# KH Instability

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 April 7, 2016, 03:32 KH Instability #1 New Member   Marija Bervida Join Date: Mar 2016 Location: Slovenia Posts: 5 Rep Power: 2 Hi all! I am new to CFD and OpenFOAM, and this is my first post here on forum. Lately I am trying to simulate Kelvin-Helmholtz instability in the atmosphere, which can be noticed in "wavy" clouds. I will describe what I did for now, and I would appreciate your help (advices, comments). My problem consist of two layers of air with different speeds and different densities. Because this is case between two layers of air in which density difference is not so big like it is in a case of KH instability between air and water I don't know if I can neglect it, because the shearing will appear due to different velocities. My domain looks like this: (box) 10 km (x dir) x 1 km (y dir) x 400 m (z dir). In y direction i divided inlet on 2 equal part, and for boundary condition I put different (uniform) velocities on this inlets. On outlet I put pressure to be zero. And for all rest I put zeroGradient conditions (and empty iz z direction, because of 2D). Is that ok? (I saw somewhere on forum when was word about KH instability between air and water someone suggested to put cyclic boundary condition in flow direction.) I am not sure which solver to use. For now I run several simulation using icoFoam (incompressible, transient) and twoLiquidMixingFoam (multiphase). For start i used icoFoam to see how it all functions (setting up case, adding temperature like passive scalar to solver, ...). How KH instability would be like transitional case between laminar and turbulent flow, I am not sure about what to use (should I use turbulent model). For considering density difference (also for laminar flow) I used twoLiquidMixingFoam solver, where density difference is considered. I assumed some little difference in densities between this two layers of air 0.13 kg/m^3. In icoFoam (laminar flow) density difference is ignored, and the waves occure due to difference in velocities. Even I've got some nice results with multiphase solver twoLiquidMixingFoam (2 mixing incompressible fluids) and icoFoam I am not sure if these are appropriate solvers for this problem. Also, I would like to add initial perturbation (some sin wave), but I don't know how. I searched way on forum but i still didn't managed to do it. It would be of use if I can get more detailed instructions. I would really appreciate your advices and comments. Thanks in advance, Andela

 April 8, 2016, 12:14 #2 New Member   Thomas Evans Join Date: Dec 2015 Posts: 12 Rep Power: 2 funkySetFields is probably a good place to start. http://openfoamwiki.net/index.php/Co...funkySetFields

 April 11, 2016, 02:32 #3 New Member   Marija Bervida Join Date: Mar 2016 Location: Slovenia Posts: 5 Rep Power: 2 Thank you for reply. I will try with it.

 April 12, 2016, 08:17 #4 New Member   Marija Bervida Join Date: Mar 2016 Location: Slovenia Posts: 5 Rep Power: 2 Anyone other have maybe some suggestion or coment? I would really appreciate anything. =)

 Tags instability, kelvin helmholtz, perturbation

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