CiteULike: l-alex's library [1092 articles]

An experimental test of Corrsin's conjecture and some related ideas

Søren Ott — 2005-06-14T20:00:34-00:00

New Journal of Physics, Vol. 7, No. 1. (June 2005), 142.

Budgets of Reynolds stress, kinetic energy and streamwise enstrophy in viscoelastic turbulent channel flow

Costas Dimitropoulos — 2007-07-19T20:23:29-00:00

Physics of Fluids, Vol. 13, No. 4. (2001), pp. 1016-1027.

The budgets of the Reynolds stress, turbulent kinetic energy and streamwise enstrophy are evaluated through direct numerical simulations for the turbulent channel flow of a viscoelastic polymer solution modeled with the Finitely Extensible Nonlinear Elastic with the Peterlin approximation (FENE-P) constitutive equation. The influence of viscoelasticity on the budgets is examined through a comparison of the Newtonian and the viscoelastic budgets obtained for the same constant pressure drop across the channel. It is observed that as the extensional viscosity of the polymer solution increases there is a consistent decrease in the production of Reynolds stress in all components, as well as in the other terms in the budgets. In particular, the effect of the flow elasticity, which is associated with the reduction in the intensity of the velocity-pressure gradient correlations, potentially leads to a redistribution of the turbulent kinetic energy among the streamwise, the wall-normal and the spanwise directions. In this work, we also show that in the presence of viscoelasticity there is a significant reduction in all components of the production of streamwise enstrophy. This is consistent with a proposed mechanism for polymer-induced drag reduction through the inhibition of vortex stretching by the high extensional viscosity of the polymer solution. ©2001 American Institute of Physics.

Imaging with terahertz radiation

Wai Chan — 2007-07-12T20:47:57-00:00

Reports on Progress in Physics, Vol. 70, No. 8. (2007), pp. 1325-1379.

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

Advances in applications of the physics of fluids to severe weather systems

Howard Bluestein — 2007-07-06T18:32:11-00:00

Reports on Progress in Physics, Vol. 70, No. 8. (2007), pp. 1259-1323.

This article is a review of the basic dynamics of severe convective storms. The dynamics are in most instances described with the aid of the vorticity and divergence equations subject to the Boussinesq approximation. The behaviour of ordinary-cell convective storms in the presence of different amounts of vertical shear and its relation to the surface cold pool are explained. The role of vertical shear in the propagation of, and rotation in, supercells is described in detail. How cold pool production, buoyancy, and environmental vertical shear control the structure of mesoscale convective systems is discussed and the mechanism for the production of vortices in them is described. The wind field in tornadoes based on laboratory experiments, simulations, and observations is explained from the dynamics of vortices that interact with an underlying surface. Various theories for tornado formation are presented and evaluated. Avenues for future research using both numerical simulations and new and emerging observing systems are highlighted.

Note on Decay of Homogeneous Turbulence

PG Saffman — 2007-06-17T12:14:59-00:00

Physics of Fluids, Vol. 10, No. 6. (1967), pp. 1349-1349.

The assumption of self-similarity and the existence of an exact invariant are combined to predict the decay rate of homogeneous turbulence. ©1967 The American Institute of Physics

Reynolds number dependence of Lagrangian statistics in large numerical simulations of isotropic turbulence

PK Yeung — 2007-06-16T16:54:43-00:00

J. of Turbulence, Vol. 7 (2006)

Lagrangian statistics are reported from a direct numerical simulation database with grid resolution up to 20483 and Taylor-scale Reynolds number approximately 650. The approach to Lagrangian Kolmogorov similarity at high Reynolds number is studied using both the velocity structure function and frequency spectrum. A significant scaling range is observed for the latter which is consistent with recent estimates of 6-7 for the scaling constant C0. In contrast to some previous results at low Reynolds number, the current results suggest that at high Reynolds number the dissipation autocorrelation is a two-scale process influenced by both the Lagrangian velocity integral time scale and Kolmogorov time scale. Results on the logarithm of the pseudo-dissipation are in support of its modeling as a diffusion process with one-time Gaussian statistics. As the Reynolds number increases, the statistics of dissipation and enstrophy become more similar while their logarithms have significantly longer time scales.

Self-similar two-particle separation model

Beat Luethi — 2007-04-27T22:45:49-00:00

Physics of Fluids, Vol. 19, No. 4. (2007)

