A hydrodynamic facility used for research, test, and evaluation, comprising a well-guided and controlled stream of water in which items for test are placed. The water tunnel is in many ways similar in appearance, arrangement, and operation to a subsonic wind tunnel. It is related to and complementary to the towing tank, in which the test item, usually a scale model of a ship or ship component, is towed through stationary water and evaluated through observation and measurement. In a water tunnel the test item is held stationary while the water is circulated around it. Many water tunnels are capable of operation with variable internal pressure to simulate the phenomenon of cavitation.