Vortex detachment and reverse flow in pulsatile laminar flow through axisymmetric sudden expansions

Incompressible, steady and pulsatile flows in axisymmetric sudden expansion s with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 less than or equ al to Re less than or equal to 400 and Womersley number 0.1 less than or eq ual to W less than or equal to 50. For steady flow the calculated recircula tion zone length increased linearly with an increase in Re, in good agreeme nt with earlier experiments. For pulsatile flows, particularly at higher va lues of W, the recirculation zone length correlated strongly with the accel eration of the flow and not with the instantaneous Reynolds number; it incr eased during the deceleration phase and decreased during the acceleration p hase. The computed mean velocity and reattachment length were in general ag reement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the v ortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstre am: at relatively high W, these vortices became, once more, attached.