Modeling transitional plane Couette flow

The Galerkin method is used to derive a realistic model of plane Couette flow in terms of partial differential equations governing the space-time dependence of the amplitude of a few cross-stream modes. Numerical simulations show that it reproduces the globally sub-critical behavior typical of this flow. In particular, the statistics of turbulent transients at decay from turbulent to laminar flow displays striking similarities with experimental findings.Comment: 33 pages, 10 figure

On the decay of turbulence in plane Couette flow

The decay of turbulent and laminar oblique bands in the lower transitional range of plane Couette flow is studied by means of direct numerical simulations of the Navier--Stokes equations. We consider systems that are extended enough for several bands to exist, thanks to mild wall-normal under-resolution considered as a consistent and well-validated modelling strategy. We point out a two-stage process involving the rupture of a band followed by a slow regression of the fragments left. Previous approaches to turbulence decay in wall-bounded flows making use of the chaotic transient paradigm are reinterpreted within a spatiotemporal perspective in terms of large deviations of an underlying stochastic process.Comment: ETC13 Conference Proceedings, 6 pages, 5 figure

Convective and absolute Eckhaus instability leading to modulated waves in a finite box

We report experimental study of the secondary modulational instability of a one-dimensional non-linear traveling wave in a long bounded channel. Two qualitatively different instability regimes involving fronts of spatio-temporal defects are linked to the convective and absolute nature of the instability. Both transitions appear to be subcritical. The spatio-temporal defects control the global mode structure.Comment: 5 pages, 7 figures (ReVTeX 4 and amsmath.sty), final versio

Transition from the Couette-Taylor system to the plane Couette system

We discuss the flow between concentric rotating cylinders in the limit of large radii where the system approaches plane Couette flow. We discuss how in this limit the linear instability that leads to the formation of Taylor vortices is lost and how the character of the transition approaches that of planar shear flows. In particular, a parameter regime is identified where fractal distributions of life times and spatiotemporal intermittency occur. Experiments in this regime should allow to study the characteristics of shear flow turbulence in a closed flow geometry.Comment: 5 pages, 5 figure

Critical exponents of directed percolation measured in spatiotemporal intermittency

A new experimental system showing a transition to spatiotemporal intermittency is presented. It consists of a ring of hundred oscillating ferrofluidic spikes. Four of five of the measured critical exponents of the system agree with those obtained from a theoretical model of directed percolation.Comment: 7 pages, 12 figures, submitted to PR

On the relevance of subcritical hydrodynamic turbulence to accretion disk transport

Hydrodynamic unstratified keplerian flows are known to be linearly stable at all Reynolds numbers, but may nevertheless become turbulent through nonlinear mechanisms. However, in the last ten years, conflicting points of view have appeared on this issue. We have revisited the problem through numerical simulations in the shearing sheet limit. It turns out that the effect of the Coriolis force in stabilizing the flow depends on whether the flow is cyclonic (cooperating shear and rotation vorticities) or anticyclonic (competing shear and rotation vorticities); keplerian flows are anticyclonic. We have obtained the following results: i/ The Coriolis force does not quench turbulence in subcritical flows; ii/ The resolution demand, when moving away from the marginal stability boundary, is much more severe for anticyclonic flows than for cyclonic ones. Presently available computer resources do not allow numerical codes to reach the keplerian regime. iii/ The efficiency of turbulent transport is directly correlated to the Reynolds number of transition to turbulence $Rg$, in such a way that the Shakura-Sunyaev parameter $\alpha\sim 1/Rg$. iv/ Even the most optimistic extrapolations of our numerical data show that subcritical turbulent transport would be too inefficient in keplerian flows by several orders of magnitude for astrophysical purposes. v/ Our results suggest that the data obtained for keplerian-like flows in a Taylor-Couette settings are largely affected by secondary flows, such as Ekman circulation.Comment: 21 pages, 17 figures, accepted in Astronomy and Astrophysic

Cytochrome c550 in the cyanobacterium Thermosynechococcus elongatus: Study of redox mutants

Cytochrome c550 is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c550 we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by +125 mV and –30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c550 is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c550 was also constructed. It showed a lowered affinity for Cl– and Ca2+ as reported earlier for the cytochrome c550-less Synechocystis 6803 mutant, but it showed a shorter lived Formula state, rather than a stabilized S2 state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c55

Evolution of turbulent spots in a parallel shear flow

The evolution of turbulent spots in a parallel shear flow is studied by means of full three-dimensional numerical simulations. The flow is bounded by free surfaces and driven by a volume force. Three regions in the spanwise spot cross-section can be identified: a turbulent interior, an interface layer with prominent streamwise streaks and vortices and a laminar exterior region with a large scale flow induced by the presence of the spot. The lift-up of streamwise streaks which is caused by non-normal amplification is clearly detected in the region adjacent to the spot interface. The spot can be characterized by an exponentially decaying front that moves with a speed different from that of the cross-stream outflow or the spanwise phase velocity of the streamwise roll pattern. Growth of the spots seems to be intimately connected to the large scale outside flow, for a turbulent ribbon extending across the box in downstream direction does not show the large scale flow and does not grow. Quantitatively, the large scale flow induces a linear instability in the neighborhood of the spot, but the associated front velocity is too small to explain the spot spreading.Comment: 10 pages, 10 Postscript figure