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Kiyosi Horiuti - One of the best experts on this subject based on the ideXlab platform.
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multimode stretched spiral vortex and nonequilibrium energy spectrum in Homogeneous shear flow Turbulence
Physics of Fluids, 2011Co-Authors: Kiyosi Horiuti, Tetsuya OzawaAbstract:The stretched spiral vortex [T. S. Lundgren, “Strained spiral vortex model for turbulent structures,” Phys. Fluids 25, 2193 (1982)] is identified in Turbulence in Homogeneous shear flow and the spectral properties of this flow are studied using direct-numerical simulation data. The effects of mean shear on the genesis, growth, and annihilation processes of the spiral vortex are elucidated, and the role of the spiral vortex in the generation of Turbulence is shown. As in Homogeneous Isotropic Turbulence [K. Horiuti and T. Fujisawa, “The multi mode stretched spiral vortex in Homogeneous Isotropic Turbulence,” J. Fluid Mech. 595, 341 (2008)], multimodes of the spiral vortex are extracted. Two symmetric modes of configurations with regard to the vorticity alignment along the vortex tube in the core region and dual vortex sheets spiraling around the tube are often educed. One of the two symmetric modes is created by a conventional rolling-up of a single spanwise shear layer. Another one is created by the conve...
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the multi mode stretched spiral vortex in Homogeneous Isotropic Turbulence
Journal of Fluid Mechanics, 2008Co-Authors: Kiyosi Horiuti, Takeharu FujisawaAbstract:The stretched spiral vortex is identified using direct numerical simulation (DNS) data for Homogeneous Isotropic Turbulence and its properties are studied. Its genesis, growth and annihilation are elucidated, and its role in the generation of Turbulence is shown. Aside from the two symmetric modes of configurations with regard to the vorticity alignment along two spiral sheets and the vortex tube in the core region studied in previous works, a third asymmetric mode is found. One of the two symmetric modes and the asymmetric mode are created not by a conventional rolling-up of a single vortex sheet but through the interaction among several sheets. The stagnation flow caused by the two sheets converges to form recirculating flow through its interaction with the vortex along the third sheet. This recirculating flow strains and stretches the sheets. The vortex tube is formed by axial straining, lowering of pressure and the intensification of the swirling motion in the recirculating region. As a result of the differential rotation induced by the tube and that self-induced by the sheet, the vortex sheets are entrained by the tube and form spiral turns. The transition between the three modes is examined. The initial configuration is in one of two symmetric modes, but it is transformed into another set of two modes due to the occurrence of reorientation in the vorticity direction along the stretched sheets. The symmetric mode tends to be more persistent than the asymmetric mode, among the two transformed modes. The tightening of the spiral turns of the spiral sheets produces a cascade of velocity fluctuations to smaller scales and generates a strongly intermittent dissipation field. To precisely capture the spiral turns, a grid resolution with at least k max η≈4.0 (k max is the largest wavenumber, η is the averaged Kolmogorov scale) is required. At a higher Reynolds number, self-similar spiral vortices are successively produced by the instability cascade along the stretched vortex sheets. A cluster consisting of spiral vortices with an extensive range of length scales is formed and this cluster induces an energy cascade.
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a classification method for vortex sheet and tube structures in turbulent flows
Physics of Fluids, 2001Co-Authors: Kiyosi HoriutiAbstract:A new classification method for structures in turbulent flow is proposed and applied to the analysis of Homogeneous Isotropic Turbulence. The criteria for the classification of the structures into three groups, namely, the group of structures similar to the core region of the Burgers’ vortex tube in which vorticity is predominant, that of the structures similar to the curved sheet in the circumference of the tube core in which strain is predominant, and that of the flat sheets similar to the Burgers’ vortex layer in which vorticity and strain are comparably large, were considered. This method was developed based on the eigenvalue solutions of the λ2 method [Jeong and Hussain, J. Fluid Mech. 285, 69 (1995)] on the basis of the principal strain eigenvectors, which were reordered according to the degrees of alignment with the vorticity vector. Assessment of the proposed method was carried out in fully developed Homogeneous Isotropic Turbulence and in the process of rolling up of the vortex layer in ABC flow....
