The small scales of turbulence in atmospheric winds at heights relevant to MAVs
Creators: name: Thompson, Mark
Publisher: name: RMIT University
Year: 2024

description: Micro Air Vehicles (MAVs) are typically of low mass and moment of inertia and have flight speeds comparable to birds and larger insects. Such craft traverse the lower levels of the Atmospheric Boundary Layer (ABL); a significantly different environment than that experienced by larger manned aircraft, which spend the majority of their time in relatively clean air and fly at speeds considerably higher than typical wind speeds in the ABL. The winds and associated turbulence in the ABL has been a subject of research for over an hundred years, however the focus of research has been on the larger scales of turbulence. For MAVs the small scales of turbulence (of the order of a wing span or chord; typically 150 mm) are the most problematic and there is little information available on atmospheric turbulence at these scales. Information on the span-wise structure of turbulence at these scales is lacking, particularly in complex/city terrain. This research aimed to fill that gap.<br><br>Four dynamically calibrated multi-port pressure probes were used by spanning them across the oncoming wind. Measurements were recorded in 4 locations; 2 open terrains locations well away local structures, 1 urban location with little local effects present and 1 urban location with significant local effects. Velocity spectra and the 2nd order velocity structure functions for the 3 orthogonal components were obtained. Spectral plots are the standard method of presenting the distribution of turbulence energy over a range of scales, however with the limited number of span-wise measurement points only the stream-wise spectra could be calculated. The calculation of the structure functions was possible in both the stream-wise and the span-wise directions, hence their use here. Both the spectra and structure functions were compared to the theoretical values for sub-inertial turbulence which is known to be “locally isotropic”.<br><br>The results show mixed agreement with the sub-inertial turbulence theory. Spectra showed excellent agreement to the Kolmogorov -5/3 spectrum and the structure functions exhibited growth rates to the theorised power of 2/3 for all locations, except for that of the urban location with local effects. The relations between the u, v and w components had poorer agreement. In general the ratios between the v and u component fluctuations were relatively close to the 4/3 ratio predicted, however the ratio between the w and u components varied between locations; this may have been result of the presence of the ground influencing the results. Lastly when the span-wise and the stream-wise measurements are considered together the u component results showed excellent agreement with the predictions; however the lateral and vertical results did not follow the same theoretically predicted relationship.<br><br>The results have also been analysed to investigate the effect each component has on the lift and roll of an aerofoil via strip theory. The vertical fluctuations were most significant and hence should be the primary concern of those attempting to recreate or simulate turbulence for testing since roll is a major concern for fixed wing MAVs.