We present a new stochastic model for relative two-particle separation in turbulence. Inspired by material line stretching, we suggest that a similar process also occurs beyond the viscous range, with time scaling according to the longitudinal second-order structure function S2(r), e.g.; in the inertial range as −1/3r2/3. Particle separation is modeled as a Gaussian process without invoking information of Eulerian acceleration statistics or of precise shapes of Eulerian velocity distribution functions. The time scale is a function of S2(r) and thus of the Lagrangian evolving separation. The model predictions agree with numerical and experimental results for various initial particle separations. We present model results for fixed time and fixed scale statistics. We find that for the Richardson-Obukhov law, i.e., r(t)2=gt3, to hold and to also be observed in experiments, high Reynolds numbers are necessary, i.e., Re>(1000), and the integral scale needs to be large compared to initial separation, i.e., /r0>30 and d/>3 need to be fulfilled, where d is the size of the field of view. Removing the constraint of finite inertial range, the model is used to explore separation dynamics in the asymptotic regime. As Re, the distance neighbor function takes on a constant shape, almost as predicted by the Richardson diffusion equation. For the Richardson constant we obtain that g0.95 as Re. This asymptotic limit is reached at Re>1000. For the Richardson constant g, the model predicts a ratio of gb/gf1.9 between backwards and forwards dispersion. ©2007 American Institute of Physics

Heavy Particle Concentration in Turbulence at Dissipative and Inertial Scales

J Bec — 2007-03-10T19:28:51-00:00

Physical Review Letters, Vol. 98, No. 8. (2007)

Spatial distributions of heavy particles suspended in an incompressible isotropic and homogeneous turbulent flow are investigated by means of high resolution direct numerical simulations. In the dissipative range, it is shown that particles form fractal clusters with properties independent of the Reynolds number. Clustering is there optimal when the particle response time is of the order of the Kolmogorov time scale . In the inertial range, the particle distribution is no longer scale invariant. It is, however, shown that deviations from uniformity depend on a rescaled contraction rate, which is different from the local Stokes number given by dimensional analysis. Particle distribution is characterized by voids spanning all scales of the turbulent flow; their signature in the coarse-grained mass probability distribution is an algebraic behavior at small densities.

Generalized detection of a turbulent front generated by an oscillating grid

Markus Holzner — 2006-10-22T17:42:29-00:00

Experiments in Fluids

An informal introduction to turbulence

A Tsinober — 2006-10-09T15:03:22-00:00

(2001)

On turbulent energy production in wall bounded flows

R Gurka — 2006-07-28T22:59:59-00:00

Physics of Fluids, Vol. 16, No. 7. (2004), pp. 2704-2707.

The main point of this Brief Communication is that the turbulent energy production is due to the compressing of material elements rather than stretching. This is understood in the sense that the positiveness of the turbulent energy production is due to the contribution of the term associated with the compressive (negative) eigenvalue/eigenvector of the mean strain. ©2004 American Institute of Physics.

Eulerian and Lagrangian time microscales in isotropic turbulence

H Tennekes — 2006-06-08T12:50:27-00:00

@JFM, Vol. 67, No. 3. (1975), pp. 561-567.

Lagrangian spectrum up to $(ε/ν)^1/2$, Eulerian extends to higher frequencies than its Lagrangian counterpart. As a consequence, $∂ u/∂ t$ is overestimated by a factor of $Re_λ^1/2$. The Eulerian spectrum has $ω^-5/3$ behaviour, which is not governed by Kolmogorov similarity.

XPIV -- Multi-Plane Stereoscopic Particle Image Velocimetry

A Liberzon — 2006-04-20T15:39:53-00:00

Exp. Fluids, Vol. 36, No. 2. (2004), pp. 355-362.

Origin of Non-Gaussian Statistics in hydrodynamics turbulence

Yi Li — 2005-09-05T16:01:04-00:00

Physical Review Letters, Vol. 95, No. 164502. (2005)

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s 1S0 to the excited state 4s 1S0 in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.

The dispersion of discrete particles in a turbulent fluid field

Richard Calabrese — 2007-08-17T08:13:28-00:00

AIChE Journal, Vol. 25, No. 6. (1979)

Performance and accuracy investigations of two Doppler global velocimetry systems applied in parallel

Christian Willert — 2007-07-12T05:26:47-00:00

Measurement Science and Technology, Vol. 18, No. 8. (2007), pp. 2504-2512.

Two Doppler global velocimetry systems were applied in parallel to assess their performance in wind tunnel environments. Both DGV systems were mounted on a common traverse surrounding the glass-walled 1.4 $×$ 1.8 m$^2$ test section of the wind tunnel. The traverse normally supports a three-component forward-scatter laser Doppler velocimetry system. The reproducible tip-vortex flow field generated by the blunt tip of an airfoil was chosen for this investigation and was precisely surveyed by LDA just prior to the DGV measurements. Both DGV systems shared the same continuous wave laser light source, laser frequency monitoring and fibre optic light sheet delivery system. The principal differences between the DGV implementations are with regard to the imaging configuration. One configuration relied on a single camera view that observed three successively operated light sheets. In the second configuration, three camera views simultaneously observed a single light sheet using a four-branch fibre imaging bundle. The imaging bundle system had all three viewpoints in a forward scattering arrangement which increased the scattering efficiency but reduced the frequency shift sensitivity. Since all three light sheet observation components were acquired onto the same image frame, acquisition times could be reduced to a minimum. On the other hand, the triple light sheet–single camera system observed two light sheets in forward scatter and one light sheet in backscatter. Although three separate images had to be recorded in succession, the image quality, spatial resolution and signal-to-noise ratio were superior to the imaging bundle system. Comparison of the DGV data with LDV measurements shows very good agreement to within 1–2 m s$^−1$. The remaining discrepancy has a variety of causes, some are related to the reduced resolving power of the fibre imaging bundle system (graininess, smoothing), exact localization of the receiver head with respect to the scene, laser frequency drift or background influences. The extensive data base available allows a detailed study of the influence of each of these factors.