Federico Toschi - One of the best experts on this subject based on the ideXlab platform.
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enhanced settling of nonheavy inertial particles in Homogeneous Isotropic Turbulence the role of the pressure gradient and the basset history force
Physical Review E, 2017Co-Authors: Van Mat Michel Hinsberg, Hjh Herman Clercx, Federico ToschiAbstract:The Stokes drag force and the gravity force are usually sufficient to describe the behavior of sub-Kolmogorov-size (or pointlike) heavy particles in Turbulence, in particular when the particle-to-fluid density ratio ${\ensuremath{\rho}}_{p}/{\ensuremath{\rho}}_{f}\ensuremath{\gtrsim}{10}^{3}$ (with ${\ensuremath{\rho}}_{p}$ and ${\ensuremath{\rho}}_{f}$ the particle and fluid density, respectively). This is, in general, not the case for smaller particle-to-fluid density ratios, in particular not for ${\ensuremath{\rho}}_{p}/{\ensuremath{\rho}}_{f}\ensuremath{\lesssim}{10}^{2}$. In that case the pressure gradient force, added mass effects, and the Basset history force also play important roles. In this study we focus on the understanding of the role of these additional forces, all of hydrodynamic origin, in the settling of particles in Turbulence. In order to qualitatively elucidate the complex dynamics of such particles in Homogeneous Isotropic Turbulence, we first focus on the case of settling of such particles in the flow field of a single vortex. After having explored this simplified case we extend our analysis to Homogeneous Isotropic Turbulence. In general, we found that the pressure gradient force leads to a decrease in the settling velocity. This can be qualitatively understood by the fact that this force prevents the particles from sweeping out of vortices, a mechanism known as preferential sweeping which causes enhanced settling. Additionally, we found that the Basset history force can both increase and decrease the enhanced settling, depending on the particle Stokes number. Finally, the role of the nonlinear Stokes drag has been explored, confirming that it affects settling of inertial particles in Turbulence, but only in a limited way for the parameter settings used in this investigation.
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droplet size distribution in Homogeneous Isotropic Turbulence
Physics of Fluids, 2012Co-Authors: Prasad Perlekar, L. Biferale, M Sbragaglia, Sudhir S Srivastava, Federico ToschiAbstract:We study the physics of droplet breakup in a statistically stationary Homogeneous and Isotropic turbulent flow by means of high resolution numerical investigations based on the multicomponent lattice Boltzmann method. We verified the validity of the criterion proposed by Hinze [AIChE J. 1, 289 (1955)] for droplet breakup and we measured the full probability distribution function of droplets radii at different Reynolds numbers and for different volume fractions. By means of a Lagrangian tracking we could follow individual droplets along their trajectories, define a local Weber number based on the velocity gradients, and study its cross-correlation with droplet deformation.
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droplet size distribution in Homogeneous Isotropic Turbulence
arXiv: Fluid Dynamics, 2011Co-Authors: Prasad Perlekar, L. Biferale, M Sbragaglia, Sudhir S Srivastava, Federico ToschiAbstract:We study the physics of droplet breakup in a statistically stationary Homogeneous and Isotropic turbulent flow by means of high resolution numerical investigations based on the multicomponent lattice Boltzmann method. We verified the validity of the criterion proposed by Hinze (1955) for droplet breakup and we measured the full probability distribution function (pdf) of droplets radii at different Reynolds numbers and for different volume fraction. By means of a Lagrangian tracking we could follow individual droplets along their trajectories, define a local Weber number based on the velocity gradients and study its cross-correlation with droplet deformation.
