Interpretation of air motion in reentrant bowl in-piston engine by estimating Reynolds stresses

The present paper aims at investigating the flow field behavior within a reciprocating engine under motoring conditions. Simultaneously two velocity components of the air velocity were acquired at different engine speeds within the cylinder at different radii from the cylinder axis. Mean motion, integral time scales and Reynolds shear stresses, for the radial and tangential components, were estimated from the instantaneous velocity data by applying an ensemble averaging technique. The integral time scale was obtained from the single point time autocorrelation function whereas, the Reynolds shear stresses were computed through the estimate of the degree of the fluctuations correlation. Tests, carried out at 1,000, 1,500, and 2,000 rpm, showed that the tangential mean motion scales approximately with engine speed whereas a radial inward motion can be observed during the last part of compression. The integral time scale shows a global decrease along the compression and an approximate inverse scaling trend with engine speed. Further, an increase of the Reynolds shears stress, at the end of compression, was observed.