Numerical simulation of three-dimensional flow in a cavity

Christopher Freitas — 2007-03-29T21:58:47-00:00

International Journal for Numerical Methods in Fluids, Vol. 5, No. 6. (1985), pp. 561-575.

Previous three-dimensional simulations of the lid-driven cavity flow have reproduced only the most general features of the flow. Improvements to a finite difference code, REBUFFS, have made possible the first completely successful simulation of the three-dimensional lid-driven cavity flow. The principal improvement to the code was the incorporation of a modified QUICK scheme, a higher-order upwind finite difference formulation. Results for a cavity flow at a Reynolds number of 3200 have reproduced experimentally observed Taylor-Görtler-like vortices and other three-dimensional effects heretofore not simulated. Experimental results obtained from a unique experimental cavity facility validate the calculated results.

Free-stream boundaries of turbulent flows

S Corrsin — 2006-09-11T23:10:52-00:00

NACA, Vol. 1244 (1954)

Noise- and inertia-induced inhomogeneity in the distribution of small particles in fluid flows

Julyan Cartwright — 2006-08-09T21:02:11-00:00

Chaos: An Interdisciplinary Journal of Nonlinear Science, Vol. 12, No. 2. (2002), pp. 489-495.

The dynamics of small spherical neutrally buoyant particulate impurities immersed in a two-dimensional fluid flow are known to lead to particle accumulation in the regions of the flow in which vorticity dominates over strain, provided that the Stokes number of the particles is sufficiently small. If the flow is viewed as a Hamiltonian dynamical system, it can be seen that the accumulations occur in the nonchaotic parts of the phase space: the Kolmogorov–Arnold–Moser tori. This has suggested a generalization of these dynamics to Hamiltonian maps, dubbed a bailout embedding. In this paper we use a bailout embedding of the standard map to mimic the dynamics of neutrally buoyant impurities subject not only to drag but also to fluctuating forces modeled as white noise. We find that the generation of inhomogeneities associated with the separation of particle from fluid trajectories is enhanced by the presence of noise, so that they appear in much broader ranges of the Stokes number than those allowing spontaneous separation. ©2002 American Institute of Physics.

Dispersion of passive tracers in the direct enstrophy cascade: Experimental observations

Marie Jullien — 2006-07-13T19:14:32-00:00

Physics of Fluids, Vol. 15, No. 8. (2003), pp. 2228-2237.

The paper reports a statistical analysis of the separation of pairs in the enstrophy cascade, in a two-dimensional flow. The flow is produced experimentally, using electromagnetic forcing. Two regimes of separation are found. At early times (i.e., within two integral times) the separation process is exponential, following Lin's law [J. T. Lin, J. Atmos. Sci. 29, 394 (1972)]. In this range of time, the probability density functions (PDFs) of separations are self-similar in time, developing stretched exponential tails, and one may define a Lagrangian correlation time. Above two integral times, an algebraic regime takes place, with exponential tails for the PDFs of the separations, and self-similar Lagrangian correlation functions. The present work thus confirms the existence of two regimes, by analyzing a number of statistical quantities including the Lagrangian PDF and the temporal Lagrangian correlation functions which so far, were left undetermined for the Batchelor regime. ©2003 American Institute of Physics.

Transport effects associated with turbulence with particular attention to the influence of helicity

HK Moffatt — 2006-06-15T07:45:13-00:00

Reports on Progress in Physics, Vol. 46, No. 5. (1983), pp. 621-664.

The action of turbulence on a passive convected scalar field (e.g. temperature) or vector field (e.g. the magnetic field in an electrically conducting fluid) is reviewed, with particular attention paid to anomalous effects that can arise through the influence of Coriolis forces in a rotating system on the statistics of the turbulence. The simplest such effect (which corresponds to a breaking of the Onsager symmetry relations) is a 'skew-diffusion' effect, i.e. the appearance of a component of turbulent heat flux perpendicular to the local mean temperature gradient. The famous alpha effect of magnetohydrodynamic dynamo theory is also in this category, as is the more subtle Radler effect (the appearance of a mean electromotive force perpendicular to the mean current in a plasma). These effects are all associated with the helicity of a turbulent flow, i.e. the correlation between the velocity field u(x,t) and the vorticity field omega (x,t)=curl u.

Introduction: mixing in microfluidics

Julio Ottino — 2006-06-15T07:43:26-00:00

Phil. Trans. R. Soc. Lond. A, Vol. 362, No. 1818. (2004), pp. 923-935.

Light-induced shape-memory polymers

Andreas Lendlein — 2005-04-14T03:55:33-00:00

Nature, Vol. 434, No. 7035. (14 April 2005), pp. 879-882.

