International Collaboration for Turbulence Research


International Collaboration for Turbulence Research

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Pumir A.,International Collaboration for Turbulence Research | Pumir A.,Ecole Normale Superieure de Lyon | Pumir A.,Max Planck Institute for Dynamics and Self-Organization | Xu H.,International Collaboration for Turbulence Research | And 10 more authors.
Physical Review X | Year: 2014

In statistically homogeneous turbulent flows, pressure forces provide the main mechanism to redistribute kinetic energy among fluid elements, without net contribution to the overall energy budget. This holds true in both two-dimensional (2D) and three-dimensional (3D) flows, which show fundamentally different physics. As we demonstrate here, pressure forces act on fluid elements very differently in these two cases. We find in numerical simulations that in 3D pressure forces strongly accelerate the fastest fluid elements, and that in 2D this effect is absent. In 3D turbulence, our findings put forward a mechanism for a possibly singular buildup of energy, and thus may shed new light on the smoothness problem of the solution of the Navier-Stokes equation in 3D.

Gibert M.,Max Planck Institute for Dynamics and Self-Organization | Gibert M.,International Collaboration for Turbulence Research | Gibert M.,CNRS Neel Institute | Xu H.,Max Planck Institute for Dynamics and Self-Organization | And 5 more authors.
Journal of Fluid Mechanics | Year: 2012

We report experimental results on the dynamics of heavy particles of the size of the Kolmogorov scale in a fully developed turbulent flow. The mixed Eulerian structure function of two-particle velocity and acceleration difference vectors delta rv· δ ra p was observed to increase significantly with particle inertia for identical flow conditions. We show that this increase is related to a preferential alignment between these dynamical quantities. With increasing particle density the probability for those two vectors to be collinear was observed to grow. We show that these results are consistent with the preferential sampling of strain-dominated regions by inertial particles. © 2012 Cambridge University Press.

Ahlers G.,University of California at Santa Barbara | Ahlers G.,International Collaboration for Turbulence Research | He X.,Max Planck Institute for Dynamics and Self-Organization | He X.,International Collaboration for Turbulence Research | And 6 more authors.
New Journal of Physics | Year: 2012

We report on the experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh-Bénard convection (RBC) in a cylindrical sample of aspect ratio F = D/L = 0.50 (D = 1.12 m is the diameter and L = 2.24 m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bar as the fluid. They are for the Rayleigh-number range 3 × 10 12 ≲ Ra ≲ 10 15 and for Prandtl numbers Pr between 0.79 and 0.86. For Ra < Ra 1 * ≃ 1.4 × 10 13 we find Nu = N 0 Ra γeff with γeff = 0.312 ± 0.002, which is consistent with classical turbulent RBC in a system with laminar boundary layers below the top and above the bottom plate. For Ra 1 * < Ra < Ra 2 * (with Ra 2 * ≃ 5× 10 14) γeff gradually increases up to 0.37 ±0.01. We argue that above Ra 2 * the system is in the ultimate state of convection where the boundary layers, both thermal and kinetic, are also turbulent. Several previous measurements for Y = 0.50 are re-examined and compared with our results. Some of them show a transition to a state with γeff in the range from 0.37 to 0.40, albeit at values of Ra in the range from 9 × 10 10 to 7 × 10 11 which is much lower than the present Ra 1 * or Ra 2 *. The nature of the transition found by them is relatively sharp and does not reveal the wide transition range observed in this work. In addition to the results for the genuine Rayleigh-Bénard system, we present measurements for a sample which was not completely sealed; the small openings permitted external currents, imposed by density differences and gravity, to pass through the sample. That system should no longer be regarded as genuine RBC because the externally imposed currents modified the heat transport in a major way. It showed a sudden decrease of γeff from 0.308 for Ra < Ra t ≃ 4× 10 13 to 0.25 for larger Ra. A number of possible experimental effects are examined in a sequence of appendices; none of these effects is found to have a significant influence on the measurements. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Salazar J.P.L.C.,Cornell University | Salazar J.P.L.C.,International Collaboration for Turbulence Research | Salazar J.P.L.C.,Federal University of Santa Catarina | Collins L.R.,Cornell University | Collins L.R.,International Collaboration for Turbulence Research
Journal of Fluid Mechanics | Year: 2012

