Geology Reference
In-Depth Information
Conventional siren designs are built with four lobes cut along radial lines.
Rotating sirens with additional lobes would surely increase frequency or data
rate, but large lobe numbers are associated with much lower 'p signals. For this
reason, they are not used in designs to the authors' knowledge. Because
constructive interference now enhances our arsenal of tools against attenuation,
we have been able to reassess the use of higher lobe numbers. Downhole and
uphole telemetry concepts are easily tested in our wind tunnels.
Wind tunnel usage enables a scale of knowledge accumulation, together
with cost, time and labor efficiencies not previously possible. Numerous
parameters can be evaluated, first by computational models, and then by testing
in air. Design parameters are numerous: lobe number, stator and rotor
thicknesses, stator-rotor gap, rotor clearance with the collar housing, rotor taper
angles, and so on.
Tests are not limited to signal strength. Torque is important, as is the ability
to pass lost circulation material - this is assessed by introducing debris at the
upstream end of the short wind tunnel and observing the resulting movement.
Erosion tendencies are determined by noting the convergence effects of threads
glued to solid surfaces - rapid streamline convergence implies high erosion,
e.g., see Figures 1.4b,c.
Two new parameters were included in our test matrix. The bottom left
photograph in the top group of Figure 1.5d.1 shows a “curved siren” with swept
blades. Research was performed to determine the degree of harmonic
generation associate with constant speed rotations. Since the sound generation
process is nonlinear, a rotation rate of Z will not only produce pressure signals
with Z, but those with frequencies r2Z, r3Z, r4Z and so on. Higher
harmonics are associated with acoustic inefficiencies we wish to eliminate, not
to mention added surface signal processing problems - the ability of blade
sweep-back in reducing undesirable energy transfer was one objective of the test
program. Sweep effects are likely to affect jamming due to lost circulation
materials, so that jamming considerations cannot be ignored - they may help by
cutting in scissor-like fashion or hinder by obvious reductions in flow area.
The second parameter considered upstream effects. Figure 1.5.d.2 shows
conical flow devices which guide inlet flow into the siren. Their effects on
torque and signal generation were studied. Convergence and divergence,
together with additional grooved helical tracks etched into the hub surface, could
possibly affect signal strength, acoustic harmonic distribution and torque, and a
variety of hub designs was defined to evaluate possible outcomes.
In order to achieve a given high data rate, one might employ sirens with
“few lobes, turning rapidly” or “many lobes, turning slowly.” Each design,
characterized by different signal strength versus torque relationships, must be
assessed in the wind tunnel. Very often, strong signals are accompanied by high
resistive torques, an undesirable situation that impedes rapid angular
reciprocation. Experiments help to identify optimal designs.
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