Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 234

and closing of a mud siren against an oncoming flow. The static 'p method
is invalid for both mud loops and wind tunnels.
x Are ' p measurements useful at all? Yes, they are, when performed and
used properly. As we will demonstrate in our “intermediate wind tunnel”
discussion, differential pressure can be used to determine signal strength for
MWD signal sources where created pressures are antisymmetric with
respect to source position - that is, for positive pulsers and sirens.
However, a nonzero 'p is not a general requirement for MWD design. For
example, the disturbance pressure field associated with a negative pulser is
symmetric with respect to the source point and 'p vanishes identically
whatever the signaling scheme. For negative pulsers, the correct
characterization would involve “delta axial velocity” because velocities at
both sides of the source are equal and opposite.
x How accurately does the short wind tunnel model erosion? The use of short
wind tunnel modeling in evaluating and correcting turbine erosion was
discussed in Chapter 8. The present chapter will develop methods for siren
flows. The author, in addition to turbines and sirens, has worked
extensively in two additional hardware classes, namely, (1) MWD flow
strainers, placed at the surface to capture debris which would otherwise
impinge upon the downhole tool, and (2) proppant transport tools used to
convey sand-laden stimulation fluids downhole for hydraulic fracturing. In
both applications, wind tunnel flow visualization correctly replicates areas
of erosion known from field results; changes to hole, orifice and slot
patterns suggested by wind tunnel analysis which would significantly
reduce erosion again, in both hardware classes, led to substantial
improvements to life-span. These subjects are beyond the scope of this
topic and will be covered separately. We emphasize that metal erosion rates
cannot be estimated from wind analysis, nor can the exact surface pattern
that erosion ultimately takes with time - these answers depend on the
details of mud and metal interaction which can only be obtained from actual
operations. The wind tunnel, however, is useful in revealing the physical
mechanisms that are likely to cause trouble, and does provide means for
averting such events. For instance, streamline flow patterns, which are
easily visualized, provide some indication of particle impingement angles,
which control ductile versus brittle fracture. Changes to these angles,
possible by aerodynamic tailoring, can significantly affect life span.
x What are potential error sources in wind tunnel measurement? Wind
tunnel analysis is useful in many ways but care must be taken to avoid
incorrect conclusions. For flow visualization and erosion analysis, actual
metal parts or wind tunnel prototypes of new models may be placed in the
wind tunnel and evaluated directly. Simple visualization is possible using
threads bonded to solid surfaces - rapid streamline convergence is an