# Structural inclination angle of scalar fluctuations in a turbulent boundary layer

## TL;DR

This study measures scalar concentration correlations in a turbulent boundary layer, revealing that scalar structures incline at a consistent angle of about 30 degrees, regardless of source height, indicating coherence in scalar organization.

## Contribution

It provides new insights into the inclination angle of scalar structures in turbulent boundary layers and confirms its invariance across different source heights.

## Key findings

- Scalar structures incline at approximately 30 degrees.
- Inclination angle is weakly dependent on source height.
- Scalar coherence is similar to velocity field coherence.

## Abstract

Two-point measurements of concentration are obtained in a meandering passive-scalar plume released at five different heights within the fully-turbulent region of a high-Reynolds number turbulent boundary layer (TBL). These measurements are obtained using a combination of two photo-ionisation detectors (PIDs). A stationary PID is positioned at the plume centreline, while the second PID traverses across the plume in the wall-normal direction. Similar to the large-scale coherence observed in the two-point correlation map of streamwise velocity fluctuations \citep[e.g. Marusic et al.][]{marusic2007reynolds}, the two-point correlation results of concentration also indicate coherence in the scalar field. Particularly, the iso-contours of correlation indicate an inclination of scalar structures in the direction of the flow. The corresponding inclination angle is found to be a weak function of the location of the plume within the boundary layer. However, the inclination angle is observed to be invariant near 30$^\circ$ for all source heights that are below $z/\delta \leq 0.33$. This observed steepness of inclination angle relative to the inclination angle of streamwise velocity is consistently explained through the physical model put forth by Talluru et al. \cite{Talluru2018} that explains the organisation of scalar around the low- and high-speed regions in the flow which inherently also have a characteristic inclination.