| The fiber-optic interferometric microcavity sensor has the advantages of small size,light weight,stable performance,water-proof,adapting for the hard environment,etc.Combining with demodulation method of low-coherence interferometry(LCI),it can be employed for fluid flow velocity measurement.In this thesis,the measurement method of flow velocity based on optical fiber sensing with interferometric microcavity is studied for the airflow and water velocity measurement requirements of water conservancy,aviation,meteorology and other fields.A signal preprocessing method based on all-phase filtering is studied to extract the LCI fringe pattern,and the phase signal is retained after filtering.The flow velocity measurement method of differential pressure based on fiber-optic interferometric microcavity sensing is studied and verified by airflow experiments.The demodulation method based on tracing peak position of a single LCI fringe pattern and phase shifting interferometry is studied,therefore,the pressure demodulation and flow velocity measurement of high accuracy are realized.The instrumentation of the velocity measurement method is studied,and preliminary field experiments are carried out.The main work of this thesis includes:1.Based on the theoretical analysis of interferometric microcavity sensor of diaphragm-type and LCI demodulation system,a signal preprocessing method based on all-phase filtering is studied.The low-frequency background intensity and high-frequency noise of the original signal are effectively removed,and the smooth LCI fringe pattern is obtained.The phase information of the original signal is retained,which is conducive to the subsequent high-precision demodulation calculation based on the position and phase information.2.The measurement method of turbulent flow velocity based on principle of differential pressure and fiber-optic interferometric microcavity sensing is proposed.Based on the theoretical analysis and simulating calculation of the pressure distribution in the turbulent flow field,the optimal positions of the sensors in the flow field are determined.The sensor holder for measuring flow velocity composed of dual fiber-optic interferometric microcavity sensors with sealed cavity is designed and fabricated,which is verified by high-speed airflow experiments.The velocity measurement range is 7.9-81m/s,and the measurement accuracy is 0.69%.3.A demodulation method of spatial domain for LCI with high accuracy and adaptability is proposed,and the method is suitable for velocity measurement of turbulent flow field with real-time changes of pressure and temperature.Based on the spatial polarized LCI demodulation system,the proposed method employs the centroid position of the fringe pattern for tracing the peak position of a single fringe,and the seven-step phase shifting interferometry is employed to improve the accuracy to sub-pixel level.At room temperature,it is proved that the demodulation accuracy is0.019% in the pressure range of 11-290 k Pa.Moreover,no parameter needs to be adjusted in advance,and the pressure demodulation accuracy is stable at different temperatures.The experimental results of airflow velocity indicate that the proposed method is suitable for the flow velocity measurement in the turbulent flow field.4.The instrumentation and field application of the flow velocity measurement method based on fiber-optic interferometric microcavity sensor and LCI demodulation are studied.The design and debugging of the hardware and software systems of the instrument are completed,the metrological traceability of the instrument is realized,and the environmental adaptability and reliability tests are passed.According to the flow velocity measurement method of differential pressure and fiber-optic interferometric microcavity sensor,the flow velocity sensor array is designed and fabricated.In the high dam scale model of hydro-junction,the field experiments of the instrument are carried out,and the velocity measurement of the underflow is realized. |
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