# divergence in MAC Method

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 December 15, 2000, 06:53 divergence in MAC Method #1 Maciej Matyka Guest   Posts: n/a hello. i have new problem with my program. i see than divergence equations fails. my divergence equation is: D[i][j] = (U[i][j] - U[i-1][j])/dx + (V[i][j] - V[i][j-1]; in paper by Harlow Welch (MAC method) he write than divergence cannot be larger than 3.5 * 10^(-4). Ok, but when divergence is too big for one of cells, then i cannot make it smaller (when i less dt, Divergence dont change). Any ideas? /// maq ps. just small question - width of output = 256 height of output = 256 number of cells x = 64 number of cells y = 64 thats means than dx,dy = 256/4 ???

 December 15, 2000, 11:00 errata :) #2 Maciej Matyka Guest   Posts: n/a sorry: of corse: D[i][j] = (U[i][j] - U[i-1][j])/dx + (V[i][j] - V[i][j-1]; and dx,dy = 256/64??? sorry. anyway anybody know?

 December 19, 2000, 10:43 Re: errata :) #3 Senthil Guest   Posts: n/a dear maq check ur D eqns, dy is missing there.if it is the typing mistake here alone, then the culprit is the way u evaluate the u and v. u(i,j) is average of u(i,j) and u(i-1,j) and u(i-1,j) is average of u(i-2,j) and u(i-1,j) and so forth.check this out by senthil.

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