Two-Point Spatial Velocity Correlations in the Near-Wall Region of a Reciprocating Internal Combustion Engine 2017-01-0613

Developing a complete understanding of the structure and behavior of the near-wall region (NWR) in reciprocating, internal combustion (IC) engines and of its interaction with the core flow is needed to support the implementation of advanced combustion and engine operation strategies, as well as predictive computational models. The NWR in IC engines is fundamentally different from the canonical steady-state turbulent boundary layers (BL), whose structure, similarity and dynamics have been thoroughly documented in the technical literature. Motivated by this need, this paper presents results from the analysis of two-component velocity data measured with particle image velocimetry near the head of a single-cylinder, optical engine. The interaction between the NWR and the core flow was quantified via statistical moments and two-point velocity correlations, determined at multiple distances from the wall and piston positions. The analysis was conducted on instantaneous and Reynolds-decomposed flow fields, enabling the assessment of mean flow effects on the results. It is proposed that the turbulence in IC engine near-wall-layers is created by both wall-shear (as in canonical BL flows) and dissipation of large-scale core-flow turbulence. In support of this notion, coupling of the wall-generated and core turbulence was observed in the canonical log-law region through a significantly higher Reynolds stress magnitude relative to the canonical prediction. In contrast to the canonical description of turbulent boundary layers, the normalized velocity profile lacks a well-defined log-law region and does not scale with the wall shear stress. Correlation coefficients (and hence length-scales) are strongly dependent on wall-normal distance. Smaller flow structures in the two-dimensional velocity fields are apparent as the wall is approached, suggesting that turbulence dissipation in the core flow reaches the near-wall layer. A roll-off in correlation strength is consistently observed in the 10 < y⁺< 20 region, suggesting that two-point velocity correlations may provide an additional metric for estimating the boundary layer thickness in engine flows. Additional analysis of higher-engine speed data will be needed to further validate this proposition.