15th European Turbulence Conference 2015
August 25-28th, 2015, Delft, The Netherlands

Invited speakers:

Prof. Marc Brachet. Ecole Normale Superieure, Paris, France

Prof. Peter G. Frick, Institute of Continuous Media Mechanics, Perm, Russia

Prof. Bettina Frohnapfel,  Karlsruher Institut fur Technology, Germany

Prof. Andrea Mazzino, Dipartimento di Fisica, University of Genova, Italy

Prof. Bernhard Mehlig. Department of Physics, University of Gothenburg, Sweden

Prof. Lex Smits, Mechanical and Aerospace Engineering, Princeton University, USA

Prof. Chao Sun Physics of Fluids, University of Twente, The Netherlands

Prof. Steve Tobias, Applied Mathematics, University of Leeds, United Kingdom

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10:30   Instability and Transition 3
Chair: Jens Fransson
15 mins
chaotic self-sustaining turbulent-laminar interface in two-dimensional channel flow
Toshiki Teramura, Sadayoshi Toh
Abstract: Another type of self-sustainable coherent structures is found in a two-dimensional channel flow. It is embedded in a turbulent-laminar interface, so it utilizes the inhomogeneity to keep alive. Its spatio-temporally chaotic behavior and sustaining mechanism are investigated using the filtered simulation. This is an example of inhomogeneity induced coherent structures, which will be necessary to understand the spatio-temporal intermittency in three-dimensional turbulent systems.
15 mins
The growth of turbulent spots in plane Couette flow
Marie Couliou, Romain Monchaux
Abstract: Using Particle Image Velocimetry (PIV) in an experimental plane Couette flow, we investigate the growth of turbulent spots invading formerly a laminar flow. We observe the existence of large scale flows appearing as soon as laminar and turbulent domains coexist. Spectral analysis is used to study the dynamical evolution of these large-scale structures as well as that of the small-scale structure associated with turbulence. Visualisations allow a study of the evolution of the spot growth rate and also the velocity of waves which we observe at the spot edges. All these results show that two mechanisms are at work when turbulent spots grow; a growth by destabilization but also in the same proportion a growth by large-scale transport.
15 mins
Stability and exact coherent structures of the asymptotic suction boundary layer with temperature gradient
Stefan Zammert, Bruno Eckhardt
Abstract: The asymptotic suction boundary layer with a temperature gradient is a good point of entry to study the dynamics of thermal boundary layers by means of dynamical systems theory. The laminar flow without heating is parallel and its properties have been studied before. We add a temperature difference between the bottom plate and the free stream flow, and study the stability in dependence on Reynolds, Rayleigh and Prandtl number. In marked contrast to the usual Rayleigh-B\'enard problem, the onset of convection is subcritical. Tracking secondary bifurcations we identify time-periodic, spanwise, and doubly-localized exact coherent states for this flow.
15 mins
Spatially-localized time dependent solutions including turbulence and their interactions in 2D Kolmogorov flow
Yoshiki Hiruta, Toshiki Teramura, Sadayoshi Toh
Abstract: In 2D Kolmogorov flow in small aspect ratio domains, spatially-localized solutions such as kink, traveling or time-dependent kink-antikink pars coexist. However, the conservation of the flow rate in the y direction strongly restrict combination of localized solutions and their positioning. We find that by adding a homogeneous flow U y their positioning is controlled and each of localized solutions including a spatially-localized chaos is isolated. Numerical results suggest that these isolated solutions can be elements constructing a whole flow.
15 mins
Mean field model for turbulence transition in plane Poiseuille flow
Bruno Eckhardt, Michael Rath, Marina Pausch
Abstract: In the pipe flow model of Dwight Barkley the main idea is to model pipe flow as an excitable, bistable medium. Using a one-dimensional FitzHugh-Nagumo-type reaction-advection-diffusion system with two variables the model captures qualitatively a surprising number of features of the turbulence transition in pipe flow. Motivated by this success, we here describe a derivation of a set of two 1+1-dimensional coupled differential equations for the closely related system of plane Poiseuille flow from the Navier-Stokes equation. The model contains terms for the production of turbulent kinetic energy, its transfer between the modes and its dissipation by viscous terms. The model shows a bifurcation to a non-trivial state and reflects some of the complex dynamics observed in direct numerical simulations.
15 mins
On the role of the helicity in the energy transfer in three-dimensional turbulence
Ganapati Sahoo, Luca Biferale
Abstract: Behavior of the turbulent flows could be changed by changing the nature of the external force or the confining geometry which essentially results in breaking some of the symmetries of the ideal homogeneous and isotropic flows. In a numerical simulation, however, it is possible to selectively break symmetries of the Navier-Stokes equations with other constraints like helicity. In a recent [1] simulation of a decimated version of the incompressible three dimensional Navier-Stokes equations, where helicity was maintained sign-definite using a helical projection, a reversal of energy cascade similar to two-dimensional Navier-Stokes equations was observed. The sign- definite helicity breaks the parity symmetry of the flow. It is one of the important symmetries of the flow that contributes to the forward energy cascade in three dimensional Navier-Stokes equations. In our study we measure the degree to which the parity symmetry controls the direction of the cascade. We introduce a mechanism in which the parity is broken stochastically but in a time frozen manner with helical constraints. We keep triadic interactions in Fourier space involving modes with definite sign of helicity and decimate the triads of other modes with opposite sign of helicity with a fixed probability. We studied the cascade of energy in three dimensional turbulence by changing the relative weight between positive and negative helicity modes. We present the results from our recent simulations.
15 mins
Experimental investigation of Taylor-Couette flow with radius ratio 0.1 to 0.3
Sebastian Merbold, Andreas Froitzheim, Christoph Egbers
Abstract: Turbulent flow of a very wide Taylor Couette flow (radius ratio 0.1 up to 0.3) is the scope of the present work. Flow visualisation shows the existing coherent structures. Laser Doppler Velocimetry is used to analyse the local velocity behaviour and understand the flow in this geometry.
15 mins
Boundary-layer-flow instability in a rapidly rotating and strong precessing sphere
Shigeo Kida
Abstract: The linear stability analysis of the steady flow is performed in a rapidly rotating sphere with strong precession. It is shown that the localized mode destabilizing the boundary-layer flow determines the stability boundary, giving the asymptote, $Po\propto Re^{2/3}$, which is consistent with the results obtained by direct numerical simulation.