Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 248

9.3 Intermediate Wind Tunnel for Signal Strength
Measurement
The prior section dealt with torque measurement and erosion, two areas
critical to good MWD design which can be successfully studied using the short
wind tunnel. Here we consider acoustic signal strength and explain how it can
be measured using an “intermediate” length wind tunnel. In general, one cannot
determine signal strength by using a single transducer, say, upstream of the
siren, without further signal processing; this transducer will record the created
signal plus all reflections and reverberations, and the end result is difficult to
decipher or interpret. A good interpretation method is thus required.
The acoustic pressure field produced by siren opening and closing is
antisymmetric with respect to source position. For example, when the siren
closes, an overpressure is developed upstream due to impacting fluid, while an
underpressure is found downstream as fluid pulls away - these pressures are
equal and opposite, a fact verifiable in both wind tunnel and mud flow loop.
Because the pressure is antisymmetric, signal strength is amenable to differential
pressure transducer measurement. The two piezoelectric sensors connected to
differential transducers are placed upstream and downstream of the siren at
equal short distances, and effectively cancel reflections, thus providing direct
indicators of 'p. But such measurements are not perfect, since one sensor
always resides downstream of the rotor, where strong rotating vortex flows with
highly transient pressures are found; and differential pressure transducers will
never characterize negative pulsers since 'p vanishes identically. Since neither
single nor differential pressure measurements is ideal, one would ideally prefer
both. In this section, we discuss the problems associated with each technique so
that potential measurement errors could be properly understood and reduced.
When a siren opens and closes, an acoustic “water hammer” signal is
created as the fluid literally crashes into a solid wall (the term “water hammer”
will be retained for air flow, as it traditionally describes fluid compressibility
effects). Were one to visualize nearfield flow details in all their detail, one
would record a wealth of small-scale three-dimensional effects. These do not,
however, propagate to the surface; only the lowest-order pressure field having
an amplitude that is uniform across the cross-section travels to the surface. This
is known as the “plane wave” mode. The acoustic plane wave is created by
effectively stopping the oncoming fluid and is recognized as a volume effect;
thus, if the rotor-stator gap is large, or the circumferential gap between the rotor
and the MWD collar is not small, or both, stoppage is reduced and the created
signals will be weak. Siren designs should minimize these gaps, but there may
be undesirable operational consequences. For example, reducing the rotor-stator
gap will increase the likelihood of debris jamming and erosion, and similarly for
the circumferential rotor gap adjacent to the collar housing. An assessment
related to jamming and erosion must be made using short wind tunnel flow
analysis and actual mud loop or field testing.