An Experimental Research Project on Sinusoidal Pulsatile Pipe Flows Part 2: Influence of Oscillation Frequency and Amplitude on Onset of Transition to Turbulence

摘要

Pulsatile air flow in the form of a sinusoidal wave generated inside a rigid pipe of diameter D=26.6 mm is the medium considered. The frequency and the velocity amplitude ratio, F and A_1, are the major controlling parameters having the respective ranges of 0.1 Hz≤f≤14 Hz and 0.05±0.0017≤A_1≤ 0.96±0.03. The generation and control of flow is described via development of instantaneous velocity waveforms and pressure signals along the axial direction. The overall flow nature is revealed based upon the measured cross-sectional velocity profiles at the measurement site. The covered range of oscillation parameters enabled an extended laminar flow range in comparison to the counterpart of steady air flow. The transition to turbulence is observed by the first turbulent bursts in the decelerating phase of the instantaneous velocity waveforms defined as the onset of transition. The experimental set-up, the coupled measurement and data acquisition system handling pressure and velocity signals in terms of a software program written in Lab View 2009 SP1~® are presented briefly. As an original contribution to the relevant literature, the critical time-averaged and oscillating Reynolds numbers at the onset of transition, Re_(ta,crit) and Re_(os,crit) are expressed as a function of Womersley Number, ω'~(1/2) and velocity amplitude ratio, A_1 .