Effect of measurement volume shift and fringe distortion due to beam refractions through the plane window for LDA applications

Using laser Doppler anemometer (LDA) to measure velocities in inner flow field, due to the refraction after the laser beams through the flat window vertically or aslant, the measuring volume position and shape change, and affect the measured velocity. For studying the refraction effect on measuring volume shift and velocity direction, a mathematical model of the laser beams through the flat window refraction was established, using this model to study the refraction effect under the different working conditions (the beams incident angle θ, the beams half angle α₁, the refractive index of the flow n₃, plane thickness d and LDA probe location); for studying the refraction effect on measurement of fringe distortion under different θ, the LDA calibration test system was established. Results show that the refraction effect can be ignored within 10° of θ, 5° of α₁. The refractive index of both the plane sides as close as possible, will reduce the refraction effect; Along the length of the measurement volume direction after refraction, fringe spacing increases linearly, and with the increase of θ, fringe spacing tend to be uniform.