Water tunnel (hydrodynamic)

It functions similar to a recirculating wind tunnel, but uses water as the working fluid, and related phenomena are investigated, such as measuring the forces on scale models of submarines or lift and drag on hydrofoils.

For many cases as long as the Reynolds number is equivalent, the results are valid, whether a submerged water vehicle model is tested in air or an aerial vehicle is tested in water.

The advantage is that the increased viscosity will allow the flow to be a higher speed (and thus easier to maintain in a stable manner) for a lower Reynolds number.

Often, a tunnel will be co-located with other experimental facilities such as a wave flume at a Ship model basin.

To ensure similarity to the full-scale propeller, the pressure is lowered, and the gas content of the water is controlled.