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.

A Turbulent Model for Gas-Particle Jets

2000 ◽  
Vol 122 (3) ◽  
pp. 505-509 ◽  
Author(s):  
J. Garcı´a ◽  
A. Crespo
Keyword(s):  
Mixing Layer ◽  
Stokes Number ◽  
Particle Dispersion ◽  
Lagrangian Model ◽  
Grid Turbulence ◽  
Mean Velocity ◽  
Dissipation Term ◽  
Particle Flows ◽  
Integral Scales ◽  
Kolmogorov Constant

This work is concerned with turbulent diffusion in gas-particle flows. The cases studied correspond to dilute flows and small Stokes number, this implies that the mean velocity of the particles is very similar to that of the fluid element. The classical k-ε method is used to model the gas-phase, modified with additional terms for the k and ε equations, that takes into account the effect of particles on the carrier phase. The additional dissipation term included in the equation for k is due to the slip between phases at an intermediate scale, far from both the Kolmogorov and the integral scales. This term has a proportionality constant equal to 3/2 of Kolmogorov constant, C0. In this paper, a value of 3.0 has been used for this constant as suggested by Du et al., 1995, “Estimation of the Kolmogorov Constant C0 for the Langarian Structure Using a Second-Order Lagrangian Model of Grid Turbulence,” Phys. Fluids 7, (12), pp. 3083–3090. The additional source term for the ε equation is taken as proportional to ε/k, as is usually done. In all experiments analyzed the particles increased the dissipation of turbulent kinetic energy. A comparison is made between the results obtained with the model proposed in this work and the experiments of Shuen et al., 1985, “Structure of Particle-Laden Jets: Measurements and Predictions,” AIAA Journal, 23, No. 3, and Hishida et al., 1992, “Experiments on Particle Dispersion in a Turbulent Mixing Layer,” ASME Journal of Fluids Engineering, 119, pp. 181–194. [S0098-2202(00)02103-9]

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.

Orthonormal wavelet decomposition of turbulent flows: intermittency and coherent structures

1997 ◽  
Vol 348 ◽  
pp. 177-199 ◽  
Author(s):  
R. CAMUSSI ◽  
G. GUJ
Keyword(s):  
Turbulent Flows ◽  
Coherent Structures ◽  
Velocity Component ◽  
Wavelet Transforms ◽  
Wavelet Decomposition ◽  
Transverse Velocity ◽  
Mixing Layer ◽  
Grid Turbulence ◽  
Jet Mixing ◽  
Vortex Tubes

Experimental data obtained in various turbulent flows are analysed by means of orthogonal wavelet transforms. Several configurations are analysed: homogeneous grid turbulence at low and very low Reλ, and fully developed jet turbulence at moderate and high Reλ. It is shown by means of the wavelet decomposition in combination with the form of scaling named extended self-similarity that some statistical properties of fully developed turbulence may be extended to low-Reλ flows. Indeed, universal properties related to intermittency are observed down to Reλ≃10. Furthermore, the use of a new conditional averaging technique of velocity signals, based on the wavelet transform, permits the identification of the time signatures of coherent structures which may or may not be responsible for intermittency depending on the scale of the structure itself. It is shown that in grid turbulence, intermittency at the smallest scales is related to structures with small characteristic size and with a shape that may be related to the passage of vortex tubes. In jet turbulence, the longitudinal velocity component reveals that intermittency may be induced by structures with a size of the order of the integral length. This effect is interpreted as the signature of the characteristic jet mixing layer structures. The structures identified on the transverse velocity component of the jet case turn out on the other hand not to be affected by the mixing layer and the corresponding shape is again correlated with the signature of vortex tubes.

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.

Influence of Inlet Velocity Condition on Unsteady Flow Characteristics in Piping With a Short-Elbow at High Reynolds Number Condition

2014 ◽  
Author(s):  
Ayako Ono ◽  
Masaaki Tanaka ◽  
Jun Kobayashi ◽  
Hideki Kamide
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Velocity Fluctuation ◽  
Complex Geometry ◽  
Perforated Plate ◽  
Secondary Flows ◽  
Curvature Radius ◽  
Mean Velocity ◽  
Inlet Velocity ◽  
Velocity Condition

