Data-driven classification of sheared stratified turbulence from experimental shadowgraphs

TL;DR

This paper introduces a novel data-driven method combining dimensionality reduction and clustering to classify and analyze different turbulence regimes in stratified fluid experiments, revealing physical insights into turbulence transitions.

Contribution

The authors develop an unsupervised classification approach that automatically identifies turbulence types from shadowgraph images, providing a new tool for analyzing complex stratified turbulence datasets.

Findings

Identified five distinct turbulence regimes with physical interpretations.

Revealed gradual variation of turbulence types across parameter space.

Discovered multiple routes to stratified turbulence through phase space analysis.

Abstract

We propose a dimensionality reduction and unsupervised clustering method for the automatic classification and reduced-order modeling of density-stratified turbulence in laboratory experiments. We apply this method to 113 long shadowgraph movies collected in a `Stratified Inclined Duct' (SID) experiment, where turbulence is generated by instabilities arising from a sheared buoyancy-driven counterflow at Reynolds numbers $Re \approx 300-5000$, tilt angles $\theta=1^\circ-6^\circ$ and Prandtl number $Pr \approx$ 700. The method automatically detects edges representative of discrete density interfaces, extracts a low-dimensional vector of statistics representative of their morphology, projects these statistics onto a two-dimensional phase space of principal coordinates, and applies the OPTICS clustering algorithm. Five clusters are detected and interpreted physically based on their typical interface morphology and an examination of representative frames, revealing distinct types of turbulence and mixing: laminarizing, braided, overturning, granular and unstructured, as well as some intermediate types. The ratio of time spent in each cluster varies gradually across the $(Re,\theta)$ space. At intermediate values of $Re\,\theta$, intermittent turbulence cycles between clusters in phase space and reveals at least two distinct routes to stratified turbulence. These insights demonstrate the potential of this method to reveal the underlying physics of complex turbulent systems from large experimental datasets.