Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 241

(drill collar) enclosure and the inner central hub are not shown. Here the siren is
shown in the initial opened position. Let us consider the situation when wind
blows past it and the rotor tends to close. If this siren is placed in a downhole
tool, the closing torque will vary linearly with mud weight and quadratically
with mud pump flow rate. We wish to determine what this torque would be so
that a proper motor can be selected for the MWD tool.
The restoring torque is easily obtained from wind tunnel analysis. The
siren is placed (without an electric motor drive) into the wind tunnel as shown in
Figure 9.4b. A thin shaft extends downstream, which may be supported at one
or more points to prevent flexing. A wire wheel or thin circular disk is attached
to this shaft and the linear force F required to keep the rotor in the fully opened
position is recorded. The product between F and the moment arm R is the
restoring torque T air = FR for the opened position considered. If the “pulling”
force is reduced, the rotor will close slightly; the resulting reduced torque is the
restoring torque associated with the partially closed azimuthal position. If this
process is repeated for all positions from opened to closed, a range of closing
torques can be determined on a static basis.
Oncoming flow
speed U air
Downstream rotor
(of stable closed siren)
attempts to close
Fluid pressures exert
a "closing torque"
which moves rotor
Figure 9.4a. Simple stable-closed siren.