A small, robust, lightweight, low-power-consumption instrumentation system has been proposed for determining the kinetic energies, masses, and other parameters of wind-borne particles. Originally intended for use in future exploration of Mars, the system might also prove useful on Earth for quantifying the erosive and penetrating characteristics of particles in sandstorms, industrial grit-blasting streams, and the like.
Thin round or square piezoelectric transducer plates with areas between 5 and 10 cm² would be mounted on the outside of the instrumentation package, so that they would be exposed to the wind. The impacts of wind-borne particles would emit acoustic signals; that is, they would cause the plates to vibrate. The acoustic signals and the resultant electrical outputs of the transducers would exhibit frequency spectra that would depend primarily on the energies of the impinging particles. (The spectra would also include minor mass-dependent components.)
The leading edge of each transducer output signal in the time domain would serve as a trigger to start analyzing the signal. The analysis would begin with Fourier transformation to convert the time-domain signal to a frequency spectrum. The spectrum would be compared with recorded known spectra to determine the impact energy. In the event that signals representing multiple particle impacts were present during the transformation time, then the system would attempt to decompose the resulting composite spectrum into component spectra associated with the impact energies individual particles.
Impact events can be counted over time to obtain an impact rate. The impact energies computed for events in the count can be used to compute an erosion quotient — a parameter that is useful for quantifying the abrasiveness of impinging dust. If wind-velocity data from ancillary instrumentation were available, and if it were assumed that particles travel at the wind velocity, then the speed and direction of impinging particles, relative to the direction perpendicular to the surface of each transducer could be calculated. The mass of each particle could be calculated from its relative velocity and impact energy. If it were assumed that all particles are of the same density, then the relative sizes of the particles could be determined from their masses. If the density were known, then the absolute sizes could be determined from the masses. One could then also compute a particle-size distribution from the aggregated data on the sizes of the particles included in the count.
This work was done by Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 115on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-20221
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Determining characteristics of wind-borne particles
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