viscous diffusion in PeleC

[jagmohas]

I am trying to understand this calculation from GradUtil.H in PeleC. Could you point me to a source where I can understand this calculation? I don't understand why we add terms for j+1 but subtract terms for j-1.
td(i, j, k, 0) = 0.25 * dx1inv * (q(i, j + 1, k, QU) + q(i - 1, j + 1, k, QU) - q(i, j - 1, k, QU) - q(i - 1, j - 1, k, QU)); // dudy
td(i, j, k, 1) = 0.25 * dx1inv * (q(i, j + 1, k, QV) + q(i - 1, j + 1, k, QV) - q(i, j - 1, k, QV) - q(i - 1, j - 1, k, QV)); // dvdy

Thanks.

[drummerdoc]

Does this help?

[jagmohas]

Thanks Marc, I sketched something similar, but when comparing it with the code I missed that we don't have a bracket for qc+qd. Thanks a ton.