Laser anemometry for in-flight flow investigations

Two laser anemometers, a differential laser Doppler anemometer (LDA) of NLR and a laser-two-focus (L2F) anemometer of DLR, were installed in a research aircraft to demonstrate their capabilities for in-flight flow investigations. Both anemometers use a diode laser as a light source and an avalanche photodiode for detecting scattered light. The aircraft was flown in different atmospheric conditions, e.g. close to temperature inversion layers, in clouds and in polluted air above industrial regions to investigate whether it is possible to measure without artificially seeding air. The optical access was provided by two glass inserts in the emergency exit of the aircraft. Measurement volumes were traversed through the flow perpendicular to the fuselage. In this manner, the anemometers measured mean velocities, turbulence levels and for L2F additionally to these quantities the flow direction in the boundary layer of the fuselage. A ground test with the anemometers was performed before the flight tests. Experience with parallel operation of the anemometers was gained in a free jet and properties of both systems were investigated. Information from the tests enabled us to analyze performance data in the free jet and in flight. The experiment shows that laser anemometry can be applied successfully for in-flight flow measurements. The inherent characteristics of the technique are exploited extensively in windtunnel tests and in other ground applications, and are also promising for in-flight experiments. In particular the absence of flow disturbance, the high bandwidth and the high accuracy are useful in flight