Free Turbulent Jet


Free Turbulent Jet

Wall turbulence is turbulence motions which are constrained by one or more boundaries. In wall turbulence, the turbulence is generated in velocity gradient caused by the no-slip condition. Free turbulence is turbulence motions which are unaffected by walls and develop and spread in an open ambient fluid. Three examples of free turbulence are free-shear layer (mixing layer), free jet, and wake behind a body immersed in a stream. 

            Free jets can be defined as a pressure driven unrestricted flow of a fluid into a quiescent ambiance, the wall ceiling or obstruction does not influence the jet. Free turbulent jet occurs when the fluid is discharged between nozzle orifices into a stationary or moving liquid. Just downstream of the disturbance that caused the velocity gradients, the flow will be developing and non-similar. Further downstream, the flow will be similar and the velocity profiles will all look alike when suitably scaled. When the fluid exits from the orifice, the fluid becomes completely turbulent at a short distance from the pint of discharge. The boundary layer at the exit of the device develops as a free shear layer, mixing with the ambient fluid thereby entraining the ambient fluid in the jet stream. 

          Thus, the mass flow at any cross section of the jet progressively increases thereby the jet spreads along the downstream direction. In order to conserve momentum, the jet centerline velocity decreases with downstream distance. So, as the free turbulent jet spreads out, the velocity decreases, but the total momentum remains constant. The velocity decreases is mainly due to the shear interaction with the surrounding fluid. Therefore, the kinetic energy will reduce and dissipated as heat or energy loss.

 As mentioned above, when fluid elements move downstream, they interact with surround fluid and their speed decreases. The speed of the fluid near the centerline will maintain nearly its initial speed at some distance downstream because it interacts less with the surrounding. Refer the figures below, the region in which the centerline speed is nearly that of the exit is called the potential core. Its radial extent, which decreases downstream, can be estimated by measuring the streamwise variation of the centerline speed. The potential core vanishes quickly at a distance of about one diameter from the exit, where the velocity profile loses its mixing-layer-flat-core shape. 

There are two types of turbulent free jet, which is momentum jet and buoyant jet. In the momentum jet, the fluid motion is as a result of kinetic energy. The jet and surrounding may be the same fluid at the same temperature such as jet engine exhaust. In the buoyant jet, the jet arises from a stationary fluid at nozzle. The jet results from a difference in nozzle and surrounding temperature or density. Typical examples are heated air rising through cold air, salt water entering fresh water.

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