Slender vortex filaments in the Boussinesq approximation

A model for the motion of slender vortex filaments is extended to include the effect of gravity. The model, initially introduced by Callegari and Ting ["Motion of a curved vortex filament with decaying vortical core and axial velocity," SIAM J. Appl. Math. 35, 148-175 (1978)], is based on a matched asymptotic expansion in which the outer solution, given by the Biot-Savart law, is matched with the inner solution derived from the Navier-Stokes equations. Building on recent work by Harikrishnan et al. ["On the motion of hairpin filaments in the atmospheric boundary layer," Phys. Fluids 35, 076603 (2023)], the Boussinesq approximation is applied such that the density variations only enter in the gravity term. However, unlike Harikrishnan et al. ["On the motion of hairpin filaments in the atmospheric boundary layer," Phys. Fluids 35, 076603 (2023)], the density variation enters at a lower order in the asymptotic expansion and, thus, has a more significant impact on the self-induced velocity of the vortex filament. In this regime, which corresponds to the regime studied by Chang and Smith ["The motion of a buoyant vortex filament," J. Fluid Mech. 857, R1 (2018)], the effect of gravity is given by an alteration of the core constant, which couples the motion of the filament to the motion within the vortical core, in addition to a change in the compatibility conditions (evolution equations), which determine the leading order azimuthal and tangential velocity fields in the vortex core. The results are used to explain certain properties of buoyant vortex rings, as well as qualitatively explore the impact of gravity on tornado-type atmospheric vortices.