Local manifold learning and its link to domain-based physics knowledge

TL;DR

This paper demonstrates that local PCA can effectively identify meaningful low-dimensional parameterizations in thermo-chemical state-space data, linking them to physical combustion processes, even in complex turbulent flames.

Contribution

The study shows that local PCA can detect intrinsic parameterizations in thermo-chemical data, extending its application to complex turbulent reacting flows.

Findings

Local PCA detects variables linked to stoichiometry, reaction progress, and soot formation.

It performs well on benchmark combustion models with known parameterizations.

It extends to complex turbulent jet flames where parameterization is not obvious.

Abstract

In many reacting flow systems, the thermo-chemical state-space is known or assumed to evolve close to a low-dimensional manifold (LDM). Various approaches are available to obtain those manifolds and subsequently express the original high-dimensional space with fewer parameterizing variables. Principal component analysis (PCA) is one of the dimensionality reduction methods that can be used to obtain LDMs. PCA does not make prior assumptions about the parameterizing variables and retrieves them empirically from the training data. In this paper, we show that PCA applied in local clusters of data (local PCA) is capable of detecting the intrinsic parameterization of the thermo-chemical state-space. We first demonstrate that utilizing three common combustion models of varying complexity: the Burke-Schumann model, the chemical equilibrium model and the homogeneous reactor. Parameterization of these models is known a priori which allows for benchmarking with the local PCA approach. We further extend the application of local PCA to a more challenging case of a turbulent non-premixed $n$-heptane/air jet flame for which the parameterization is no longer obvious. Our results suggest that meaningful parameterization can be obtained also for more complex datasets. We show that local PCA finds variables that can be linked to local stoichiometry, reaction progress and soot formation processes.