On the behavior of a large scale tumbling vortex flow submitted to compression

Abstract

An analysis of the disruption of a compressed tumbling vortex is proposed here. This situation is typical of in-cylinder flows developing in combustion chambers of premixed spark ignition engines. In an inviscid fluid, we discuss analytically the motion of a point vortex of constant intensity submitted to compression in a time dependant rectangular domain. A relevant non-dimensional number comparing the vortex time scale and the compression time scale is obtained. We show that the compression unsteadiness is expected to delay the viscous response of the near-wall flow. The temporal evolution of a tumbling motion is then measured with high-speed particle image velocimetry in a model square piston compression machine. The experimental setup and the measurement procedure are presented. The data provide a clear picture of the evolution and disruption of the compressed vortex. A physical analysis is proposed and we prove that elliptical instability and interaction with walls are the main mechanisms that lead to the generation of a three-dimensional unstructured velocity field.