Long-range Schmidt-Cassegrain laser velocimeter for large wind-tunnel applications

Abstract

A two-component dual-beam laser velocimeter was designed and fabricated for use in the NASA Ames 40 X 80 ft wind tunnel. The instrument design was predicated on the use of inexpensive, high-quality, large-aperture Schmidt-Cassegrain telescope optics to perform both the laser beam focusing and scattered light collection functions. The instrument was configured to measure the streamwise and cross-stream components of velocity and enclosed in a streamlined optical assembly mounted on a two-dimensional traverse on the floor of the test section. The laser velocimeter probe volume was translated in the vertical direction by means of zoom optics. Coaxial backscattered light was collected through the same optical system and analyzed for Doppler shift. Single-mode polarization-maintaining fiber optics transmitted light from the remotely located laser into the optical assembly. A multimode fiber conveyed the scattered light to remotely located photodetectors. Standard frequency domain processing permitted analysis of Doppler signals with very poor signal-to-noise ratio. The system was evaluated in an optics laboratory in preparation for wind-tunnel test program applications. Sample data from this evaluation were presented as representative of the capability of the instrument. Statement of the Problem I N recent decades the laser Doppler velocimeter1 (LV) has seen increasing application to the study of aerodynamic phenomena. The LV provides a noninvasive measurement capability that is critical for the study of many phenomena. The dual-beam or differentialDoppler configuration of the instrument2 has been shown to have many practical advantages and is used extensively in small-scale wind-tunnel testing. Although several new techniques offer increased potential for global measurement capability,3'4 the LV remains a very important instrument for any aeronautical research facility. Although significant progress has been made,5 LV has not been widely used in large-scale wind-tunnel testing. To maintain the noninvasive quality of the measurement, it is necessary to place the instrument outside the wind-tunnel test section, or at least far enough away to limit its effect on the flowfield of interest. In largescale wind tunnels, this means large separation between the probe volume (laser beam intersection volume) where Doppler shifted light is scattered and the receiving optics where that light is collected and focused on a photodetector. This large separation or longrange operation requirement demands high laser power density in the probe volume to achieve high scattered intensity of the Doppler shifted light and a large collection solid angle to concentrate it on the photodetector. For a closed jet wind tunnel, it is further necessary to use windows of sufficient quality that the laser beams launched into the test section as well as the scattered light collected from the probe volume are not significantly degraded. In particular, the laser beams must not be refracted by striae in the windows so as to significantly reduce beam overlap in the probe volume. The window aperture must be sufficient to access the entire flow region of interest. The National Full-Scale Aerodynamics Complex at NASA Ames Research Center is composed of two large wind tunnels and a static (hover) test facility. The 40 x 80 ft wind tunnel is the smaller of the two wind tunnels, with a speed capability up to 300 kn (154 m/s).