Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

TL;DR

This paper investigates how the geometrical features of the turbulent/turbulent interface evolve spatially in a cylinder wake, revealing differences from the turbulent/non-turbulent interface and the influence of flow conditions on interface complexity.

Contribution

It provides the first detailed analysis of the spatial variation of the TTI's fractal dimension and its dependence on background turbulence characteristics.

Findings

TTI spreads faster than TNTI towards ambient flow.

A transition at x/d ≈ 15 affects interface propagation rates.

TNTI PDF becomes Gaussian downstream after large-scale motions dissipate.

Abstract

This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are independently varied and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high-Schmidt-number ($Sc$) scalar and a planar laser induced fluorescence (PLIF) experiment was carried out to capture the interface between the wake and the ambient flow from $x/d$ = 5 to 40 where $x$ is the streamwise coordinate from the centre of the cylinder and $d$ is the cylinder's diameter. It is found that the TTI generally spreads faster toward the ambient flow than the TNTI. A transition region of the interfaces' spreading is found at $x/d \approx 15$, after which the interfaces propagate at a slower rate than previously (upstream) and the mean interface positions of both TNTI and TTI scale with the local wake half-width. The location of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time we explore the spatial variation of the ``roughness'' of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity only becomes important for the fractal dimension farther downstream.