Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R21: Turbulence: DNS |
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Chair: David Goldstein, University of Texas at Austin Room: 209 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R21.00001: Energy spectrum in high Reynolds number turbulence - high resolution DNS results Koji Morishita, Takashi Ishihara, Yukio Kaneda, Mitsuo Yokokawa, Atsuya Uno The energy spectrum and energy flux in high Reynolds number ($Re$) forced incompressible turbulence are investigated by using high-resolution DNS in a periodic box. We used negative viscosity (at a wavenumber range $kL<3$) to keep the total energy constant, and used well-developed turbulence fields as the initial conditions ($L$ is the integral length scale). The DNS with up to $6144^3$ grid points show that, after a transient period of the order of eddy turnover time, the standard deviation of the energy spectrum and that of the energy flux are largest at $kL\sim1$ and is an algebraically decreasing function of $kL$. As in previous studies, the energy spectra are insensitive to the values of $k_ {max}\eta$ when $k_{max}\eta\ge1$ ($\eta$ is the Kolmogorov length scale). The time-averaged, normalized energy spectra of high $Re$ turbulence at high $k$ overlap well with each other when they are plotted against $k\eta$. The normalized spectra have a slope steeper than -5/3 (the Kolmogorov scaling law) by factor 0.1 at $k\lambda\sim1$ ($\lambda$ is Taylor micro-scale). The DNS suggest that there is another wavenumber range ($k\lambda<1$), in which the spectrum has a slope close to -5/3, and also that the latter range increases with $Re$ and the Kolmogorov constant is $1.8\pm0.1$. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R21.00002: Direct numerical simulation of incompressible acceleration-driven variable-density turbulence Ilana Gat, Georgios Matheou, Daniel Chung, Paul Dimotakis Fully developed turbulence in variable-density flow driven by an externally imposed acceleration field, e.g., gravity, is fundamental in many applications, such as inertial confinement fusion, geophysics, and astrophysics. Aspects of this turbulence regime are poorly understood and are of interest to fluid modeling. We investigate incompressible acceleration-driven variable-density turbulence by a series of direct numerical simulations of high-density fluid in-between slabs of low-density fluid, in a triply-periodic domain. A pseudo-spectral numerical method with a Helmholtz-Hodge decomposition of the pressure field, which ensures mass conservation, is employed, as documented in Chung {\&} Pullin (2010). A uniform dynamic viscosity and local Schmidt number of unity are assumed. This configuration encapsulates a combination of flow phenomena in a temporally evolving variable-density shear flow. Density ratios up to 10 and Reynolds numbers in the fully developed turbulent regime are investigated. The temporal evolution of the vertical velocity difference across the shear layer, shear-layer growth, mean density, and Reynolds number are discussed. Statistics of Lagrangian accelerations of fluid elements and of vorticity as a function of the density ratio are also presented. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R21.00003: A lattice-Boltzmann scheme of the Navier-Stokes equations on a 3D cuboid lattice Haoda Min, Cheng Peng, Lian-Ping Wang The standard lattice-Boltzmann method (LBM) for fluid flow simulation is based on a square (in 2D) or cubic (in 3D) lattice grids. Recently, two new lattice Boltzmann schemes have been developed on a 2D rectangular grid using the MRT (multiple-relaxation-time) collision model, by adding a free parameter in the definition of moments or by extending the equilibrium moments. Here we developed a lattice Boltzmann model on 3D cuboid lattice, namely, a lattice grid with different grid lengths in different spatial directions. We designed our MRT-LBM model by matching the moment equations from the Chapman-Enskog expansion with the Navier-Stokes equations. The model guarantees correct hydrodynamics. A second-order term is added to the equilibrium moments in order to restore the isotropy of viscosity on a cuboid lattice. The form and the coefficients of the extended equilibrium moments are determined through an inverse design process. An additional benefit of the model is that the viscosity can be adjusted independent of the stress-moment relaxation parameter, thus improving the numerical stability of the model. The resulting cuboid MRT-LBM model is then validated through benchmark simulations using laminar channel flow, turbulent channel flow, and the 3D Taylor-Green vortex flow. