A water tunnel is an experimental facility used for testing the
hydrodynamic behavior of submerged bodies in flowing water. 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. Water tunnels are sometimes used in place of wind tunnels to perform measurements because techniques like
particle image velocimetry (PIV) are easier to implement in water. 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. For low Reynolds number flows, tunnels can be made to run oil instead of 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.
Whereas in
wind tunnels the driving force is usually sophisticated multiblade
propellers with adjustable
blade pitch, in water and oil tunnels the fluid is circulated with pumps, effectively using a net
pressure head difference to move the fluid rather than imparting
momentum on it directly. Thus the return section of water and oil tunnels does not need any flow management; typically it is just a pipe sized for the pump and desired flow speeds. The upstream section of a water tunnels generally consists of a pipe (outlet from the pump) with several holes along its side and with the end open followed by a series of coarse and fine screens to even the flow before the contraction into the test section. Wind tunnels may also have screens before the contraction, but in water tunnels they may be as fine as the screen used in window openings and screen doors.
Additionally, many water tunnels are sealed and can reduce or increase the internal static
pressure, to perform
cavitation studies. These are referred to as cavitation tunnels.
Methods
Because it is a high-speed phenomenon, a special procedure is needed to visualize cavitation. The propeller, attached to a
dynamometer, is placed in the inflow, and its thrust and torque is measured at different ratios of propeller speed (number of revolutions) to inflow velocity. A
stroboscope synchronized with the propeller speed "freezes" the cavitation bubble. By this means, it is possible to determine if the propeller would be damaged by cavitation. To ensure
similarity to the full-scale propeller, the pressure is lowered, and the gas content of the water is controlled.
Often, a tunnel will be co-located with other experimental facilities such as a
wave flume at a
Ship model basin.
Multiple cavitation tunnels at the Versuchsanstalt für Wasserbau und Schiffbau,[6] Berlin
Cavitation tunnel at the University Duisburg-Essen, Institute of Ship Technology, Ocean Engineering and Transport Systems,[7] University Duisburg-Essen
Cavitation tunnel at Potsdam Ship Model Basin,[8] Potsdam
Large Cavitation tunnel at Hamburg Ship Model Basin,[9] Hamburg
Multiple cavitation tunnels at the Oskar von Miller Institut,[10] Technical University of Munich
A miniature water tunnel whose return section is simply a reservoir tub with a
sump pump. Flow rate is controlled by constricting the flow on the outlet of the pump with a valve.