Obstruction-scaling Model

Amsden, B, An Obstruction-Scaling Model for Diffusion in Homogeneous Hydrogels, Macromolecules 32, 874, 1999. 

Amsden, B. (2001) Diffusion in polyelectrolyte hydrogels application of an obstruction-scaling model to solute diffusion in calcium alginate. Macrortrolecules, 34, 1430-1435. 

The Obstruction-Scaling Model for a cross-linked polymer network... 

Amsden B. (1999) An obstruction-scaling model for diffusion in homogeneous hydrogels. Macromolecules, 32, pp. 874-879. 

Data are available only for simple building geometries. In Allard," a tool for the calculation of wind pressure coefficients for simple geometries is made available, and another tool is described in Knoll et al. Existing wind pressure data have to be examined carefully, because many data represent peak pressure values needed for static building analysis. Real cases with obstructions and buildings in the close surroundings are difficult to handle. Wind-tunnel tests on scale models or CFD analysis will be required. 

Physical (scale) models employing wind tunnels or water channels have been used for dense gas dispersion simulation, especially for situations with obstructions or irregular terrain. Exact similarity in all scales and the re-creation of atmospheric stability and velocity distributions are not possible— very low air velocities are required to match large scale results. Havens et al (1995) attempted to use a 100-1 scale approach in conjunction with a finite element model. They found that measurements from such flows cannot be scaled to field conditions accurately because of the relative importance of the molecular difiusion contribution at model scale. The use of scale models is not a common risk assessment tool in CPQRA and readers are direaed to additional reviews by Mcroncy (1982), and Duijm et al. (1985). 

The interaction of dispersing clouds with vapor fences is a complex physical process. When a flow meets an obstruction, turbulence levels are increased downstream because of vorticities introduced into the flow field, and increased velocity gradients are induced by flow momentum losses. Detailed modeling of such a process is very difficult and requires a combination of small-scale experiments and computational fluid dynamics. 

The problem with the TNT equivalency model is that litde, if any, correlation exists between the quantity of combustion energy involved in a VCE and the equivalent weight of TNT required to model its blast effects. This result is clearly proven by the fact that, for quiescent clouds, both the scale and strength of a blast are unrelated to fiiel quantity present. These faaors are determined primarily by the size and nature of the partially confined and obstructed regions within the cloud. 

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