Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 239

From the pressure measurement for each pitot probe at a radial location
“r,” the oncoming axial flow velocity u(r) is calculated using Bernoulli's
equation. This is not the final form of the flow rate information required. In our
downhole applications, we require the integrated volume flow rate Q inside the
drill collar and not the individual velocities in u(r). These quantities are related
by the formula
R
Q = ³ u(r) 2Sr dr

(9.1)

0
which integrates u(r) across the circular area of the inside of the test section. If a
pitot tube rake is used, all the required velocity inputs are available immediately
and much time can be saved. Otherwise, if a single probe is used, then that
probe will need to be moved across the test section and measurements are time-
consuming. To make the testing procedure as efficient as possible, the integral
can be calculated immediately by a personal computer that automatically accepts
the input velocity data, which then computes and stores Q along with the
measured velocity profile u(r). Again we emphasize the need to record Q each
time the model is changed or the blower or rotation speeds are altered. To
ensure accuracy in the measurement procedure, physical checks should be
employed. For turbines, doubling Q should double the no-load rotor speed and
quadruple the torque. For sirens, doubling Q should quadruple the opening or
closing rotor torque (this measurement is described below).
Figure 9.3 also indicates “flow straighteners” (those used in the CNPC
wind tunnel are displayed in Figure 9.2h). These must be placed inside the wind
tunnel immediately after the air exits from the blower so that the streamlines
impinging on the turbine or siren are straight. They are necessary because the
blower produces very “confused” rotating air masses that do not flow in a
straight manner as the flow would in a downhole situation. These flow
straighteners also destroy the rotating air masses that may be formed at turns in
the closed-loop system. These can be constructed very simply by gluing
together a number of thin-walled plastic tubes and anchoring them in place just
downstream of the blower outlet so that they cannot be blown away. The length
of these tubes should be about six to eight inches.
The above discussion emphasizes the importance of clean straight flows
upstream of the turbine or siren so that spurious rotating air masses do not bias
erosion and torque results. When we discuss “intermediate wind tunnel” use in
acoustic signal strength testing, we will explain the use of flow straighteners
downstream of the rotating siren rotor as well. Essentially, strong rotating air
masses are developed behind the siren rotor that are associated with swirling
vortex transients. These pressures are of a hydraulic, incompressible flow
nature, and are identical in frequency to the acoustic pressures generated by
opening and closing the rotor relative to the stator. As compressible pressures