Most of the hydrodyriamic or aerodynamic components are simulated on reduced scale models. Tests are conducted in blowdown tunnels, towing tanks, launching tanks on the whole or part of a naval vessel, aeroplanes or civil works whose performance is to be predicted. While applying the standard similitude in fluid mechanics, an effort is made to arrange the non-dimensional parameters for the model to be equal to those for the prototype. For single-phase flows the relevant parameters are the Euler, Reynolds, Froude and Mach numbers. In all such tests the general practice is to work at the highest possible Reynolds number. Even cryogenic blowdown tunnels are being developed in which it would be possible to obtain on a reduced scale the Reynolds number of transport aircrafts. Small cavitation tunnels are found in universities, engineering colleges as well as in industrial laboratories working on cavitation problems.