On transition of the pulsatile pipe flow.

Transition in a pipe flow with a superimposed sinusoidal modulation was studied in a straight circular water pipe using laser Doppler anemometer techniques. The study determined the stability transition boundary in the three dimensional parameter space defined by the mean and modulation Reynolds numbers and the frequency parameter. It also documented the mean passage frequency of turbulent plugs as functions of the Reynolds numbers and the frequency parameter. The study also delineated the conditions when plugs occur randomly in time (as in the steady flow) or phase locked with the excitation. The periodic flow requires a new definition of the transitional Reynolds number identified on the basis of the rate of change of the mean passage frequency and the mean Reynolds number. (from authors' abstract)