The interactive influences of oscillation frequency and velocity amplitude ratio on onset of transition in pulsatile pipe flow are investigated experimentally in the ranges of time-averaged and oscillating Reynolds numbers of 1019±35≤Reta≤4817±164 and 107±4≤Reos≤4261±145 with velocity amplitude ratio of 0.05±0.0017≤A1≤0.96±0.03 for 227 runs. Oscillation frequency is in the range of 0.1 Hz≤ƒ≤14 Hz corresponding to Womersley number range of 2.72≤√ω’≤32.21 covering quasi-steady, intermediate and beginning of inertia dominant regimes of pulsatile laminar flow.
The onset of transition in laminar pulsatile pipe flow can be retarded up to Reta,crit=4817 at √ω’=3.85 in the covered experimental range. No more effect of A1 on Reta,crit is seen for √ω’>8.61 although there is a considerable effect of on Reta,crit. The effect of √ω’ on Reta,crit and Reta,crit is not observed for A1<0.30. The interactive influences of √ω’ and are also investigated on Reta,crit as functions of Reta,crit = Reta,crit (A1√ω’) and Reta,crit= Reta,crit (√ω’/A1). The interactive influences of √ω’ and A1 are not seen on Reta,crit for √ω’>8.61. This shows that the time-periodicity (√ω’,A1) does not affect significantly the onset of transition at the beginning of inertia dominant region of pulsatile laminar flow (√ω’>8.61) when compared with their interactive effects in the quasi-steady and intermediate regions (√ω’≤8.61). For flow classification of pulsatile flow at the onset of transition, a new non-dimensional interactive oscillation number, Ios=√ω’/A1 is proposed. According to this non-dimensional number, the pulsatile flow at the onset of transition can be sub-classified such as quasi-steady for Ios < 5, intermediate for 5 ≤ Ios ≤ 30 and inertia dominant regions of pulsatile pipe flows for Ios > 30 considering the effects of both √ω’ and A1 on contrary to the classifications available in the literature.