On transition of the unsteady pipe flow.

The stability of a fully developed laminar pipe flow with a superimposed sinusoidal modulation was studied in a straight water tube using laser Doppler anemometer techniques. The study identified the stability/instability boundary as the mean and modulating Reynolds numbers and the frequency parameter space between the laminar and transitional flows. Experimental results from the study were compared with theoretical results. At high frequency parameter values agreement was found between experiment and stability theories. A study of 'turbulent puffs' documents the mean frequency of occurrence of puffs as functions of the Reynolds numbers and the frequency parameter, and delineates the conditions when puffs occur randomly in time as in the unexited flow or phase locked with excitation. (from authors' abstract)