In the present study, we investigate the scaling of relative velocity structure functions, of order two and higher, for inertial particles, both in the dissipation range and the inertial subrange using direct numerical simulations (DNS). Within the inertial subrange our findings show that contrary to the well-known attenuation in the tails of the one-point acceleration probability density function (p.d.f.) with increasing inertia (Bec et al., J. Fluid Mech., vol. 550, 2006, pp. 349-358), the opposite occurs with the velocity structure function at sufficiently large Stokes numbers. We observe reduced scaling exponents for the structure function when compared to those of the fluid, and correspondingly broader p.d.f.s, similar to what occurs with a passive scalar. DNS allows us to isolate the two effects of inertia, namely biased sampling of the velocity field, a result of preferential concentration, and filtering, i.e. the tendency for the inertial particle velocity to attenuate the velocity fluctuations in the fluid. By isolating these effects, we show that sampling is playing the dominant role for low-order moments of the structure function, whereas filtering accounts for most of the scaling behaviour observed with the higher-order structure functions in the inertial subrange. In the dissipation range, we see evidence of so-called "crossing trajectoriesa", the "sling effect" or "caustics", and find good agreement with the theory put forth by Wilkinson et al. (Phys. Rev. Lett., vol. 97, 2006, 048501) and Falkovich & Pumir (J. Atmos. Sci., vol. 64, 2007, 4497) for Stokes numbers greater than 0.5. We also look at the scaling exponents within the context of the model proposed by Bec et al. (J. Fluid Mech., vol. 646, 2010, pp. 527-536). Another interesting finding is that inertial particles at low Stokes numbers sample regions of higher kinetic energy than the fluid particle field, the converse occurring at high Stokes numbers. The trend at low Stokes numbers is predicted by the theory of Chun et al. (J. Fluid Mech., vol. 536, 2005, 219-251). This work is relevant to modelling the particle collision rate (Sundaram & Collins, J. Fluid Mech., vol. 335, 1997, pp. 75-109), and highlights the interesting array of phenomena induced by inertia. © 2012 Cambridge University Press.

Saw E.-W.,Michigan Technological University | Saw E.-W.,Max Planck Institute for Dynamics and Self-Organization | Saw E.-W.,International Collaboration for Turbulence Research | Salazar J.P.L.C.,Cornell University | And 6 more authors.
New Journal of Physics | Year: 2012

Particles that are heavy compared to the fluid in which they are embedded (inertial particles) tend to cluster in turbulent flow, with the degree of clustering depending on the particle Stokes number. The phenomenon is relevant to a variety of systems, including atmospheric clouds; in most realistic systems particles have a continuous distribution of sizes and therefore the clustering of 'polydisperse' particle populations is of special relevance. In this work a theoretical expression for the radial distribution function (RDF) for mono- and bidisperse inertial particles in the low Stokes number limit (Chun et al 2005 J. Fluid Mech. 536 219-51) is compared with the results of a direct numerical simulation of particle-laden turbulence. The results confirm the power-law form of the RDF for monodisperse particles with St ≲ 0.3. The clustering signature occurs at scales ≲ 10-30 times the Kolmogorov scale, consistent with a dissipation-scale mechanism. The theory correctly predicts the decorrelation scale below which bidisperse particles cease to cluster because of their distinct inertial response. A 'saturation' effect was observed, however, in which the power-law exponent is limited by the least clustered particle population. An expression is presented with which a polydisperse RDF can be obtained from the mono- and bidisperse RDFs and the particle size distribution. The DNS data clearly show that the effect of polydispersity is to diminish clustering, and place a bound on the level of polydispersity required to approximate a monodisperse system; this result is of relevance to experimental studies and realistic systems. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Bragg A.D.,Cornell University | Bragg A.D.,International Collaboration for Turbulence Research | Bragg A.D.,Los Alamos National Laboratory | Ireland P.J.,Cornell University | And 3 more authors.
Physics of Fluids | Year: 2016

In this paper, we investigate both theoretically and numerically the Forward-In-Time (FIT) and Backward-In-Time (BIT) dispersion of fluid and inertial particle-pairs in isotropic turbulence. Fluid particles are known to separate faster BIT than FIT in three-dimensional turbulence, and we find that inertial particles do the same. However, we find that the irreversibility in the inertial particle dispersion is in general much stronger than that for fluid particles. For example, the ratio of the BIT to FIT mean-square separation can be up to an order of magnitude larger for the inertial particles than for the fluid particles. We also find that for both the inertial and fluid particles, the irreversibility becomes stronger as the scale of their separation decreases. Regarding the physical mechanism for the irreversibility, we argue that whereas the irreversibility of fluid particle-pair dispersion can be understood in terms of a directional bias arising from the energy transfer process in turbulence, inertial particles experience an additional source of irreversibility arising from the non-local contribution to their velocity dynamics, a contribution that vanishes in the limit St → 0, where St is the particle Stokes number. For each given initial (final, in the BIT case) separation, r0, there is an optimum value of St for which the dispersion irreversibility is strongest, as such particles are optimally affected by both sources of irreversibility. We derive analytical expressions for the BIT, mean-square separation of inertial particles and compare the predictions with numerical data obtained from a Reλ ≈ 582 (where Reλ is the Taylor Reynolds number) Direct Numerical Simulation (DNS) of particle-laden isotropic turbulent flow. The small-time theory, which in the dissipation range is valid for times ≤max[St τη, τη] (where τη is the Kolmogorov time scale), is in excellent agreement with the DNS. The theory for long-times is in good agreement with the DNS provided that St is small enough so that the inertial particle motion at long-times may be considered as a perturbation about the fluid particle motion, a condition that would in fact be satisfied for arbitrary St at sufficiently long-times in the limit Reλ → ∞. © 2016 AIP Publishing LLC.

