13:30
Vortex Dynamics 4
Chair: Javier Jimenez
13:30
15 mins
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Statistics of Streamline Segments in a Turbulent Channel Flow with a Wavy Wall
Fabian Hennig, Jonas Boschung, Norbert Peters
Abstract: We investigate the statistical properties of so called streamline segments in a turbulent channel flow with one plane and one wavy wall. We give a short overview on the concept of streamline segments and recent results in description and modelling in this field. Finally, we show the length distribution and conditional moments of streamline segments in the wavy channel flow and compare the statistics to those in homogeneous isotropic turbulence.
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13:45
15 mins
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External and Internal interfacial turbulent shear layers
Julian Hunt, Takashi Ishihara, Jerke Eisma, Wim-Paul Breugen, Jerry Westerweel, Marianna Braza
Abstract: Simulation, PIV data, and local models show characteristics and conditional statistics of turbulence either side and within interfacial layers [I] depending on the mean profile and the presence of resistive/porous walls.
Key words; turbulence, interface structure, conditional statistics, numerical models
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14:00
15 mins
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REVERSIBILITY IN THE 3D INERTIAL TURBULENT CASCADE
Alberto Vela-Martin, Javier Jiménez
Abstract: The inviscid nature of the of the inertial range of the turbulent energy cascade suggests that it should be reversible, and that reverse cascade effects should remain in normal turbulence. Using a reversible LES model, we study some properties of the direct and reverse cascades in isotropic turbulence. The reverse cascade is fairly stable and resilient to perturbations. The study of the Lyapunov
exponents and eigenvectors of both cascades allows us to compute a ’viscous’ limit below which the system is enslaved to larger scales,
and to characterize the most unstable solutions of the forward cascade. They appear to correspond to stretched vortices.
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14:15
15 mins
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RAYLEIGH NUMBER DEPENDENCE OF THE ARCHIMEDES NUMBER DEPENDENT LARGE-SCALE FLOW STRUCTURE FORMATION IN MIXED CONVECTION
Max Körner, Christian Resagk, André Thess
Abstract: We report on experimental investigations of large-scale flow structure formation in mixed convection. We characterize the flow field by measuring the velocity fields within a rectangular model room using 2D2C PIV. The control parameters are the Reynolds number Re, the Rayleigh number Ra and the Prandtl number Pr. All parameters are linked through the Archimedes number Ar. In 6.410-2 ≤ Ar ≤ 1.39101, 4.2103 ≤ Re ≤ 6.35104 and Ra = 3.1107, Ra = 1.8108 and Pr = 0.713 we found flow 3 different flow structures. While keeping Ra and Pr constant and varying Ar through Re variations, we found an Ar dependence of the largescale flow structure formation within 6.410-2 ≤ Ar ≤ 1.39101. Furthermore, we found a Ra dependence of the structure formation, which shifts the transition points between the structures to higher Archimedes numbers and reduces the mean velocities within the
investigated domain.
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14:30
15 mins
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Kraichnan-Leith-Batchelor similarity theory and two-dimensional inverse cascades
B. Helen Burgess, Richard Scott, Theodore Shepherd
Abstract: We study the scaling properties and Kraichnan-Leith-Batchelor (KLB) theory of forced inverse cascades in generalized two-dimensional (2D) fluids ($\alpha$-turbulence models) simulated at resolution $8192^2$. We consider $\alpha=1$ (surface quasigeostrophic flow), $\alpha=2$ (2D vorticity dynamics) and $\alpha=3$. The forcing scale is well-resolved, a direct cascade is present and there is no large-scale dissipation. Coherent vortices spanning a range of sizes, most larger than the forcing scale, are present for both $\alpha=1$ and $\alpha=2$. The active scalar field for $\alpha=3$ contains comparatively few and small vortices. The energy spectral slopes in the inverse cascade are steeper than the KLB prediction $-(7-\alpha)/3$ in all three systems. Since we stop the simulations well before the cascades have reached the domain scale, vortex formation and spectral steepening are not due to condensation effects; nor are they caused by large-scale dissipation, which is absent. One- and two-point pdfs, hyperflatness factors and structure functions indicate that the inverse cascades are intermittent and non-Gaussian over much of the inertial range for $\alpha=1$ and $\alpha=2$, while the $\alpha=3$ inverse cascade is much closer to Gaussian and non-intermittent. For $\alpha=3$ the steep spectrum is close to that associated with enstrophy equipartition. Continuous wavelet analysis shows approximate KLB scaling $\mathcal{E}(k) \propto k^{-2}$ ($\alpha=1$) and $\mathcal{E}(k) \propto k^{-5/3}$ ($\alpha=2$) in the interstitial regions between the coherent vortices. Our results demonstrate that coherent vortex formation ($\alpha=1$ and $\alpha=2$) and non-realizability ($\alpha=3$) cause 2D inverse cascades to deviate from the KLB predictions, but that the flow between the vortices exhibits KLB scaling and non-intermittent statistics for $\alpha=1$ and $\alpha=2$. The results will appear in \cite{BurgessEA2015}, which has been accepted to the \emph{Journal of Fluid Mechanics}.
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14:45
15 mins
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Invariant solutions in large eddy simulation of homogeneous shear turbulence
Atsushi Sekimoto, Javier Jiménez
Abstract: The unstable invariant solutions in the large eddy simulation of homogeneous shear turbulence with vanishing kinematic viscosity are obtained by Newton-Krylov-hookstep method.
The small scale is represented by the standard Smagorinsky model with a constant Cs.
It is shown that these solutions appear by a saddle-node bifurcation as decreasing Cs and have the same symmetry with Nagata's equilibrium solutions in Couette flow (JFM 217, 519-527 (1990)).
Both lower- and upper- branch solutions are characterized by staggered
streamwise-inclined vortex pairs. Also, lower-branch solutions are localized in the vertical direction, while upper-branch solutions are characterized by taller flow structures, which is consistent with the asymptotic theory at high-Reynolds numbers (K. Deguchi \& P. Hall, Phil. Trans. R. Soc A, 372:20130352 (2014)).
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