CERC — Environmental Software and Services

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Advanced software for modelling dense gas dispersion

Model options

GASTAR allows the modelling of instantaneous (puff), continuous (plume), time-varying (transient) or jet source, for isothermal, thermal or aerosol release types.

The model includes the following options:

Initial air entrainment[top]

For instantaneous and continuous releases only. Because of the nature of some releases, particularly explosive instantaneous, it is desirable to allow an initial mixing with air for the source term. It is unlikely in a real incident that the exact amount of air entrainment at the start would be known, however you are required to give the mass (flux) of air entrained at the start. The model assumes this to be at the air temperature and will use this together with the mass (flux) and temperature of released material to recalculate an overall cloud temperature and density.

Flashing calculation[top]

Pressurised liquids which flash upon release to the atmosphere require calculation of the flash fraction i.e. the proportion of the release changing to gas; the remaining material is assumed to be suspended aerosol. No allowance for rainout is made.

Hazardous fraction specification[top]

This parameter gives the fraction of the release that is considered hazardous. For general use this is the whole release.

By changing this you may model the release of a dense gas (say CO2) in which a small amount of a contaminant (say H2S) was present. In this case, the dynamics of the cloud will depend on the main dense gas (CO2), but the important concentration levels will be those of the contaminant in the release. The concentration of the contaminant will therefore be given directly in the model output.

Initial momentum mixing (puff releases)[top]

The default is for the momentum to be initially well mixed. This option is used to determine the initial conditions for puff momentum mixing.

Typically, instantaneous releases are a result of some catastrophic event such as a tank rupture or explosion. In these cases it is easy to see that internally the puff will have a well mixed momentum. For some situations this is not true, for example the Thorney Island instantaneous heavy gas dispersion trials. Here the cloud was created inside a large tent-like construction that dropped to the ground to release the puff. The material effectively appeared as a large stationary puff which slowly picked up speed as the wind advected it away. It would be more appropriate to model this case assuming the momentum was not well mixed initially. The effect of this is to make the cloud advection velocity start from zero and gradually grow. When the Momentum initially well mixed option is chosen, this reduction factor is not used and the cloud advection velocity is non-zero from the start of the modelling process.

Automatic plume width calculation (plume releases)[top]

For continuous releases, you can check Internally calculate initial plume width option if you wish GASTAR to determine the initial conditions for continuous release calculations. You must supply the source release rate and the physical source width. The effective (i.e. the actual plume) width, height and density are calculated from the source mass flux, temperature and prevailing Meteorological conditions. This option produces a physically realistic plume aspect ratio.

Pool uptake model (time-varying releases)[top]

The Pool uptake model provides a simple way for you to create a time-varying source term based on the evaporation of material from a pool. The model uses the supplied evaporation rates to calculate the subsequent cloud development above the pool which can then be used as a source term inside GASTAR.

Azimuthal and elevation angles (jet releases)[top]

Azimuthal angle: this is the horizontal bearing of the jet at the source. This is measured in the same manner as the wind bearing, clockwise from north in degrees and represents the direction from which the jet is coming.

Elevation angle: this is the elevation angle of the jet at the source. This is measured from the horizontal and is positive if the jet is pointing upwards.

Being able to specify any jet direction with respect to the wind, and also the jet height above the ground, makes GASTAR much more flexible than some other models.

Effect of obstacles (buildings, fences and walls)[top]

Denser-than-air clouds may be significantly affected by the interaction of the cloud with solid (or porous) obstacles such as buildings, tanks or pipe arrays or the source structure itself.

Our approach is to consider a small number of relevant and commonly occurring situations and to seek to provide models for those cases. Within the spirit of integral models we look for algorithms that will reflect the influence of obstacles on advection (speed and direction), dilution and, for time-varying or instantaneous releases, the fluid hold-up near the obstacle.

The model algorithms are not intended to describe complex flow processes near an obstacle but to quantify the net change in cloud features as the cloud interacts with the obstacle, thereby providing a step adjustment to the cloud variables at the obstacle position. This may be used to estimate the concentrations of the cloud approaching the front face or leaving the rear face of the obstacle, although care would be required in assessing the concentrations in the immediate neighbourhood of the obstacle.

Of particular note is the porosity factor which allows estimates of the effects of porous structures such as pipe racks on industrial sites.

Effect of slopes[top]

Variations in the elevation of the underlying surface will influence the buoyancy driven motion of the dense gas. Topography, whether in the form of a general slope, isolated hills or more complex terrain, will alter or divert the cloud or plume. The topography may enhance plume dilution and divert the plume away from regions of elevated terrain. Alternatively, the dense plume may be channelled into valleys or low-lying areas and then be protected from the diluting influence of the ambient flow. There is extensive treatment of the interaction of topography with buoyancy influenced flows in the geophysical literature, but little use has been made of this information source.

Topographic features that are small compared with the size of the release may be considered in much the same way as buildings or structures but without any substantial flow separation unless the topography is very abrupt.

When the topographic feature is large compared with the scale of the release, the topography reduces to the local slope. Somewhat surprisingly, the downslope velocity of a dense fluid released on a slope under calm conditions is not a strong function of the slope. Larger slopes lead to increased entrainment and dilution, with the entrained fluid acting as an effective drag on the downslope flow.

When the wind is upslope, the cloud widens and its dilution is enhanced. When the wind is downslope, the cloud is narrower and the dilution is decreased. The variation of the lateral growth of the plume results from the effective summation of the wind and the buoyancy induced motion down the slope. The entrainment is influenced by the velocity shear and will therefore be enhanced by an upslope wind and reduced by a downslope wind. The ambient velocity required to reverse a downslope flow of a plume or cloud is a weak function of a slope and is typically twice the downslope flow under calm conditions. In the case of cross winds, the dilution is not greatly affected.

Effect of ground roughness changes[top]

If you have slopes turned on and choose not to use the Meteorological screen data, you will need to enter meteorological data for each slope segment and these data will be displayed in the table summary. This feature can be utilised to allow for varying ground roughness without actually altering the ground slope. This allows you to account for changes in land use in the modelling.

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