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cloud droplet growth by condensation in Homogeneous Isotropic Turbulence
Journal of the Atmospheric Sciences, 2009Co-Authors: A. Lanotte, Agnese Seminara, Federico ToschiAbstract:The growth of cloud droplets by diffusion of water vapor in a three-dimensional Homogeneous Isotropic turbulent flow is considered. Within a simple model of advection and condensation, the dynamics and growth of millions of dropletsare integrated. A droplet-size spectra broadening is obtainedandit is shown to increase with the Reynolds number of Turbulence by means of two series of direct numerical simulations at increasing resolution. This is a key point toward a proper evaluation of the effects of Turbulence for condensation in warm clouds, where the Reynolds numbers typically achieve extreme values. The obtained droplet spectral broadening as a function of the Reynolds number is shown to be consistent with dimensional arguments. A generalization of this expectation to Reynolds numbers not accessible by direct numerical simulation (DNS) is proposed, yielding upper and lower bounds to the actual size spectra broadening. It is argued that the lower bound is the relevant limit at high Reynolds numbers. A further DNS matching the large scales of the system suggests consistency of the picture drawn. The assumptions underlying the model are expected to hold up to spatial scales on the order of 100 m; no direct comparison with in situ measures is possible. Additional effortis needed to evaluate the impact of this effect for condensation in more realistic cloud conditions.
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condensation of cloud microdroplets in Homogeneous Isotropic Turbulence
arXiv: Chaotic Dynamics, 2007Co-Authors: Alessandra S Lanotte, Agnese Seminara, Federico ToschiAbstract:The growth by condensation of small water droplets in a three-dimensional Homogeneous Isotropic turbulent flow is considered. Within a simple model of advection and condensation, the dynamics and growth of millions of droplets are integrated. A droplet-size spectra broadening is obtained and it is shown to increase with the Reynolds number of Turbulence, by means of two series of direct numerical simulations at increasing resolution. This is a key point towards a proper evaluation of the effects of Turbulence for condensation in warm clouds, where the Reynolds numbers typically achieve huge values. The obtained droplet-size spectra broadening as a function of the Reynolds number is shown to be consistent with dimensional arguments. A generalization of this expectation to Reynolds numbers not accessible by DNS is proposed, yielding upper and lower bounds to the actual size-spectra broadening. A further DNS matching the large scales of the system suggests consistency of the picture drawn, while additional effort is needed to evaluate the impact of this effect for condensation in more realistic cloud conditions.
Martin R Maxey - One of the best experts on this subject based on the ideXlab platform.
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settling velocity and concentration distribution of heavy particles in Homogeneous Isotropic Turbulence
Journal of Fluid Mechanics, 1993Co-Authors: Lianping Wang, Martin R MaxeyAbstract:The average settling velocity in Homogeneous Turbulence of a small rigid spherical particle, subject to a Stokes drag force, has been shown to differ from that in still fluid owing to a bias from the particle inertia (Maxey 1987). Previous numerical results for particles in a random flow field, where the flow dynamics were not considered, showed an increase in the average settling velocity. Direct numerical simulations of the motion of heavy particles in Isotropic Homogeneous Turbulence have been performed where the flow dynamics are included. These show that a significant increase in the average settling velocity can occur for particles with inertial response time and still-fluid terminal velocity comparable to the Kolmogorov scales of the Turbulence. This increase may be as much as 50% of the terminal velocity, which is much larger than was previously found. The concentration field of the heavy particles, obtained from direct numerical simulations, shows the importance of the inertial bias with particles tending to collect in elongated sheets on the peripheries of local vortical structures. This is coupled then to a preferential sweeping of the particles in downward moving fluid. Again the importance of Kolmogorov scaling to these processes is demonstrated. Finally, some consideration is given to larger particles that are subject to a nonlinear drag force where it is found that the nonlinearity reduces the net increase in settling velocity.
