What is jet entrainment?
Blow a thin stream of air across a room and it will not stay thin. It widens steadily, and by the time it reaches the far wall it is moving a great deal more air than left your lips. That is entrainment: a moving jet grips the still fluid around it through friction and drags it along, so the stream gathers fluid as it travels.
Reading the diagram
The nozzle on the left turns a slow, wide supply into a fast, narrow jet. The dashed cone is the mixing layer, the boundary where the fast jet and the still ambient fluid rub together. The arrows show that fluid being entrained, pulled in from the sides. The three shaded shapes are velocity profiles: near the nozzle the jet is narrow and fast, and further downstream it is broad and slower.
Why it matters
Entrainment is the trick behind jet pumps and ejectors that move fluid with no moving parts, behind air amplifiers that turn a little compressed air into a lot of room air, and behind carburettors, chimneys and spray nozzles. It also explains why a plume of smoke spreads as it rises and why an open window feels drafty far from the gap.
1. The nozzle makes a fast jet
A converging nozzle forces a wide, slow supply through a small opening, so the fluid leaves as a narrow, fast jet. All its momentum is packed into a small cross-section, which is what makes it able to grip the fluid around it.
2. The jet grips the fluid beside it
At the edge of the jet, the fast fluid and the still fluid rub against each other. Friction drags the still fluid along and slows the fast fluid down, so a mixing layer forms between the two.
3. Surrounding fluid is dragged in
Once the edge fluid starts moving, it leaves a low-pressure gap that fresh fluid flows in to fill. Fluid is drawn in from every side — this steady intake is entrainment, and it means the jet is never the same stream it started as.
4. The jet widens into a cone
Every extra parcel of fluid the jet collects has to be carried downstream, so the stream grows wider with distance. The angle is small but steady, and the jet becomes a spreading cone.
5. Momentum is shared, so peak speed drops
Nozzle thrust stays roughly the same, but it is now spread over more fluid. The centreline speed falls as the jet widens, while a shrinking potential core at the very centre still holds the fastest flow.
What is jet entrainment?
It is the way a fast-moving jet of fluid drags the still fluid around it along with it. The jet and the surrounding fluid rub against each other at the edges, and that mixing pulls more and more of the surrounding fluid into the moving stream.
Why does a jet spread out as it travels?
Because it keeps entraining fluid from the sides. Every bit of extra fluid it picks up has to be carried somewhere, so the jet widens into a spreading cone the further it travels from the nozzle.
Does the jet get stronger as it grows?
No. The total momentum stays roughly the same, but it is now shared among more fluid, so the peak speed along the centreline falls. A jet carries more fluid but moves it more slowly the further downstream you look.
Why do only the edges mix?
The centre of the jet stays fastest and unbroken — it is called the potential core. Mixing happens in the shear layer where fast jet fluid meets slow surrounding fluid, so the edges are where entrainment actually occurs.
Where is jet entrainment used?
It drives jet pumps and ejectors that move fluid with no moving parts, air amplifiers that multiply a small compressed-air supply, chimney and carburettor flows, and the way a spray nozzle pulls in air to break up a liquid.