Geology Reference
In-Depth Information
While dynamic similarity is achievable, testing errors are possible, which
are fortunately avoidable. Because air is about one thousand times less dense
than mud, force, torque and power are also one thousand time less for the same
model and flow speed. If low wind speeds are used to study higher mud flow
rates, which is common in practice, the differences increase further. To reduce
experimental error, siren and turbine models must be installed using low friction
bearings. Also, the highest possible wind speeds should be used, meaning that
large blowers are best. Volume flow rate ratings stated by HVAC
manufacturers may not be not meaningful - these apply to calibrated flows in
rectangular ducts where resistance is small compared to ducts containing large
sirens with high blockage. In wind tunnel applications, flow resistance will
generally remain high - and for the same blower settings, flow rates will vary
with the particular siren or turbine in the test section. So three simple rules
apply: low-friction bearings; powerful electric blowers for high test speeds and,
importantly, accurate measurement of volume flow rate from test to test.
9.1.2 Three types of wind tunnels.
There are three types of wind tunnels useful to MWD testing, namely,
“short,” “intermediate,” and “long.” Short tunnels are used to study physical
properties associated with the constant density, incompressible nature of the
fluid, e.g., torque, power, streamline pattern, erosion, drag, viscous flow
separation, free-spinning turbine speeds, and so on. Longer wind tunnels are
used to study acoustic wave propagation in high-data-rate applications, where
the wavelengths are conveniently shorter than those in mud flows, but still long
when compared to the wind tunnel diameter of the flow cross-section. By short
wavelengths, we mean several to about one hundred feet - the literature on
“short wave” or “high frequency” acoustics, e.g., ultrasonics, does not apply,
because wavelengths are many orders of magnitude smaller.
Long wind tunnels allow us to simulate sound reflections at the surface and
downhole near the pulser, and also to measure signal strength produced by
particular siren designs. They also provide a convenient laboratory for
evaluating single and multiple-transducer signal processing schemes. They
allow us to test different telemetry schemes under more or less ideal conditions
before field tests with real mudpumps, desurgers and mud motors are present.
However, acoustic data reduction, interpretation and extrapolation require more
sophisticated mathematical methods than those used for turbine torque and
power testing in Chapter 8. We now discuss the design and use of short wind
tunnels, and later, extend our methods to long wind tunnel acoustic analysis.
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