Experimental characterization of the velocity boundary layer in a motored IC engine

The velocity fields near the piston wall are recorded in a motored IC engine during the end of the compression stroke for two different engine speeds and two intake pressure values. Thanks to the use of a Particle Tracking Velocimetry algorithm, the resolution at the wall is sufficient to allow for a study of the boundary layer. The velocity is found to be parallel to the piston with decreasing levels of fluctuations close to the piston surface. Nevertheless these fluctuations are much higher than in canonical boundary layer studies. Even though the values of the momentum-thickness-based Reynolds numbers are extremely low, the boundary layers present shape factors corresponding to turbulent conditions. Using the friction velocity uτ to plot the velocity profiles in wall coordinates, a viscous layer can be observed for all cases but a logarithmic region is never found. This is explained by the fact that the friction Reynolds number δ+ is always too low, meaning that the viscosity-affected near-wall region and the outer region of the boundary layer overlap and prevent the creation of a logarithmic layer. This has strong implications on the use of wall models in numerical simulations. The velocity-defect in the outer region of the boundary layer is shown to scale with δ*δu∞ rather than with the expected uτ from canonical models. These results are attributed to the fact that the turbulence in the core flow in IC engines is not generated by the friction at the wall on the contrary to classical cases. By exchanging momentum with the outer flow, the boundary layer can thus exhibit turbulent properties even though the friction at the wall is rather low.