Water confined between two cylinders, when the inner one rotates, first flows in smooth layers. But once the speed crosses a certain threshold, the layers crumple into ring-shaped rolls, resembling a stack of bagels. Polymer additives, even in tiny amounts, behave like little springs: they stretch and compress, giving the fluid elasticity. In the new work, a numerical experiment tested how such springs rearrange the stability threshold and the very dance of the vortices.

The author modeled the flow in the gap between cylinders, where the radius of the inner cylinder is twenty times smaller than the outer one. Into the steady flow, a finite-duration white noise was introduced: random kicks at all frequencies, as if the fluid were struck simultaneously by many tiny hammers. The computer then watched whether these perturbations decayed or grew. For ordinary water, the critical Reynolds number at which growth begins turned out to be about 265. For a weakly elastic solution, the threshold dropped to 202: the springs destabilize the flow.

But the biggest surprise came next. In ordinary water, after the loss of stability, the familiar Taylor vortices appeared as rings. In the polymer solution, helical vortices formed: spiral structures twisted along the cylinder axis, like the threads of a screw. Previously, such vortices in ordinary fluids appeared only when both cylinders rotated in opposite directions. The elasticity of the polymer molecules mimics this effect, even though the outer cylinder remains stationary.