I. BACKGROUNDAlthough much has been done to apply fiber optic technology to design a variety of sensors, few are truly commercially available. Classical temperature, pressure, and strain sensors have not been displaced by this new technology to any significant extent.There could be important advantages to using fiber optic (FO) sensors over resistance, piezoelectric, or thermoelectric sensors, but these cannot be realized until FO sensors are demonstrated to be "better" than classical sensors, if only for some applications.Foil Gages are widely used to measure strain. They are small, and have gained wide acceptability across the scientific and engineering community. However, FG have some shortcomings where FO sensor technology may offer, perhaps, a better alternative.A change in ambient temperature produces four effects on a foil gage and specimen [2]: (1) These effects produce a thermally induced mechanical strain in the gage that does not occur in the specimen. In contrast, the fiber optic sensing element is simply an empty cavity that strictly follows dimensional changes in the specimen. A combination of specially engineered materials and circuit designs have been developed to deal with the apparent strains in FG, but they have the effect of reducing the operating temperature range and the sensitivity of the strain gage-Wheatstone bridge. The temperature effects exhibited by FG may also be produced by self-heating of the resistance element.
A feasibility study on the effects of injecting water into the exhaust plume of an altitude rocket diffuser for the purpose of reducing the far-field acoustic noise has been performed. Water injection design parameters such as axial placement, angle of injection, diameter of injectors, and mass flow rate of water have been systematically varied during the operation of a subscale altitude test facility. The changes in acoustic far-field noise were measured with an array of free-field microphones in order to quantify the effects of the water injection on overall sound pressure level spectra and directivity. The results showed significant reductions in noise levels were possible with optimum conditions corresponding to water injection at or just upstream of the exit plane of the diffuser. Increasing the angle and mass flow rate of water injection also showed improvements in noise reduction. However, a limit on the maximum water flow rate existed as too large of flow rate could result in un-starting the supersonic diffuser.
Nomenclature
D DE= exit diameter of altitude diffuser D WJ = diameter of noise suppressor water jets J = momentum flux ratio of a single water jet to the altitude diffuser exhaust jet OSPL = overall sound pressure level, dB (reference pressure = 20e-6 Pa) r = radial location SDT = sub-scale diffuser test facility V WJi = exit velocity of an individual water jet from the noise suppressor V SDT = exit velocity of SDT altitude diffuser W A3 = total mass (weight) flow rate of A-3 altitude diffuser W SDT = total mass (weight) flow rate of SDT altitude diffuser W WJ = total mass (weight) flow rate of water jets x = axial location ρ = density
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