Near-Wall Coherent Structures: History, Identification and Detection - Transport and Coherent Structures in Wall Turbulence - page 129

3
Near-Wall Coherent Structures:
History, Identification and Detection
3.1. Introduction
Coherent eddy structures play a fundamentally important
role in the transport of momentum and scalar quantities (e.g.
temperature). They take the form of quasi-streamwise
vortices (QSVs), which are the “spaghetti”-like structures
visible in Figure 3.1. We can clearly see the predominant
effect of these structures on the turbulent mixing: the
turbulent activity, which is reflected by temperature
fluctuations in three successive planes in Figure 3.1,
increases significantly when the QSVs “pass through”.
The “discovery” of a certain coherence in the structure of
wall turbulence, 50 years ago, certainly attracted a huge
amount of research interest, but at the same time it raised a
number of questions and posed numerous problems. The first
of these relates directly to the identification of the
structures, as the study of their dynamic characteristics
deeply depends on this. This chapter aims to shed light on
this aspect of the process. We begin, though, with a brief look
at the history of the practice, before analyzing single-point
detections, as carried out when measuring with probes. The
topology of wall turbulence will then be examined,