Wall-layer structure and drag reduction

WG Tiederman — 2007-07-25T09:41:00-00:00

J. Fluid Mech, Vol. 156 (1985), pp. 419-437.

When drag-reducing additives are confined entirely to the linear sublayer of a turbulent channel flow of water, both the spanwise spacing and bursting rate of the wall-layer structure are the same as those for a water flow and there is no evidence of drag reduction. Drag reduction is measured downstream of the location where the additives injected into the sublayer begin to mix in significant quantities with the buffer region (10 < y+ < 100) 2 of the channel flow. At streamwise locations where drag reduction does occur and where the injected fluid is not yet uniformly mixed with the channel flow, the dimensionless spanwise streak spacing increases and the average bursting rate decreases. The decrease in bursting rate is larger than the corresponding increase in streak spacing. The wall-layer structure is like the structure in the flow of a homogeneous, uniformly mixed, drag-reducing solution. Thus, the additives have a direct effect on the flow processes in the buffer region and the linear sublayer appears to have a passive role in the interaction of the inner and outer portions of a turbulent wall layer.

On the mechanism of turbulent drag reduction in dilute polymer solutions: dynamics of vortex filaments

AL Yarin — 2007-07-25T09:38:53-00:00

Journal of Non-Newtonian Fluid Mechanics, Vol. 69, No. 2-3. (April 1997), pp. 137-153.

The localized-induction approximation (LIA) is generalized in the framework of the quasi-one-dimensional approach to include the effect of polymer additives on stretching of thin vortex filaments. It is shown that in dilute polymer solutions vorticity diffusion is negligible for vortices with longwave perturbations, and their strength remains constant in spite of the fact that high longitudinal stress and elongational viscosity may be generated in the course of vortex stretching in shear flow. General governing equations for vortex filaments in a background flow of dilute polymer solution are proposed. Their numerical solution is obtained. It shows that interaction between a spanwise perturbed vortex filament and a background shear flow leads to strong vortex stretching and to emergence of configurations similar to horseshoe or hairpin vortices. In dilute polymer solutions, strong stretching of vortex filaments yields high longitudinal stress (high elongational viscosity). The latter retards or completely arrests vortex stretching, which is similar to the findings of [1] that frequency of turbulence generation in `bursts' decreases in dilute polymer solutions. Therefore, a more gentle mean velocity profile is expected in the boundary layer, which results in drag reduction.

Velocity measurements made with laser dopplermeter on the turbulent pipe flow of a dilute polymer solution

MJ Rudd — 2007-07-25T09:29:07-00:00

J. Fluid Mech, Vol. 51, No. 4. (1972), pp. 673-685.

This paper presents some new measurements which have been made on a drag-reducing polymer solution in pipe flow. A novel type of laser dopplermeter, which has been developed by the author, is briefly described and the measurements which have been obtained are given. These results and their implications are then discussed in terms of conventional models for turbulent flow in a pipe. These suggest that the polymer has very little effect upon the turbulent core of the flow, but thickens and stabilizes the viscous sublayer. The turbulent intensity inside the sublayer is unchanged but, owing to its thickening, the velocity fluctuations just outside are greater. There is not a general suppression of turbulence within the sublayer although well inside the sublayer the spanwise velocity component is found to be reduced.

New Answers on the Interaction Between Polymers and Vortices in Turbulent Flows

Yves Dubief — 2007-07-24T15:03:35-00:00

Flow, Turbulence and Combustion, Vol. 74, No. 4. (28 June 2005), pp. 311-329.

Abstract Numerical data of polymer drag reduced flows is interpreted in terms of modification of near-wall coherent structures. The originality of the method is based on numerical experiments in which boundary conditions or the governing equations are modified in a controlled manner to isolate certain features of the interaction between polymers and turbulence. As a result, polymers are shown to reduce drag by damping near-wall vortices and sustain turbulence by injecting energy onto the streamwise velocity component in the very near-wall region.

Stabilization of hydrodynamic flows by small viscosity variations

Rama Govindarajan — 2007-07-24T14:59:18-00:00

Physical Review E, Vol. 67, No. 2. (18 February 2003), 026310.

Motivated by the large effect of turbulent drag reduction by minute concentrations of polymers; we study the effects of a weakly space-dependent viscosity on the stability of hydrodynamic flows. In a recent paper [Phys. Rev. Lett. 87 ; 174501; (2001)]; we exposed the crucial role played by a localized region where the energy of fluctuations is produced by interactions with the mean flow (the “critical layer”). We showed that a layer of a weakly space-dependent viscosity placed near the critical layer can have a very large stabilizing effect on hydrodynamic fluctuations; retarding significantly the onset of turbulence. In this paper we extend these observations in two directions: first we show that the strong stabilization of the primary instability is also obtained when the viscosity profile is realistic (inferred from simulations of turbulent flows with a small concentration of polymers). Second; we analyze the secondary instability (around the time-dependent primary instability) and find similar strong stabilization. Since the secondary instability develops around a time-dependent solution and is three dimensional; this brings us closer to the turbulent case. We reiterate that the large effect is not due to a modified dissipation (as is assumed in some theories of drag reduction); but due to reduced energy intake from the mean flow to the fluctuations. We propose that similar physics act in turbulent drag reduction.

