United States Department of Transportation
i
Direct measurement of initial wake separation (bo) and initial circulation (????) using pulsed lidars
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2013-06-17
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Abstract:The initial separation distance (bo) between a counter-rotating vortex pair generated by an aircraft is a fundamental parameter affecting wake turbulence decay. For the past decade Pulsed Doppler Lidars have emerged as the primary remote sensors for measurement and characterization of wake vortices1-5. Unfortunately, bo measurements are challenging using Pulsed Lidars in a traditional side-looking configuration. Such configuration introduces large errors due to poor range resolution but it is necessitated by the large standoff distance of pulsed Lidars6. In order to mitigate for this low range resolution the measured descent speed is usually used to infer the wake separation7 bo. This is however an inferred measurement that relies on a number of assumptions such as the weight and air speed of the aircraft. More recently, compact pulsed Lidars with short standoff distances for wind energy applications were introduced8,9. This paper describes a simple approach that makes use of this short standoff distance attribute by positioning the scanner directly under the aircraft’s path in an upward looking Range Height Indicator (RHI) scan. Unlike the side-looking configuration, this scanning strategy leverages the high cross-range resolution and eliminates the range induced coupling between the aircraft vortex pair. This allows for a higher accuracy direct measurement of bo and therefore minimizes the number of assumptions of unknown airplane parameters. When estimating the parameters of a vortex decay curve, a measurement of initial circulation ???? will further reduce the number of assumptions such as weight and air speed of the generating aircraft. The paper starts with the description of the measurement configuration and the algorithm used to extract bo and initial circulation ???? A simulation of the vortices wind field and the Lidar signal is then used to validate the approach, as well as performing sensitivity analysis. Finally, an uncertainty analysis is performed to determine the minimum number of measurements needed to achieve a 2% accuracy for the bo, ???? estimate.
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