THE MEASUREMENT OF TURBULENCE USING A LASER DOPPLER VELOCIMETER

IN 1964, YEH AND CUMMINS DEMONSTRATED THAT COHERENT LIGHT SOURCES COULD BE USED FOR THE MEASUREMENT OF VELOCITIES IN FLUIDS BY OBSERVING THE DOPPLER SHIFT IN THE FREQUENCY OF LIGHT SCATTERED FROM SMALL PARTICLES MOVING WITH THE FLUID.MANY INVESTIGATORS HAVE ATTEMPTED TO EXTEND THE TECHNIQUE TO THE MEASUREMENT OF RAPIDLY FLUCTUATING VELOCITIES (OR TURBULENCE).THE FUNDAMENTAL LIMITATION OF THIS TYPE OF VELOCIMETER IS THE SO-CALLED DOPPLER AMBIGUITY.THE AMBIGUITY INTRODUCED BY THE FINITE LIFETIME OF THE SIGNALS GENERATED BY INDIVIDUAL SCATTERING PARTICLES MOVING THROUGH THE BEAM WAS ANALYZED BY LUMLEY.THESE RESULTS WERE LATER EXTENDED TO INCLUDE THE EFFECTS OF THE TURBULENCE ITSELF AND NOISE ARISING FROM THE DETECTION PROCESS.A UNIFIED ACCOUNT OF THE AMBIGUITY INTRODUCED BY FINITE LIFETIME, TURBULENCE, APERTURE, VELOCITY GRADIENTS, AND NOISE IS PRESENTED AND RESULTS ARE INTERPRETED USING THE POWER SPECTRUM.AN OPERATIONAL LASER-DOPPLER VELOCIMETER IS DESCRIBED AND MEASUREMENTS OF POWER SPECTRA IN BOTH LAMINAR AND TURBULENT FLOW ARE PRESENTED.RESULTS ARE SEEN TO BE IN EXCELLENT AGREEMENT WITH THEORY.TECHNIQUES FOR OPTIMIZATION OF THE RESPONSE ARE PRESENTED.(A)