How do I model atmospheric flows?
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This is a complex subject. This Solution aims principally at microscale atmospherics, on timescales of minutes or hours and spatial scales of kilometres, although some of the concepts apply at larger scales too. Moreover, attention is confined to simple modelling of neutral flows. Atmospheric stability is a complicated phenomenon, and will be dealt with in another or an updated Solution. Usually, the user will need to set profiles of velocity and turbulence (e.g. k and epsilon) at the upstream wind inlet. At the ground, roughness should be specified (CFX expects the 'sandgrain' roughness, which is usually a large multiple, 30 typically, of the 'aerodynamic' roughness). For constant roughness, the specified ground roughness should be consistent with the roughness used at the wind inlet. Elsewhere, use may be made of symmetry conditions, openings (pressure boundaries, where the user should specify turbulence values equal to the undisturbed values), or sliding walls. If a jet or plume is expected the penetrate the top of the domain, that boundary will need to be an opening. For some applications, a Boussinesq model of the air suffices. If a compressible medium is being modelled, care needs to be taken in setting the pressure at openings; a separate Solution is being prepared on that subject. A note is attached here, which provides the underlying theory for the simple approach. This includes sample CEL. |
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