Spatial and Temporal Nonuniformity

The thermal parameters for comfort should be relatively uniform both spatially and temporally. Variations in heat flow from the body make the physiological temperature regulation more difficult. Nonuniform thermal conditions can lead to nonuniform skin temperatures. The active elements of the regulatory system may need to make more adjustments and work harder in order to keep thermal skin and body temperatures stable. To avoid discomfort from environmental nonuniformities, the temperature difference between feet and head should be less than about 3 °C (Fig. 5.9) and the mean surface temperature or radiant difference from one side of the body to the other should not he greater then about 10 °C. 

Similarly, with cycling temperatures, large fast cycles can cause discomfort. To avoid this, if the time to complete one cycle is less than 15 minutes and the peak-to-peak temperature variation is greater than 1.1 °C, the average rate of temperature change should be less than 2.2 °C/h (Fig. 5.9). Very slow rates of temperature change dT/dt 0.5 °C/h) are much less difficult to adjust to and the change can go unnoticed until the temperature is beyond the comfort zone temperature. 

Local air motion is another thermal nonuniformity that can cause a local cooling of the skin and the feeling of a draft. Draft discomfort from local air motion increases as the air temperature decreases below skin temperature. Fluctuations in the local air motion increase the perception of drafts and should be avoided. The unsteadiness of air motion is often described in terms of its turbulence intensity (Tu)  

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Computations based on the extended 3D model of the hot zone have shown, in particular, that the distribution of the temperature and of the heat at the melt/cmcible and the crystal/melt boundaries are essentially nonstationary and greatly deviate from the axisymmetric ones [37, 38]. Note that strong spatial and temporal nonuniformity of the heat flux at the crystal/melt interface (and, hence, of the growth rate) is probably a cause of striations in the resistivity and the oxygen level in the grown crystals. 

A uniform electric field distribution across the sample is extremely important for achieving device quality materials. Unfortunately, real chromophore materials do not always behave as uniform insulator materials. We have already demonstrated that ionic impurities can dramatically reduce the effective electric field felt by chromophores. The presence of spatially and temporally varying nonuniform space charge distributions leads to nonuniform poling fields. The resulting nonuniform chromophore order can lead to light scattering. 

Of conrse, the illumination is more commonly of nonuniform spatial and temporal qnality, leading to more complicated expressions for Tnl (44). Instead, it may be easier to nse relative measures of nonlinear absorption by splitting the beam between the sample and a reference, such as ZnSe (285). 

In the stabilizing mode, all branches keep away from illumination for maintaining a solution. In the destabilizing mode, under equal illumination conditions, some branches enterprisingly invade aversive illuminated regions contrary to their photoavoidance, while others remain shrunk as usual. This implies that the amoeba s photoavoidance response is nonuniform spatially and temporally. This flexibly variable stimulus-response would be essential for the survival of the amoeba required to search for food in a harsh environment, because it enables the amoeba surrounded by aversive stimuli to break through the stalemated situation with enterprising responses. In our experiment, the deadlock-like critical situation in which the amoeba is stuck in its starved condition was flexibly broken by the amoeba s spontaneous destabilization. That is, unstable phenomena have a positive aspect to produce the flexibility. 

Localized forms of corrosion (such as pitting, crevice, and galvanic corrosion) arise when the metal surface is not compositionally uniform and/or when there is not a uniform exposure of the metal surface to the corrosive environment [6], Such localized forms of corrosion lead to nonuniform current density distribution across the metal surface as well as nonuniform distribntions of species concentration (such as metal ions, H+, and Oj). For these localized forms of corrosion, the scanning probe techniques provide valuable spatial and temporal information not available from the surface-averaging (or global) techniques mentioned earlier. A brief review of scanning probe techniques as applied to localized corrosion studies has recently appeared [7]. We have recently prepared a very extensive review chapter for Volume 24 of Electroanalytical Chemistry A Series of Advances [8], and the reader is referred to that chapter for more in-depth discussion of the application of these techniques in corrosion research. In Section 14.2, we provide an overview of several scanning electrochemical probe techniques used in corrosion research. In Section 14.3, we describe... 

This type of nonequilibrium results from the existence of a heterogeneous flow domain. Spatial (or temporal) heterogeneities in such properties as hydraulic conductivity or sorption capacity can result in nonuniform velocity fields. Conditions for diffusive mass transfer of solute may develop because of concentration gradients created by the nonuniform velocities. If this diffusive mass transfer is rate limited, nonequilibrium behavior (e.g., asymmetrical BTC) results. The influence of macroscopic heterogeneities (e.g., aggregates, macropores) on solute transport in soil has been well-documented [see Brusseau and Rao, (1990) for a recent review]. It should be noted that transport-related nonequilibrium (TNE) affects both sorbing and nonsorbing solute. 

Kai, S., Muller, S.C., and Ross, J., Measurements of temporal and spatial sequences of events in periodic precipitation processes, J. Chem. Phys., 76, 1392, 1982. Kenning, D.B.R., Two-phase flow with nonuniform surface tension, Appl. Mech. Rev., 21, 1101, 1968. 

When we think of diffusion acting on a system in which there are concentration inhomogeneities, our intuition suggests that diffusion should act to lessen, and eventually eliminate, the inhomogeneities, leaving a stable pattern with concentrations that are equal everywhere in space. As in the case of temporal oscillation, for a closed system the laws of thermodynamics require that this intuition be valid and that the eventual concentration distribution of the system be constant, both in time and in space. In an open system, however, just as appropriate nonlinear rate laws can lead to temporal structure, like oscillations and chaos, the interaction of nonlinear chemical kinetics and diffusion can produce nonuniform spatial structure, as suggested schematically in Figure 14.1. 

For some systems, there is no resonance Raman or SERS effect to be utilized, and the sensitivity becomes the main problem. In this case, a potential difference method will be of great help [11], Here, a spectrum is acquired at potentials where there is no or only a weak surface signal which is subtracted from that at the potential of interest. In addition, a change in the composition of the electrolyte or an isotopic labeling experiment may be considered to identify the surface species and verify its orientation and structure. For temporally resolved studies, electrochemical transient techniques are helpful to understand the surface dynamics and the reconstruction processes of surfaces. For nonuniform surfaces, spatially resolved measurements provide more reliable and complete information on the surface. This is also useful for electrode surfaces that change either chemically or topographically in a microzone upon variation of potential. 

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