LASER BASED NON-INTRUSIVE MEASUREMENT OF JET ENGINE FLOWS: FLOW VELOCITY

LASER BASED NON-INTRUSIVE REMOTE SENSING TECHNIQUES ARE BEING DEVELOPED TO CHARACTERIZE SUPERSONIC FLOWS AND THRUSTS IN HIGH PERFORMANCE JET ENGINE EXHAUSTS. THE MOST PROMISING APPROACH TO MAKING A VELOCITY MEASUREMENT INVOLVES THE PRECISE MEASUREMENT OF A DOPPLER SHIFTED MOLECULAR ABSORPTION OR FLUORESCENCE SIGNAL AND RELATING THE MAGNITUDE OF THE SHIFT TO THE CARRIER FLOW VELOCITY. THE MOST SIGNIFICANT OBSTACLE TO THIS APPROACH IS IN THE DETERMINATION OF THE SMALL SPECTRAL SHIFT TO AN ACCURACY SUFFICIENT TO BE USEFUL. WE PROPOSE TO DEVELOP A RESONANCE FLUORESCENCE - DOPPLER SHIFT APPROACH WHICH CAN PROVIDE TWO DIMENSIONAL VELOCITY AND DENSITY DATA WITH KHZ DATA RATES. IN OUR APPROACH THE FREQUENCY PRECISIION IS PROVIDED BY A NARROWBAND CONTINUOUS DYE LASER WHICH IS FREQUENCY MODULATED OVER A SMALL RANGE (NEAR A MOLECULAR ABSORPTION). THE FREQUENCY MODULATION TECHNIQUE TO BE USED HAS BEEN DEVELOPED AND DEMONSTRATED AT KHZ CYCLE RATES. THE LASER BEAM IS PASSED THROUGH A REFERENCE CELL CONTAINING THE MOLECULAR SPECIES TO BE PROBED AND IS THEN PASSED THROUGH THE EXHAUST FLOW TO BE DETERMINED. SINCE THE DOPPLER SHIFTED MOLECULES IN THE HIGH SPEED FLOW WILL BE EXCITED AND FLUORESCENCE AT A SLIGHTLY DIFFERENT FREQUENCY FROM THOSE IN THE REFERENCE CELL, THE TWO FLUORESCENCE SIGNALS WILL OCCUR AT DIFFERENT TIMES IN THE LASER MODULATION CYCLE. BY MEASURING THE TIME DIFFERENCE BETWEEN THESE SIGNALS THE DOPPLER SHIFT CAN BE DETERMINED WITH GREAT PRECISION. THE MAGNITUDE OF THE RAYLEIGH SCATTER FROM THE OBSERVATION POINT, WHICH IS CONSTANT WITH LASER FREQUENCY, CAN BE MONITORED AND USED TO DETERMINE THE GAS DENSITY AS WELL. THE ABSOLUTE INTENSITY OF THE FLUORESCENCE SIGNAL (OR THE RELATIVE SIGNALS OF TWO TRANSITIONS) MAY ALSO PROVE USEFUL FOR THE SIMULTANEOUS DETERMINATION OF OTHER FLOW FIELD PROPERTIES SUCH AS SPECIES CONCENTRATION AND TEMPERATURE.