Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 232

rotation rate, geometric details and air density. The differential pressure
transducer is important and useful because, provided the two taps are close, the
measurement is unaffected by flow loop reflections that may be traveling back
and forth (these traveling waves simply cancel). It therefore represents the
siren's “true strength.”
On the other hand, the pressure signal “p” that ultimately leaves the MWD
collar and travels to the surface is measured by the single transducer at the
bottom right of Figure 1.5m (this is located just outside the test shed). It is this
“p/'p” that is optimized in the six-segment acoustic waveguide analysis of
Chapter 2. We noted that the taps of the differential transducer should be
“close” so that the effects of traveling waves (due to reflections) are subtracted
out identically. The required distance depends on fluid sound speed and
transducer sensitivity. To determine this separation, one simple test is all that is
needed. Shout “hello” down the plastic tubing: if the differential transducer
does not respond, then the separation distance is acceptable. The short wind
tunnel system measures the foregoing “'p.”
Chapter 1 displays other wind tunnel components, e.g, electronic
flowmeter, differential pressure transducer, piezoelectric transducers, flow
straighteners, motor drive, torque gauge, angular position and rpm counter
assembly. Also shown are shown instruments for control and simultaneous
recording of flow and rotation rate, differential pressure, single transducer
pressures at four locations in the long flow loop. These measurements are taken
as time progresses. Single transducer data is used to assess the degree of
constructive or destructive interference due to downhole reflections, and also, to
provide data for multiple transducer signal processing (noise is introduced at the
opposite end of the long wind tunnel). Data relating 'p to angular position are
useful in developing feedback control loops ultimately needed for telemetry.
The reader interested in developing a wind tunnel facility for high-data-rate mud
pulse telemetry testing should review Chapter 1. Other wind tunnel systems
have since been built which improve upon the advances cited there.