On the generation of helical waves in circular pipe flow

The question of the existence of helical waves in fluid flow through a circular pipe is discussed. In the case of pipe flow forced only by an axial pressure gradient, direct simulations have failed to find any self-sustaining helical waves. The effect of residual swirl in the inlet flow is investigated, by performing linear stability calculations and direct simulations of a forced basic flow. The results are qualitatively similar to that of flow in a rotating pipe, in which stable helical waves are known to exist. These results suggest that residual rotation may be an important mechanism in triggering laminar-turbulent transition in circular pipe flow.