10:30
Geophysical and astrophysical turbulence 1
Chair: Steve Tobias
10:30
15 mins
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Geostrophic convective turbulence: the effect of boundary layers
Rodolfo Ostilla-Mónico, Erwin van der Poel, Rudie Kunnen, Roberto Verzicco, Detlef Lohse
Abstract: We conduct computations of rotating Rayleigh-Bénard convection in the so-called geostrophic regime, characterized by strong thermal forcing (high Rayleigh numbers) and strong rotation (small Ekman numbers). We employ the full Navier-Stokes equations in our computations and compare no-slip and stress-free boundaries for the plates. The Ekman boundary layers, that exist in the no-slip case but not for stress-free, enhance convective heat transfer and prevent the formation of large-scale flow structures.
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10:45
15 mins
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Non local resonances in weak turbulence of gravity-capillary waves
Quentin Aubourg, Nicola Mordant
Abstract: We report a laboratory investigation of weak turbulence of water surface waves in the gravity-capillary crossover. By using time-space resolved profilometry and a bicoherence analysis, we study the 3-wave resonant interactions that are responsible for energy transfer among waves. We show that the energy transfer occurs through non local coupling between capillary and gravity waves.
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11:00
15 mins
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DIRECT NUMERICAL SIMULATIONS OF TURBULENT FLOW THROUGH POROUS CHANNELS AND DUCTS
Arghya Samanta, Ricardo Vinuesa, Iman Lashgari, Philipp Schlatter, Luca Brandt
Abstract: Direct numerical simulations of the fully developed turbulent flow through a porous channel and duct are performed based on the spectral element code Nek5000. The volume-averaged Navier-Stokes (VANS) equations are implemented in order to describe the flow in the composite medium. The numerical simulations of the VANS equations are carried out at a constant value of the bulk Reynolds number when the porosity, or equivalently, the permeability of the medium varies successively. The mean and turbulent
energy budgets are computed and the effect of porosity on the secondary flow in a duct is examined.
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11:15
15 mins
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DIRECT NUMERICAL SIMULATIONS OF PARTICLE-DRIVEN GRAVITY CURRENTS IN A BASIN CONFIGURATION
Luis Espath, Leandro Pinto, Sylvain Laizet, Jorge Silvestrini
Abstract: Three-dimensional highly resolved Direct Numerical Simulations (DNS) of a particle-driven gravity current are presented for the lock-exchange problem in a basin configuration. Two Reynolds numbers are investigated in order to identify differences in
the flow structures and dynamics. For this numerical study, we limit our investigations to gravity currents over a flat bed in which density differences are small enough for the Boussinesq approximation to be valid. The concentration of particles is described in an Eulerian fashion by using a transport equation combined with the incompressible Navier-Stokes equations, with possibility of particles deposition but no erosion and re-suspension. For this original flow configuration, it is found that the Reynolds number has a strong influence on the flow, in particular on the deposition pattern over the flat bed. Furthermore, we found out that the well-known lobe-and-cleft patterns at the head of the current have a different shape than what is usually observed for the lock-exchange problem in a channel configuration. The curvated shape of the front has a significant twisting effect on the structures at the head of the current.
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11:30
15 mins
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INVERSE CASCADE IN SPACE TURBULENCE DURING CURRENT DISRUPTION
Anthony Lui
Abstract: Wavelet analysis of magnetic fluctuations during space turbulence observed in current disruption events of the Earth’s magnetotail shows that energy is released at progressively lower frequency with time. This feature departs from the expectation of fluid turbulence and arises from the excitation of a kinetic instability in the Earth’s magnetotail current sheet. From insights gained from two-dimensional particle simulation of the instability, it is found that the inverse cascade phenomenon can be identified as the basic property of the excited plasma instability. It is caused by excited waves associated with the instability having slower growth rates with increasing wavelength.
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11:45
15 mins
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DNS of inertial wave attractors in a librating annulus with height-dependent gap width
Marten Klein, Abouzar Ghasemi Varnamkhasti, Torsten Seelig, Ion Dan Borcia, Uwe Harlander, Andreas Will
Abstract: Direct numerical simulations (DNS) of inertial wave attractors have been carried out in a librating Taylor-Couette system with broken mirror symmetry in the radial-axial cross-section. The inertial wave excitation mechanism and its localisation at the edges was clarified by applying boundary layer theory. Additional resonance peaks in the simulated response spectra were found to agree with low-order wave attractors obtained by geometric ray tracing. Numerics and theory are in qualitative agreement with recent lab experiments.
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12:00
15 mins
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Direct and inverse energy cascades in a forced rotating turbulence experiment
Frédéric Moisy, Pierre-Philippe Cortet, Basile Gallet, Antoine Campagne
Abstract: We present experimental evidence for a double cascade of kinetic energy in a statistically stationary rotating turbulence experiment. Turbulence is generated by a set of vertical flaps which continuously injects velocity fluctuations towards the center of a rotating water tank. The energy transfers are evaluated from two-point third-order three-component velocity structure functions, which we measure using stereoscopic particle image velocimetry in the rotating frame. Without global rotation, the energy is transferred from large to small scales, as in classical three-dimensional turbulence. For nonzero rotation rates, the horizontal kinetic energy presents a double cascade: a direct cascade at small horizontal scales and an inverse cascade at large horizontal scales. By contrast, the vertical kinetic energy is always transferred from large to small horizontal scales, a behavior reminiscent of the dynamics of a passive scalar in two-dimensional turbulence. At the largest rotation rate the flow is nearly two-dimensional, and a pure inverse energy cascade is found for the horizontal energy. To describe the scale-by-scale energy budget, we consider a generalization of the Kármán-Howarth-Monin equation to inhomogeneous turbulent flows, in which the energy input is explicitly described as the advection of turbulent energy from the flaps through the surface of the control volume where the measurements are performed.
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