The Propagation of Turbulent Slugs in a Pulsatile Pipe Flow

Abstract

Hot wire measurements were made on the propagation of turbulent slugs in a pulsatile pipe flow. The pulsatile pipe flow was produced by imposing a sinusoidal oscillation on a steady flow. Two types of slug behavior are observed according to whether the pulsation frequency f is low or high, and this paper presents two typical examples with f=0.300 and 2.082 Hz. In both examples the ap·pear·ance of the turbulent slug is periodic, in contrast to the randomness that is characteristic of transition in steady flows. Such periodicity is also known in transitional pulsatile boundary layers on a flat plate. At f=0.300 Hz a turbulent slug generates near the entrance (x/D≒3) and propagates downstream, where x is the axial coordinate and D is the pipe diameter. The velocities of leading and trailing edges of the slug can be predicted by steady flow data. Meanwhile at f=2.082 Hz numerous slugs are generated in the region downstream of x/D≒25 in the middle stage of the accelerating period. After a short time, a new slug appears at x/D≒3 and propagates downstream. Later its leading edge merges with the preceding slugs at x/D≒40 and then these slugs become a single slug. This slug disappears at almost the same time as in an earlier stage of the following accelerating period, due to the combined effect of viscous dissipation and acceleration. The velocity and tur·bu·lence intensity downstream of x/D≒50 were found not to change axially.