Analysis of the turbulent in-cylinder flow in an IC engine using tomographic and planar PIV measurements

In this work, planar PIV and TPIV are utilized to analyze the turbulent in-cylinder flow field within a singlecylinder optical spark-ignition engine. This data extends from comprehensive experimental velocimetry database [Baum et al. 2014], which is analyzed to resolve important statistical turbulent quantities that are important for modeling engine flows. In particular, the local distribution of Reynolds-stress gradient tensors (RSGT) and anisotropic invariant tensors were analyzed and discussed with relevance to homogeneous and isotropic turbulence levels within IC engines. Distributions are analyzed for several crank-angles and engine speeds to assess the sensitivity of the distributions for common engine operations. TPIV provides access to all nine components of the velocity gradient tensors and allowed analysis of the z-component turbulent velocities and their gradients. The local RSGT distribution within central tumble plane reveals regions of large negative and positive values, which are locally influenced by the flow topology. Such regions deviate from the classical definition of homogeneous turbulence and are shown to vary in magnitude with crank-angle and engine speed. Anisotropic invariants are additionally dependent on the flow topology. During intake most regions within the flow recirculation region approximated isotropic turbulence levels, while flows within the intake-jet approximated anisotropic contraction and flows within the stagnation zone approximated anisotropic expansion. During compression, as the flow approached the tumble center location, the turbulence showed a clear trend towards 3D isotropy. TPIV is further used to identify the spatial distribution of 3D turbulent vortical structures. Strong vortical structures are found based on a threshold criterion (Q-criterion). A box-counting method is utilized to identify the individual 3D vortical structures as an independent volumetric object and classify their shape within the flow topology. The aspect ratios of each individual structure is plotted on the ξ-η-map to characterize their geometry. Although many vortical geometries are realized, predominant structures resembled elongated structures during intake, while structures resembled small compact geometries during compression.