Revisiting "bursts" in wall-bounded turbulent flows

TL;DR

This paper introduces a new event-based framework and a burstiness index to analyze turbulent bursting activities across multiple scales, revealing their dependence on flow structures and Reynolds number in wall-bounded turbulence.

Contribution

The study develops a novel burstiness index and an event-based analysis method to characterize multi-scale turbulence bursts and their Reynolds number dependence.

Findings

Large amplitude fluctuations are governed by flow structures regardless of Reynolds number.

Burstiness features of small-scale turbulence depend on Reynolds number.

Burstiness index varies with height and scale in wind tunnel data, but not in atmospheric flows.

Abstract

Turbulent signals are known to exhibit burst-like activities, which affect the turbulence statistics at both large and small scales of the flow. In our study, we pursue this problem from the perspective of an event-based framework, where bursting events are studied across multiple scales in terms of both their size and duration. To illustrate our method and assess any dependence on the Reynolds number, we use two datasets - from the Melbourne wind tunnel and from SLTEST - an atmospheric surface layer experiment. We show that an index, namely the burstiness index, can be used successfully to describe the multi-scale nature of turbulent bursting while accounting for the small-scale intermittency effects. Through this index, we demonstrate that irrespective of the Reynolds number, the presence of large amplitude fluctuations in the instantaneous velocity variance and momentum flux signals are governed by the coherent structures in the flow. Concerning small-scale turbulence, a Reynolds number-dependence is noted while studying the scale-wise evolution of the burstiness features of second-order streamwise velocity increments. Specific to the wind-tunnel dataset, the burstiness index of streamwise velocity increment signal displays a strong dependence on height and also decreases as the scales increase with the maximum being obtained at scales comparable to the dissipative structures. However, such features are nearly absent in the atmospheric flows. To conclude, this research paves a novel way to evaluate the effect of bursts on the turbulence statistics at any specified scale of the flow.