激光多普勒测速圆环壁面窗口折射效应的影响

Refraction Effect from Beam Refraction by Annular Wall Window in LDA Applications

采用转盘标定系统和理论分析,对激光多普勒测速仪（Laser Doppler anemometer,LDA）圆环壁面窗口折射效应进行研究,建立描述两束激光轨迹的折射效应理论计算模型,应用此模型推导了测量体位置和速度矢量偏差修正公式,分析不同光束半夹角α1、内流场介质折射率n3、圆环壁面厚度d、探头位置对折射效应的影响。应用转盘标定系统对计算模型进行验证,并且进行了条纹畸变折射效应研究。使用相对平移系数kx、ky和速度误差Δu来评估折射效应对于测量体位置和测速误差的影响。结果表明:控制α1在10°以内可以忽略其折射效应影响;壁厚d=30时,折射效应产生的kx、ky和Δu近似为恒定值;壁面两侧折射率尽可能的接近,可减小n3的折射效应;Δu试验数据略小于计算数据,结果相对比较吻合。

The influence of laser beam refraction resulting from the annular wall window on refraction effect is investigated both in a theoretical analysis and laser Doppler anemometry(LDA) using a calibration disc.A mathematical procedure,without any simplifications,describing the propagation of two laser beams through an annular window was established to study the effect of refraction on measuring volume(MV) shift in terms of laser beam half angle,annular window thickness,refractive index of the fluid,and LDA probe location.A test system used for LDA calibration was assembled to study this effect of refraction to fringe distortion.The coefficients of relative motion k_x and k_y,and the velocity bias Δu relative to the real fluid velocity are used to estimate the refraction influence on MV position and velocity bias.Results show that the effect can be ignored within 10° of the half angle.k_x,k_y,and Δu are nearly constant if the thickness of the annular wall window is more than 30 mm.The refractive index of the fluid should be matched as near possible with that of the annular wall to minimalize the refractive influence.The velocity bias arising from the change in fringe spacing is smaller than that estimated by the mathematical procedure.