ABSTRACT
When a body is placed in stationary fluid, it is subjected to a force normal to the element of surface. This normal force acting on a unit surface area is called pressure. If the gravity effect is neglected, the pressure is uniform everywhere. For example, a human body feels a force of 1013 mbar all over the surface, that is, the so-called atmospheric pressure. Compressibility is a measure of the change of volume or density of a fluid under the change of pressure and temperature. The modulus of compressibility is defined as the ratio of the relative volume change of fluid to the change of pressure. The performances of aircraft, ships, water turbines, and other big machines are usually predicted by laboratory experiments that use geometrically similar scaled-down models. A fundamental, important question is how can get two dynamically similar flows about geometrically similar bodies with different fluids and different velocities.
