73th Annual Meeting of the APS Division of Fluid Dynamics (November 22, 2020 — November 24, 2020)

P0019: The curious nature of hairpin vortices

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Authors

• Abhishek Paraswarar Harikrishnan, Free University of Berlin
• Cedrick Ansorge, University of Cologne
• Rupert Klein, Free University of Berlin
• Nikki Vercauteren, Free University of Berlin

DOI: https://doi.org/10.1103/APS.DFD.2020.GFM.P0019

The Direct Numerical Simulation (DNS) of an Ekman flow is shown here. It describes the Atmospheric Boundary Layer (ABL) which is the lowest part of the troposphere in direct contact with the Earth's surface. 

Applying the vorticity magnitude on xy or xz plane reveals large regions where turbulence is absent in the highly stratified (HS) case (first column of images), a feature which is not present in the neutrally stratified (NS) case (second column of images). 

Curiously, if we randomly extract Q-criterion structures from both cases, the hairpins from the HS case appear to be more organized i.e. the head of the hairpin points towards a certain direction. 

The final two rows shows several hairpins extracted with Multilevel Percolation (MLP). The first row shows their size measured in terms of 𝛿 (boundary layer height under neutral conditions). The second row shows their position in terms of y+ (viscous wall units). 

The Reynolds number is described in terms of the geostrophic wind velocity (G) and 𝛿 as Re = G𝛿/ν.