There're lots of different GI methods, most of them use spatial acceleration structures such as BVH and KD-Trees after triangulation. Fluids use a grid for disc

There're lots of different GI methods, most of them use spatial acceleration structures such as BVH and KD-Trees after triangulation. Fluids use a grid for discretisation, behind the scenes the PDEs are solved be means of the finite difference methods, it has nothing to do with ray intersection tests.