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

than against water hammer. Cushioning of the water hammer impact is not excessive if
the bunch diameter is substantially greater than the film thickness, Ref. (12).
Furthermore, the target is also attacked by cavitation produced at nuclei within the liquid
film by the water hammer pressure wave, Ref. (13).
Since the pressure levels generated on a target surface can be expected to be a
major factor in determining the effectiveness of the cutting operation, estimating these
pressure levels is important. With a steady-state flow of water from a conventional
steady-state jet impinging normally on a flat surface, the level of stress generated at the
stagnation point is given by:
Ps = 1 2
V
2
where:
ρ is the water density
V is the jet velocity.
However in applications using the Percussive Jet where the water jet tends to
break up to form discrete liquid droplets or bunches, the impact stress can be order of
magnitude higher P s. Numerous studies have indicated that the transient stress between a
liquid droplet and rigid plane surface can be given approximately by the simplified water
hammer equation.
where
C is the acoustive wave speed.
P a = ρ C. V
Using the above equation the following table was constructed showing the manner in
which the steady-state stagnation pressure, P s, and intermittent “water hammer” pressure
,P a, vary with nozzle discharge pressure, P. The discharge velocity varies with nozzle
characteristics but can be estimated from:
V = 12.0 P
P (psig) V (ft/sec) P s (psig) P a (psig)
350 220 340 15,000
550 280 530 19,000
10,000 1,200 9,700 81,000
30,000 2,100 29,000 140,000
In considering the significance of this table, examine a 0.065-inch diameter
nozzle firing 10 gallons of water per minute at a discharge pressure of 10,000 psig. The
steady state stress on the target material will be 9,700 psig. However, for a Percussive jet,
in which the bunches have completely separated the intermittent stress will be 81,000
psig. Incidentally, most rock materials have uniaxial tensile strengths that vary between
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