Fluid dynamically caused cycle-to-cycle variations in piston-driven pipe expansion flows

Abstract

A piston-driven pipe flow with a sudden expansion in cross-sectional area was studied experimentally using flow visualization and laser-Doppler anemometry as basic experimental techniques. To drive the flow, a piston was moved in the larger of the two pipes. It drew in the fluid into the pipe assembly during the upward stroke and removed fluid during the downward stroke. The time-varying flow field was investigated, yielding detailed information on the flow structure, its development, and variation with time. In the present paper, particular attention is given to those flow conditions that resulted in cycle-to-cylce variations of the flow field. For some of the conditions, repetitive cycle-to-cycle variations of the flow field occurred, yielding identical flows for even-numbered flow cycles and also for odd numbered cycles. However, the latter differed from the former. The presented data show that non-repetitive cycle-to-cycle variations of the flow field also exist and that these can be driven into a state where the flow appears to be nearly chaotic. The results of the investigation clearly demonstrate that fluid dynamically caused cycle-to-cycle flow variations can exist in piston-driven internal flows with separation.