16
Nov
06

Kelvin-Helmholtz Instability Waves

Kelvin-Helmholtz Instability Waves are formed when one fluid layer is on top of another fluid layer moving with a different velocity. These instabilities take the form of small waves that can eventually grow into vortex rollers. This is a purely potential flowirrotational) phenomenon. The following Mathematica code was adapted from Zheming Zheng’s Fortran program. It uses vortex blobs to simulate smooth rollers and periodic point vortices to simulate the far field. A simple predictor-corrector scheme is used to integrate the differential equations:

(* runtime: 9 minutes *) n = 40; dt = 0.05; L = 2Pi; gamma = L/n; rcore = 0.5; plist = Table[{x, 0.01 Sin[2Pi x/L]}, {x, 0, L(1 - 1/n), L/n}]; vcalc[plist_] := Table[Sum[{x, y} = plist[[j]] - plist[[i]]; gamma/(2Pi) Sum[If[i == j && k == 0, 0, r2 = (x + k L)^2 +y^2; {-y, x + k L}/r2(1 - Exp[-r2/rcore^2] - 1)], {k, -3, 3}] + If[i ==j, 0, gamma/(2L) {-Sinh[2Pi y/L], Sin[2Pi x/L]}/(Cosh[2Pi y/L] - Cos[2Pi x/L])], {j, 1, n}], {i, 1, n}]; Do[vlist = vcalc[plist]; vlist0 =vlist; vlist = vcalc[plist + dt vlist]; plist += dt(vlist0 + vlist)/2; ListPlot[plist, PlotJoined -> True, PlotRange -> {-2, 2}, AspectRatio -> Automatic], {i, 1, 400}];

Here is some Mathematica code to plot streamlines assuming periodic point vortices: (* runtime: 2 seconds *) Clear[psi]; psi[x_, y_] := Plus @@ Map[-gamma/(4 Pi) Re[Log[Cos[2Pi(x – #[[1]])/L] – Cosh[2Pi (y – #[[2]])/L]]] &, plist]; ContourPlot[psi[x, y], {x, 0, L}, {y, -L/3, L/3}, PlotRange -> All, ContourShading -> False, PlotPoints -> 20, AspectRatio -> Automatic]

Link: Kelvin-Helmholtz waves in the clouds

Here is another variation showing vortex pairing and pathlines.

Link: movie – famous experiment by Bernal

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