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Making Aircraft Vortices Visible to Radar by Spraying Water into the Wake

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  • 1 NASA Ames Research Center, Moffett Field, California
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Abstract

Aircraft trailing vortices pose a danger to following aircraft during takeoff and landing. This necessitates spacing rules, based on aircraft type, to be enforced during approach in instrument flight regulation (IFR) conditions; this can limit airport capacity. To help choose aircraft spacing based on the actual location and strength of the wake, it is proposed that wake vortices can be detected using conventional ground-based precipitation and cloud radars. This is enabled by spraying a small quantity of water into the wake from near the wing. The vortex strength is revealed by the Doppler velocity of the droplets. In the present work, droplet size distributions produced by nozzles used for aerial spraying are considered. Droplet trajectory and evaporation in the flow field is numerically calculated for a heavy aircraft, followed by an evaluation of radar reflectivity at 6 nautical miles (n mi) behind the aircraft. Small droplets evaporate, while larger droplets fall out of the wake. In the humid conditions that typically prevail during IFR, a sufficient number of droplets remain in the wake and give good signal-to-noise ratios (SNR). For conditions of average humidity, higher-frequency radars combined with spectral processing give good SNR.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JTECH-D-16-0066.s1.

Corresponding author address: Karim Shariff, NASA Ames Research Center, Mail Stop 258-1, Moffett Field, CA 94035. E-mail: karim.shariff@nasa.gov

Abstract

Aircraft trailing vortices pose a danger to following aircraft during takeoff and landing. This necessitates spacing rules, based on aircraft type, to be enforced during approach in instrument flight regulation (IFR) conditions; this can limit airport capacity. To help choose aircraft spacing based on the actual location and strength of the wake, it is proposed that wake vortices can be detected using conventional ground-based precipitation and cloud radars. This is enabled by spraying a small quantity of water into the wake from near the wing. The vortex strength is revealed by the Doppler velocity of the droplets. In the present work, droplet size distributions produced by nozzles used for aerial spraying are considered. Droplet trajectory and evaporation in the flow field is numerically calculated for a heavy aircraft, followed by an evaluation of radar reflectivity at 6 nautical miles (n mi) behind the aircraft. Small droplets evaporate, while larger droplets fall out of the wake. In the humid conditions that typically prevail during IFR, a sufficient number of droplets remain in the wake and give good signal-to-noise ratios (SNR). For conditions of average humidity, higher-frequency radars combined with spectral processing give good SNR.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JTECH-D-16-0066.s1.

Corresponding author address: Karim Shariff, NASA Ames Research Center, Mail Stop 258-1, Moffett Field, CA 94035. E-mail: karim.shariff@nasa.gov
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