Analysis and identification of vortices within a turbulent channel boundary layer flow

摘要

Vortical structures are regarded as the dominant organized patterns in wallturbulence. They play a key role in physical phenomena of practical importance such asenergy and momentum transport, combustion, mixing, and noise and drag production.Considerable investigations have been performed in drag and noise phenomena studies,with a main purpose of controlling and reducing them. Various techniques to control thedrag reduction have been studied for over last five decades; however, the detailedunderstanding of the drag reduction mechanism is still lacking. Vortices play animportant role in turbulence structure. Nevertheless, the identification of vortices is stillunclear, not even a universal definition of a vortex is accepted.In the present study, several vortex feature extraction schemes are implemented.The methods are applied to analyze instantaneous two-dimensional velocity fieldsobtained by particle tracking Velocimetry (PTV) measurements of a turbulent channelflow with and without microbubble injection within the boundary layer. Microbubbleinjection is one of the drag reduction techniques, first studied in early 1970s, that hasundergone extensive research in past years, and the generated information has aided into drag reduction understanding.As a general rule, vortex extraction methods can be either a simple visualizationscheme or more sophisticated identification tools. The Reynolds decomposition and itsvariants are suitable due to their capacity to mark vortices advecting at differentvelocities. In the case of identification techniques, which yield a scalar field calculatedfrom either the velocity vector field or the velocity gradient tensor, both the modifiedswirling strength Λci or the λ2 criteria were found to be well suited for vortexidentification.