Vorticity crawlers and other strange animals in near wall turbulence

What is it about?

In many key areas of technology, the interaction of a turbulent fluid with a solid boundary (the wall) is of paramount importance because it controls key global features of the flow, such as flow separation and reattachment, recirculation, heat transfer rates etc. For example, this kind of interaction is important during the stall of over an airplane wing. While our understanding of near-wall turbulence has improved over the last couple of decades, we are still discovering new aspects of the mechanisms that take place in near-wall turbulence. For these reasons, over the last two decades, there has been many notable efforts by an active research community to visualize and understand key structures that appear in the turbulent flow of a fluid over a solid boundary (the wall). In this work, we perform direct numerical simulation of fully developed turbulent pipe flow and compute the one-point turbulence tensors. We also introduce a new eduction method for the visualization of near-wall structures and this leads to the reporting of new structures, the "vorticity crawlers".

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Why is it important?

In this work,we carry direct numerical simulations of fully developed pipe flow and we use a new eduction method based on the instantaneous values of the turbulence structure tensors to shed light on large-scale but low-energy turbulence structures that populate the extreme vicinity of the wall. We have called these structures, "vorticity crawlers", because in visualizations of the turbulence the vorticity lines that penetrate through them give them the appearance of insects crawling on the wall. Using conditional averaging in our direct numerical simulations, we are currently exploring the contribution of these structures to the statistics of near-wall turbulence, an area with implications for engineering turbulence models.

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