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 R33: Turbulence: Strongly Anisotropic Flows 
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Chair: Fabien Godeferd, Ecole Centrale de Lyon Room: Oregon Ballroom 202 

R33.00001: ABSTRACT WITHDRAWN 
Tuesday, November 22, 2016 1:43PM  1:56PM 
R33.00002: Vorticity alignment and spectral statistics in a variabledensity turbulent flow Ilana Gat, Georgios Matheou, Daniel Chung, Paul Dimotakis Turbulent flows with high density gradients subject to an externally imposed acceleration field, such as gravity, occur in many phenomena, ranging from geophysics to astrophysics. This study investigates turbulence in fluids over a range of density ratios, from small (R=1.005) to large (R=10). The investigation relies on direct numerical simulation using the incompressible variabledensity NavierStokes equations, in a triply periodic domain. The flow is initialized with density gradients perpendicular to the acceleration field. This configuration induces baroclinic torques with shear and buoyancy contributing to the evolution of turbulence and turbulent mixing. Of interest in fluid modeling is vorticity alignment, which is presented for the broad density ratio range studied. Prominent variabledensity contributions to the vorticity field such as baroclinic torques are discussed. Kineticenergy spectra are compared to specific kinetic energy spectra to illustrate aspects of variabledensity effects. [Preview Abstract] 
Tuesday, November 22, 2016 1:56PM  2:09PM 
R33.00003: Scale by scale energy flux in rotating homogeneous turbulence Fabien Godeferd, Donato Vallefuoco, Aurore Naso Homogeneous rotating turbulence is strongly anisotropic and exhibits vortices elongated along the rotation axis, and reduced downscale energy cascade w.r.t. isotropic turbulence. We characterize its dynamics by the KarmanHowarthMonin equation for 2ndorder 2point velocity correlations $R(\mathbf{r},t)$, where $\mathbf{r}$ is the $vector$ separation: $\partial_t R/2=\nabla\cdot\mathbf{F}/4+\nu\nabla^2 R+\phi_{inj}$ showing the balance between energy flux $\mathbf{F}$ (3rdorder moment of velocity increment $\mathbf{u}(\mathbf{x}+\mathbf{r})\mathbf{u}(\mathbf{x})$), dissipation, and injected energy. From Direct Numerical Simulations of forced rotating turbulence, we get estimates of all terms in the KHM eq. at each scale $\mathbf{r}$. A map of $\mathbf{F}$ components in the axisymmetric frame is obtained, and compared with experimental data (Lamriben et al. 2011) for two components measured in an azimuthal plane. We evaluate the role of the unmeasured azimuthal component of energy flux, at different Rossby numbers. We also explain why experiments and inertial wave turbulence theory by Galtier (2013) predict opposing trends in the dependence of the radial energy flux with the direction of $\mathbf{r}$, by identifying two separate regimes in different scale ranges. [Preview Abstract] 
Tuesday, November 22, 2016 2:09PM  2:22PM 
R33.00004: The structure of MHD turbulence under an external magnetic field: results from simulations on elongated domains X.M. Zhai, P.K. Yeung Turbulence in an electrically conducting fluid in the limit of low magnetic Reynolds number is, because of the Lorentz force due to an external magnetic field, very different from classical turbulence at both the large scales and the small scales. The importance of minimizing finite domainsize effects on the large scale development has often tended to limit the Reynolds number reached in the past. In this work we use periodic domains stretched along the magnetic field with aspect ratio up to 8 and beyond. The initial state is obtained from decaying isotropic turbulence with largeeddy length scales of order 1\% of the length of the domain. After a transient period the kinetic energy returns to a power law decay while the integral length scales in the direction parallel to the magnetic field show preferential growth. At early times the parallel velocity component becomes stronger than the other two but this anisotropy is subsequently reversed under the combined effects of anisotropic Joule dissipation and viscous dissipation. The small scales show characteristics of quasi twodimensional behavior in the transverse plane. Results over a range of magnetic interaction parameters and Reynolds numbers are compared with known theoretical predictions. [Preview Abstract] 
Tuesday, November 22, 2016 2:22PM  2:35PM 