Perlekar P.,TU Eindhoven | Perlekar P.,International Collaboration for Turbulence Research | Benzi R.,University of Rome Tor Vergata | Nelson D.R.,Harvard University | And 2 more authors.
Physical Review Letters | Year: 2010

We study the statistical properties of population dynamics evolving in a realistic two-dimensional compressible turbulent velocity field. We show that the interplay between turbulent dynamics and population growth and saturation leads to quasilocalization and a remarkable reduction in the carrying capacity. The statistical properties of the population density are investigated and quantified via multifractal scaling analysis. We also investigate numerically the singular limit of negligibly small growth rates and delocalization of population ridges triggered by uniform advection. © 2010 The American Physical Society.

van der Bos F.,TU Munich | Geurts B.J.,University of Twente | Geurts B.J.,TU Eindhoven | Geurts B.J.,International Collaboration for Turbulence Research
Computer Methods in Applied Mechanics and Engineering | Year: 2010

A computational error-assessment of large-eddy simulation (LES) in combination with a discontinuous Galerkin finite element method is presented for homogeneous, isotropic, decaying turbulence. The error-landscape database approach is used to quantify the total simulation error that arises from the use of the Smagorinsky eddy-viscosity model in combination with the Galerkin discretization. We adopt a modified HLLC flux, allowing an explicit control over the dissipative component of the numerical flux. The optimal dependence of the Smagorinsky parameter on the spatial resolution is determined for second and third order accurate Galerkin methods. In particular, the role of the numerical dissipation relative to the contribution from the Smagorinsky dissipation is investigated. We observed an 'exchange of dissipation' principle in the sense that an increased numerical dissipation implied a reduction in the optimal Smagorinsky parameter. The predictions based on Galerkin discretization with fully stabilized HLLC flux were found to be less accurate than when a central discretization with (mainly) Smagorinsky dissipation was used. This was observed for both the second and third order Galerkin discretization, suggesting to emphasize central discretization of the convective nonlinearity and stabilization that mimics eddy-viscosity as sub-filter dissipation. © 2009 Elsevier B.V. All rights reserved.

Perlekar P.,TU Eindhoven | Perlekar P.,The Interdisciplinary Center | Perlekar P.,International Collaboration for Turbulence Research | Benzi R.,University of Rome Tor Vergata | And 5 more authors.
Physical Review Letters | Year: 2014

We study the competition between domain coarsening in a symmetric binary mixture below critical temperature and turbulent fluctuations. We find that the coarsening process is arrested in the presence of turbulence. The physics of the process shares remarkable similarities with the behavior of diluted turbulent emulsions and the arrest length scale can be estimated with an argument similar to the one proposed by Kolmogorov and Hinze for the maximal stability diameter of droplets in turbulence. Although, in the absence of flow, the microscopic diffusion constant is negative, turbulence does effectively arrest the inverse cascade of concentration fluctuations by making the low wavelength diffusion constant positive for scales above the Hinze length. © 2014 American Physical Society.

Liberzon A.,Tel Aviv University | Liberzon A.,International Collaboration for Turbulence Research
International Journal of Heat and Fluid Flow | Year: 2011

Effects of dilute polymer solutions on a lid-driven cubical cavity turbulent flow are studied via particle image velocimetry (PIV). This canonical flow is a combination of a bounded shear flow, driven at constant velocity and vortices that change their spatial distribution as a function of the lid velocity. From the two-dimensional PIV data we estimate the time averaged spatial fields of key turbulent quantities. We evaluate a component of the vorticity-velocity correlation, namely 〈ω 3v〉, which shows much weaker correlation, along with the reduced correlation of the fluctuating velocity components, u and v. There are two contributions to the reduced turbulent kinetic energy production -〈u v〉S uv, namely the reduced Reynolds stresses, -〈u v〉, and strongly modified pointwise correlation of the Reynolds stress and the mean rate-of-strain field, S uv. The Reynolds stresses are shown to be affected because of the derivatives of the Reynolds stresses, ∂〈u v〉/∂y that are strongly reduced in the same regions as the vorticity-velocity correlation. The results, combined with the existing evidence, support the phenomenological model of polymer effects propagating from the polymer scale to the velocity derivatives and through the mixed-type correlations and Reynolds stress derivatives up to the turbulent velocity fields. The effects are shown to be qualitatively similar in different flows regardless of forcing type, homogeneity or presence of liquid-solid boundaries. © 2011 Elsevier Inc.

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