Topographic steering
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In fluid mechanics, topographic steering is the effect of potential vorticity conservation on the motion of a fluid parcel. This means that the fluid parcels will not only react to physical obstacles in their path, but also to changes in topography or latitude. The two types of 'fluids' where topographic steering is mainly observed in daily life are air (air can be considered a compressible fluid in fluid mechanics) and water in respectively the atmosphere and the oceans. Examples of topographic steering can be found in, among other things, paths of low pressure systems and oceanic currents.
In 1869, Kelvin published his circulation theorem, which states that a barotropic, ideal fluid with conservative body forces conserves the circulation around a closed loop.[1] To generalise this, Bjerknes published his own circulation theorem in 1898.[2] Bjerknes extended the concept to inviscid, geostrophic and baroclinic fluids, resulting in addition of terms in the equation.