In design of the Japan Sodium-cooled Fast Reactor (JSFR), mean velocity of the coolant is approximately 9 m/s in the primary hot leg (H/L) piping which diameter is 1.27 m. The Reynolds number in the H/L piping reaches 4.2×107. Moreover, a short-elbow which has Rc/D = 1.0 (Rc: Curvature radius, D: Pipe diameter) is used in the hot leg piping in order to achieve compact plant layout and reduce plant construction cost. In the H/L piping, flow-induced vibration (FIV) is concerned due to excitation force which is caused by pressure fluctuation on the wall closely related with the velocity fluctuation in the short-elbow. In the previous study, relation between the flow separation and the pressure fluctuations in the short-elbow was revealed under the specific inlet condition with flat distribution of time-averaged axial velocity and relatively weak velocity fluctuation intensity in the pipe. However, the inlet velocity condition of the H/L in a reactor may have ununiformed profile with highly turbulent due to the complex geometry in reactor vessel (R/V). In this study, the influence of the inlet velocity condition on unsteady characteristics of velocity in the short-elbow was studied. Although the flow around the inlet of the H/L in R/V could not simulate completely, inlet velocity conditions were controlled by installing the perforated plate with plugging the flow-holes appropriately. Then expected flow patterns were made at 2D upstream position from the elbow inlet in the experiments. It was revealed that the inlet velocity profiles affected circumferential secondary flow and the secondary flows affected an area of flow separation at the elbow, by local velocity measurement by the PIV (particle image velocimetry). And it was found that the low frequent turbulence in the upstream piping remained downstream of the elbow though their intensity was attenuated.

LES and Hybrid RANS/LES of a Fundamental Trailing Edge Slot

2014 ◽  
Author(s):  
Elizaveta Ivanova ◽  
Gregory M. Laskowski
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Numerical Study ◽  
Mixing Layer ◽  
Detached Eddy Simulation ◽  
Trailing Edge ◽  
Adiabatic Wall ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation

This paper presents the results of a numerical study on the predictive capabilities of Large Eddy Simulation (LES) and hybrid RANS/LES methods for heat transfer, mean velocity, and turbulence in a fundamental trailing edge slot. The geometry represents a landless slot (two-dimensional wall jet) with adjustable slot lip thickness. The reference experimental data taken from the publications of Kacker and Whitelaw [1] [2] [3] [4] contains the adiabatic wall effectiveness together with the velocity and the Reynolds-stress profiles for various blowing ratios and slot lip thicknesses. The simulations were conducted at three different lip thickness and several blowing ratio values. The comparison with the experimental data shows a general advantage of LES and hybrid RANS/LES methods against unsteady RANS. The predictive capability of the tested LES models (dynamic ksgs-equation [5] and WALE [6]) was comparable. The Improved Delayed Detached Eddy Simulation (IDDES) hybrid method [7] also shows satisfactory agreement with the experimental data. In addition to the described baseline investigations, the influence of the inlet turbulence boundary conditions and their implication for the initial mixing layer and heat transfer development were studied for both LES and IDDES.

scholarly journals Bidirectional cyclical flows increase energetic costs of station holding for a labriform swimming fish, Cymatogaster aggregata

2020 ◽  
Vol 8 (1) ◽  
Author(s):  
Sarah M Luongo ◽  
Andreas Ruth ◽  
Connor R Gervais ◽  
Keith E Korsmeyer ◽  
Jacob L Johansen ◽  
...  
Keyword(s):  
Steady Flow ◽  
Flow Direction ◽  
Energy Budgets ◽  
Constant Flow ◽  
Energetic Costs ◽  
Mean Velocity ◽  
Unidirectional Flow ◽  
In The Wild ◽  
Additional Costs ◽  
Temperate Fish

Abstract Wave-induced surge conditions are found in shallow marine ecosystems worldwide; yet, few studies have quantified how cyclical surges may affect free swimming animals. Here, we used a recently adapted respirometry technique to compare the energetic costs of a temperate fish species (Cymatogaster aggregata) swimming against a steady flow versus cyclical unidirectional and bidirectional surges in which unsteady swimming (such as accelerating, decelerating and turning) occurs. Using oxygen uptake (ṀO2) as an estimate of energetic costs, our results reveal that fish swimming in an unsteady (i.e. cyclical) unidirectional flow showed no clear increase in costs when compared to a steady flow of the same average speed, suggesting that costs and savings from cyclical acceleration and coasting are near equal. Conversely, swimming in a bidirectional cyclical flow incurred significantly higher energetic costs relative to a steady, constant flow, likely due to the added cost of turning around to face the changing flow direction. On average, we observed a 50% increase in ṀO2 of fish station holding within the bidirectional flow (227.8 mg O2 kg−1 h−1) compared to a steady, constant flow (136.1 mg O2 kg−1 h−1) of the same mean velocity. Given wave-driven surge zones are prime fish habitats in the wild, we suggest the additional costs fish incur by station holding in a bidirectional cyclical flow must be offset by favourable conditions for foraging and reproduction. With current and future increases in abiotic stressors associated with climate change, we highlight the importance of incorporating additional costs associated with swimming in cyclical water flow in the construction of energy budgets for species living in dynamic, coastal habitats.

