Research On Viscosity Measurement Method Based On Ultrasonic Modulation Effect Of Fiber Bragg Gratings

Viscosity,which reflects the internal friction between fluid molecules during exercise,is one of the important physical properties of a fluid.And it is of great significance for product quality assessment in industrial production and fluid properties assessment in scientific research.Generally,for low-viscosity fluids such as water,carbon tetrachloride,glucose solution etc.,the capillary method is used to measure the viscosity.However,this measurement method has the disadvantage that the inner wall of capillary is easily blocked by the measured sample particles.Moreover,this method also takes long time and complicated cleaning and drying operation.For low-viscosity fluids,this paper proposes a new viscosity measurement method based on FBG(Fiber Bragg Grating)flexural wave modulation,and the research work on the viscosity measurement method based on FBG flexural wave modulation is introduced in the following aspects:1.Based on the mode coupling theory,this paper analyzes the effects of longitudinal and flexural waves on the ultrasonic modulation of FBG.When the longitudinal wave is transmitted in the FBG,a periodic strain-modulating FBG cycle is generated along the grating direction,so that the sides of the central wavelength of the FBG generate the respective order of sidebands.While the flexural wave is transmitted in the FBG,Vertical grating direction generates periodic strain modulation FBG refractive index.If the acoustic wave frequency meets the phase matching condition,the FBG core and the cladding mode are excited in the same direction,and then modulates the reflectivity of the center wavelength and modulates the induced channel on its left side.2.Numerical analysis of FBG ultrasonic modulation characteristics is conducted.Through the simulation of the acoustic vibration characteristics of FBG,the influence of acoustic frequency and power on the periodic strain of FBG is analyzed.And the effects of acoustic frequency and acoustic power on FBG spectra are numerically analyzed by combining the acoustic vibration characteristics with the ultrasonic modulation principle.Besides,the effect of liquid viscosity on the vibration characteristics of FBG is simulated and analyzed upon the fluid-solid coupling principle.3.A FBG ultrasonic modulation experiment platform was set up to verify the FBG ultrasonic modulation phenomenon.The verification shows that the greater the longitudinal wave frequency,the greater the difference between the modulation sideband and the center wavelength.The greater the longitudinal wave power,the smaller the reflected light power of the FBG while the greater the reflected light power of the modulation sideband.When the flexural wave frequency satisfies the phase matching condition,the greater the flexural wave power,the smaller the center wavelength reflected light power,and the larger the modulation sideband reflected light power.Based on the FBG flexural wave modulation effect,the change of the FBG reflection spectrum at different viscosities was measured,and the relationship between the liquid viscosity and the reflected light power of the FBG under flexural wave modulation was obtained.Experiments verify the feasibility of the method.