A comparison between different asymmetric abdominal aortic aneurysm morphologies employing computational fluid-structure interaction analysis

Zhonghua Li — 2007-07-17T11:37:23-00:00

European Journal of Mechanics - B/Fluids, Vol. 26, No. 5. ( 2007), pp. 615-631.

Considering representative asymmetric aneurysms in the abdominal aorta, the transient 3-D blood flow and pressure distributions as well as aneurysm wall stresses were numerically analyzed. To obtain more realistic and accurate results for blood flow fields and wall stress distributions, a coupled fluid-flow and solid-structure solver was employed. Geometric abdominal aortic aneurysm (AAA) variations studied included the degree of asymmetry, neck angle and bifurcation angle, and hence their impacts on the hemodynamics and biomechanics. The simulation results indicated that the assumption of symmetric AAA geometry may underestimate AAA-wall stress considerably. The neck angle influences the blood flow field substantially. A large neck angle, resulting in strong wall curvatures near the proximal neck, can produce aggravating blood flow patterns and elevated wall stresses (Von Mises). The iliac bifurcation angle affects blood flow patterns insignificantly but plays an important role in wall-stress concentrations. The wall stress of lateral asymmetric AAAs is higher than for the anterior-posterior asymmetric types. The maximum wall stress-site is located near the anterior distal side for the anterior-posterior asymmetric AAA and the distal side towards the asymmetric bulge in the lateral asymmetric AAA.

Decaying grid-generated turbulence in a rotating tank

C Morize — 2007-06-18T15:03:00-00:00

Physics of Fluids, Vol. 17, No. 9. (2005)

The decay of initially three-dimensional homogeneous turbulence in a rotating frame is experimentally investigated. Turbulence is generated by rapidly towing a grid in a rotating water tank, and the velocity field in a plane perpendicular to the rotation axis is measured by means of particle image velocimetry. During the decay, strong cyclonic coherent vortices emerge, as the result of enhanced stretching of the cyclonic vorticity by the background rotation, and the selective instability of the anticyclonic vorticity by the Coriolis force. This asymmetry towards cyclonic vorticity grows on a time scale –1 ( is the rotation rate), until the friction from the Ekman layers becomes dominant. The energy spectrum perpendicular to the rotation axis becomes steeper as the instantaneous Rossby number Ro=/2 decreases below the value 2±0.5 ( is the root-mean square of the vertical vorticity). The spectral exponent increases in time from its classical Kolmogorov value 5/3 up to values larger than 2. Below the threshold Ro<2, the velocity derivative skewness decreases as |S|Ro, reflecting the inhibition of the energy transfers by the background rotation, with a net inverse energy cascade that develops at large scales. ©2005 American Institute of Physics

Three-dimensional modes in a periodically driven elongated cavity

Jonathan Leung — 2007-06-17T19:36:29-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 2. (2005)

Three-dimensional instability modes of the periodic flow in a rectangular cavity driven by the harmonic sliding oscillation of its floor are explored experimentally. Theory for a cavity with infinite span predicts two synchronous modes and a quasiperiodic traveling-wave mode as primary transitions from two-dimensional to three-dimensional flow for different combinations of floor oscillation amplitude and frequency. Previously, only one of the two synchronous modes had been found experimentally. Here, we provide experimental details of both synchronous modes and a quasiperiodic mode. All three modes appear in the parameter regimes predicted by the theory; however, in the finite-span experiments, the traveling wave nature of the quasiperiodic mode is replaced by a nonpropagating mode with spatial features similar to those of the traveling mode.

An improved measure of strain state probability in turbulent flows

Thomas Lund — 2007-06-17T19:33:50-00:00

Physics of Fluids, Vol. 6, No. 5. (1994), pp. 1838-1847.

Probability density functions (PDFs) of the strain-rate tensor eigenvalues are examined. It is found that the accepted normalization used to bound the intermediate eigenvalue between ±1 leads to a PDF that must vanish at the end points for a non-singular distribution of strain states. This purely kinematic constraint has led previous investigators to conclude incorrectly that locally axisymmetric deformations do not exist in turbulent flows. An alternative normalization is presented that does not bias the probability distribution near the axisymmetric limits. This alternative normalization is shown to lead to the expected flat PDF in a Gaussian velocity field and to a PDF that indicates the presence of axisymmetric strain states in a turbulent field. Extension of the new measure to compressible flow is discussed. Several earlier results concerning the likelihood of various strain states and the correlation of these with elevated kinetic energy dissipation rate are reinterpreted in terms of the new normalization. Most importantly, it is found that regions of axisymmetric expansion play a much more dominant role in the turbulent dissipation process than was previously believed. Physics of Fluids is copyrighted by The American Institute of Physics.