scholarly journals Experimental Investigation of Nozzle Spacing Effects on Characteristics of Round Twin Free Jets

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Andrew Laban ◽  
Seyed Sobhan Aleyasin ◽  
Mark Francis Tachie ◽  
Mike Koupriyanov
Keyword(s):  
Symmetry Plane ◽  
Shear Layers ◽  
Significant Rise ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Potential Core ◽  
Turbulent Statistics ◽  
Spacing Ratio ◽  
Stress Ratios ◽  
Nozzle Spacing

The objective of this paper is to investigate the effects of nozzle spacing on the mean velocity and higher-order turbulent statistics of free twin round jets produced from sharp contraction nozzles. The experiments were performed in an air chamber where four nozzle spacing ratios, S/d = 2.8, 4.1, 5.5, and 7.1, were investigated at a fixed Reynolds number of 10,000. A planar particle image velocimetry (PIV) system was used to conduct the velocity measurements. The results show that downstream of the potential core, a reduction in spacing ratio leads to an earlier and more intense interaction between the jets, indicated by enhanced half-velocity width spread rate in the inner shear layers and a significant rise of turbulent intensities and vorticity thickness along the symmetry plane. A reduction in spacing ratio, however, confines the ambient fluid entrainment along the inner shear layers leading to a reduced core jet velocity decay rate. The closer proximity of the jets also leads to the decrease of Reynolds stresses in the inner shear layers but not in the outer shear layers. The Reynolds stress ratios along the jet centerline reveal the highest anisotropy in the potential core region.

Compressibility effects in a turbulent annular mixing layer. Part 1. Turbulence and growth rate

2000 ◽  
Vol 421 ◽  
pp. 229-267 ◽  
Author(s):  
JONATHAN B. FREUND ◽  
SANJIVA K. LELE ◽  
PARVIZ MOIN
Keyword(s):  
Growth Rate ◽  
Mach Number ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Velocity Difference ◽  
Mean Velocity ◽  
The Mean ◽  
Mach Numbers ◽  
High Mach ◽  
Compressibility Effects

This work uses direct numerical simulations of time evolving annular mixing layers, which correspond to the early development of round jets, to study compressibility effects on turbulence in free shear flows. Nine cases were considered with convective Mach numbers ranging from Mc = 0.1 to 1.8 and turbulence Mach numbers reaching as high as Mt = 0.8.Growth rates of the simulated mixing layers are suppressed with increasing Mach number as observed experimentally. Also in accord with experiments, the mean velocity difference across the layer is found to be inadequate for scaling most turbulence statistics. An alternative scaling based on the mean velocity difference across a typical large eddy, whose dimension is determined by two-point spatial correlations, is proposed and validated. Analysis of the budget of the streamwise component of Reynolds stress shows how the new scaling is linked to the observed growth rate suppression. Dilatational contributions to the budget of turbulent kinetic energy are found to increase rapidly with Mach number, but remain small even at Mc = 1.8 despite the fact that shocklets are found at high Mach numbers. Flow visualizations show that at low Mach numbers the mixing region is dominated by large azimuthally correlated rollers whereas at high Mach numbers the flow is dominated by small streamwise oriented structures. An acoustic timescale limitation for supersonically deforming eddies is found to be consistent with the observations and scalings and is offered as a possible explanation for the decrease in transverse lengthscale.

Experimental Investigation of Perforated Plate Turbulent Flow Past a Solid Sphere

2004 ◽  
Author(s):  
Himanshu Tyagi ◽  
Rui Liu ◽  
David S.-K. Ting ◽  
Clifton R. Johnston
Keyword(s):  
Perforated Plate ◽  
Solid Sphere ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Comprehensive Investigation ◽  
Mean Velocity ◽  
Kolmogorov Length Scale ◽  
Turbulence Effect ◽  
Plastic Sphere ◽  
The Mean

As the first stage of a comprehensive investigation of turbulence effect on the aerodynamics of sphere, the effect of freestream turbulence on the wake generated behind a sphere is investigated in this study. A 10 cm (4 in) diameter plastic sphere was placed in a wind tunnel of cross section 75 cm by 75 cm. The freestream turbulence was generated by fixing a perforated plate to the entrance of the test section. The wake was characterized using a normal constant-temperature hotwire at 30D, 40D and 50D (11.25d, 15d & 18.75d respectively) downstream of the sphere (where D = 3.75 cm is the diameter of the perforated plate hole, and d is the diameter of the sphere). The Reynolds number of the flow, based on the mean velocity and the diameter of the sphere, was 6.6 × 104. Based on the instantaneous velocity measurement, Kolmogorov length scale, integral length scale and relative turbulence intensity in the wake were deduced.

Sign in / Sign up

Export Citation Format

Share Document