Abstract
For research in the atmospheric boundary layer and in the vicinity of wind turbines, the turbulent 3D wind vector can be measured from fixed-wing unmanned aerial systems (UAS) with a five-hole probe and an inertial navigation system. Since non-zero vertical wind and varying horizontal wind causes variations in the airspeed of the UAS, and since it is desirable to sample with a flexible cruising airspeed to match a broad range of operational requirements, the influence of airspeed variations on mean values and turbulence statistics is investigated. Three calibrations of the five-hole probe at three different airspeeds are applied to the data of three flight experiments. Mean values and statistical moments of second order, calculated from horizontal straight level flights are compared between flights in a stably stratified polar boundary layer and flights over complex terrain in high turbulence. Mean values are robust against airspeed variations, but the turbulent kinetic energy, variances and especially covariances, and the integral length scale are strongly influenced. Furthermore, a transect through the wake of a wind turbine and a tip vortex is analyzed, showing the instantaneous influence of the intense variations of the airspeed on the measurement of the turbulent 3D wind vector. For turbulence statistics, flux calculations, and quantitative analysis of turbine wake characteristics, an independent measurement of the true airspeed with a pitot tube and the interpolation of calibration polynomials at different Reynolds numbers of the probe’s tip onto the Reynolds number during the measurement, reducing the uncertainty significantly.
Highlights
For environmental science in the atmospheric boundary layer (ABL), airborne measurements supplement the limited coverage of ground-based measurements or tethered balloons.Manned research aircraft have intensively investigated a wide range of processes and phenomena [1,2,3]and unmanned aerial systems (UAS) adopted the measurement techniques of quantities such as temperature, humidity, and the wind vector [4,5,6,7], but are not limited to these, as shown by Bärfuss et al [8] for aerosol and radiation measurements, or by Schuyler and Guzman [9] for gas concentrations
This study investigates the magnitude of deviations of mean values and turbulence quantities, when measuring with a calibration that does not exactly match the airspeed during the experiment
It is impossible to setup an experimental study in the wind tunnel, since atmospheric turbulence cannot be artificially produced in a representative way
Summary
For environmental science in the atmospheric boundary layer (ABL), airborne measurements supplement the limited coverage of ground-based measurements or tethered balloons.Manned research aircraft have intensively investigated a wide range of processes and phenomena [1,2,3]and unmanned aerial systems (UAS) adopted the measurement techniques of quantities such as temperature, humidity, and the wind vector [4,5,6,7], but are not limited to these, as shown by Bärfuss et al [8] for aerosol and radiation measurements, or by Schuyler and Guzman [9] for gas concentrations. Atmosphere 2019, 10, 124 attitude, position, and velocity of the vehicle, measured by an inertial navigation system (INS), multiple coordinate transformations yield the wind vector. This method is widely used in manned aircraft [1,10] and on fixed-wing UAS [4,5,6,7]. The accuracy of the wind vector measurement is crucial, and the propagation of errors have many influencing factors, originating in the attitude and ground speed measurement of the aircraft, the flow angles and flow magnitude (true airspeed vector) measurement with the multi-hole probe, and in the measurement of the thermodynamic state of the air. Extensive studies for various systems and subsystems of the wind vector measurement with manned research aircraft [11,12,13,14,15,16], including in-flight calibration procedures and uncertainty analysis [10,17], and with UAS (e.g., for the M2 AV [7]) were performed
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