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signal strength is useless unless turning torques are low and erosion is minimal.
On the other hand, one might select a low-torque siren shape that by itself
creates weaker signals, but which provides strong signals by virtue of a
telemetry scheme which takes advantage of constructive wave interference.
Or, possibly, one might consider how two sirens placed in series might be
phased in order to develop a single coherent reinforced signal. Multiple sirens
address pressing issues confronting existing single-siren systems. Single-sirens
require very small rotor-stator gaps for signal generation which are associated
with high erosion and large torques. Multiple sirens, while complicating
mechanical design, offer increased lifespan and reduced turbine power demand.
As an additional example for long wind tunnel analysis, consider turbine-
siren interactions. In existing siren-based MWD tools, the siren is placed above
the turbine - the usual reason offered, namely that an upstream turbine would
block the MWD signal, is fallacious - simple tests in a long wind tunnel have
shown that turbines do not reflect the long acoustic waves important to signal
transmission. This indicates that the siren can be placed closer to the bottom of
the MWD collar, and nearer to the drillbit, without incurring any penalties. This
shortened distance, obviously, implies that constructive interference based on
drillbit reflections can occur in less time, and thus, data rates will be higher.
The list of potential important applications goes on and on. For this
reason, it is important to understand the concepts behind wind tunnel
methodologies fully. The need for short wind tunnels to evaluate siren torque,
power and erosion is clear, as is the need to use long wind tunnels to study wave
interference absent of the complicating reflections plaguing shorter tunnel
lengths. Wind tunnel turbine design was addressed in Chapter 8 because it
supports mud siren integration - modern MWD systems cannot function flexibly
unless they can effectively generate the power needed.
9.1 Early Wind Tunnel and Modern Test Facilities
We consider wind tunnel analysis in greater detail and review basic
principles explaining why wind tunnels work. According to the principle of
“dynamic similarity,” applicable to all scientific disciplines including fluid
mechanics, the dimensionless parameters governing two separate experiments
must be close in order that they describe identical physical phenomena. For
example, this is how aerospace companies perform experiments using more
convenient and cheaper models. In commercial jetliner design, inexpensive
small-scale models are tested in small wind tunnels powered by low-speed
blowers, thus eliminating the need for jet engines and full-scale prototypes.
Then, flight characteristics are extrapolated to large size aircraft flying at faster
speeds or at altitudes with different air densities.
The test engineer's judgement is important when more than one
dimensionless parameter governs the problem and it is not possible to match all
of the parameters. Wind tunnel results for lift, i.e., the upward force keeping the
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