Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 39

Figure 1.5s. Short wind tunnel, with long wind tunnel connection.
Comments: When long wind tunnel testing is required, the siren is driven
by the motor with its shaft penetrating the end of the short section at the junction
shown. This leads to a small amount of tolerable air leakage. Note that ninety-
degree turns, while requiring higher pressures to move air, do not induce
reflections, since the wavelengths are long. This was in fact validated by
monitoring reflection arrival times associated with “firecracker” blasts at an
inlet. For example, if a signal is created at an open end, and the hollow pipe is
1,000 ft long, the reflection time will be two seconds since the sound speed is
1,000 ft/sec. Any arrivals prior to this would indicate undesirable reflections.
Figure 1.5t. Short wind tunnel “alone.”
Comments: When the “short wind tunnel alone” is required without the
“black tube” of Figure 1.5s, e.g., for 'p signal strength, harmonic distribution
and torque testing for a given frequncy, or for erosion flow visualization, it is
not necessary to connect its long wind tunnel appendage. Thus, air blowers may
operate at much lower pressures, although high pressure wind tunnel fracturing
is still possible under “stable closed” conditions. At the above right, a six lobe
siren is shown closing in the “stable open” position. Its low torque and good
stability characteristics make it a front-running candidate for use in “multiple
sirens in series” designs.