11
Jan
05

### Von Kármán Vortex Street

When fluid passes an object, it can leave a trail of vortices called a Von Kármán Vortex Street. This animation shows the vorticity where blue is clockwise and red is counter-clockwise. This simulation assumes unsteady, incompressible, viscid, laminar flow. For solenoidal Jos Stam’s C Source Code and paper which is patented by Alias.
```(* Note: Something is wrong with this code. Please tell me if you find out what it is. runtime: 14 seconds *) n = 64; dt = 0.3; mu = 0.001; v = Table[{0, 0}, {n}, {n}]; Do[Do[If[i < 5, v[[i, j]] = {0.1, 0}]; If[(i - n/4)^2 + (j -n/2)^2 < 4^2, v[[i, j]] = {0, 0}], {i, 1, n}, {j, 1, n}]; ui = ListInterpolation[v[[All, All,1]]]; vi = ListInterpolation[v[[All, All, 2]]]; v = Table[{i2, j2} = {i, j} - n dt v[[i, j]]; {ui[i2, j2], vi[i2, j2]}, {i, 1, n}, {j, 1, n}]; v = Transpose[Map[Fourier[v[[All, All, #]]] &, {1,2}], {3, 1, 2}]; v = Table[x = Mod[i + n/2, n] - n/2; y = Mod[j + n/2, n] - n/2; k = x^2 + y^2; Exp[-k dt mu]If[k > 0, (v[[i, j]].{-y, x}/k){-y, x}, v[[i, j]]], {i, 1, n}, {j, 1, n}]; v = Transpose[Map[Re[InverseFourier[v[[All, All, #]]]] &, {1, 2}], {3, 1, 2}]; ListDensityPlot[Table[((v[[i + 1, j, 2]] - v[[i - 1, j, 2]]) - (v[[i, j + 1, 1]] - v[[i, j - 1, 1]]))/2, {j, 2, n - 1}, {i, 2, n - 1}], Mesh -> False,Frame -> False, ColorFunction -> (Hue[2#/3] &)], {t, 1, 25}];```

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