Jellyfish Jet Propulsion: Bell Contraction & Vortex Thrust (2D)

This 2D companion drives a real jet-propulsion model instead of a decorative pulsing bell: an asymmetric contraction/relaxation cycle changes the subumbrellar cavity's volume, the rate of that volume change sets a water-ejection velocity through the bell margin, and a momentum-flux thrust (F = ρ·A·v²) accelerates the animal against quadratic drag and its own added-mass inertia — producing the sharp thrust-then-coast "pulse-glide" pattern real medusae use, with contraction frequency, duty cycle, contraction strength, bell diameter and drag coefficient all verified to move swim speed and thrust in the physically correct direction.