Uncomplicated Setting Apparatus for Measurement of Fluid Flow Rate using Laser Doppler Technique: Physics Teaching

Laser Doppler velocimetry (LDV) or laser Doppler anemometry (LDA) is the technique of using the Doppler shift in a laser beam to determine the fluid velocity. In this work, dual beam mode (two incident beams, single observation location) was selected to use. A cost effective and easy module for measurement of water flow rate was designed and constructed. A He-Ne laser with a wavelength of 632.8 nm was used as a light source. The laser was passed through a beam splifter and divided into two beams with identical intensity and coherence. Subsequently, the two laser beams travelled to a focusing lens with a focal length of 100 mm and focused on a center of water flow channel in quartz cuvefte with a dimension of 1 x 1 x 5 cm(3). The beam angle was set at 4.96 degrees. When the seeding particles, bubbles and microorganism in water, were moving through the intersection point of two beams, the light was scattered. The scattered light was collected by photodetector connected to processing system. The frequency of scatterred light is shifted according to the Doppler shift relations due to effect of Rayleigh scattering. The water flow rate can be calculated from Doppler shift frequency.