Analysis of In-cylinder Flow Structures and Turbulence in a Laboratory Scale Engine using Direct Numerical Simulations

TL;DR

This study uses direct numerical simulations to analyze in-cylinder flow and turbulence in a laboratory engine at different speeds, revealing cycle variability, turbulence evolution, and anisotropy characteristics relevant for engine modeling.

Contribution

It provides detailed DNS-based insights into flow structures, turbulence evolution, and anisotropy in a laboratory engine at realistic speeds, enhancing turbulence modeling accuracy.

Findings

Cycle-to-cycle variability decreases at higher engine speeds.

Tumble breakdown and turbulence peaks differ between speeds.

Higher engine speed shows more isotropic turbulence during mid-compression.

Abstract

In-cylinder flow structures and turbulence characteristics are investigated using direct numerical simulations (DNS) in a laboratory-scale engine at technically relevant engine speeds (1500 and 2500 rpm at full load). The data is computed for 12 compression-expansion cycles at each engine speed with initial conditions derived from precursor large eddy simulations (LES) validated against experimental data. Analysis of the tumble ratio indicates significant cycle-to-cycle variation, with lower variability found at higher engine speed. The process of tumble breakdown, quantified by the evolution of mean and turbulent kinetic energy, reveals distinct features between operating conditions, with delayed turbulence peaks observed at lower engine speed. Analysis of the Reynolds stress tensor demonstrates higher stress values at higher engine speed, with pronounced anisotropy near the walls and higher values around the tumble vortex core. Examination of the anisotropic Reynolds stress invariants through Lumley triangles reveals predominantly isotropic turbulence during mid-compression, transitioning to distinct anisotropic states near top dead center (TDC). The lower engine speed exhibits a stronger tendency toward one-component turbulence due to partial tumble dissipation, while the higher speed maintains a more balanced anisotropy. These findings extend previous numerical and experimental studies on turbulence development and in-cylinder flow structures during the compression stroke, providing insights for improving turbulence modeling in practical engine simulations.