On the mean-velocity scaling in the turbulent piston boundary layer of a motored internal-combustion engine

• The evolution of the mean velocity profile within the boundary layer developing over the piston surface of a motored internal-combustion research engine is examined on the basis of a wall-resolved Large-Eddy Simulation database. • A systematic assessment of the applicability of mean-velocity scaling methods commonly used for wall-bounded turbulent flows has been carried out under the complex conditions of internal-combustion engines, where this complexity manifests particularly in the wall-bounded regions, causing the velocity profiles to deviate substantially from classical turbulent boundary-layer behaviour. • Several scaling and transformation approaches are examined with respect to their ability to produce universal mean-velocity profiles relative to the classical logarithmic wall law. • The results show that none of the tested transformations achieves a universal collapse of the velocity profiles across different piston locations and crank-angle positions. Moreover, none of the transformations fully conforms to the classical logarithmic law, although the TL transformation provides the closest agreement. • This work establishes a comprehensive framework for evaluating velocity-scaling methods under engine-relevant, non-canonical flow conditions, thereby advancing the understanding of momentum boundary layers in such complex systems. The turbulent flow in internal combustion engines is inherently complex due to moving geometries and time-varying topologies. This complexity is reflected in the wall-bounded regions, where experimental and numerical studies have reported velocity profiles that deviate substantially from classical turbulent boundary-layer behavior, often lacking a distinct logarithmic region. However, a systematic assessment of the applicability of established mean-velocity scaling methods under such conditions has not yet been performed. Here, we investigate the mean velocity within the boundary layer developing on the piston surface of a motored internal combustion research engine, for which experimental and numerical high-resolution near-wall boundary layer data have recently become available. Several scaling and transformation approaches are examined in terms of their ability to provide universal mean-velocity profiles relative to the classical logarithmic wall law. The results show that none of the tested transformations achieves a universal collapse of the velocity profiles across different piston locations and crank-angle positions. At the piston center, however, the semi-local scaling and the Trettel–Larsson (TL) transformation yield an effective collapse of the mean profiles. Nevertheless, none of the transformations fully aligns the profiles with the classical logarithmic law, with the TL transformation performing best in this regard. These findings confirm and quantify that the absence of a distinct logarithmic region is an inherent feature of the engine boundary layer.