Due to limitations in available sensor technology, unmanned aerial vehicles (UAVs) lack an active sensing capability to measure turbulence, gusts, or other unsteady aerodynamic phenomena. Conventional in situ anemometry techniques fail to deliver in the harsh and dynamic multirotor environment due to form factor, resolution, or robustness requirements. To address this capability gap, a novel, fast-response sensor system to measure a wind vector in two dimensions is introduced and evaluated. This system, known as `MAST' (for MEMS Anemometry Sensing Tower), leverages advances in microelectromechanical (MEMS) hot-wire devices to produce a solid-state, lightweight, and robust flow sensor suitable for real-time wind estimation onboard a UAV. The MAST uses five pentagonally-arranged microscale hot-wires to determine the wind vector's direction and magnitude. The MAST's performance was evaluated in a wind tunnel at speeds up to 5 m/s and orientations of 0°-360°. A neural network sensor model was trained from the wind tunnel data to estimate the wind vector from sensor signals. The average error of the sensor is 0.14 m/s for speed and 1.6° for direction. Furthermore, 95% of measurements are within 0.36 m/s for speed and 5.0° for direction. With a bandwidth of 570 Hz determined from square-wave testing, the MAST stands to greatly enhance UAV wind estimation capabilities and enable capturing relevant high-frequency phenomena in flow conditions.
The wake of a three-bladed horizontal axis wind turbine was studied at aerodynamic conditions similar to what is experienced by commercially available turbines. Field relevant Reynolds numbers and tip speed ratios were obtained through the use of a high-pressure wind tunnel, at relatively low velocities. Measurements of the streamwise velocity were acquired through the use of the novel nano-scale thermal anemometry probe (NSTAP), which yields very high spatial and temporal resolution, enabling unattenuated turbulence measurements. Profiles of the mean velocity and turbulent fluctuations are presented, as they demonstrate important features of the wake development, such as wake recovery and tip vortex evolution. One dimensional energy spectra are also presented to provide details on the dominant flow features present in the wake. Reynolds number invariance is shown for mean velocity deficit and streamwise variance profiles for all downstream positions presented. Downstream evolution of streamwise variance profiles provides insight to the dynamic interactions between the tip and root vortex, such as their eventual coalescence. Spectral analysis show that the near wake flow-structures are dominated by the tip vortex, but that other larger structures are present as well, which may be related to the wake meandering phenomenon.
Vortical impulse theory is used to investigate the relationship between turbine thrust and the near-wake velocity and vorticity fields. Three different hypotheses regarding the near-wake structure allow the derivation of novel expressions for the thrust on a steadily rotating wind turbine, and these are tested using stereoscopic particle-image velocimetry (PIV) data acquired just behind a rotor in a water channel. When one assumes that vortex lines and streamlines are aligned in a rotor-fixed frame of reference, one obtains a PIV-based thrust estimate that fails even to capture the trend of the directly measured thrust, and this failure is attributed to an implicit assumption that most of the generated thrust does useful work. When one neglects the axial gradients of radial velocity, the PIV-based thrust estimate captures the measured thrust trend, but underpredicts its magnitude by approximately
$33\,\%$
. The third and most promising physical proposition treats the trailing vortices as purely ‘rolling’ structures that exhibit zero-strain rate in their cores, with the corresponding thrust estimates in close agreement with direct thrust measurements. This best-performing expression appears as a correction to the classical thrust expression from momentum theory, possessing additional squared-velocity terms that can account for the high-thrust regime of turbine operation that is typically addressed empirically.
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