10:30
Vortex Dynamics 1
Chair: Helge Andersson
10:30
15 mins
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REVISITING THE PARADIGM OF PREDOMINANT VORTEX STRETCHING AND RELATED
Arkady Tsinober
Abstract: We address two issues of paradigmatic nature. The first one, among other things, concerns the fact how it is possible to have predominant vortex stretching mostly due to the largest eigen-strain without predominant alignment of vorticity ω with the eigen-vector λ1 corresponding to the largest eigen-strain Λ1. The second is about the paradigmatic qualitative difference between the fields of vorticity and that of rate of strain.
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10:45
15 mins
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Large scale organization of near wall turbulent boundary layer
Raoul Dekou, Michel Stanislas, Jean-Marc Foucaut
Abstract: An experimental database at a Reynolds number based on momentum thickness Re close to 9800, was obtained in the Laboratoire de Mécanique de Lille wind tunnel with stereo-PIV (SPIV) and hot wire anemometry (HWA). With a Linear Stochastic Estimation procedure based on correlations computation, a 3 component field is reconstructed at high
frequency from stereo-PIV at 4Hz and hot wire data at 30kHz. This paper describes methods for extracting large scale coherent structures (streaks and vortices) from the reconstructed PIV field. The outcoming features are characterized (size, intensity and life time), and the results are discussed with emphasis on the origin of the structures, their spatio-temporal organization and energetic contribution to the flow.
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11:00
15 mins
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Skin-friction field in turbulent convection
Anastasiya Kolchynska, Vinodh Bandaru, Janet Scheel, Jörg Schumacher
Abstract: The dynamics of the boundary layers of temperature and velocity are the key to deeper understanding of turbulent transport of heat and momentum in thermal convection. Here, the structure of the skin friction field at the bottom and top plates of a Rayleigh-B\'{e}nard convection setup is investigated. We therefore analyze data obtained in direct numerical simulations of Rayleigh-B\'{e}nard convection in a cylindrical cell of aspect ratio $\Gamma = 1$. Our analysis is focused to critical points of the two-dimensional skin friction field at the walls. We analyze the statistics of the critical points and relate them to the thermal plumes which detach from the wall and move up into bulk.
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11:15
15 mins
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Vortex Dynamics in The Transitional and Turbulent Wake of 6:1 Prolate Spheroid at 45-deg incidence angle
Fengjian Jiang, José P. Gallardo, Helge I. Andersson
Abstract: The incompressible flow past a 6:1 prolate spheroid with an inclination angle of 45o at Re = 3,000 has been studied by means of direct numerical simulations (DNS). The Reynolds number is based on the inflow velocity and minor-axis length. The preliminary results presented here are focused mainly on vortex dynamics and vortical structures in the wake. The wake behind this configuration starts almost symmetric but is soon strongly deflected and bent as it evolves to the intermediate wake. A pair of unequal-strength vortices dominates the intermediate wake, of which one exhibits the shape of a long vortex tube while the other rapidly breaks down into turbulent-like vortical structures.
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11:30
15 mins
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TOMOGRAPHIC PARTICLE-IMAGE VELOCIMETRY MEASUREMENTS IN A TURBULENT WAVY CHANNEL FLOW
Alexander Rubbert, Wolfgang Schröder, Michael Klaas
Abstract: To investigate small scale turbulent structures and their statistical properties in non-isotropic turbulent flow subjected to favorable and adverse pressure gradients, a novel method to divide the instantaneous flow field into strictly monotonic elements is applied to tomographic particle-image velocimetry data. Especially the scaling regimes of velocity differences within each element with respect to their lengths are considered.
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11:45
15 mins
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Large-scale patterns in turbulent Rayleigh-Benard convection in very large aspect ratio cells
Joerg Schumacher, Mohammad Emran
Abstract: Large-scale patterns, which are well-known from the spiral defect chaos regime of thermal convection at Rayleigh numbers Ra < 1e4, continue to exist in three-dimensional numerical simulations of turbulent Rayleigh-Benard convection
in extended cylindrical cells with an aspect ratio Gamma=50 and Ra>1e5. They are uncovered when the turbulent fields are averaged in time and turbulent
fluctuations are thus removed. We apply the Boussinesq closure to calculate turbulent viscosities and diffusivities, respectively. The resulting turbulent Rayleigh number Ra*, that describes the convection of the mean patterns, is indeed in the spiral defect chaos range. Interestingly, the turbulent Prandtl numbers are smaller than one with 0.2 <= Pr* <= 0.4 for Prandtl numbers 0.7 <= Pr <= 10. Finally, we demonstrate that these mean flow patterns are robust to an additional finite-amplitude side wall-forcing when the level of turbulent fluctuations in the flow is sufficiently high.
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12:00
15 mins
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Assessing late-time singular behaviour in symmetry-plane models of 3D Euler flow
Dan Lucas, Rachel Mulungye, Miguel Bustamante
Abstract: Motivated by work on stagnation-point type exact solutions of the 3D Euler fluid equations by Gibbon [Gibbon et. al. Phys. D, 132, 497, (1999)] and the subsequent demonstration of finite-time blowup by Constantin [Constantin, Math. Res. Notices, 9, 455, (2000)] we introduce a one-parameter family of models of the 3D Euler equations on a 2D symmetry plane. These models provide a collection of blow-up scenarios which admit analytical solutions and are computationally inexpensive in comparison to the full 3D Euler equations. We take advantage of these features to examine the efficacy of novel methods which aid the assessment of finite-time blow-up in numerical simulations. The principal of these is the mapping to regular systems [Bustamante, Phys. D, 240, 1092, (2011)]; a bijective nonlinear mapping of time and the prognostic variables based on a Beale-Kato-Majda (BKM) type supremum norm regularity condition [Beale et. al. Commun. Math. Phys. 94, 61, (1984)]. We show a 3 order of magnitude increase of accuracy of the singularity time when employing the mapping with negligible additional computational expense. An investigation of the spectra of the primary field (vortex stretching rate) allows us to confirm a power law decrement of the analyticity-strip width with time in agreement with rigorous bounds bridging between the global spatial behaviour and BKM theorems [Bustamante & Brachet, Phys. Rev. E. 86, (2012)].
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12:15
15 mins
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The spatial origin of -5/3 spectra in grid-generated turbulence
Sylvain Laizet, Jovan Nedic, John Christos Vassilicos
Abstract: A combined wind tunnel and computational study of grid-generated turbulence along the centreline shows that the close to -5/3 power law signature of energy spectra in the frequency domain originates in the production region close to the grid where the velocity derivative statistics become quite suddenly isotropic but also where the turbulent fluctuating velocities are very intermittent and non-Gaussian. As the inlet flow velocity increases, these power laws are increasingly well defined and increasingly close to -5/3 over an increasing range of frequencies. However, this range continuously decreases with streamwise distance from the grid even though the local Reynolds number first increases and then decreases along the same streamwise extent. The intermittency at the point of origin of the close to -5/3 power spectra consists of alternations between intense vortex tube clusters with shallow broad-band spectra and quiescent regions where the velocity fluctuations are smooth with steep energy spectra.
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