Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D21: Turbulence Simulation: Isotropic/Homogenous Shear |
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Chair: James Riley, University of Washington Room: 30B |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D21.00001: On the kinematics of scalar iso-surfaces in turbulent flow Weirong Wang, James J. Riley, John C. Kramlich The behavior of scalar iso-surfaces in turbulent flows is of fundamental interest and also of importance in certain applications, e.g., the stoichiometric surface in nonpremixed, turbulent reacting flows. Of particular interest is the average area per unit volume of the surface, ${\Sigma}$. We report on the use of direct numerical simulations to directly compute ${\Sigma}$ and to model its evolution in time for the case of isotropic turbulence. Using both a direct measurement technique, and also Corrsin's (1955) suggestion of surface-crossing, we find the iso-surface in space and also measure ${\Sigma}$ as the surface evolves in time. This allows us to follow the growth of the surface due to local surface stretching and its ultimate decrease due to molecular destruction. We are also able to measure the principal terms in the evolution equation for ${\Sigma}$, including the surface stretching term ${\cal S}$ and the molecular destruction term ${\cal M}$. For example, for the scalar $Z$ we find that its spatial derivative quantities are approximately statistically independent of $Z$ itself, so that ${\cal S}$ and ${\cal M}$ are approximately statistically independent of $Z$ as well. Finally, a model is proposed which fairly accurately predicts the evolution of ${\Sigma}$. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D21.00002: Local topology of energy transport in isotropic turbulence Jonas Boschung, Charles Meneveau Similar to the velocity vector field, whose tangent (stream) lines represent how fluid volume (or mass in constant density flows) is transported in the flow, it is of interest to consider the vector field corresponding to the transport of mechanical energy (Meyers \& Meneveau, 2012). The transport includes advection and viscous diffusion. In order to characterize the local topology of this vector field in turbulence, we examine statistical properties of its gradient field. This energy transport field is not divergence-free, due to dissipation and unsteady changes of kinetic energy. Therefore, the first invariant (the trace) of its gradient tensor is not zero, as in compressible flow. The three invariants $P_E$, $Q_E$ and $R_E$ of the energy transport gradient tensor are analyzed using concepts developed earlier for analysis of compressible flows. Data from DNS of isotropic turbulence is used, from the JHU database (Li et al. 2008, JoT), as well as other sources. Contracting node-like topology occurs very frequently, consistent with the dissipative nature of fluid turbulence. Further topological properties are established based on conditional PDFs of the invariants, and flow visualizations are used to develop insights into the local structure of the energy transport vector field [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D21.00003: Scaling of Lyapunov Exponents in Homogeneous, Isotropic DNS Nicholas Fitzsimmons, Myoungkyu Lee, Nicholas Malaya, Robert Moser In order to study the nature of the chaos in turbulence, we investigate Lyapunov exponents. These exponents measure the rate of separation of initially infinitesimally close trajectories in phase space. Lyapunov exponents are examined for two purposes: to investigate the scaling of the exponents with respect to the parameters of forced homogeneous isotropic turbulence, and to locate the chaotic features of turbulence. Specifically, we explore the scaling of the Lyapunov exponents with respect to the Taylor Reynolds number, $Re_\lambda$, and with respect to the ratio of the integral length scale and the computational domain. The latter is varied through manipulation of the Uhlenbeck Ornstein process, which forces the DNS. The exponents are measured by introducing a linear disturbance, evolving it with the linearized Navier-Stokes equation, and normalizing it at each step. Using this disturbance and the velocity field one calculates the instantaneous growth rate of the disturbance at each time step. We will show how these exponents may then be used to measure the predictability in turbulent flows and allow for the study of the instabilities of the chaotic field. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D21.00004: ABSTRACT MOVED TO M17.00010 |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D21.00005: Inverse energy cascade in rotational turbulence Huidan (Whitney) Yu, Rou Chen, Hengjie Wang Rotation influences large-scale motions in the Earth's atmosphere and oceans and it is also important in many industrial applications such as turbo machinery, rotor-craft, and rotating channel etc. We study rotation effects on decaying isotropic turbulence through direct numerical simulation using lattice Boltzmann method. A Coriolis force characterized by the angular velocity of the frame of reference $\Omega $ is included in the lattice Boltzmann equations. The effects of rotation on fundamental turbulence features such as kinetic energy and enstrophy decay, energy spectrum, etc. are studied. The decay laws are quantitatively captured. Inverse energy cascade are observed in the 3D turbulence with and without rotation. The scaling of the inverse energy cascade and its relation to initial energy spectrum are explored. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D21.00006: Simulation of Homogeneous Turbulence Subjected to Plane Strain Chris Zusi, J. Blair Perot Direct numerical simulation is used at a resolution of 512$^{3}$ to investigate the behavior of turbulence subjected to rotation, as well as plane and axi-symmetric strain. The initial isotropic turbulence is generated by the stirring action of many small randomly placed cubes, rather than imposed as an initial condition. Anisotropic turbulent structure is then generated by rotation, dimensionless plane strain or axi-symmetric strain. Multiple simulations are used to investigate the influence of initial conditions, rotation rate, strain rate and Reynolds number on the strained turbulence structure and its subsequent anisotropic decay. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D21.00007: The effect of aspect ratio on statistically-stationary homogeneous shear flow Siwei Dong, Atsushi Sekimoto, Javier Jim\'enez The effect of the aspect ratio of the numerical domain is investigated in homogeneous shear turbulence at moderate $Re_\lambda=40\mbox{-}100$, over long times for which its length scales are constrained by the numerics. For $L_y/L_z>1$, the cross-shear box size, $L_y$, has a negligible influence on the statistics. For $A=L_x/L_z\le 2$, the flow contains a relatively steady streamwise-velocity $(u)$ streak, and the r.m.s. $u'$ dominates the energy. For $A\ge 2$, the integral length is proportional to the spanwise box size $L_z$ and the r.m.s. velocities are proportional to $SL_z$. That regime is dominated by strong bursting. In $2 [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D21.00008: Direct numerical simulations of statistically-stationary homogeneous shear turbulence Atsushi Sekimoto, Siwei Dong, Javier Jim\'enez The long-term behaviour of homogeneous shear turbulence is studied using a new direct simulation code. The incompressible Navier-Stokes and continuity equations are formulated in terms of the vertical vorticity and of the Laplacian of the vertical velocity. The domain is periodic in the streamwise $(x)$ and spanwise $(z)$ directions, and periodic between shifting points of the lower and upper $y$-boundaries. The discretization is dealiased Fourier in $(x,z)$ and compact finite differences in $y$, with the shear-periodic boundary conditions embedded in the finite-difference matrices for each Fourier mode. There is no recurrent remeshing, thus avoiding the secular loss of enstrophy during long integration times. The code was validated using linear theory, as well as the initial shearing of isotropic turbulence. The results depend only weakly on the vertical box dimension. Over long times it develops the streaks and quasi-periodic bursting behaviour typical of wall-bounded turbulence, with mean shear parameters, $Sq^2/\epsilon$, of the same order as those in the logarithmic layer of turbulent channels, suggesting that it can be used as a model for the self-sustaining mechanism of inertial wall turbulence. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D21.00009: Turbulence generation through concentrated momentum sources Agustin Maqui, Diego Donzis In many applications, thermal non-equilibrium can be a major contributor to the dynamics of turbulence. One example amenable to laboratory experiments is that of laser induced photo-dissociation of molecules, which can create fragments with very large velocities. It is of fundamental and practical interest to investigate whether this large momentum excess can be used to generate realistic turbulence. Consequently, direct numerical simulations (DNS) with concentrated momentum sources that reproduce the photo-dissociation process have been conducted to study the creation and evolution of turbulence.Two critical times are found between which the flow quickly reorganizes into fully developed turbulence. The characteristic time scales are successfully scaled with parameters related to the initial conditions and the establishment of turbulence is studied through the evolution of the velocity gradients, spectra, and anisotropy measures. Preliminary results indicate that compressible turbulence is reached at an earlier stage than incompressible. Furthermore, it is shown that higher Taylor Reynolds numbers ($R_\lambda$) can be reached for compressible flows with weaker momentum sources. Further results and consequences for particular cases realizable in laboratories will be discussed. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D21.00010: Mechanism of axis-switching in low aspect-ratio rectangular jets Nan Chen, Huidan (Whitney) Yu Axis-switching (AS) refers to the change in the orientation of the major axis of the jet from initial spanwise to later lateral direction. This phenomenon is of great interest both from fundamental physics and practical application points of view. AS is most noticeable in square and low aspect-ratio (AR) rectangular jet flows. It has been reported experimentally and computationally that square jet and rectangular jets switch the major axis 45$^{0}$ and 90$^{0}$ respectively. In this work we explore the mechanism of AS phenomenon through direct numerical simulation using lattice Boltzmann method for 5 rectangular jets with different aspect ratios at relatively low Reynolds numbers. We identified the three characteristic regions of jet flow that are potential core (PC), characteristic decay (CD), and axisymmetric decay (AD) regions. It is found that 45$^{0}$ (square jet) and 45$^{0}$ first then 90$^{0}$ (rectangular jets) AS occur in the CD region. The correlation between jet propagate velocity and coherent structure shows that the 45$^{0}$ AS occurs close to the entrance of CD region where the correlation gets the maximum value, indicating that the 45$^{0 }$AS is driven mainly by the jet propagatation. The 90$^{0 }$AS appears close to the end of the CD region and doesn't show unified relation to the correlation for different jets. The mechanism of 90$^{0 }$AS is more complicated because both jet propagation and boundary condition contribute the driven. Meanwhile, flow pattern and vortices are closely looked into to reveal the mechanism of AS phenomena. [Preview Abstract] |
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