MEASUREMENT OF FLUID VELOCITY BY LASER-DOPPLER TECHNIQUES

LASER-DOPPLER TECHNIQUES FOR FLUID VELOCITY MEASUREMENT HAVE ALREADY PROVEN THEMSELVES BOTH THEORETICALLY AND IN PRACTICE.THEY PROMISE TO OPEN NEW VISTAS IN EXPERIMENTAL FLUID MECHANICS DUE TO THEIR GREAT VERSATILITY.THE ADVANTAGES OF SUCH MEASUREMENTS INCLUDE ABSENCE OF A PROBE IN THE FLOW, RELATIVE INDEPENDENCE OF THE MEASUREMENT ON VARYING FLUID PROPERTIES OTHER THAN LOCAL VELOCITY, ISOLATION OF INDIVIDUAL VELOCITY COMPONENTS, AND EXACTNESS AND LINEARITY OF THE RELATION BETWEEN THE VELOCITY AND THE MEASURED ELECTRICAL QUANTITY (FREQUENCY).THESE ARE TO A CERTAIN EXTENT OFFSET BY THE NEED OF HAVING A TRANSPARENT FLUID AND LIGHT-SCATTERING PARTICLES WHOSE VELOCITY IS ACTUALLY MEASURED AND WHICH MAY NOT FOLLOW THE FLOW PERFECTLY.IN ADDITION, THERE IS THE SIGNAL BROADENING WHICH AFFECTS TURBULENCE MEASUREMENT AND THE NEED OF HAVING, AND PERHAPS DEVELOPING, COSTLY AND COMPLICATED ELECTRONIC INSTRUMENTATION TO MAKE THE MEASUREMENTS.OPTICAL INSTRUMENTATION HAS MANY POTENTIAL APPLICATIONS.THIS IS NOT TO SUGGEST THAT ONE SHOULD DISCARD OTHER, OLDER, INSTRUMENTATION.HOT-WIRE ANEMOMETERS HAVE BEEN USED FOR MANY DECADES, BUT WE STILL RETAIN PITOT TUBES AND SIMPLE U-TUBE MANOMETERS FOR FLOW MEASUREMENT.THE NEW INSTRUMENTATION SHOULD BE CONSIDERED AS AN IMPORTANT ADDITION TO THE TOOLS OF FLUID MECHANICS RESEARCH THAT ABLY COMPLEMENTS OTHER TECHNIQUES.(A)