The rapid development of fiber optic sensing technology has enriched the types of wind tunnel strain balances. Numerous wind tunnel tests and research institutions have carried out various work on the fiber optic balance (FOB) based on the fiber optic strain gage (FOSG). In hypersonic wind tunnel tests, the total temperature of airflow is so high that both the model and the balance will be heated by the heating airflow, resulting the thermal output of the FOSG which affects the accuracy of the FOB. In order to reduce the thermal output effect, a new self-temperature compensation (STC) method has been proposed in this paper, by subtracting thermal expansions between the balance and a STC structure. In addition, the structure can not only reduce the thermal output, but also achieve a strain amplification of the FOSG. The formulas of both strain amplification and thermal compensation of the FOSG with the STC structure were deduced, as well as finite element simulation and tests were carried out, which verified the effectiveness of the compensation structure. The results of theoretical derivation, simulation and tests showed that the strain amplification decreases with the increase of the distance of installation points of the FOSG, and there is a zero point in the thermal output. However, due to the simplification of the STC structure, there are deviations in the theoretical derivation which can only be used in the preliminary design. The agreements between experiments and simulations, verified the temperature compensation effectiveness of the STC structure.
Hypersonic wind tunnel experiment technologies are involved to many subjects such as aerodynamic forces, aerothermodynamics, thermal protection of aircraft structures, heat-fluid-solid coupling, hypersonic boundary layer, airbreathing propulsion system and light-weighted and high-strength material. In comparison with traditional electromechanical or electronic sensors, the fiber optic sensors have relatively high potential to work in hypersonic wind tunnel. This article has classified and summarized the research status and the representative achievement on the fiber optic sensing technologies, giving special attention to the summary of research status on the popular Fabry-Perot interferometric, fiber Bragg gratings and (quasi) distributed fiber optic sensors working in hypersonic wind tunnel environment, and discussed the current problems in special optical fiber sensing technologies.
In general, conventional resistance strain balances are used to obtain aerodynamics forces in hypersonic wind tunnels. Owing to the advantages of fast response, high sensitivity and anti-electromagnetic interference, fiber optic sensors provide a new technical approach to wind tunnel strain balance. In this paper, a three-component fiber optic balance based on MEMS Fabry-Perot strain sensor has been developed. The MEMS strain sensor based on Fabry-Perot interference was fabricated by surface and bulk MEMS techniques. The MEMS FP strain sensor has a high finesse factor by adapting high-reflection films deposited on the surfaces of FP cavity. The three-component fiber optic balance has been calibrated, and evaluated in a hypersonic low density wind tunnel. The results of static calibration and wind tunnel tests have declared a good performance compared to the results of resistance strain balance.
This paper presents high-sensitivity, micro machined all fiber Fabry-Perot Interferometric (FFPI) strain gauges, as well as their applications in water tunnel environment for hydrodynamic measurements. The FFPI strain gauge has a short Fabry-Perot cavity and a long hollow hole next to the cavity, formed by two step laser etching process. Such configuration enables the sensitivity to be enlarged as many as eight times. The deformation of the FFPI strain gauges is measured by the spectrum shift of the reflected optical signal, using a white light optical demodulator. The strain sensitivity is measured to be 0.015 nm/με, and the minimum detectable strain alteration is about 0.07 με in our set-up. A force balance, using the proposed FFPI strain gauges as sensing elements, has been fabricated, calibrated and evaluated in a water tunnel flow, to measure the hydrodynamic loading. Experimental results indicate that, the proposed balance based on the FFPI strain gauge is reliable and robust and is potentially suitable for water environment.