R33.00005: Anisotropic grid adaptation in LES Siavash Toosi, Johan Larsson The modeling errors depend directly on the grid (or filter) spacing in turbulenceresolving simulations (LES, DNS, DES, etc), and are typically at least as significant as the numerical errors. This makes adaptive gridrefinement complicated, since it prevents the estimation of the local error sources through numerical analysis. The present work attempts to address this difficulty with a physicsbased errorsource indicator that accounts for the anisotropy in the smallest resolved scales, which can thus be used to drive an anisotropic gridadaptation process. The proposed error indicator is assessed on a sequence of problems, including turbulent channel flow and flows in more complex geometries. The formulation is geometrically general and applicable to complex geometries. [Preview Abstract] 
Tuesday, November 22, 2016 2:35PM  2:48PM 
R33.00006: Crossover between two and threedimensional turbulence in spatial mixing layers Luca Biancofiore We investigate how the domain depth affects the turbulent behaviour in spatially developing mixing layers by means of largeeddy simulations (LES) based on a spectral vanishing viscosity technique. Analyses of spectra of the vertical velocity, of Lumley's diagrams, of the turbulent kinetic energy and of the vortex stretching show that a twodimensional behaviour of the turbulence is promoted in spatial mixing layers by constricting the fluid motion in one direction. This finding is in agreement with previous works on turbulent systems constrained by a geometric anisotropy, pioneered by Smith, Chasnov \& Waleffe [\emph{Phys. Rev. Lett.}, {\bf 77}, 24672470]. We observe that the growth of the momentum thickness along the streamwise direction is damped in a confined domain. A full twodimensional turbulent behaviour is observed when the momentum thickness is of the same order of magnitude as the confining scale. [Preview Abstract] 
Tuesday, November 22, 2016 2:48PM  3:01PM 
R33.00007: Advection and the Efficiency of Spectral Energy Transfer in TwoDimensional Turbulence Nicholas Ouellette, Lei Fang We report measurements of the geometric alignment of the smallscale turbulent stress and the largescale rate of strain that together lead to the net flux of energy from small scales to large scales in twodimensional turbulence. We find that the instantaneous alignment between these two tensors is weak, and thus that the spectral transport of energy is inefficient. We show, however, that the strain rate is much better aligned with the stress at times in the past, suggesting that the differential advection of the two is responsible for the inefficient spectral transfer. We provide evidence for this conjecture by measuring the alignment statistics conditioned on weakly changing stress history. Our results give new insight into the relationship between scaletoscale energy transfer, geometric alignment, and advection in turbulent flows. [Preview Abstract] 
Tuesday, November 22, 2016 3:01PM  3:14PM 
R33.00008: Janus spectra: cascades without local isotropy Chienchia Liu, Rory Cerbus, Pinaki Chakraborty Twodimensional turbulent flows host two disparate cascades: of enstrophy and of energy. The phenomenological theory of turbulence, which provides the theoretical underpinning of these cascades, assumes local isotropy. This assumption has been amply verified via computational, experimental and field data amassed to date. Local isotropy mandates that the streamwise ($u$) and transverse ($v$) velocity fluctuations partake in the same cascade; consequently, the attendant spectral exponents ($\alpha_u$ and $\alpha_v$) of the turbulent energy spectra are the same, $\alpha_u = \alpha_v$. Here we report experiments in soapfilm flows where $\alpha_u$ corresponds to the energy cascade, but concurrently $\alpha_v$ corresponds to the enstrophy cascade, as if two mutually independent turbulent fields of disparate dynamics were concurrently active within the flow. This species of turbulent energy spectra, which we term the Janus spectra, has never been observed or predicted theoretically. Remarkably, the tools of phenomenological theory can be invoked to elucidate this manifestly anisotropic flow. [Preview Abstract] 
Tuesday, November 22, 2016 3:14PM  3:27PM 
R33.00009: Direct and inverse energy cascades in strongly rotating turbulent flows Ganapati Sahoo, Irene Mazzitelli, Prasad Perlekar, Fabio Bonaccorso, Luca Biferale \\ Rotation plays a key role in many geophysical and astrophysical flows. Under a strong rotation rate (low Rossby numbers), threedimensional turbulent flows show a tendency to develop fluctuations in a plane perpendicular to the rotation axis leading to a twodimensional and threecomponents (2D3C) evolution. By using high resolution direct numerical simulations up to $4096^3$ collocation points we present a systematic analysis of the 2D3C field and of the energy transport both concerning direct and inverse cascades using a decomposition in helicalFourier modes. [Preview Abstract] 
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