Absorbing boundary layers

Lane, H., Kettil, R, Enelund, M., Ekevid, T. Wiherg, N.-E. 2007b. Absorbing Boundary Layers for Elastic Wave Propagation. Presented at the 8th International Conference on Computational Plasticity, Barcelona, Spain, September 5-8, 2007. 

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

Fluid motion acts to decrease the diffusion boundary layer thickness. Strategies of the microorganism to increase solute flux by decreasing its size or surface concentrations of the solute, c°, will be examined in Section 6. In this section, the solute concentration at the surface of the organism, c°, is assumed to be zero, i.e. the cell is a perfect absorber (sink), since this will provide an upper limit for the importance of fluid motion. It is clear that if fluid motion has no effect for a perfect absorber, it will have no effect for an imperfect one. 

In combustion of liquid oils such as heavy diesel fuels, the fuel is sprayed through nozzles into air. After ignition, a flame forms around the evaporating drops, driven by the vaporization of the drop in the hot boundary layer. The latent heat of vaporization strongly affects the temperature of the drop so that the heat release by the reaction is compensated by the heat absorbed in vaporizing the fuel and the heat lost to the cooler gas in the air. The composition profiles we might expect are shown in Figure 12-11, but the temperature will also vary around the particle. 

The essential difference between the homogeneous model and the heterogeneous one is that the latter model takes into account the fact that the diffusion of the absorbed component alternately occurs through continuous- and dispersed phases in the liquid boundary layer at the gas-hquid interface. The mass transport through this heterogeneous phase is a nonUnear process, one can get explicit mathematical expression for the absorption rate only after its simpHfica-tion. 

The albedo (R) of a thick, boundary layer cloud that does not absorb solar radiation over a surface with zero albedo can be approximated (Twomey, 1991 Schwartz and Slingo, 1996 Baker, 1997) by... 

For beading oils, the most probable pathway is mass transfer in series. Assuming a shuttle effect of the oil phase, investigators consider that the solute absorbed in the oil droplets near the gas-aqueous phase interface is given up to the water phase outside the boundary layer. 

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