So I was reading through the Theory Guide for GPUSPH, and I stumbled across this bit;
And what I noticed is that the absolute value of the gradient is used. In theory the gradient of the kernel is anti-symmetric, so particles placed equally distanced on opposite sides, would contribute with opposite signs - of course this will not happen in equation 15, since it only focuses on the normalized distance, q.
I think my confusion might stem from the fact, that I have usually seen the gradient defined as this;
Where the nominator would return a vector with three values, indicating xi - xj, yi - yj and zi - zj etc. In this case, two particles placed on opposite sides, would get a different sign, so the gradient values would be anti-symmetric.
I am probably misunderstanding something along the way, just thought I might ask here, since this is the first time I have seen that the absolute value of the gradient is used.
Hope someone can clear my confusion, kind regards