Jets can be classified as submerged if they discharge into an ambient fluid of similar physical properties (e.g., air in air) and unsubmerged if the properties of the two fluids are quite different (e.g., water in air). Several configurations have been tested such as: jets issuing from orifices or round and slot nozzles different types of jet arrays; flat and curved impingement surfaces, normal and inclined impingement, etc. Only the simple geometry of a single jet impinging on a flat surface is treated below. Figure 1 shows the typical fluid-dynamic features of a submerged (top) and an unsubmerged (bottom) jet. For both, three main regions can be identified: