A jet is formed if fluid flow ceases to be guided on all sides by walls - and especially if it issues from a nozzle into an unbounded space. We discriminate between a free jet in the case of liquid issuing into space filled with gas, and a submerged jet in the case of comparable specific volumes (liquid into liquid, or gas into gas)
 |
Fig.H-16
|
where the flow is characterised by strong entrainment of the surrounding fluid (Fig.H-16). In both cases the range of the jet is limited. In the free jet case the limiting factor is breakup caused by surface tension. The relative distance from nozzle to breakup point
/
- where
is the nozzle exit diameter, is in a unique manner dependent upon the Weber number (1931)


..
[m/s] being the nozzle exit velocity
and
[N/m] being the surface tension.
A submerged jet is slowed down by a different mechanism: the entrained fluid increases the mass flow rate
with the downstream distance and this must lead to decrease of velocity
=
/
. This is due to the fact that the momentum flow rate
is a constant quantity along the jet path - as long as the jet does not impinge upon an obstacle. If such impingement takes place, then the momentum flow rate decrement equals the force with which the jet acts upon the body. This is shown in Fig.H-17 and is based upon the derivation of the force effect in [H](a) - with the typical simiplification of pressure being basically equal everywhere in the flowfield, so that the pressure forces disappear from the force balance equation.
Going to another page:
click
This is page Nr. H06 from textbook
Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author
to his address
tesar@fsid.cvut.cz
WWW server administrators: Jiri Kvarda, Zdenek Maruna
...... Contact: webmaster@vc.cvut.cz
Last change : 25.03.1997