This is one of my favorite experiment. In this experiment we take advantage of Doppler effect to measure speed of flow of liquid. A He-Ne Laser is used and laser beam is split into two parallel beams using a beam splitter and mirror. The two rays are then allowed to converge at a point in liquid where they interact with particles(silver-coated glass beads) in liquid. The diverging beams coming out of liquid are again converged at a point and a photo diode sensor is placed at that point to sense intensity. The intensity vs time pattern is analyzed and frequency is determined using Fourier transform. I have attached a sample Intensity pattern and its Fourier transform. Data and results are also included. You can view the actual documentation of this experiment here: http://www.phywe.com/461/apg/359/pid/30764/LDA-laser-Doppler-anemometry-with-Cobra3-.htm
Although it’s a nice experiment to measure speed of liquid without actually disturbing it. It cannot determine direction of flow.