Here is the flow inside a square box where the flow across the top of the box is moving to the right at Reynolds number 1000. This program uses the finite volume method to solve the Navier-Stokes equations assuming steady, incompressible, viscous, laminar flow. This was calculated on a 300×300 non-uniform grid and took 3 hours to run on my laptop. The animation shows the motion along the streamlines/pathlines.

Driven Cavity – calculated in Fortran 90, plotted in Mathematica 4.2, 11/2/05

The following Mathematica code is not completely accurate at the boundaries, but it gives the basic idea:

(* runtime: 20 minutes *)
n = 50; dx = 1.0/(n - 1); RE = 1000; relax = 0.4; residu = residv = residp = 1;
u = Table[If[j == 1 || i == 2 || i == n, 0, 1], {i, 1, n}, {j, 1, n}];
v = p = pstar = ap = an = as = aw = ae = sc = sp = du = dv = Table[0, {n}, {n}];
lisolv[i0_, j0_, phi0_] := Module[{phi = phi0, p = q =Table[0, {n}]}, Do[q[[j0 - 1]] = phi[[i, j0 - 1]]; Do[p[[j]] = an[[i, j]]/(ap[[i, j]] -as[[i, j]]p[[j - 1]]); q[[j]] = (as[[i, j]]q[[j - 1]] + ae[[i, j]] phi[[i + 1, j]] + aw[[i, j]]phi[[i - 1, j]] + sc[[i, j]])/(ap[[i, j]] - as[[i, j]]p[[j - 1]]), {j, j0, n - 1}]; Do[phi[[i,j]] = p[[j]]phi[[i, j + 1]] + q[[j]], {j, n - 1, j0, -1}], {i, i0, n - 1}]; phi];
calc[i0_, j0_] := Module[{}, {cn, cs, ce, cw} = 0.5 {v[[i,j + 1]] + v[[i0, j0 + 1]], v[[i, j]] + v[[i0, j0]], u[[i + 1, j]] + u[[i0 + 1, j0]], u[[i, j]] + u[[i0, j0]]}dx; cp = Max[0, cn - cs + ce - cw]; {an[[i, j]], as[[i, j]], ae[[i, j]], aw[[i, j]]} = Map[Max[0, 1 - 0.5 Abs[#]] &, RE{cn, cs, ce, cw}]/RE + Map[Max[0, #] &, {-cn, cs, -ce, cw}]; sp[[i, j]] = -cp; ap[[i, j]] = an[[i, j]] + as[[i, j]] + ae[[i, j]] + aw[[i, j]] - sp[[i, j]]];
While[Max[residu, residv, residp] > 0.001, residu = residv = residp = 0; Do[calc[i - 1, j]; sc[[i, j]] = cp u[[i, j]] + dx(p[[i - 1, j]] - p[[i, j]]); du[[i, j]] = relax dx/ap[[i, j]]; residu += Abs[an[[i, j]]u[[i, j + 1]] + as[[i, j]]u[[i, j - 1]] + ae[[i, j]]u[[i + 1, j]] + aw[[i, j]]u[[i - 1, j]] - ap[[i, j]]u[[i, j]] + sc[[i, j]]], {i, 3, n - 1}, {j, 2, n - 1}]; ap /= relax; sc += (1 - relax) ap u; u = lisolv[3, 2, u]; Do[calc[i, j - 1]; sc[[i, j]] = cp v[[i, j]] + dx(p[[i, j - 1]] - p[[i, j]]); dv[[i, j]] = relax dx/ap[[i, j]];residv += Abs[an[[i, j]]v[[i, j + 1]] + as[[i, j]]v[[i, j - 1]] + ae[[i, j]]v[[i + 1, j]] + aw[[i, j]]v[[i - 1, j]] - ap[[i, j]]v[[i, j]] + sc[[i, j]]], {i, 2, n - 1}, {j, 3, n - 1}]; ap /= relax; sc += (1 - relax)ap v; v = lisolv[2, 3, v];Do[an[[i, j]] = dx dv[[i, j + 1]]; as[[i, j]] = dx dv[[i, j]]; ae[[i, j]] = dx du[[i + 1, j]]; aw[[i, j]] = dx du[[i, j]]; sp[[i, j]] = 0; sc[[i, j]] = -((v[[i, j + 1]] - v[[i, j]])dx + (u[[i + 1, j]] - u[[i, j]]) dx); residp += Abs[sc[[i, j]]]; ap[[i, j]] = an[[i, j]] + as[[i, j]] + ae[[i, j]] + aw[[i, j]] - sp[[i, j]]; pstar[[i, j]] = 0, {i, 2, n - 1}, {j, 2, n - 1}]; Do[pstar = lisolv[2, 2, pstar], {2}]; Do[If[i != 2, u[[i, j]] += du[[i, j]](pstar[[i - 1, j]] - pstar[[i, j]])]; If[j != 2, v[[i, j]] += dv[[i, j]](pstar[[i, j - 1]] - pstar[[i, j]])]; p[[i, j]] += 0.3(pstar[[i, j]] - pstar[[n - 1, n - 1]]), {i, 2, n - 1}, {j, 2, n - 1}]];

Here is some Mathematica code to plot streamlines. The blue lines are rotating clockwise and the red lines are rotating counter-clockwise. The plot on the left agrees fairly well with Ercan Erturk’s results.
psi = Table[0, {n - 1}, {n - 1}]; Do[psi[[i, j]] = psi[[i, j - 1]] + dx(u[[i + 1, j - 1]] + u[[i + 1, j]])/2, {j, 2, n - 1}, {i, 1, n - 1}];
psi = ListInterpolation[psi, {{0, 1}, {0, 1}}];
ContourPlot[psi[x, y], {x, 0, 1}, {y, 0, 1}, PlotPoints -> 50, PlotRange -> All, ContourShading -> False,Contours -> {-0.08, -0.077, -0.07, -0.06, -0.045, -0.025, -0.01, -0.0025, 0, -8*^-6, 2*^-6, 3*^-5, 8*^-5, 1*^-4, 3*^-4, 6*^-4, 9*^-4}, ContourStyle -> Table[{Hue[2(1 - x)/3]}, {x, 0, 1, 1/16}]]

