David Dritschel
University of St Andrews
Publications:
Dritschel D. G., Sokolovskiy M. A., Stremler M. A.
Celebrating the 200th Anniversary of the Birth of Hermann Ludwig Ferdinand von Helmholtz (31.08.1821–08.09.1894)
2021, vol. 26, no. 5, pp. 463466
Abstract

Riccardi G., Dritschel D. G.
Evolution of the Singularities of the Schwarz Function Corresponding to the Motion of a Vortex Patch in the Twodimensional Euler Equations
2021, vol. 26, no. 5, pp. 562575
Abstract
The paper deals with the calculation of the internal singularities of the Schwarz
function corresponding to the boundary of a planar vortex patch during its selfinduced motion
in an inviscid, isochoric fluid. The vortex boundary is approximated by a simple, timedependent
map onto the unit circle, whose coefficients are obtained by fitting to the boundary computed in
a contour dynamics numerical simulation of the motion. At any given time, the branch points of
the Schwarz function are calculated, and from them, the generally curved shape of the internal
branch cut, together with the jump of the Schwarz function across it. The knowledge of the
internal singularities enables the calculation of the Schwarz function at any point inside the
vortex, so that it is possible to check the validity of the map during the motion by comparing
left and right hand sides of the evolution equation of the Schwarz function. Our procedure
yields explicit functional forms of the analytic continuations of the velocity and its conjugate
on the vortex boundary. It also opens a new way to understand the relation between the time
evolution of the shape of a vortex patch during its motion, and the corresponding changes in
the singular set of its Schwarz function.

Dritschel D. G.
Ring Configurations of Point Vortices in Polar Atmospheres
2021, vol. 26, no. 5, pp. 467481
Abstract
This paper examines the stability and nonlinear evolution of configurations of equalstrength point vortices equally spaced on a ring of constant radius, with or without a central vortex, in the threedimensional quasigeostrophic compressible atmosphere model. While the ring lies at constant height, the central vortex can be at a different height and also have a different strength to the vortices on the ring. All such configurations are relative equilibria, in the sense that they steadily rotate about the $z$ axis. Here, the domains of stability for two or more vortices on a ring with an additional central vortex are determined. For a compressible atmosphere, the problem also depends on the density scale height $H$, the vertical scale over which the background density varies by a factor $e$. Decreasing $H$ while holding other parameters fixed generally stabilises a configuration. Nonlinear simulations of the dynamics verify the linear analysis and reveal potentially chaotic dynamics for configurations having four or more vortices in total. The simulations also reveal the existence of staggered ring configurations, and oscillations between single and double ring configurations. The results are consistent with the observations of ring configurations of polar vortices seen at both of Jupiter’s poles [1].
