Swirl Center Precession in Engine Flows

The origin and development of swirl center precession in engine flows has been investigated in a steady flow rig, with and without a porous plate simulating a stationary piston, and in a model engine motored at 200rpm; swirl, in all cases, was generated by means of 60° vanes located in the axisymmetric inlet port. The swirl center performs a helical motion that originates as an instability in the forced-vortex core from its interaction with the axial flow at a free stagnation point and develops in the engine from the piston towards the cylinder head; an opposite trend has been observed in the steady flow case with the open-ended cylinder. In the ensemble-averaged measurements, swirl center precession has been identified by the increased tangential velocity fluctuations around the off-centre zero swirl velocity. The characteristic frequency of the periodic swirl center motion was found to scale linearly with induction air mass flowrate over the investigated range of engine speeds (180-700rpm).