Here is some Mathematica code to plot vorticity contours. The plot on the left agrees fairly well with Ghia’s results.
omega = ListInterpolation[Table[(v[[i, j]] - v[[i - 1, j]])/dx - (u[[i, j]] - u[[i, j - 1]])/dx, {i, 2, n - 1}, {j, 2, n - 1}], {{0, 1}, {0, 1}}];
ContourPlot[omega[x, y], {x, 0, 1}, {y, 0, 1},PlotPoints -> 50, PlotRange -> All, ContourShading -> False, Contours -> Range[-14, 7],ContourStyle -> Table[{Hue[2(1 - x)/3]}, {x, 0, 1, 1/21}]]

Computational Fluid Dynamics (CFD) Links

• CFD Gallery – I especially like the simulations of turbulence
• Turbulent Boundary Layer – large simulation by Said Elghobashi
• More CFD Movies – nice simulations of fire
• Colorful Fluid Mixing Gallery
• Gallery of Fluid Dynamics – excellent web site on fluid dynamics by Mark Cramer
• Gallery of Fluid Motion – from the American Institute of Physics
• CFD code – simple Fortran codes by Abdusamad Salih
• FlowLab – educational resource by Fluent

Driven Cavity – Mathematica 4.2, 11/16/06

Here is the same driven cavity using the finite difference method. This code uses the Alternating Direction Implicit (ADI) method to solve the vorticity transport equation assuming unsteady, incompressible, viscous flow.
(* runtime: 1 minute *)
<< LinearAlgebra`Tridiagonal`; n = 20; RE = 1000; dx = 1.0/(n - 1); dt = 0.25; cxy = dt/(4dx); sxy = dt/(2RE dx^2); lambda = 1.5; free = Range[2, n - 1];
omega = v = psi = Table[0, {n}, {n}]; u = Table[If[j == n, 1, 0], {n}, {j, 1, n}];
Do[omega[[All, 1]] = 2psi[[All, 2]]/dx^2; omega[[All, n]] = 2 (psi[[All, n - 1]] + dx)/dx^2; omega[[1, All]] = 2 psi[[2, All]]/dx^2; omega[[n, All]] = 2 psi[[n - 1, All]]/dx^2; omega[[free, free]] = Transpose[Partition[TridiagonalSolve[Drop[Flatten[Table[If[i == n - 1, 0, -(cxy u[[i, j]] + sxy)], {j, 2, n - 1}, {i, 2, n - 1}]], -1], Flatten[Table[1 + 2sxy, {j, 2, n - 1}, {i, 2, n - 1}]], Drop[Flatten[Table[If[i == n - 1,0, cxy u[[i + 1, j]] - sxy], {j, 2,n - 1}, {i, 2, n - 1}]], -1], Flatten[Table[(cxy v[[i, j - 1]] + sxy)omega[[i, j - 1]] + (1 - 2sxy)omega[[i, j]] + (-cxy v[[i, j + 1]] + sxy)omega[[i, j + 1]] + If[i == 2, (cxy u[[i - 1, j]] + sxy)omega[[i - 1, j]],0] - If[i == n - 1, (cxy u[[i + 1, j]] - sxy) omega[[i + 1, j]], 0], {j, 2, n - 1}, {i, 2, n - 1}]]], n - 2]]; omega[[free, free]] = Partition[TridiagonalSolve[Drop[Flatten[Table[If[j == n - 1, 0, -(cxy v[[i, j]] + sxy)], {i, 2, n - 1}, {j, 2, n - 1}]], -1], Flatten[Table[1 + 2sxy, {i, 2, n - 1}, {j, 2, n - 1}]], Drop[Flatten[Table[If[j == n - 1, 0, cxy v[[i, j + 1]] - sxy], {i, 2, n - 1}, {j, 2, n - 1}]], -1], Flatten[Table[(cxy u[[i - 1, j]] + sxy)omega[[i - 1, j]] + (1 - 2sxy)omega[[i, j]] + (-cxy u[[i + 1, j]] +sxy)omega[[i + 1, j]] + If[j == 2, (cxy v[[i, j - 1]] + sxy)omega[[i, j - 1]], 0] - If[j == n - 1, (cxy v[[i, j + 1]] - sxy) omega[[i, j + 1]], 0], {i, 2, n - 1}, {j, 2, n - 1}]]], n - 2]; resid = 1; While[resid > 0.001, resid = 0; Do[resid += Abs[psi[[i, j]] - (psi[[i,j]] = lambda(psi[[i, j - 1]] + psi[[i - 1, j]] + psi[[i + 1, j]] + psi[[i, j + 1]] - dx^2omega[[i, j]])/4 + (1 - lambda)psi[[i, j]])], {i, 2, n - 1}, {j, 2, n - 1}]]; Do[u[[i, j]] = (psi[[i, j + 1]] - psi[[i, j - 1]])/(2dx); v[[i, j]] = -(psi[[i + 1, j]] - psi[[i - 1, j]])/(2dx), {i, 2, n - 1}, {j, 2, n - 1}]; ListContourPlot[Transpose[psi], PlotRange -> All, ContourShading -> False], {100}];

Link: Driven Cavity – simple Fortran program by Zheming Zheng that uses this method