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August 23, 2007, 06:11 |
Einestine summation - help
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#1 |
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Can someone explain to me which one of this is right for;
d(u_i u_i)/dt = nu(d(u_i)/d(x_j))^2 for x - momentum is it A. d(u_1)^2/dt = nu(d(u_1)/d(x_1) + d(u_1)/d(x_2))^2 or B. d(u_1)^2/dt = nu(d(u_1)/d(x_1))^2 + (d(u_1)/d(x_2))^2. I am really confused, please help. With regards, Tilek |
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August 23, 2007, 08:45 |
Re: Einstein summation - help
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#2 |
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With Einstein Summation, you will only sum over the repeated index (indices), (i.e. 'i' in this case)
For X-momentum (j=1) you would sum the i's (i=1,2,3) But since this sounds like a homework assignment I won't put up the answer just yet. |
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August 23, 2007, 21:14 |
Re: Einestine summation - help
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#3 |
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Yeah thanks for the comment and I still look forward to your feedback.
A colleague here said it is (A). But I saw esp. the RHS to be possibly written as nu(d(u_i)/d(x_j))(d(u_i)/d(x_j)) in which case (B) looks the case applicable here. In my first thought also; if I have to write it in code (like Fortran), with a do loop, it sounds to be case (B). But in the line of the rule "sum first the repeated indices", it would look case (A) applies. I think this is a simple problem for those in turbulence research, which I am struggling right now. I appreciate feedback still. Cheers, TAW |
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August 24, 2007, 08:47 |
Re: Einestine summation - help
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#4 |
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Well for strictly ESN (Einstein Summation Notation), both are wrong. You will not sum the J's, since it is not a repeated index, so leave the J as is.
And if you want just i=1, just substitute in. This isn't really ESN if you just want i=1. |
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