NEA-1040 CRUNCH.
CRUNCH, Dispersion Model for Continuous Dense Vapour Release in Atmosphere
NAME OR DESIGNATION OF PROGRAM, COMPUTER, DESCRIPTION OF PROGRAM OR FUNCTION, METHOD OF SOLUTION, RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM, TYPICAL RUNNING TIME, UNUSUAL FEATURES OF THE PROGRAM, RELATED AND AUXILIARY PROGRAMS, STATUS, REFERENCES, MACHINE REQUIREMENTS, LANGUAGE, OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED, OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS, NAME AND ESTABLISHMENT OF AUTHORS, MATERIAL, CATEGORIES
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| Program name | Package id | Status | Status date |
|---|---|---|---|
| CRUNCH | NEA-1040/01 | Tested | 01-JUN-1987 |
Machines used:
| Package ID | Orig. computer | Test computer |
|---|---|---|
| NEA-1040/01 | ICL 2980 | IBM 3090 |
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3. DESCRIPTION OF PROGRAM OR FUNCTION
The situation modelled is as follows. A dense gas emerges from a source such that it can be considered to emerge through a rectangular area, placed in the vertical plane and perpendicular to the plume direction, which assumes that of the ambient wind. The gas flux at the source, and in every plane perpendicular to the plume direction, is constant in time and a stationary flow field has been attained. For this to apply, the characteristic time of release must be much larger than that for dispersal of the contaminant. The plume can be thought to consist of a number of rectangular elements or 'puffs' emerging from the source at regular time intervals. The model follows the development of these puffs at a series of downwind points. These puffs are immediately assumed to advect with the ambient wind at their half-height. The plume also slumps due to the action of gravity and is allowed to entrain air through its sides and top surface. Spreading of a fluid element is caused by pressure differences across this element and since the pressure gradient in the wind direction is small, the resulting pressure differences and slumping velocities are small also, thus permitting this convenient approximation. Initially, as the plume slumps, its vertical dimension decreases and with it the slumping veloicity and advection velocity. Thus the plume advection velocity varies as a funtion of downwind distance. With the present steady state modelling, and to satisfy continuity constraints, there must be consequent adjustment of plume height. Calculation of this parameter from the volume flux ensures this occurs. As the cloud height begins to grow, the advection velocity increases and the plume height decreases accordingly. With advection downwind, the cloud gains buoyancy by entraining air and, if the cloud is cold, by absorbing heat from the ground. Eventually the plume begins to disperse as would a passive pollutant,through the action of ambient atmospheric turbulence, and to follow the dispersion processes down to low concentrations, especially important for toxic gases, a virtual source passive dispersion model is fitted to the slumping plume.
The situation modelled is as follows. A dense gas emerges from a source such that it can be considered to emerge through a rectangular area, placed in the vertical plane and perpendicular to the plume direction, which assumes that of the ambient wind. The gas flux at the source, and in every plane perpendicular to the plume direction, is constant in time and a stationary flow field has been attained. For this to apply, the characteristic time of release must be much larger than that for dispersal of the contaminant. The plume can be thought to consist of a number of rectangular elements or 'puffs' emerging from the source at regular time intervals. The model follows the development of these puffs at a series of downwind points. These puffs are immediately assumed to advect with the ambient wind at their half-height. The plume also slumps due to the action of gravity and is allowed to entrain air through its sides and top surface. Spreading of a fluid element is caused by pressure differences across this element and since the pressure gradient in the wind direction is small, the resulting pressure differences and slumping velocities are small also, thus permitting this convenient approximation. Initially, as the plume slumps, its vertical dimension decreases and with it the slumping veloicity and advection velocity. Thus the plume advection velocity varies as a funtion of downwind distance. With the present steady state modelling, and to satisfy continuity constraints, there must be consequent adjustment of plume height. Calculation of this parameter from the volume flux ensures this occurs. As the cloud height begins to grow, the advection velocity increases and the plume height decreases accordingly. With advection downwind, the cloud gains buoyancy by entraining air and, if the cloud is cold, by absorbing heat from the ground. Eventually the plume begins to disperse as would a passive pollutant,through the action of ambient atmospheric turbulence, and to follow the dispersion processes down to low concentrations, especially important for toxic gases, a virtual source passive dispersion model is fitted to the slumping plume.
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5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM
Acceleration of the plume to the wind velocity is not considered, since an analysis of inertial effects has shown that the time for which these are important is short, compared to the dispersion time. Additionally, wind shear effects on cloud structure are not included; for a puff release producing a cloud of finite extent, this may not be valid but for a plume, extending to large downwind distances, they can be argued to have only a minor influence at the advancing front.
Acceleration of the plume to the wind velocity is not considered, since an analysis of inertial effects has shown that the time for which these are important is short, compared to the dispersion time. Additionally, wind shear effects on cloud structure are not included; for a puff release producing a cloud of finite extent, this may not be valid but for a plume, extending to large downwind distances, they can be argued to have only a minor influence at the advancing front.
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NEA-1040/01
NEA-DB ran the chlorine release test case on IBM 3090 and VAX-11/780 computers. On the IBM, 0.10 CPU seconds of execution time were required.[ top ]
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NEA-1040/01, included references:
- S.F. Jagger:Development of CRUNCH: A Dispersion Model for Continuous Releases
of a Denser-than-Air Vapour into Atmosphere.
SRD R 229 (January 1983)
- D02 - Ordinary Differential Equations
Reprint of D02YAF from NAG Fortran Library (February 1983)
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NEA-1040/01
To run the test case included in this package on an IBM 3090 computer, 224K bytes of main storage were required.[ top ]
NEA-1040/01
MVS (IBM 3090); VMS (VAX-11/780).[ top ]
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NEA-1040/01
| File name | File description | Records |
|---|---|---|
| NEA1040_01.001 | Information file | 90 |
| NEA1040_01.002 | CRUNCH Fortran File | 2620 |
| NEA1040_01.003 | Interface D02YAF Fortran File | 97 |
| NEA1040_01.004 | Clorine Release sample problem input | 18 |
| NEA1040_01.005 | LNG Release sample problem input | 13 |
| NEA1040_01.006 | LNG Release sample problem output | 96 |
| NEA1040_01.007 | Clorine Release sample problem output | 191 |
Keywords: accidents, dispersions.