Characteristics of the turbulent/nonturbulent interface in boundary layers, jets and shear-free turbulence

Carlos B Da Silva,Rodrigo R Taveira,Guillem Borrell

doi:10.1088/1742-6596/506/1/012015

Abstract

The characteristics of turbulent/nonturbulent interfaces (TNTI) from boundary layers, jets and shear-free turbulence are compared using direct numerical simulations. The TNTI location is detected by assessing the volume of turbulent flow as function of the vorticity magnitude and is shown to be equivalent to other procedures using a scalar field. Vorticity maps show that the boundary layer contains a larger range of scales at the interface than in jets and shear-free turbulence where the change in vorticity characteristics across the TNTI is much more dramatic. The intermittency parameter shows that the extent of the intermittency region for jets and boundary layers is similar and is much bigger than in shear-free turbulence, and can be used to compute the vorticity threshold defining the TNTI location. The statistics of the vorticity jump across the TNTI exhibit the imprint of a large range of scales, from the Kolmogorov micro-scale to scales much bigger than the Taylor scale. Finally, it is shown that contrary to the classical view, the low-vorticity spots inside the jet are statistically similar to isotropic turbulence, suggesting that engulfing pockets simply do not exist in jets.

Highlights

In many turbulent shear flows such as boundary layers, jets, mixing layers and wakes there is a sharp interface that divides the flow field into two distinct regions
In one region the flow is turbulent while in the other, the flow is largely irrotational (Corrsin and Kistler [1]). This sharp interface - the turbulent/nonturbulent interface (TNTI) – is continually deformed over a wide range of scales and the flow dynamics in its vicinity determines many of the most important flow features: the growth and spreading rate of wakes, the exchanges of mass across mixing layers, and the mixing and reaction rates in jets are some of the flow features that are largely determined by the characteristics of the TNTI and the flow dynamics in its vicinity
Direct numerical simulations of a boundary layer, a planar jet and shear-free turbulence at Reynolds numbers slightly higher than Reλ = 100 are used to compare the characteristics of the TNTI in different flows

Summary

Introduction

In many turbulent shear flows such as boundary layers, jets, mixing layers and wakes there is a sharp interface that divides the flow field into two distinct regions. In one region the flow is turbulent while in the other, the flow is largely irrotational (Corrsin and Kistler [1]). This sharp interface - the turbulent/nonturbulent interface (TNTI) – is continually deformed over a wide range of scales and the flow dynamics in its vicinity determines many of the most important flow features: the growth and spreading rate of wakes, the exchanges of mass across mixing layers, and the mixing and reaction rates in jets are some of the flow features that are largely determined by the characteristics of the TNTI and the flow dynamics in its vicinity. Past studies described the entrainment as being caused by large-scale eddy motions (engulfment) occurring from time to time at particular locations along the TNTI [2], but recent works suggest instead that the entrainment results from small scale motions (nibbling) acting along the entire TNTI (Mathew and Basu [3], Hunt et al.[4]), as originally described by Corrsin and Kistler [1]

Objectives

Findings

Conclusion