The Hydrodynamics of the Free-Surface Water Tunnel

Free-surface water tunnels are used to determine the hydrodynamic characteristics of bodies which move in water near the surface. To develop the basic principles for the design and operation of such a tunnel the conditions for dynamic similitude near an air-water interface are analyzed. Although the tests should determine how the hydrodynamic characteristics are affected by the surface waves produced by the body itself, the dimensions and conditions of operation of the test channel are sometimes responsible for disturbances that interfere with the interpretation of the local wave effect. In order to record the type of interfering waves that might be expected and the conditions responsible for their occurrence, the Free-Surface Water Tunnel at the Hydrodynamics Laboratory of the California Institute of Technology was temporarily operated under these conditions. Since the local wave pattern produced by the body under study depends primarily on the criterion for inertial-to-gavitational similitude between model tests and prototype operation, the relation between this criterion and the conditions that produce the undesirable channel waves is discussed. Application of gravitational similitude usually results in model tests that are different from prototype operation in respect to viscous and surface-tension effects. A chart is developed showing the relations for gravitational similitude and indicating the resulting dissimilitude for viscous and surface-tension phenomena. This chart is also used to indicate the complete range of operation of a free-surface test channel as well as the range in which undesirable waves are produced in the working section. Through recognition of the nature of the interactions between the hydrodynamic characteristics of the tunnel and the body under investigation, design and operation principles for a free-surface water tunnel are developed showing how to decrease or avoid the wave difficulties and minimize the unavoidable dissimilitude.