Exact coherent structures in pipe flow in the presence of wall transpiration

The linear instability of the flow in a pipe subjected to a wavelike transpiration velocity at the walls is considered. The fully nonlinear problem is formulated at high Reynolds numbers and small transpiration velocities. Solutions of the nonlinear system describing the bifurcation of disturbances caused by the transpiration are calculated and a complex bifurcation structure is uncovered with several nonlinear states possible at some transpiration amplitudes. The symmetries and structure of the nonlinear solutions are discussed.