A 2-D axisymmetric CFD model of oscillatory flow with separation

A 2-D axisymmetric computational model was developed to simulate a UMN test rig and encompassing: a piston/cylinder and 90 degree flow turn to a radial flow between two discs. Three cases were studied representing a combination of two disc spacings (54 & 127 mm) and two oscillating frequencies (30 & 70 CPM). The CFD-ACE+ commercial CFD code was utilized for this study. Both laminar and turbulent simulations were attempted. The CFD code was validated by comparing with experimental data for a unidirectional jet impinging on a flat plate. The agreement was good. The laminar and turbulent flow cases mimic closely the flow visualization obtained in the UMN rig. The CFD simulations for the three oscillatory flow cases revealed close similarity of flow features. However, distinct differences were noticed, in both computations and experiments, between small and large disc spacing in the channel. The small channel shows the presence of vortices only in a portion of the cycle. The large channel shows a large vortex throughout the whole cycle. This vortex gets stronger during jetting into the disc space (exhaust stroke) and weaker during suction into the cylinder.