Syscrusher. In a nutshell as to why you want the jet to be stood off slightly can be explained in the following way.
When the jet is really close to the surface, the what happens is the water just squirts out the side of the jet tube, but in the middle of the tube it doesn't really move at all. i.e. the point of central stagnation is fairly large. Imagine filling a glass with water and sticking a flat piece of something on it and turning it upside down. Now lift the glass slightly away from the surface. The water that flows out mostly flows out the small gap, but the water in the middle is barely moving at all, i.e. the stagnation region.
Now do the same experiment but lift the glass away quickly and all the water pours out and strikes the whole area under the glass, rather than merely leaking out the sides.
By standing the jet off a certain distance we greatly reduce that central stagnation effect where the water is barely moving at all. The actual best distance to stand the jet off by is linked to the velocity of the jet stream, as one can well imagine. It gets a little more complicated in submerged jet scenarios where the jet loses power as it moves through the liquid around it, but also gains added turbulence as a bit of a bonus.
The actual math of all that is the subject of a large number of research papers into impingement and its effects. I won't pretend that I could construct a mathematical answer for any scenario, and in fact I don't think that many researchers could either, but the theory that's out there does provide guidelines for good starting points, and unless your jet velocity is extremely low, having the jet really close (<2d) often turns out to be worse.
Have a poke through that paper I linked to above for more information.
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