Re: How does gravity affect a water splash?

Date: Mon Feb 10 12:56:27 2003
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Physics
ID: 1042638783.Ph
Message:
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To come up with an analytical answer to the problem is really difficult.
The mechanics of a splash are best understood by experimentation, or
numerical simulation (by a computer model), as the paths and forces
operating on gazzillions of particles have to be traced for a good
understanding of what's going on. This is all beyond capabilities...

However, we can still answer the question: "would the splash be
unaffected by the force of gravity" by looking at how the various
relevant parameters of a splash are affected by gravity.

The change in density for water under gravity at 1G or 2G is probably not
significant. Water at 0G (in a free fall) does not appear signifficantly
less dense than water sitting still. Neither does water sitting in a
container being accelerated (shaking a plastic bottle does not show
evidence of a significant change in water density because of the
accelerations, i.e., it does not dent inwards or explode)

Water is a very hard fluid to compress so as long as the gravitational
constant is kept within reasonable limits (I would imagine within one
order of magnitude from 1G), the assumption is valid.

We could hope for the height of the splash to be unchanged by gravity, if
the mechanism for a splash was determined by two relevant parameters that
were both affected by the force of gravity in exactly opposing ways.
Let's look at the various relevant parameters in a splash:

Speed of impact
Rate of energy disipation in the water
Surface tension

When the surface of water splashes due to an object suddenly penetrating
it, the splash is a result of a ripple that could not propagate away from
the penetration point fast enough. A secondary splash happens when the
water surrounding the hole left by the impact, rushes to fill the hole.

The acceleration of the ball is affected positively with increased G, and
the speed of waves in water (its ability to disipate the impact) is also
increased with increased G. So far so good...

However, surface tension, another relevant parameter of a water splash,
is not affected by the force of gravity. In fact, water in microgravity
tries to maintain a spherical shape (as a unit) by its own surface
tension, until disturbed.

Simply knowing that there is one relevant parameter of the splash that is
not affected by gravity, is enough to answer: the height of the splash
will have to be different at different values of G.

-Aurelio R. Ramos

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