Defect turbulence

Figure 13.3 a) Climb velocity of a single defect vs. wave number mismatch. Parameters of the OLE are either from experiments [32] (solid curve) or from hydrodynamical calculations [12] (broken line) b) Snapshot of a defect turbulent pattern. 

The situation is reminiscent of Rayleigh-Benard convection in isotropic fluids where stable roll attractors apparently compete with complex patterns, spiral defect turbulence [115-117], It has been shown very recently that if anisotropy is introduced into this system by inclining the convection cell, a normal roll pattern with dislocation defect turbulence occurs, which looks quite similar to patterns observed in EHC [118]. 

Some statistical properties of defect turbulent states in EHC were studied experimentally [42, 133, 134] and, as pointed out before, the role of thermal fluctuations slightly below threshold was measured and analyzed. Of the numerous investigations of far-off threshold effects we only mention the study of phase waves in the oscillatory bimodal state [135, 136] and the transition between strongly turbulent states [60]."... 

S. Kai, Y. Adachi, S. Nasimo Stability diagram, defect turbulence, and new patterns in electroconvection in nematics, in RE. Cladis, P. Palffy-Muhoray (eds.) Spatio-Temporal Patterns in Nonequilibrium Complex Systems, SFI Studies in the Sciences of Complexity, Addison-Wesley, (1994). 

Figure C3.6.10 Defect-mediated turbulence in tire complex Ginzburg-Landau equation, (a) The phase, arg( ), as grey shades, (b) The amplitude [A], witli a similar color coding. In tire left panel topological defects can be identified as points around which one finds all shades of grey. Note tire apparently random spatial pattern of amplitudes.

Figure C3.6.11 Defect-mediated turbulence in tire BZ reaction, (a) Spatial stmcture close to tire instability, (b) Fully developed spatio-temporal turbulence. The control parameter is tire concentration of H2SO4 in tire feed reactor. Reproduced by pennission from Ouyang and Flesselles [501.

One may also observe a transition to a type of defect-mediated turbulence in this Turing system (see figure C3.6.12 (b). Here the defects divide the system into domains of spots and stripes. The defects move erratically and lead to a turbulent state characterized by exponential decay of correlations [59]. Turing bifurcations can interact with the Hopf bifurcations discussed above to give rise to very complicated spatio-temporal patterns [63, 64]. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Hacker, D. S., 1963, Comment on Velocity Defect Law for a Transpired Turbulent Boundary Layer, AIAA J. 7(11) 2676. (5)... 

Enhanced chemical reactivity of solid surfaces are associated with these processes. The cavitational erosion generates unpassivated, highly reactive surfaces it causes short-lived high temperatures and pressures at the surface it produces surface defects and deformations it forms fines and increases the surface area of friable solid supports and it ejects material in unknown form into solution. Finally, the local turbulent flow associated with acoustic streaming improves mass transport between the liquid phase and the surface, thus increasing observed reaction rates. In general, all of these effects are likely to be occurring simultaneously. 

The dead-end setup is by far the easiest apparatus both in construction and use. Reactor and separation unit can be combined and only one pump is needed to pump in the feed. A cross-flow setup, on the other hand, needs a separation unit next to the actual reactor and an additional pump to provide a rapid circulation across the membrane. The major disadvantage of the dead-end filtration is the possibility of concentration polarization, which is defined as an accumulation of retained material on the feed side of the membrane. This effect causes non-optimal membrane performance since losses through membrane defects, which are of course always present, will be amplified by a high surface concentration. In extreme cases concentration polarization can also lead to precipitation of material and membrane fouling. A membrane installed in a cross-flow setup, preferably applied with a turbulent flow, will suffer much less from this... 

In general, it can be very difficult to determine the nature of the boundary terms. A specific result in an exactly solvable case is discussed in Section IV.A.2. Equation (55) is the Gallavotti-Cohen FT derived in the context of deterministic Anosov systems [28]. In that case, Sp stands for the so-called phase space compression factor. It has been experimentally tested by Ciliberto and co-workers in Rayleigh-Bemard convection [52] and turbulent flows [53]. Similar relations have also been tested in athermal systems, for example, in fluidized granular media [54] or the case of two-level systems in fluorescent diamond defects excited by light [55]. 

In addition to this, the use of a direct gate means that a number of other points also have to be observed, without which perfect moulding production is not possible either. In particular, attention has to be paid to using a mixing head that works absolutely free of prefeed and overrun and conveys the mix into the mould cavity completely free of turbulence. And yet it is above all these problems of prefeed and overrun which have still not been very satisfactorily solved today, continually resulting in moulding defects under production conditions. 

The superposition of individual tree wakes results in the under-forest and aboveforest velocity features found in extensive areas of forests or woods. The initial growth of wake deficits and the subsequent decay at greater downwind distances are characteristics of both individual tree and forest measurements. Yano [663] developed a concept of momentum defect superposition in the wakes of an array of roughness elements to reproduce velocity, turbulence and shear distributions within and above canopies. 

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