Self-similar development of a turbulent layer in a homogeneous fluid

VS Maderich — 2007-06-17T12:08:51-00:00

Okeanologiia, Vol. 17 (October 1977), pp. 784-790.

The self-similar behavior of zero-shear turbulence behind an oscillating grid in a homogeneous fluid is studied on the basis of a model composed of a turbulent energy balance equation and a dissipation rate equation, written in the dimensionless form employed by Hanjalic and Launder (1972). Analysis of the similar solution obtained reveals a strong dependence of the parameters of the turbulent layer on the constants of the turbulence model.

Use of a simple model for the interpretation of experimental data on particle resuspension in turbulent flows

L Biasi — 2007-06-15T11:57:45-00:00

pp. 1175-1200.

A simple mechanistic model for resuspension assuming a log-normal distribution of adhesive forces is used to analyse the resuspension measurements of a series of experiments reported in the literature covering a range of particle size from sub-micron to 30m in both mono-layer and multi-layer deposits. Correlations are derived for the geometric mean and spread of the adhesive force as a function of particle size. These show that adhesion is much less than that for smooth contact between particle and surface, consistent with the effect of surface roughness. Furthermore, sub-micron particles in a bed of particles appear to be more tightly bound with a much reduced spread than when the coverage of particles is less than a mono-layer. The model including these correlations is easily implementable and suitable for incorporation into severe nuclear accident codes and industrial gas/particle codes.

On the dispersion of small particles suspended in an isotropic turbulent fluid

MW Reeks — 2007-06-15T11:49:26-00:00

J. Fluid Mech., Vol. 83 (1977), pp. 529-546.

A solution to the dispersion of small particles suspended in a turbulent fluid is presented, based on the approximation proposed by Phythian for the dispersion of fluid points in an incompressible random fluid. Motion is considered in a frame moving with the mean velocity of the fluid, the forces acting on the particle being taken as gravity and a fluid drag assumed linear in the particle velocity relative to that of the fluid. The probability distribution of the fluid velocity field in this frame is taken as Gaussian, homogeneous, isotropic, stationary and of zero mean. It is shown that, in the absence of gravity, the long-time particle diffusion coefficient is in general greater than that of the fluid, approaching with increasing particle relaxation time a value consistent with the particle being in an Eulerian frame of reference. The effect of gravity is consistent with Yudine's effect of crossing trajectories, reducing unequally the particle diffusion in directions normal to and parallel to the direction of the gravitational field. To characterize the effect of flow and gravity on particle diffusion it has been found useful to use a Froude number defined in terms of the turbulent intensity rather than the mean velocity. Depending upon the value of this number, it is found that the particle integral time scale may initially decrease with increasing particle relaxation time though it eventually rises and approaches the particle relaxation time. It is finally shown how this analysis may be extended to include the extra forces generated by the fluid and particle accelerations.

Spatial characterization of the numerically simulated vorticity fields of a flow in a flume

Alex Liberzon — 2007-06-13T07:08:27-00:00

Theoretical and Computational Fluid Dynamics, Vol. 19, No. 2. (1 May 2005), pp. 115-125.

The topology of large scale structures in a turbulent boundary layer is investigated numerically. Spatial characteristics of the large scale structure are presented through an original method, proper orthogonal decomposition (POD) of the three-dimensional vorticity fields. The DNS results, obtained by Tiselj et al. [23] for a fully developed turbulent flow in a flume, are used in the present work to analyze coherent structures with the proposed methodology. In contrast to the reconstruction methods that use instantaneous flow quantities, this approach utilizes the whole dataset of the numerical simulation. The analysis uses one thousand 3D vorticity fields from 50000 time steps of the simulation for the Reynolds number of 2600 (the turbulent Reynolds number Re*=171). The computational domain is 2146×171×537 wall units and the grid resolution is 128×65×72 points (in streamwise, wall-normal and spanwise directions, respectively). Experimental results obtained by using particle image velocimetry (PIV) in a fully developed turbulent boundary layer in a flume, which were analyzed with the same statistical characterization method, are in agreement with the DNS analysis: the dominant vortical structure appears to have a longitudinal streamwise orientation, an inclination angle of about 8°, streamwise length of several hundred wall units, and a distance between the neighboring structures of about 100 wall units in the spanwise direction.

Reduced-order description of fluid flow with moving boundaries by proper orthogonal decomposition

Yogen Utturkar — 2007-06-13T07:07:58-00:00

International Journal of Heat and Fluid Flow, Vol. 26, No. 2. (April 2005), pp. 276-288.

