A laser Doppler velocimeter (LDV) system has been developed for use on practical gas-turbine engines. The system has been used to measure inlet and exhaust velocities on an F-100 EMD engine from an F-15 airplane (see Figure 1). To perform this work successfully, it was necessary to develop several novel subsystems, including a rugged LDV transceiver, a high-performance frequency-domain signal processor, and equipment for adding seed particles to the inlet and exhaust flows. In addition, it was necessary to provide for remote control of the system from a blockhouse at a distance of 30 m from the engine.

The LDV transceiver features a special ruggedized design: The main structural component of the transceiver was machined from a billet of aluminum, and all optics were hard-mounted on this component. This was necessary to enable the LDV transceiver to survive the intense vibrational and acoustical fields that surround a practical gas-turbine engine.

Figure 1. The LDV System includes a rugged LDV transceiver mounted near the engine and connected via optical fibers to optical and electronic instrumentation in the blockhouse.

A 40-MHz Bragg cell provides frequency shifting for the LDV. The laser beam is generated by an argon-ion laser in the blockhouse and delivered to the LDV transceiver by a 30-m-long, single-mode, polarization-preserving optical fiber (see Figure 1). The intensity of the laser beam emerging from the end of the fiber-optic link in the LDV transceiver is monitored remotely; that is, from within the blockhouse. A second 30-m-long multimode optical fiber delivers the scattered light received from seed particles passing through the interferometric LDV probe volume to a photodetector in the blockhouse. This photodetector is a photomultiplier/preamplifier combination developed specially to perform at signal frequencies >120 MHz - well in excess of characteristic response frequencies of typical photodetectors in older LDV systems.

Figure 2. These Plots Show Axial Speeds in inlet and exhaust flows as measured by use of the LDV system during a transient from idle to full military power then back to idle.

The frequency-domain signal processor, known as the Real-Time Signal Analyzer™(RSA), was developed to provide an easy-to-operate, extremely capable processor of LDV signals. The RSA can perform up to 107 measurements per second on LDV signals and is thus capable of performing at rates well in excess of any expected data rates. Not only is the potentially noisy LDV signal measured in the frequency domain by use of discrete Fourier transforms, but the Doppler burst is also detected in the frequency domain, enabling operation at signal-to-noise ratios well below 0 dB. The output of the RSA is delivered to a laptop computer, where the results are displayed in real time and stored. All control over the RSA is exercised via this computer.

Two seeders were developed. One was an evaporation/condensation seeder that introduced a propylene glycol smoke, as a nonhazardous seeding material, into the inlet flow. This seeder was specially designed to minimize perturbation of the inlet flow and eliminate a possibility of introduction of foreign objects that could damage the engine. The other seeder - of the fluidized-bed type - introduced refractory seed particles into a moderate-pressure engine bypass airflow downstream of the engine to enable LDV measurement of the exhaust flow. Both seeders were required to provide copious amounts of seed to obtain adequate data rates at the high flow rates of a practical gas-turbine engine.

Figure 2 presents some results from a sample test run, showing inlet and exhaust axial speeds for a transient ramp from idle to full military power, then back to idle. The success in using this system to perform ground-based measurements raises the hope of accomplishing such measurements in flight on a practical aircraft in the future.

This work was done by Kimberly Ennix, Tim Conners, and Dean Webb of Dryden Flight Research Center and Roger Rudoff, John Hanscom, Robert Shearrer, and William D. Bachalo of Aerometrics, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Aerometrics, Inc.
755 N. Mary Avenue
Sunnyvale, CA 94086

Refer to DRC-98-08, volume and number of this NASA Tech Briefs issue, and the page number.


NASA Tech Briefs Magazine

This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine.

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