So, what's the big deal?
External flow is one of the most wide-spread topics in engineering. Every day vehicles from cars, to planes, even boats, are all significantly impacted by the study of external flow. This experiment is focused on the fluid dynamic properties of a rectangular air jet. A jet is the flow of a fluid that exits a nozzle into an ambient fluid of different velocity. The nozzle is used to increase the pressure which in turn will increase the velocity of the jet. If the surrounding ambient fluid is at rest then the jet is called a free jet. If the ambient fluid is moving the jet is called coflowing because the jet fluid and ambient fluid are both moving. Due to having a difference in velocity between the jet and the surrounding ambient fluid a thin unstable shear layer is created. The shear layer and the velocity profile of the jet spread normal to the flow. The velocity at the exit of the nozzle of a typical laboratory jet has a smooth profile and a low turbulence level, about 0.1% - 0.5% of the mean velocity. The growth is due to the instability of the shear layer and the waves begin to grow.
The waves grow because of the shear layer being highly unstable and when it faces the instabilities in the flow this eventually leads to the formation of the large-scale vortical structures. These large-scale vortical structures enhance the mixing of the jet and ambient fluid. As these large-scale vertical structures are formed it entrains the ambient fluid by pulling it into the flow where the interface mixing takes place. The wake of the jet will continue to grow further away from the jet nozzle, the flow will stop and the velocity gradient between to two fluids will approach zero beyond the potential core.
The potential core is said to be a central region of the jet that has almost uniform mean velocity. The potential core eventually disappears because of the spreading of the shear layer normally at a distance between four to six diameters downstream from the nozzle. However the entrainment process continues to grow much further than the end of the potential core. The velocity distribution of the jet eventually relaxes to an asymptotic bell-shaped velocity profile. The peak of this profile is referred to as the centerline velocity Vc which is the local maximum. The distance between half of the centerline velocity from the centerline velocity on the bell-shaped velocity profile is referred to as the jet half-width. The increase in the jet half-width downstream distance provides a measure of the spreading rate of the jet.