The approach of proper orthogonal decomposition (POD) has been extensively adopted for fluid dynamics in fixed geometries. This technique is examined here for fluid flow with moving boundaries; in the context of cavitating and phase change flows, and fluid-membrane interaction. The purpose is to assess the capability of POD in extracting the salient features and offering a compact representation to the CFD solutions associated with boundary movement. The cavitating flow simulations are investigated to distill the effect of turbulence modeling, between the Launder-Spalding and a filter-based turbulence models. The lower-order eigenmodes of the flow field, for both turbulence models, show different flow structures and global parameters between higher and lower cavitation numbers. The effect of multi-timescales produced by the filter-based turbulence model is discerned by POD analysis. For 3-D, membrane wing flows, very few POD modes seem sufficient for accurate representation of the velocity field. However, reduced-order analysis of the aerodynamic performance, which is strongly dictated by pressure, may be coarsened by moving membrane dynamics. The flow with fusion is further considered for its solid-liquid phase front propagation. While few modes can sufficiently construct the flow field for the later interval of the flow, a larger number of POD modes are required to provide the flow scales for the initial part of the phase change process.

Possible origin of Prandt's mixing-length theory

P Bradshaw — 2007-04-04T14:27:47-00:00

Nature, Vol. 249, No. 5453. (10 May 1974), pp. 135-136.

On the physical mechanisms of two-way coupling in particle-laden isotropic turbulence

A Ferrante — 2006-09-13T11:29:32-00:00

Physics of Fluids, Vol. 15, No. 2. (2003), pp. 315-329.

The objective of the present study is to analyze our recent direct numerical simulation (DNS) results to explain in some detail the main physical mechanisms responsible for the modification of isotropic turbulence by dispersed solid particles. The details of these two-way coupling mechanisms have not been explained in earlier publications. The present study, in comparison to the previous DNS studies, has been performed with higher resolution (Re = 75) and considerably larger number (80 million) of particles, in addition to accounting for the effects of gravity. We study the modulation of turbulence by the dispersed particles while fixing both their volume fraction, v = 10–3, and mass fraction, m = 1, for three different particles classified by the ratio of their response time to the Kolmogorov time scale: microparticles, p/k1, critical particles, p/k1, large particles, p/k>1. Furthermore, we show that in zero gravity, dispersed particles with p/k = 0.25 (denoted here as "ghost particles") modify the spectra of the turbulence kinetic energy and its dissipation rate in such a way that keeps the decay rate of the turbulence energy nearly identical to that of particle-free turbulence, and thus the two-way coupling effects of these ghost particles would not be detected by examining only the temporal behavior of the turbulence energy of the carrier flow either numerically or experimentally. In finite gravity, these ghost particles accumulate, via the mechanism of preferential sweeping resulting in the stretching of the vortical structures in the gravitational direction, and the creation of local gradients of the drag force which increase the magnitudes of the horizontal components of vorticity. Consequently, the turbulence becomes anisotropic with a reduced decay rate of turbulence kinetic energy as compared to the particle-free case. ©2003 American Institute of Physics.

Identification method for vortex sheet structures in turbulent flows

Kiyosi Horiuti — 2006-09-12T11:50:48-00:00

Physics of Fluids, Vol. 17, No. 12. (2005)

A new identification method was proposed for an eduction of vortex sheet structures in turbulent flows. This method took advantage of a prominent feature of a sheet, i.e., comparable dominance of both strain rate and vorticity and their strong correlation. The effectiveness of the proposed method was presented in the assessment using direct numerical simulation data for homogeneous isotropic turbulence. Both strain rate and vorticity were indeed large and correlated in the region identified using the proposed method. As a result, intense dissipation took place in the educed region. The relationship between the eigenvalue solution used in the present method and the invariants of fourth-order moments of velocity gradients was discussed. It was shown that the proposed method performed better than other invariants and previous identification methods for educing the vortex sheets. ©2005 American Institute of Physics

The structure of turbulence in a rod-roughened channel

A Ashrafian — 2006-09-12T07:40:16-00:00

Int. J. Heat Fluid Flow, Vol. 27 (2006), pp. 65-79.

roughness increases the intensity of the vorticity fluctuations in the roughness layer, but not in the outer. The Reynolds stress anisotropy invariant maps showed that the states of the near-wall turbulence are substantially different. It was observed that the wall-ward transport of teh kinetic energy is substantially increased very close to the wall while the away-from-the-wall transport of kinetic energy is relatively reduced at the edge of the roughness layer.

The transport of small particles by a fluid

A Babiano — 2006-08-09T22:15:21-00:00

Lecture notes in physics, Vol. 567 (2001), pp. 114-124.

Effects of Long-chain Molecule Additives in Water on Vortex Streets

GE Gadd — 2006-07-25T22:21:50-00:00

Nature, Vol. 211, No. 5045. (July 1966), pp. 169-170.

The large eddies of turbulent motion

HL Grant — 2006-07-25T13:42:55-00:00

J. Fluid Mech, Vol. 4 (1958), pp. 149-190.

Spatially decaying turbulence and its relation to mixing across density interfaces

EJ Hopfinger — 2006-07-21T23:46:45-00:00

J. Fluid Mech., Vol. 78, No. 1., pp. 155-175.

Mechanical models of dilute polymer solutions in strong flows

EJ Hinch — 2006-07-12T06:35:45-00:00

Physics of Fluids, Vol. 20, No. 10. (1977), pp. S22-S30.

In strong flows, such as turbulence, it is suggested that randomly coiled macromolecules might become nearly fully extended. To investigate this extended state, four models are considered which show the importance of the inextensibility of the polymer chain and the variation of the friction coefficient with the extension. The rheological consequences of these processes are then explored in an appropriately modified dumb-bell model. A dilute polymer solution is predicted to have a high extensional viscosity but a low shear viscosity, a stress hysteresis, and a slow stress relaxation. The action of the novel rheology is shown in two simple flows. Finally, speculations are made on the possible way dilute polymer solutions can modify turbulence. Physics of Fluids is copyrighted by The American Institute of Physics. doi:10.1063/1.861735 PACS: 47.25.Fj Additional Information Full Text: [ PDF (954 kB)

The turbulent diffusion of drag-reducing polymer solutions from a point source in flow through a pipe

WD Mccomb — 2006-07-04T08:48:16-00:00

Chemical Engineering Science, Vol. 37, No. 12. (1982), pp. 1759-1770.

Solutions containing salt(NaCI) were injected into water flow through a fine tube at the centre of a pipe. Tests were made to ensure that the flow was turbulent and well developed at the injection point; and that neither the injection process nor the injector caused any appreciable flow disturbance. Radial concentration profiles were measured at various downstream distances by withdrawing samples from the flow and measuring their electrical conductivity. A comparison was made between water, and aqueous solutions of polyethyleneoxide and polyacrylamide. Results for the eddy diffusivity in water agreed with those from previous investigations. Turbulent diffusivities were found to be greatly reduced in the polymer solutions (even after allowance had been made for the reduced wall shear stress) compared to the values for water alone. There were detailed differences in the effects of the two different kinds of polymer and these were attributed to their different viscoelasdc properties in the concentration ranges studied.

Effect of polymer additives on the small-scale structure of grid-generated turbulence

WD Mccomb — 2006-07-03T23:05:05-00:00

Physics of Fluids, Vol. 20, No. 6. (1977), pp. 873-879.

The effect of polyethyleneoxide (Polyox grade WSR 301) on grid-generated turbulence was investigated using laser anemometry and flow-visualization techniques. It was found that the polymer additive reduced both the turbulent intensity and the rate of decay behind the grid. At typical drag-reducing concentrations, turbulent energy spectra were qualitatively the same as those in water, in agreement with the results of other investigations. However, at higher additive concentrations, the dissipation-range spectra showed noticeable attenuation. This seemed to be a threshold effect with onset at a polymer concentration between 100 and 250 ppm. This result was supported by photographs of dye-injection tracer but in this case the onset concentration for small-eddy suppression was between 50 and 100 ppm. Physics of Fluids is copyrighted by The American Institute of Physics. doi:10.1063/1.861977 PACS: 47.25.-c Additional Information Full Text: [ PDF (730 kB)

Structural changes during elongation of polymer solutions

J Ferguson — 2006-07-02T21:39:02-00:00

Journal of Non-Newtonian Fluid Mechanics, Vol. 23 (1987), pp. 49-72.

During elongational flow experiments on polymer solutions certain anomalous examples of flow behaviour have been noted. These are, 1. (1) a non uniform velocity profile in the radial direction in strongly non-Newtonian fluids being elongated at high rates.2. (2) the loss of tack in the filament as rate of elongation increased,3. (3) the bouncing of the filament off a rotating drum at high elongation rates, and4. (4) the development of sustained rubber-like lateral vibrations in the fluids elongated at high rate.In addition, droplet formation occurred on the surface of the solutions which were elongated rapidly then suddenly stopped. Photography showed that the filaments continued through these droplets. When captured and dried, it was observed that the filament contained the bulk of the solute. The droplets appeared to have only a low concentration of polymer.On the basis of the photographic evidence and the thermodynamics of complex formation between elongated polymer chains, it would appear that complexation of the chains is occurring at high elongation rates. This could partly account for the strain hardening encountered during elongation of many polymeric fluids. Where solubility parameter difference between polymer and solvent was large, the complex formed could come out of solution forming a gel and a solvent rich phase.

Eulerian and Lagrangian studies in surface flow turbulence

John Cressman — 2006-07-02T20:09:51-00:00

New Journal of Physics, Vol. 6 (2004)

Experimental and numerical studies of turbulent fluid motion in a free surface are presented. The flow is realized experimentally on the surface of a tank filled with water stirred well below the surface. Numerically, it is modelled by free-slip boundary conditions. The surface flow is unconventional: it is not incompressible and neither kinetic energy nor enstrophy is conserved in the limit of zero fluid viscosity and in the absence of external driving as is the case for incompressible two-dimensional turbulent flows. The dynamics of passive Lagrangian tracers that are advected in such flows are dominated by rapidly changing patches of the surface flow divergence. Owing to compressibility, particles form clusters within multifractal mass distributions. Also studied is the motion of pairs and triplets of particles. The mean square separation shows an extended range with a reduced scaling exponent in comparison with the classical Richardson value. Clustering is also manifest in strongly deformed triangles spanned within triplets of tracers.