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GASTAR
Section 8: Theory
The model uses observations from the field and laboratory to justify the vertical concentration
profile of the form exp(−z) when the puff or plume is exhibiting dense gas effects.
8.2.8 Averaging time
An averaging time option is available for plumes and this accounts for meander. This has a
minimum default value, to be consistent with puff dispersion parameters.
The use of smaller averaging times is not justified for determining 'peak' puff or plume
concentrations. A specific concentration fluctuation module is required.
8.2.9 Current model limitations and uncertainties
The general philosophy behind the modelling has been to ensure that the model is physically
correct and as simple (transparent) as possible. Several physical effects have not been included
initially because it is commonly the case that their inclusion produces a vastly more complicated
model with no commensurate improvement in model performance. Thus while we have no
qualms about minor changes in some model constants, or incorporation of other, better, physical
descriptions, this will only be done when it is apparent that the model is severely lacking in
performance in some particular area. We believe that it is essential for there to be strong
experimental evidence that demands a change to the model before any alteration is considered.
Listed below are some points of concern to the developers of this model which are currently
under consideration:
• Along-wind shear dispersion has been included for instantaneous releases but not for the timevarying releases.
• The vertical concentration profiles will change with cloud stability, being nearly top-hat when
Ri* is large and the buoyancy driven flows are larger than the ambient flows, with a negative
experimental coefficient of 1.5 when Ri* is of order unity or less and with a negative
experimental coefficient of 1.0 over the extensive range of Ri* . The current dispersion model
only allows for a value of unity. This may be changed if enough evidence is available that this
is necessary.
• The near-source description in this model for very negatively buoyant release is thought to be
more correctly based than in other models, however, this complicated region may require
more attention. There is, in fact, very limited experimental data available on which to base a
more sophisticated model of this region.
• The influence of surface roughness and atmospheric stability on the dispersion code enters,
essentially, through the effect on u* . There is little evidence available to confirm or negate
this
approach.
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