Questions regarding the flow of a free airjet.

[CaspianTiger]

Hi i have some coursework which has some discussion points i don't quite understand.

Describing the experiment. There was an airjet and we have to investiage the velocity distribution at a number of stations along the length to reveal how a uniform jet mixes with its surroundings.


The nozzle is 30mm in diamater and we used a pitot tube which could be moved along the jets dimater and also along it length. The pitot tube was moved to 3 distances away from the air jet, 2D, 10D and 15D. At these distances the velocity of the airjet was measured at incremental radius to the centrepoint.

The flow rate at x=10D has significantly increased compared to x=0 (right at the beginning of the airjet) why does the volume flow rate increase and where does the additional flow rate come from.

And why does the jet spread out as it goes downstream?

Any information or resources that could help me with these topics, would be greatly appreciated.

If you need any extra information i will happily supply it.

Thanks

 

[minger]

The flow rate doesn't increase, it should be exactly the same. Through the law of continuity, the mass coming into the nozzle equals the mass going out of the nozzle equals the fluid flow outside of the nozzle.

What should happen is this. As viscosity takes effect, the jet should disperse from the diameter of the nozzle to a much larger diameter as larger and larger turbulent eddies form. Since mass flow is constant, it should slow down as the area increases. As the flow slows down, the pressure should rise.

 

[CaspianTiger]

minger the report i have been given basically states that the flow rate increases and asks why does it increase and where has the additional flow rate come from.