That violates conservation of momentum. To get realistic flow, you need to use continuity equation to preserve the energy of the flow and let the shear dissipate and diffuse into whorls. Which is a lot of extra calculation, as pointed out in the article, so you need to ask yourself if that level of realism is really necessary for your use case.
is as good as conserving momentum as the original model. (Assuming you calculate all the new values using a new matrix instead of overwriting the old ones.) It breaks energy conservation, but that simulates the friction between oposite direction currents, without going to the whorls level.
I agree that this is not 100% realistic, but I think it's a good and simple addition to the model in the article, and tweaking the coefficients it may give a good enough visual result.
