. These components include several bands of propagating signals (Rossby waves) and have fractional covariances that are larger in the Pacific and Indian Oceans, while in the Atlantic they can explain at most 20% of the total variance. All wind sources show a shift in the variability regime around 15° of latitude, with the mean and large-scale prevailing over meso- and small-scale variability within the Tropics and vice versa in the extratropical regions.

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