Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 238

short wind tunnel outlet can open directly into the room. Volume flow rate
measurements are generally taken upstream of the siren and downstream of the
flow straighteners.
For short wind tunnels, area discontinuities in the tubing can be tolerated.
However, we emphasize that in designing a combined short and long wind
tunnel system, sudden changes or mismatches in area - even small changes -
will result in spurious reflections that degrade the signal traveling up the
drillpipe. Consider the following “thought experiment.” Suppose that the
MWD collar area available for flow (that is, the area associated with the inner
diameter minus the cross-sectional area of the central hub which supports the
siren) and the drillpipe area are not equal. Then signals created by the siren
travel toward the drillpipe, and at the collar-pipe junction, part progresses
upward while part reflects downward (the latter travel to the drillbit, reflect
upward to the same junction, and so on). All the time, new signals are created
only to repeat this behavior. The net effect is a “blurry” signal which must be
smoothed using the signal processing methods discussed earlier in this topic.
For this reason, one ideally matches the internal collar area to the drillpipe area
if possible. Similarly, unintentional mismatches at short and long wind tunnel
junctions result in noisy data. Such effects are often lost in data reduction. For
example, one long flow loop used in industry MWD tests contains area
discontinuities in certain buried parts of the piping that had long been forgotten.
As part of the flow rate control system, we might have the manometer and
pitot probe system in Figure 9.3. The manometer and pitot probe system uses
the Bernoulli principle to determine the local flow speed at the inlet of the pitot
probe; this principle is explained in elementary fluid mechanics topics. A single
probe is shown in Figure 9.3, but ideally, in the interest of saving time during
the experiments, a “rake” consisting of several probes extending completely
across the diameter of the inside of the test section is preferable. It is not
necessary to purchase a sophisticated system. A simple system can actually be
built for a very low price. The pitot probe itself may be acquired from a
laboratory supply house (many university aerospace engineering departments
may supply information on vendor sources), while the pressure measurement
itself may use arrays of U-tube fluid manometers than can be constructed by
hand for very low cost. The U-tube manometer operates on a simple principle
taught in freshman physics classes: the difference in the height of two fluid
columns is converted into a pressure measurement, which is in turn translated
into a velocity reading via Bernoulli's equation. This manometer-pitot tube
array should be used for velocity determination. Wind anemometers may be
acceptable if they actually measure local speeds and do not rely on calibration
using ducts of given sizes without blockages. Whether automated or manual
velocity measurements are used, it is important to set up the test procedure to
record velocities at several positions in the cross-section.