OBSERVATIONS ON TRANSITION OF THE UNSTEADY PIPE FLOW.

The stability of a fully-developed laminar pipe flow with a superimposed sinusoidal modulation has been studied in a straight water tube. This study has identified the stability-instability boundary as that demarcating laminar and transitional flows in the three-dimensional parameter space defined by the mean and modulation Reynolds numbers Re//m, Re//w and the Stokes parameter lambda . Furthermore, this study documents the mean frequency of occurrence of 'turbulent puffs' as functions of Re//m, Re//w and lambda . This study also delineates the conditions when 'puffs' occur randomly in time as in the unexcited flow or phase-locked with the excitation.