Fiber optic coupled Rayleigh scattering diagnostic for point measurement of gas density, velocity and temperature

A laser technique using molecular Rayleigh scattering for density, temperature and velocity measurements in unseeded flows is described. Laser light is focused at a point in the flow under study. The Rayleigh scattered light is collected and transmitted to a planar mirror Fabry-Perot interferometer. The interferometer fringe pattern is recorded with a cooled CCD camera. Analysis of the fringe pattern gives the density, velocity, and temperature of the gas. Results from two experiments are presented. One is a lowspeed heated air jet. Good agreement is shown between the temperature obtained from the Rayleigh scattering and temperatures measured with a thermocouple. The second experiment is a study of a subsonic free jet. In this experiment the laser light is transmitted to the test cell via a single-mode polarization-maintaining optical fiber. The temperature and velocity obtained from the Rayleigh scattering measurements agree well with values obtained from isentropic flow relations. The results indicate that this molecular Rayleigh scattering technique can provide temperature measurements with better than 3% accuracy and velocity measurements with better than 10 m/sec accuracy. Introduction Molecular Rayleigh scattering offers a means to measure gas flow parameters including density, temperature, and velocity. The Rayleigh scattered power is proportional to the gas density, the spectral width is related to the gas temperature, and the shift in the frequency of the spectral peak is proportional to one component of the bulk velocity (fig. la). Since Rayleigh scattering is a molecular scattering process, no seeding of the flow is necessary. It thus finds application in measurement situations where it is not possible to seed the flow, or where seeding is not Senior Research Engineer, Member AIAA Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royaltyfree license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. feasible because of contamination concerns. The technique has been successfully used in a variety of applications including free jets'', rocket exhaust plumes, and wind tunnels'. Many applications of Rayleigh scattering diagnostics are in test facilities, where the environment near the test section frequently has high acoustic noise levels, mechanical vibrations, and varying temperatures. This environment is not conducive to the stable operation of some of the equipment used for measuring the Rayleigh scattering spectrum. In particular, the Fabry-Perot interferometer is very sensitive to vibration and temperature changes. One way to avoid this problem is to locate the delicate optical equipment in a remote location having a more benign environment. For example, in a previous test program in the Lewis 9x15 foot subsonic wind tunnel, the laser (CW argonion) was located about 200 meters from the test section and the laser light was transmitted through an optical fiber. The Rayleigh scattered light was transmitted through an optical fiber to a separate room, where a scanning Fabry-Perot interferometer was used to measure the Rayleigh scattering spectrum. Use of the Fabry-Perot in the scanning mode means that only time average measurements can be obtained. Other Rayleigh scattering diagnostics use the Fabry-Perot in a static imaging mode", where the Rayleigh scattering spectrum can be obtained with a single laser pulse. In this paper, we present new results based on a technique where the Rayleigh scattered light from the test location is transmitted through an optical fiber to a remote location. Light exiting the fiber at the remote location is imaged through a static Fabry-Perot interferometer onto an image detector. An argon-ion laser is used for this work. However, if instantaneous measurements are desired, an injection-seeded Nd:YAG laser can be used, but optical fibers cannot then be used to transmit high energy pulses from the laser to the test region. The accuracy of the resulting temperature and velocity measurements depend critically on the alignment of Fabry-Perot interferometer. In previous work, good results were obtained for velocity measurements. Results for temperature measurements, however, were not as good. In the work reported in this paper, better results were also obtained for temperature measurements. These improvements are a result of better alignment of the Fabry-Perot interferometer and camera system, as demonstrated by the measured finesse approaching the ideal reflective finesse. An automatic alignment system was used to maintain the Fabry-Perot plates parallel and to maintain a preset fringe diameter.