Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 237

The above “short wind tunnel” consists of a high-power squirrel cage
blower, a DC motor and an electric controller to regulate blower speed. Volume
flow rate measurements, which should be performed before each change in test
conditions, should ideally be obtained from “old fashion” manometer rake
procedures which apply Bernoulli's principle. Use of electronic flow meters
with digital readouts, while convenient, should proceed with caution, and then,
only when their results are carefully calibrated against manometer results. We
will discuss a simple error-detection procedure later.
Figure 9.3 also shows flow straighteners placed ahead of the siren or
turbine test section. Without flow straighteners, siren and turbine torque
measurements can be highly inaccurate, as they are affected by rotating air
masses that originate from the blower or which discharge from corner turns in
the closed-loop mode discussed next. “Closed loop” operations are enabled by
attaching secondary tubing highlighted by the gray ducting shown. This mode is
useful in assessing the tendency of sirens or turbines to jam in the presence of
lost circulation materials. Styrofoam pellets, glass beads, string snippets,
neutrally buoyant soap bubbles and so on, can be introduced into the air stream
and forced to recirculate around the wind tunnel without dispersal into the
laboratory setting (sticky styrofoam pellets, e.g., from bean-bag stuffing, can be
unstuck by using static removal agents like StaticGuard TM easily found in
supermarkets). One effective evaluation technique includes the use of a
stroboscope (with room lights turned off) with stop-action photography so that
debris entrapment and escape in narrow gaps can be observed with the test
apparatus rotating. More quantitative assessment is possible by measuring
current fluctuations in the motor drive needed to turn at constant speed. If the
particles introduced are heavy and segregate toward the bottom, the wind tunnel
in Figure 9.3 can be pivoted to operate vertically, so that debris are uniformly
dispersed throughout the cross-section as wind flows downward.
In some applications, e.g., determination of siren stable-open or stable-
closed behavior, or turbine no-load rotation rate, the model is not driven by an
electric motor. Here, a specially crafted thin-wall test section can easily slide
into the wind tunnel tubing or form part of the main tubing itself (in the former
case, it is secured by diametrically opposed pins). In other problems, the siren
must be driven by an electric motor and operated at a given rotation speed. It is
preferable if a powerful motor is used that is small enough to fit within the
tubing. However, if wind speeds are large (so that torques, which vary as the
square of the oncoming flow speed, are likewise large), small motors may be
difficult to obtain or are expensive. The solution used in the CNPC wind tunnel
is simple. The test section shown in Figure 9.2f is installed as in Figure 9.2a.
The siren is rotated by a connecting shaft that extends outside the wind tunnel
that is in turn connected to the motor. Care is taken to avoid leaks, using
appropriate seals, which would compromise measurements taken in the long
wind tunnel. However, if long wind tunnel measurements are not required, the