A water tunnel breaks the Reynolds number conundrum.
http://ecomodder.com/forum/showthrea...ies-31047.html shows how it's done. Upside down, with hydrogen bubbles.
It does involve large quantities of water. Here's my design. Version 0.2:
The opposing paddlewheels are interleaved so they move a large volume with minimal turbulence going into the diffuser. Version 0.4 would have a single return and a single paddle wheel at the far end from the diffuser. Big return channel, small test section.