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R21.00004: A numerical study of turbulence under time-dependent axisymmetric contraction and subsequent relaxation M.P. Clay, P.K Yeung, Z. Warhaft Turbulence subjected to axisymmetric strain is a fundamental problem which is common in engineering equipment with variable cross-section, but is not yet fully understood. We have performed direct numerical simulations on a deforming domain with grids up to $1024^3$ and a time-dependent strain history designed to mimic spatial gradients in wind-tunnel experiments (Ayyalasomayajula \& Warhaft {\em J. Fluid Mech.} {\bf 566}, 273-307 (2006)). Isotropic turbulence with a specified energy spectrum is allowed to decay and then passed through a numerical conduit of 4:1 contraction ratio. The Reynolds stress tensor, velocity gradient variances, and longitudinal and transverse one-dimensional (1D) spectra are studied during both the contraction and subsequent relaxation. Contraction leads to amplification of energy in the compressed directions and departures from local isotropy. When the strain is removed local isotropy returns quickly while the energy decays with a power law exponent smaller than for decaying isotropic turbulence. The evolution of 1D spectra including changes in shape is consistent with experiments, but a large solution domain is important. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R21.00005: Extreme events and small-scale structure in computational turbulence X.M. Zhai, P.K. Yeung, K.R. Sreenivasan Detailed analyses have been made of data from a direct numerical simulation of turbulence on a periodic domain with $8192^3$ grid points designed to improve our understanding of small-scale structure and intermittency. At the Reynolds number of this simulation (1300 based on the Taylor scale) extreme events of dissipation and enstrophy as large as $10^5$ times the mean value are observed. These events are shown to possess a form that is different from similar events at low Reynolds numbers. Extreme vorticity appears to be ``chunky'' in character, in contrast to elongated vortex tubes at moderately large amplitudes commonly reported in the literature. We track the temporal evolution of these extreme events and find that they are generally short-lived, which suggests frequent sampling on-the-fly is useful. Extreme magnitudes of energy dissipation rate and enstrophy are essentially coincident in space and remain so during their evolution. Numerical tests show sensitivity to small-scale resolution and sampling but not machine precision. The connections expected between indicators of fine-scale intermittency such as acceleration statistics and the anomalous scaling of high-order velocity structure functions are also investigated. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R21.00006: Optimization of flow initialization and perturbation forcing for fast transition towards fully developed turbulent channel flow Xin Wen, Cheng Peng, Lian-Ping Wang The turbulent channel flow has been used as the simplest setup to study the flow structure and dynamics in wall-bounded single-phase and multiphase turbulence. An aspect that has not been well studied in direct numerical simulation of such flow is how to initiate such flow simulation and force the flow so that a fully developed turbulence can be achieved relatively quickly. Often it may take $\sim$ 50 eddy turnover times (defined in terms of the channel half width and wall friction velocity) for the flow to evolve to a fully developed stage, due to different time scales involved in the wall region and the center of the channel and coupling of the flows between the two. In this talk, we explore different ways to initialize the flow and to excite the flow at the early stage. The initialization typically consists of a mean flow and a disturbance flow. The excitations could be done by adding external perturbation forcing. The parameters in the initial flow and the forcing affect the speed of transition to realistic fully developed turbulence. We will discuss how to control these parameters so that realistic flow structures, Reynolds stress and rms velocity profiles can be generated quickly while maintaining a nearly constant mean flow speed. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R21.00007: The effect of spatial discretization upon traveling wave body forcing of a turbulent wall-bounded flow Soyoung You, David Goldstein DNS is employed to simulate turbulent channel flow subject to a traveling wave body force field near the wall. The regions in which forces are applied are made progressively more discrete in a sequence of simulations to explore the boundaries between the effects of discrete flow actuators and spatially continuum actuation. The continuum body force field is designed to correspond to the ``optimal'' resolvent mode of McKeon and Sharma (2010), which has the L2 norm of $\sigma_{1}$. That is, the normalized harmonic forcing that gives the largest disturbance energy is the first singular mode with the gain of $\sigma_{1}$. 