# TOD

## Seminar

### Velocity, energy and helicity of vortex knots and unknots

Seminar Room 1, Newton Institute

#### Abstract

In this talk we examine the effect of several geometric and topological aspects on the dynamics and energetics of vortex torus knots and unknots. The knots are given by small-amplitude torus knot solutions [1] to the Localized Induction Approximation (LIA) law. Vortex evolution is thus studied in the context of the Euler equations by direct numerical integration of the Biot-Savart law. Earlier stability results on vortex knots and unknots [2] are here extended [3]-[4], and the velocity, helicity and kinetic energy of different vortex knots and unknots are presented for comparison. Vortex complexity is parametrized by the winding number w given by the ratio of the number of meridian wraps to that of longitudinal wraps. We find that for w < 1 vortex knots and toroidal coils move faster and carry more energy than a reference vortex ring of same size and circulation, whereas for w > 1 knots and poloidal coils have approximately same speed and energy of the reference vortex ring. Kinetic helicity is dominated by writhe contributions and increases with knot complexity. The stabilizing effect of the Biot-Savart law for all knots and unknots tested is also confirmed. Our results provide information on relationships between geometry, topology and dynamics of complex vortex systems and apply to quantized vortices in superfluid 4He.References

[1] Ricca, R.L. (1993) Torus knots and polynomial invariants for a class of soliton equations. Chaos 3, 83-91. [1995 Erratum. Chaos 5, 346.]

[2] Ricca, R.L., Samuels, D.C. & Barenghi, C.F. (1999) Evolution of vortex knots. J. Fluid. Mech. 391, 29-44.

[3] Maggioni, F., Alamri, S.Z., Barenghi, C.F. & Ricca, R.L. (2009) Kinetic energy of vortex knots and unknots. Il Nuovo Cimento C, 32(1), 133–142.

[4] Maggioni, F., Alamri, S., Barenghi, C.F. & Ricca R.L. (2010) Velocity, energy and helicity of vortex knots and unknots. Phys. Rev. E., 82(2), 026309–026317.

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