Vilhelm Bjerknes generalized Helmholtz's vorticity equation (1858) and Kelvin's circulation theorem (1869) to inviscid, geostrophic, and baroclinic fluids, i.e., fluids of varying density in a rotational frame which has a constant angular speed.
Vilhelm Bjerknes expanded Helmholtz's vorticity equation (1858) and Kelvin's circulation theorem (1869), applying them to fluids of varying density within a rotating frame with a constant angular velocity.