Roll cell pattern

Fig. 6. (a) The flow pattern assumed in the physical model for the motion near the wall, (b) A more realistic flow pattern of the motion, (c) The roll cell pattern near the wall in a section normal to the main direction of flow. 

Figure 10.19 Velocity field (a) and director pattern (b) in roll cells that form in a tumbling nematic initially oriented in the vorticity direction of a shearing flow. (From Larson 1993, with permission from the Journal of Rheology.)...

Figure 11.16 Stripe and band patterns produced by shearing PBG solutions between glass plates under crossed polaroids in a microscope. The field of view is 890 m, and the flow direction is horizontal. The two stripe patterns form at steady state the more irregular of the two (upper left) is produced by roll cells at a low shear rate (around 0.07 sec ), while the regular stripe pattern (lower left) occurs at high shear rate, 25 sec. The perpendicular band patterns are transients that occur either during start-up of shearing (upper right), or after cessation of shearing (lower right). The detailed conditions under which these patterns are formed are discussed by Larson (1994).

A recent theoretical study has suggested that persistent activity in the PFG is considered to be an attractor state, in that relatively small amounts of variation in this state lead it back to the same state. This idea has been examined in detail theoretically, especially by Amit, who described persistent activity in terms of dynamical attractors (Amit and Brunei, 1997 Rolls et al., 2008). The spontaneous state and stimulus-selective memory states are assumed to represent multiple attractors, such that a memory state can be switched on or off by transient inputs. This formulation is plausible, insomuch as stimulus-selective persistent firing patterns are dynamically stable in time. These properties of attractors result from interactions in neuronal circuits. Neural synchrony is a general mechanism for dynamically linking together cells coding task-relevant information (Salmas and Sejnowski, 2001). The dynamics of neuronal activities and the representations they reflect are two sides of a coin. 

Four principal patterns of convection were distinguished when pure liquids were employed cells, streamers, ribs, and vermiculated rolls. These names were chosen in an attempt to describe the actual appearance of the convection patterns and in accordance with historical designations. Examples are shown in Fig. 21. The patterns depicted there were exhibited in all of the liquids under various conditions. In particular, cells appeared to be the dominant patterns in all liquids for depths of 2 mm or less, and the cell size for the various liquids at the 1-mm and 2-mm depths is shown in Table VI. For a thin (< 2 mm) layer of given liquid evaporating into still air, the cell size increased with the depth of the liquid layer, and the flow which the cellular schlieren pattern represented was the same as that observed by Benard (see Fig. 3). These cells were quite immobile and generally neither grew nor decayed in size with time. A direct stream of dry nitrogen onto the surface of the liquid sharpened the cell peripheries and tended to reduce the cell size. 

Kukkola et al. (2011) used another technique compatible with roU-to-roll processing gravure printing. They deposited WO3 sensing films on interdigitated electrodes patterned on Kapton HN PI foil from DuPont. However, the fabrication of an integrated heating element was not addressed in this study. For gas response measurement, the sensor was placed in a heated gas cell at 200°C. A gas response was obtained for a concentration of 5 ppm. 

It will be seen in the next and subsequent chapters that a wide variety of cell geometries (e.g. parallel plates, concentric cylinders, Swiss roll), types of electrode (e.g. plates, beds, porous, expanded metals and gauzes) and flow patterns are used in industrial electrochemistry. In most the flow is too complex to warrant a detailed fluid-mechanical calculation. Rather the normal approach to mass transport in electrolytic cells is to treat the cell as a unified whole and to seek expressions in terms of space-averaged quantities which permit some insight into the mass transport conditions within the cell. 

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