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Description
INTRODUCTION
Interest in unmanned aerial vehicle (UAV) technology has been rapidly growing given its applicability across a wide range of fields, including disaster risk management, agricultural monitoring, and commercial services.
UAVs tipically employ small-scale propellers operating at low Reynolds, making them susceptible to the detrimental effects of Laminar Separation Bubbles (LSBs) during flight.
The presence of LSBs may adversely affect the flowfield around propeller airfoils, leading to potential risks during missions, such as increased noise emissions from tonal disturbances in the shear layer, premature stall and higher drag penalties.
The present work examines the flow field around a 610 mm-diameter propeller, operated at rotational speeds $\Omega = $ between 840 and 1680 RPM, corresponding to tip chord-based Reynolds numbers of $2.8\times 10^4$ and $5.6\times 10^4$.
Key features of the flowfield are reconstructed from surface temperature maps obtained via Infrared Thermography (IRT), paired with heated thin film method to extract the Stanton number $St$ distribution over the propeller.
In addition, the effects of leading edge shape modification, as a mean of passive flow control, are also investigated. The leading edge is modified by introducing a sinusoidal waviness through modulation of the local chord (tubercles). Six configurations are tested by varying two amplitude values (7 and 15 $\%$ of the local chord) and three wavelengths (10, 20 and 30 mm).
EXPERIMENTAL SETUP
The flight stand used in the experiment is the Flight Stand 15 Pro from Tyto Robotics, equipped with a Force measurement unit (FMU) for thrust (up to 8 kgf), torque (up to 8 N $\cdot$ m) and rotational speed measurements.
Propellers' blades are 3D printed in PA12 Nylon and glued together. The baseline propeller mounts a NACA 4412 airfoil and has a maximum and tip chords equal to 54 mm and 15.5 mm, respectively.
The propeller also features a variable pitch, decreasing from the root ($40 ^{\circ}$) to the tip ($12 ^{\circ}$).
In the thermal setup, the suction side of the model’s surface was radiatively heated by two 2 kW halogen lamps to enhance thermal contrast among regions where the shear stresses work differently . Thermal images are acquired in the long wave infrared range (1.5 - 5.0 $\mu$m) with a FLIR X6981-HS InSb high-speed infrared camera equipped with a 100mm lens, achieving a spatial resolution of 110 pix/$c_{max}$.
RESULTS
Thermographic analysis identify a LSB spanning through the leading edge, marked by a low-$St$ number band close to the leading edge, followed by a general increase in heat transfer, consistent with the turbulent reattachment of the LSB. At inboard stations, the flow undergoes separation shortly downstream of the reattachment of the laminar separation bubble, whereas the flow remains predominantly attached towards the tip region.
A combined assessment of performance and thermographic data of the wavy configurations highlights significant modifications in the structure and extent of the LSB compared with the baseline case.
The sinusoidal leading edge alters the $St$ distribution, by generally promoting earlier transition in the valley regions and an extension of the LSB over the peak regions.
The resulting effect on aerodynamic performance is quantified through the gain in the non-dimensional thrust-to-torque ratio $C_T/C_Q$, which is found to depend primarily on the tubercles amplitude, while the wavelength plays a secondary role.
