Oscillatory flows in straight tubes with an axisymmetric bulge

A laser-Doppler anemometer has been used to study oscillatory flow of a Newtonian viscous fluid in straight circular tube with an axisymmetric bulge of two different sizes. The axial velocities were measured at successive cross-sectional planes for sinusoidal waveforms having Reynolds numbers (based on Stokes layer thickness at the inlet) from 445 to 806 and Womersley numbers ranged from 7.2 to 12.2. The cyclic flow development inside the bulge at different phases within a cycle was determined. Stability analysis obtained by solving the Orr–Sommerfield equation on instantaneous velocity profiles showed instability grows progressively during the acceleration phase and transition to turbulence in the bulge happened shortly after the commencement of the deceleration phase. Depending on the bulge geometry, the turbulent region was initially confined either to the proximal or the distal end of the bulge. This region would spread larger as the deceleration phase furthered and the smaller bulge had a larger spread than the bigger bulge. The differences could be attributed to the vortical structures development inside the bulge. Relaminarisation for the flow appeared in the subsequent acceleration phase. Finally, some comparisons had been made with results obtained from using the physiological waveform.