Tumbling Vortex Flow in a Model Square Piston Compression Machine: PIV and LDV Measurements

A model experimental set-up dedicated to the study of a compressed tumbling motion is presented in this paper. Measurements are obtained by using Laser Doppler Velocimetry and Particle Image Velocimetry in a complementary way. A tumbling motion representative of high tumble research engines develops in the square chamber. We quantify effects of cycle-to-cycle variations on ensemble mean and fluctuating velocity fields at BDC. PIV is shown to be an optimal technique in order to understand the evolution of the confined vortex during the compression stroke. The breaking down of the tumbling vortex is a gradual process and the vortex/wall interaction is proved to be an essential mechanism responsible for abrupt modifications of the flow fields and for the generation of 3D turbulence. A link is made with the present development of tumble control pistons. The problem of turbulence level estimation appears very complex as cyclic variations are enhanced during the breakdown phase. This point is discussed by analysing LDV results, PIV instantaneous velocity fields and by computing both ensemble and in-cycle rms velocities from PIV velocity fields. The influence of initial charge rotation and compression on turbulence is discussed. We show that the turbulent velocity field at TDC is nearly homogeneous and isotropic. The turbulent integral length scales are computed. When comparing the predictions of the models with experiments, one might not focus on intermediate stages where cyclic variations are prominent but on the final turbulent field obtained after the tumble “breakdown”.