Geology Reference
In-Depth Information
Oncoming flow
speed U
air
Closing torque
Restoring (opening)
torque T
air
= FR
R
Force
gauge
Force F
Figure 9.4b.
Conceptual measurement of restoring torque.
It is not necessary to use a linear force gauge. If a set of calibrated weights
were available, restoring torques are just as easily determined versus azimuthal
angle. For example, the use of a block with weight W
1
will give a torque of
W
1
R associated with relative rotor-stator angle A
1
. Measurements are repeated
for different weights W
n
to give restoring torques T
n
associated with different
degrees of relative closure A
n
. Note that we are measuring
static
torque, that is,
torque when the rotor is
not
moving; the electrical drive motor is not attached to
the rotor shaft and there is no resistance to rotor movement except bearing
friction. This is generally the worst case torque in practice; if the motor chosen
can overcome this torque, it will perform satisfactorily downhole under dynamic
rotating conditions. We emphasize that, as the siren closes or opens, blower flow
rates can change substantially and need to be monitored continuously. If
inaccurate flow rate measurements are used, torque predictions will obviously
degrade in quality. If cost were not a factor in wind tunnel construction, one
would naturally turn to automated flow rate and torque measurement systems.
For the flow in Figure 9.4a, we now assume that T
air
and Q
air
are available
from wind tunnel measurements when the air has a mass density of U
air
(Q
air
is
the volume flow rate determined from u(r) as discussed). Again, good data is
important; ensure that this torque data is correct by increasing Q
air
to some
higher level and verifying that the new T
air
increases quadratically. If this
quality test is passed, we would like to use their values to predict the
corresponding torque T
mud
in a flow with density U
mud
and flow rate Q
mud
. The
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