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the evolution of small scale structures in Homogeneous Isotropic Turbulence
Physics of Fluids, 1992Co-Authors: G R Ruetsch, Martin R MaxeyAbstract:Using direct numerical simulations of Homogeneous Isotropic Turbulence, the temporal evolution of small‐scale vorticity and passive scalar structures has been examined. Visualization of small‐scale vorticity structures indicates a process where vortex sheets may roll up into intense vortex tubes through a Kelvin–Helmholtz‐type instability. The ability of these vortex structures to mix a passive scalar has also been examined. The regions of intense scalar gradient form sheets which are found to occur in regions of persistent straining flow, such as between neighboring vortex structures of approximately equal circulation. The magnitude of vorticity of these structures is usually not large, as persistent straining is most commonly found in connection with regions of moderate vorticity. Scalar gradient sheets are also found between low intensity regions of antiparallel vorticity, which are also observed to produce persistent straining.
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small scale features of vorticity and passive scalar fields in Homogeneous Isotropic Turbulence
Physics of Fluids, 1991Co-Authors: G R Ruetsch, Martin R MaxeyAbstract:Small‐scale structures of the vorticity and passive scalar fields have been examined by means of direct numerical simulations of Homogeneous Isotropic Turbulence with 963 grid points and Rλ≊60. Both statistical and visual techniques have been used to examine the structure of certain quantities from the evolution equations for enstrophy and the scalar gradient. Tubelike regions of intense enstrophy contain large positive and sometimes large negative enstrophy production, and mostly moderate‐valued energy dissipation regions surround these tubes. The most intense regions of the scalar gradient are dissociated from the vortex tubes, and occur as large flat sheets. Within these sheets the scalar gradient production is large, the energy dissipation is small, and in their vicinity only moderate‐valued sheetlike enstrophy regions exist. The statistical techniques show that although activity in these intense regions is strong, on a volume normalized basis, by far the largest contributions to the terms in the evolution equations, along with the energy dissipation, are from low‐level ‘‘background’’ activity.
Stephen M De Bruyn Kops - One of the best experts on this subject based on the ideXlab platform.
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area of scalar isosurfaces in Homogeneous Isotropic Turbulence as a function of reynolds and schmidt numbers
Journal of Fluid Mechanics, 2020Co-Authors: Kedar Prashant Shete, Stephen M De Bruyn KopsAbstract:A fundamental effect of fluid Turbulence is turbulent mixing, which results in the stretching and wrinkling of scalar isosurfaces. Thus, the area of isosurfaces is of interest in understanding Turbulence in general, with specific applications in, for example, combustion and the identification of turbulent/non-turbulent interfaces. We report measurements of isosurface areas in 28 direct numerical simulations (DNS) of Homogeneous Isotropic Turbulence with a mean scalar gradient resolved on up to grid points with Taylor Reynolds number ranging from 24 to 633 and Schmidt number ranging from 0.1 to 7. More precisely, we measure layers with very small but finite thickness. The continuous equation we evaluate converges exactly to the area in the limit of zero layer thickness. We demonstrate a method for numerically integrating this equation that, for a test case with an analytical solution, converges linearly towards the exact solution with decreasing layer width. By applying the technique to DNS data and testing for convergence with resolution of the simulations, we verify the resolution requirements for DNS recently proposed by Yeung et al. (Phys. Rev. Fluids, vol. 3 (6), 2018, 064603). We conclude that isosurface areas scale with the square root of the Taylor Peclet number between approximately 50 and 4429, with some departure from power-law scaling evident for . No independent effect of either or is observed. The excellent scaling of area with occurs even though the probability density function of the scalar gradient is very close to exponential for but approximately lognormal when .
