NASA-LLIS-0514

Abstract

<strong>Description of Driving Event:</strong> <br>The measurement of pressure fluctuations in hypersonic flow fields is much more demanding on dynamic pressure sensors than in traditional acoustical applications. Essential sensor specifications for measurement of pressure fluctuations in hypersonic flow are small size (&lt;0.02 in.), height temperature (20000F), frequency response (200 kHz), and 0.01-10 PSI dynamic range. The recent availability of high-temperature optical fibers makes the intensity-modulated fiber optic microphone a promising instrument for obtaining these measurements. <br>Small sensor size is desirable to obtain high frequency response and to reduce spatial averaging errors due to the finite size of the sensor. In order to minimize the correction factors needed to account for the latter, it is desirable to have a sensor diameter at least as small as 0.02 in. For measurements in hypersonic flow. <br>One of the predominant considerations in hypersonic flow is the high-temperature environment to which the sensors will be exposed. Thus, materials used to fabricate such sensors must be stable in the attendant environment. <br>Sensors whose transduction is based on a material (piezoelectric, piezoresistive, magnetostrictive, etc.) Cannot meet the high-temperature requirement with current technology. The condenser microphone has an inherent small-size limitation due to loadings by cable capacitance; further, if a polarization voltage is used, then the necessarily close proximity of the preamplifier to the microphone cartridge will preclude operation at high temperatures. The recent availability of hightemperature optical fibers makes the intensity-modulated fiber optic microphone a promising instrument to fulfill the above requirements.