proceedings of the fourth us water jet conference - Waterjet ...

Our approach to improving the performance of conventional water jets is to
fundamentally modify the physical fluid behavior of an ordinary, continuous water jet.
We term these jets "Percussive Jets."
PERCUSSIVE JETS
General
The photograph in Figure 1 can be used to illustrate Percussive Jets in their
simplest form. The special free-stream characteristics of Percussive Jets shown may be
obtained by modulating the discharge of water through the jet nozzle, i.e., by cycling the
discharge flow above and below its average value with some particular frequency,
amplitude, and waveform. Details have been discussed in References (1), (2), and (3).
Figure 1. Example of Percussive Jets. A small cyclic variation or modulation impressed
on a steady discharge of water causes the free jet to become bunched. Consequently the
jet will strike a target in a sequence of sharp impacts rather than steadily.
In modulated free jet discharge, the slow and fast portions of each discharge cycle
tend to flow together or bunch in the free stream. The stream thus becomes a train of
bunches of water which eventually separate. Bunch diameter increases with downstream
distance until the axial velocity becomes uniform within each bunch.
At any particular distance from the nozzle, the free flow produced by modulation
is periodically thicker and thinner than the nozzle discharge. Maximum and minimum
diameters depend on downstream distance according to modulation characteristics and to
aerodynamic and surface tension effects. The diameter is a minimum of zero for part of
the time if the stream bunches have separated.
When this varying free flow strikes a target, the momentum flux through the
nozzle is not transmitted as a steady force, but as a possibly discontinuous sequence of
force peaks or percussive impacts. The maximum stream impact area and force at any
distance from the nozzle correspond to the maximum cross-sectional area produced by
stream bunching up to that distance.
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