Geology Reference
In-Depth Information
indicator of possible erosion. Observation may be facilitate by coating
threads with florescent dye and photographing their trajectories in the dark.
The use of lightweight beads introduced into the flowing air is very often
successful, although smoke (often seen in television commercials) presents
a problem since smoke filaments dissipate too rapidly. To ensure accuracy,
one observes that the air emerging from the blower is generally turbulent
and may contain rotating air masses associated with turns in the wind tunnel
tubing. Generally, the placement of flow straighteners, e.g., as shown in
Figure 9.2h, upstream of the siren or turbine would suffice for most testing
purposes. In turbine and siren torque measurement, it is essential to use low
friction bearings so that the torque needed to overcome bearing resistance is
much less than the aerodynamic torque. If such bearings are unavailable,
relative measurement error can be reduced by operating the blower at high
speeds. Whether blower speeds are high or low, it is absolutely essential to
be able to measure volume flow rate accurately. Digital or analog wind
anemometers available from laboratory supply houses should be used with
care since pre-programmed calibration data may be inapplicable. For the
same blower electrical setting, different volume flow rates will be obtained
depending on the blockage offered by the turbine, siren, and other
appendages, such as the central hub, the electric motor used, the manner in
which the model is supported, and so on. Accurate volume flow rate
measurements are important because air data is used to infer performance
characteristics for muds of arbitrary density flowing at general speeds - if
wind measurements are inaccurate, so will our predictions. To assign
numbers to these uncertainties, we note that torques measured at the same
speed in water versus air are different by a factor of about eight hundred,
that is, the density ratio. Thus, a small error in wind tunnel torque
measurement implies a large error in drilling mud applications. If low wind
speed data is extrapolated to high mud velocities, the potential for error is
even greater. In a discussion below, we discuss how known properties for
torque versus flow rate can be used for error-checking the experimental
procedure. For example, if measured siren or turbine torques do not vary
quadratically with flow rate, either the flow rate measurement is flawed, the
torque determination is inaccurate, or both. Such simple tests should
always be performed at the start of the day to ensure data integrity. In
almost all tests with siren or turbine rotation, shaft vibration and unsteady
flow will contribute to test error and uncertainty, and will invariably require
some type of data smoothing.
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