This is what Tuff is talking about "I think". Here's a couple of links.
Link 1
link 2
link 3
It's totally impractical for our use. Where would you get 20m/s velocity, 4ml/min flow rate from.:shrug: Then you have to consider that it would be submerged which changes the whole game.
Cheesball:
A nozzle with a gradual taper (hyperbolic shape) is what you would want just as Bill stated. How you can machine that I have no idea as I'm no pro metal machinist. You could also make it what ever shape you want too as long as it has that gradual taper. Of course, you could just plug the end of a barb then drill, cut what ever hole in it and be done with it. It'll work but not as good. The height, diameter of the nozzle, and the surface geometry on the base of the block play important roles and need to be considered. Also, don't forget the pump and it's performance as that will dictate what you can actually achieve with a nozzle. I have no sure answers for you but just to experiment for your own learning adventure. I'm sure myv65 and other more educated people in this field here could help you better. BillA also has some good info from his testing that can help you make a slight educated guess. All I know is what works best with a flat, slight convex base.