Investigation of the Boundary Layer Flow Under Engine-Like Conditions Using Particle Image Velocimetry

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Abstract
The turbulent boundary layer flow in internal combustion (IC) engines has a significant effect on the in-cylinder flow and the wall heat transfer. A detailed analysis of the in-cylinder near-wall flow was carried out on an optical steady flow test bench by using high-resolution particle image velocimetry (PIV) in order to characterize the in-cylinder boundary layer flow in this study. The difference between the in-cylinder boundary layer and the canonical turbulent boundary layer was analyzed. The experimental results show that small-scale vortices with a length scale of about 1–2 mm in the instantaneous flow fields appeared in the wall jet region due to the entrainment of the free jet in the outer region of the wall jet. The viscous sublayer thickness decreased from 0.5 mm to 0.3 mm as the valve lift increased from 2.32 mm to 7.975 mm and the pressure drop from 0.5 kPa to 1 kPa. The dimensionless velocity profile is in good agreement with the law of the wall in the viscous sublayer. However, no obvious logarithmic law distribution region was observed in the logarithmic layer. The distribution of the Reynolds stress and the turbulent kinetic energy is similar to that of the canonical turbulent boundary layer. But the Reynolds stress had a much larger magnitude because the turbulent velocity measured in this boundary layer included not only the turbulence generated by wall shear but also the large-scale turbulent vortices caused by the wall jet.

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