2D and 3D resolvent modes are examined at a modest Re$_{\tau}$ of 180. For code validation, nominal flow simulations without discretized forcing are compared to previous work by Sharma and Goldstein (2014) in which we find that as we increase the forcing amplitude there is a decrease in the mean velocity and an increase in turbulent kinetic energy. The same force field is then sampled into isolated sub-domains to emulate the effect of discrete physical actuators. Several cases will be presented to explore the dependencies between the level of discretization and the turbulent flow behavior. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R21.00008: Turbulence structure subjected to ''precession-like'' rotation Kartik Iyer, Irene Mazzitelli, Luca Biferale, Fabio Bonaccorso We report results from a series of numerical experiments in which the orientation of the rotation axis of a turbulent flow simulated in a periodic domain is arbitrarily changed. It is well known that rotation weakens spectral transfer and renders the flow anisotropic across all scales. However, when the orientation of rotation is changed, the spectral transfer becomes stronger and the flow becomes more isotropic. The large scale vortical structures aligned with the rotation are destroyed by the change in rotation axis. Based on these findings we attempt to discuss the dynamics of rotating turbulence subjected to precession. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R21.00009: The intense vorticity structures in isotropic turbulence with a ''Carreau-Yasuda'' fluid Afonso Ghira, Carlos Silva Direct numerical simulations of isotropic turbulence are carried out to assess the flow topology and the dynamics of the intense vorticity structures in a shear-thinning fluid. Specifically, the Carreau-Yasuda fluid model is used to describe the shear-thinning viscosity while the intense vorticity structures are tracked using a numerical algorithm. The eddy characteristics are compared to the ones observed in Newtonian turbulence and the effects of the shear-thinning are assessed in relation to the small scale dynamics of the flow. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R21.00010: Direct Numerical Simulations of Sound-Orifice-Boundary Layer Interaction Qi Zhang, Daniel Bodony We report on a series of direct numerical simulations (DNS) of the interaction of a monochromatic incident acoustic field with a cavity-backed circular orifice in the presence of laminar and turbulent boundary layers of freestream Mach number 0.5 and momentum thickness Reynolds number 2,300, with application to acoustic liners. The simulations show that the addition of the orifice increases the drag and can induce laminar-to-turbulent transition at sufficiently high acoustic levels. Furthermore, the sound-orifice-boundary layer system support three distinct timescales whose spatial distributions change with the phase of the incident sound. Details of the near-orifice interaction are studied to create a model of the orifice discharge coefficient that is part of a time-domain, nonlinear reduced-order model (ROM) for the liner impedance. Comparisons between the ROM-predicted and DNS-measured near-orifice flow and acoustic impedance are given. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R21.00011: A constant-energy physical-space forcing method for steadier statistics and faster convergence to homogeneous-isotropic turbulence Maxime Bassenne, Javier Urzay, George I. Park, Parviz Moin We investigate a new constant-energy forcing method for homogeneous-isotropic turbulent flows forced linearly in physical space. The method bears no computational overhead and it consists of a proportional controller embedded in the forcing coefficient. Comparisons of this forcing method are made with other existing variable-energy approaches, using direct numerical simulations (DNS) and large-eddy simulations (LES). We find that the proposed forcing method shortens the transient period from an user-defined artificial flow field to forced turbulence while maintaining steadier statistics. For illustration, the proposed forcing method is applied to a dilute particle-laden homogeneous-isotropic turbulent flow to highlight some of the influences of the forcing strategies on the statistics of the disperse phase. [Preview Abstract] |
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