Analysis of laminar-turbulent transition in time-dependent pipe flows

The detection of transition to turbulence in sinusoidal pulsatile pipe flow and the flow dynamics at the onset of transition are analyzed in this study. The experimental study is conducted in the ranges of the time averaged and oscillating Reynolds numbers of 1019?Reta?4817 and 107?Reos?4261. The velocity amplitude ratio of 0.05?A1?0.96 and oscillation frequency of 0.1 Hz? f ?14 Hz corresponding to Womersley numbers of 2.72???'?32.21 cover the so-called intermediate region of pulsatile flow. The generation and control of the pulsatile flow, the acquisition and processing of the measured data and the analyses of the flow dynamics are carried out by means of the devised program, TDFC.vi in LabView 2009-SP1 environment.An original detection method is devised in TDFC.vi instead of the transition detection by visual observation in velocity waveforms, cited in the literature. Two dimensionless, dynamic turbulence detection parameters, TDP , of (R(dU(r,t)/dt)^2)/(Utav?^2) for f ? 1 Hz and ((dU(r,t)/dt)^2)/((Uta^2)( ?^2)) for f >1 Hz are defined. Moreover, two dimensionless, dynamic threshold parameters, TP , of (1/??')^n (n=| f |) for f ? 1 Hz and (1/??')^4 for f > 1 Hz are defined as a function of ??'. The detection is based on the comparison of the magnitudes of TDP and TP.It is concluded that there is a distinct difference in flow dynamics for ??'?8.61 and ??'>8.61. Namely, at the onset of transition, despite of the noticeable influence of A1 on Reta for ??' ?8.61, no effect of A1 is observed for ??'>8.61. There is no effect of ??' on Reta for A1=0.10 and A1=0.20 although the significant effect of ??' on Reta is observed for A1>0.30 at the onset of transition.