The acceleration of the incompressible fluid into the nozzle is
what generates thrust. If you consider the body of the pressure
vessel as a pipe full of fluid that is travelling along it, a certain
volume will be flowing past any arbitrary point during a given
period.
If you attach a smaller diameter pipe to it, that same volume
will still flow along it but by being a smaller diameter, it will
now be travelling faster. In other words, it will have been
accelerated.
We know how much is travelling through the pipe and we know
the diameters of the two pieces of pipe, along with the density
of the fluid so now, we also know the mass which means that
we can use the formula...
F = ma
...where the force F is equal to the mass multiplied by the
acceleration.
However, it is not as simple as that. If you look along the axis
of the pipe constriction, and imagine the flow of the water into
the constriction, the water flows in from the outsides towards
the middle and as it does so, you can imagine it getting
more 'crowded'.
Now, imagine viewing the constriction from the side so that
the water flows from left to right. If the constriction occurs
over a long distance, the water has a chance to speed up
gradually and any imaginary 'chunk' of water only has its
shape distorted over a long distance and the difference of
speed within it is only going to be small. The amount of
distance the 'chunk' has to move sideways is only going to
be small.
However, if the change is abrupt, some of it will be moving a
lot faster than other bits of it, distorting the 'chunk' and the
more it is distorted, the more energy is turned to heat. The
water still has to flow into the constriction is it just that with
less of its energy ending up as kinetic energy, for a given
pressure, less liquid flows through it.
So, you can see that both the amount of constriction and the
type of entry into the smaller pipe affect the efficiency of the
constriction and that is what this value represents.
