Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A34: Turbulence: Analysis of DNS Models |
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Chair: Charles Meneveau, Johns Hopkins University Room: Oregon Ballroom 203 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A34.00001: Analysis of Lagrangian stretching in turbulent channel flow using a database task-parallel particle tracking approach Charles Meneveau, Perry Johnson, Stephen Hamilton, Randal Burns An intrinsic property of turbulent flows is the exponential deformation of fluid elements along Lagrangian paths. The production of enstrophy by vorticity stretching follows from a similar mechanism in the Lagrangian view, though the alignment statistics differ and viscosity prevents unbounded growth. In this paper, the stretching properties of fluid elements and vorticity along Lagrangian paths are studied in a channel flow at $Re_\tau = 1000$ and compared with prior, known results from isotropic turbulence. To track Lagrangian paths in a public database containing Direct Numerical Simulation (DNS) results, the task-parallel approach previously employed in the isotropic database is extended to the case of flow in a bounded domain. It is shown that above $100$ viscous units from the wall, stretching statistics are equal to their isotropic values, in support of the local isotropy hypothesis. Normalized by dissipation rate, the stretching in the buffer layer and below is less efficient due to less favorable alignment statistics. The Cram\'{e}r function characterizing cumulative Lagrangian stretching statistics shows that overall the channel flow has about half of the stretching per unit dissipation compared with isotropic turbulence. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A34.00002: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A34.00003: Small-scale anisotropy in turbulent boundary layers Alain Pumir, Haitao Xu, Eric Siggia In a channel flow, the velocity fluctuations are inhomogeneous and anisotropic. Yet, the small-scale properties of the flow are expected to behave in an isotropic manner in the very large Reynolds number limit. We consider the statistical properties of small-scale velocity fluctuations in a turbulent channel flow at moderately high Reynolds number (Re$_{\mathrm{\tau \thinspace }}\approx $ 1000), using the Johns Hopkins University turbulence database. Away from the wall (y$^{\mathrm{+\thinspace }}$\textgreater 200), the skewness of the normal derivative of the streamwise velocity fluctuation is approximately constant, of order 1, while the Reynolds number based on the Taylor scale is R$_{\mathrm{\lambda \thinspace }}\approx $ 150. This defines a small-scale anisotropy that is stronger than in turbulent homogeneous shear flows at comparable values of R$_{\mathrm{\lambda }}$. In contrast the vorticity-strain correlations that characterize homogenous isotropic turbulence are nearly unchanged in channel flow even though they vary with distance from the wall with an exponent that can be inferred from the local dissipation. The statistical properties of the fluctuating velocity gradient in turbulent channel flow are therefore characterized, on one hand, by observables which are insensitive to the anisotropy, and behave as in homogeneous isotropic flows, and on the other hand by quantities which are sensitive to the anisotropy. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A34.00004: Entropy Production and Fluctuation Relation in Turbulent Convection Sergio Chibbaro, Francesco Zonta We report on a numerical experiment performed to analyze fluctuations of the entropy production in turbulent thermal convection. Using Direct Numerical Simulations (DNS), we estimate the entropy production from instantaneous measurements of the local temperature and velocity fields sampled along the trajectory of a large number of point-wise Lagrangian tracers. Entropy production is related to the work made by buoyancy force. The entropy production is characterized by large fluctuations and becomes often negative. This represents a sort of "finite-time" violation of the second principle of thermodynamics, since the direction of the energy flux is opposite to that prescribed by the external gradient. We provide a physical-sound definition of energy-scale characterizing the sytem, based upon Kolmogorov theory. Then, we link our results with recent theory of statistical mechanics of nonequilibrium systems, notably the results obtained by Evans, Cohen, Morris and Gallavotti for generic reversible dynamical systems. We show that the fluctuations of entropy production observed in the present system verify neatly the Fluctuation Relation (FR), cornerstone of that theory, even though the system is time-irreversible. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A34.00005: On the establishment of fully developed turbulence in direct numerical simulations driven by stochastic forcing Sualeh Khurshid, Diego Donzis, Katepalli Sreenivasan Turbulent statistics are commonly described within the classical Richardson-Kolmogorov paradigm which aims at characterizing flows at very high Reynolds numbers. Recent work, however, has shown that certain aspects observed and predicted by high-Reynolds-number theories are present at very low Reynolds numbers. This motivates our investigation of the emergence of turbulent behavior using highly resolved direct numerical simulations (DNS) on a periodic box starting from zero initial conditions and driven by stochastic forcing at the largest scales. In particular, we focus on the evolution of different wavenumber bands in time to study short-time behavior. Detailed analysis of our DNS data shows limitations of classical approaches in explaining the development of energy cascade. We show how energy is transferred to smaller scales and the rate at which this transfer proceeds. A description of the emergence of small scale intermittency and moments of velocity gradients is also presented. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A34.00006: The importance of 3D local averaging in turbulence theory: some examples from high-resolution DNS Pui-Kuen Yeung, X.M Zhai, K.P. Iyer, K.R. Sreenivasan Dissipation fluctuations in turbulence become increasingly intermittent as the Reynolds number increases. Both theoretical and practical reasons then force us to consider the fluctuations averaged locally over three-dimensional (3D) volumes of various sizes. Often, the practice has been to supplant 3D averages by 1D averages, and to replace proper 3D quantities by convenient 1D surrogates. We examine the consequence of these practices using DNS data on a large grid of $8192^3$ at a Taylor-microscale Reynolds number 1300. We show that these common practices can often lead to erroneous results and significant ambiguities. For instance, both the dissipation and enstrophy turn out to possess the same inertial-range intermittency exponent; moments of locally-averaged dissipation and enstrophy become closer to each other with increasing order (because extreme events in both are spatially co-located); the longitudinal and transverse velocity increments scale similarly---all in contrast to results obtained using the simplifying practices mentioned above. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A34.00007: Near-wall Behavior of a Scale Self-Recognition Mixed SGS Model Mizuki Kihara, Yuki Minamoto, Yoshitsugu Naka, Naoya Fukushima, Masayasu Shimura, Mamoru Tanahashi A Scale Self-Recognition Mixed SGS Model was developed in terms of GS-SGS energy transfer in homogenius isotropic turbulence by Fukushima et al. (2015). In the present research, the near-wall characteristics of the Smagorinsky coefficient, $C_S$ are investigated in terms of GS-SGS energy transfer by analyzing DNS data of turbulent channel flows at $Re_\tau =$ 400, 800 and 1270. $C_S$ is dependent on grid anisotropy, and this cause dependences of $C_S$ on $Re_\tau$. It is revealed that $C_S$ obtained directly from the DNS data is independent of $Re_\tau$ and dependent on only dimensionless wall distance, $y^+$ and filter-width to Kolmogorov scale ratio corrected by $f$, $f \cdot {\it \Delta}/ \eta$, when the grid anisotropy is isolated from $C_S$ by using the correction function $f$ proposed by Scotti et al. (1993). The contributions of Leonard, cross and Reynolds terms to total energy transfer are also independent of $Re_\tau$ and dependent on only $y^+$ and $f \cdot {\it \Delta} / \eta$ in the near-wall region. These results suggest that $C_S$ can be determined dynamically from $f \cdot {\it \Delta}/ \eta$ in the wall turbulence if $\eta$ is sufficiently predicted from the grid scale quantities. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A34.00008: Extreme accelerations in turbulent flows John Lawson, Cristian Lalescu, Michael Wilczek, Eberhard Bodenschatz Even in weakly turbulent flows, fluid tracers routinely undergo strong accelerations tens of standard deviations in excess of the mean. These extreme events are thought to influence everyday phenomena such as rain formation in wet clouds or the turbulent combustion of fuel in engines. We report results on high resolution particle tracking experiments in a vigorously stirred turbulent flow between $R_\lambda = 130$ and $450$. These are matched with high resolution direct numerical simulations of isotropic turbulence. By acquiring very large datasets we quantify the distribution of rare, strong acceleration events (as infrequent as one in $10^8$ and in excess of 30 standard deviations) and their scaling with Reynolds number. We present back-to-back comparisons between the two to quantify the statistics of accelerations at unprecedented accuracy and discuss their consequences for turbulent flows. [Preview Abstract] |
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