Derivation of Gradient in SPH
I'm having a tricky time with the gradient using the SPH method. The standard SPH formulation makes sense:
To find the gradient, integrate by parts:
The first term is zero, since W(r,h)=0 for r>=h. So, we are left with
However, the papers I am looking at  state
That is, there is no minus sign. I feel that I'm missing some stupid mistake, but I've been looking at it for a while, and I don't see it.
Thanks in advance!
 J.J. Monaghan. "Smoothed Particle Hydrodynamics and Its Diverse Applications." Annual Review of Fluid Mechanics, Vol. 44: 323 -346. January 2012
Dear Scott Kulp
It came from a little astute trick, I advise you to read page 43 and 44 of the book "Smoothed particle hydrodynamic particle a meshfree particle method" by G.R.Liu and M.B Liu in order to understand what happened.
This book is really nice and very informative. I see that equation 2.22 is basically the same as I wrote above (though 2.22 is the divergence), with the negative, and I do understand that. However, in the summary on page 44, equation 2.25 is not negative. I don't see any explanation on why the negative was removed... Any thoughts?
I understood your question first time,however I think the answer is in page 44 where the authors wrote : "it should be noted that grad(i)Wij is taken with respect to particle i so the negative sign in equation (2.21) is removed in equation (2.25)."
I agree with you there is an vagueness in the notation formulas,maybe we can say the mistake, but the concept is true, W is an even function so grad(W) is an odd one
it means W in the equation 2.25 is the symmetry as W in equation 2.22 because the first is about all neighbors(which detected by j counters) and second is about the determined particle itself (i), maybe if you put Wji instead of Wij in eq 2.22 everything become correct.
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