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area of scalar isosurfaces in Homogeneous Isotropic Turbulence as a function of reynolds and schmidt numbers
Journal of Fluid Mechanics, 2020Co-Authors: Kedar Prashant Shete, Stephen M De Bruyn KopsAbstract:A fundamental effect of fluid Turbulence is turbulent mixing, which results in the stretching and wrinkling of scalar isosurfaces. Thus, the area of isosurfaces is of interest in understanding Turbulence in general, with specific applications in, for example, combustion and the identification of turbulent/non-turbulent interfaces. We report measurements of isosurface areas in 28 direct numerical simulations (DNS) of Homogeneous Isotropic Turbulence with a mean scalar gradient resolved on up to $14\,256^{3}$ grid points with Taylor Reynolds number $Re_{\unicode[STIX]{x1D706}}$ ranging from 24 to 633 and Schmidt number $Sc$ ranging from 0.1 to 7. More precisely, we measure layers with very small but finite thickness. The continuous equation we evaluate converges exactly to the area in the limit of zero layer thickness. We demonstrate a method for numerically integrating this equation that, for a test case with an analytical solution, converges linearly towards the exact solution with decreasing layer width. By applying the technique to DNS data and testing for convergence with resolution of the simulations, we verify the resolution requirements for DNS recently proposed by Yeung et al. ( Phys. Rev. Fluids , vol. 3 (6), 2018, 064603). We conclude that isosurface areas scale with the square root of the Taylor Peclet number $Pe_{\unicode[STIX]{x1D706}}$ between approximately 50 and 4429, with some departure from power-law scaling evident for $2.4
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area of scalar isosurfaces in Homogeneous Isotropic Turbulence as a function of reynolds and schmidt numbers
arXiv: Fluid Dynamics, 2019Co-Authors: Kedar Prashant Shete, Stephen M De Bruyn KopsAbstract:A fundamental effect of fluid Turbulence is turbulent mixing, which results in the stretching and wrinkling of scalar isosurfaces. Thus, the area of isosurfaces is of interest in understanding Turbulence in general with specific applications in, e.g., combustion and the identification of turbulent/non-turbulent interfaces. We report measurements of isosurface areas in 28 direct numerical simulations (DNSs) of Homogeneous Isotropic Turbulence with a mean scalar gradient resolved on up to $14256^3$ grid points with Taylor Reynolds number $Re{_\lambda}$ ranging from 24 to 633 and Schmidt number $Sc$ ranging from 0.1 to 7. More precisely, we measure layers with very small but finite thickness. The continuous equation we evaluate converges exactly to the area in the limit of zero layer thickness. We demonstrate a method for numerically integrating this equation that, for a test case with an analytical solution, converges linearly towards the exact solution with decreasing layer width. By applying the technique to DNS data and testing for convergence with resolution of the simulations, we verify the resolution requirements for DNS recently proposed by \citet{yeung18}. We conclude that isosurface areas scale with the square root of the Taylor P\'eclet number $Pe_{\lambda}$ between approximately 50 and 4429 with some departure from power law scaling evident for $2.4 < Pe_{\lambda} < 50$. No independent effect of either $Re_{\lambda}$ or $Sc$ is observed. The excellent scaling of area with $Pe_{\lambda}^{1/2}$ occurs even though the probability density function (p.d.f.) of the scalar gradient is very close to exponential for $Re_{\lambda}=98$ but approximately lognormal when $Re_{\lambda}=633$.
Tetsuya Ozawa - One of the best experts on this subject based on the ideXlab platform.
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multimode stretched spiral vortex and nonequilibrium energy spectrum in Homogeneous shear flow Turbulence
Physics of Fluids, 2011Co-Authors: Kiyosi Horiuti, Tetsuya OzawaAbstract:The stretched spiral vortex [T. S. Lundgren, “Strained spiral vortex model for turbulent structures,” Phys. Fluids 25, 2193 (1982)] is identified in Turbulence in Homogeneous shear flow and the spectral properties of this flow are studied using direct-numerical simulation data. The effects of mean shear on the genesis, growth, and annihilation processes of the spiral vortex are elucidated, and the role of the spiral vortex in the generation of Turbulence is shown. As in Homogeneous Isotropic Turbulence [K. Horiuti and T. Fujisawa, “The multi mode stretched spiral vortex in Homogeneous Isotropic Turbulence,” J. Fluid Mech. 595, 341 (2008)], multimodes of the spiral vortex are extracted. Two symmetric modes of configurations with regard to the vorticity alignment along the vortex tube in the core region and dual vortex sheets spiraling around the tube are often educed. One of the two symmetric modes is created by a conventional rolling-up of a single spanwise shear layer. Another one is created by the conve...