A combined experimental (PIV) and numerical (LES) study of the tumble formation during the intake stroke of an experimental single-cylinder optical engine

The focus of this paper is on the verification and validation of computational approaches commonly used for the simulation of in-cylinder processes involving intense turbulent flow and combustion. Furthermore, the tumble behavior during the intake stroke at different engine operating points is investigated since the cycle-to-cycle variability of the subsequent tumble breakdown at the end of compression stroke is reported to negatively affect the engine efficiency. In this respect, measurements with advanced laser diagnostics-based experimental techniques (TR-PIV) in an optically accessible engine have been performed and compared with LES for flow in the same engine, under identical operating conditions. The study focused on the intake stroke under wide open throttle conditions and engine speed of 1000 RPM and 1500 RPM. The ensemble averaged velocity fields of the two methods were analyzed and compared. A counter-clockwise motion and some secondary recirculation zones were identified and quantified in both methods. The same trend for tumble ratio was observed between experiments and simulations and a good agreement was found for most CAD especially at 1500 RPM. At 1000 RPM comparison showed significant variations, which after a detailed analysis of the instantaneous fields, are attributed to the strong cycle-to-cycle variability of the instantaneous tumble center. It is observed, that for relevance index values below 0.75 the ensemble averaged flow fields that the two methods predict significantly differ.