A fluid flow in which a stream of one fluid mixes with a surrounding medium, at rest or in motion. Such flows occur in a wide variety of situations, and the geometries, sizes, and flow conditions cover a large range. Jet flows vary greatly, depending on the values of two numbers. The first is the Reynolds number, defined in Eq. (1),
1. 
where ρ is the density, V is a characteristic velocity (for example, the jet exit velocity), L is a characteristic length (for example, the jet diameter), and μ is the viscosity. The second is the Mach number, defined in Eq. (2), where a is the speed of sound.
2. 
See also Mach number; Reynolds number.
For conditions where Re < 2300 and M ≪ 1, jet flows take on a simple character. An example is the water jet formed by a household tap when the valve is partially opened to produce a low flow. If the flow or the diameter is increased or the viscosity is decreased so that Re > 2300, the jet will change dramatically. For example, a water jet exiting into water at rest with Re ≈ 2300 is initially in the simple laminar state, but at this Reynolds number that state is unstable and the flow undergoes a transition to the more chaotic turbulent state. Turbulent structures called eddies are formed with a large range of sizes. The large-scale structures are responsible for capturing fluid from the surroundings and entraining it into the jet. However, the jet and external fluids are not thoroughly mixed until diffusion is completed by the small-scale structures. See also Diffusion; Laminar flow; Turbulent flow.
When the velocities in the jet are greater than the speed of sound (M > 1) the flow is said to be supersonic, and important qualitative changes in the flow occur. The most prominent change is the occurrence of shock waves. For example, a supersonic air jet exhausting from a nozzle at low pressure into higher-pressure air at rest is said to be overexpanded. As the jet leaves the nozzle, it senses the higher pressure around it and adjusts through oblique shock waves emanating from the edges of the nozzle. See also Shock wave; Supersonic flow.
Another class of jet flows is identified by the fact that the motion of the jet is induced primarily by buoyancy forces. A common example is a hot gas exhaust rising in the atmosphere. Such jet flows are called buoyant plumes, or simply plumes, as distinct from the momentum jets, or simply jets, discussed above. See also Fluid flow.
