Low Reynolds Number Aerodynamics of a UAV Wing under Icing Condition

Abstract

Small size Unmanned Aerial Vehicles (UAVs) are designed to operate in various types of weather conditions. Of these, flying under icing conditions is one of the most challenging tasks because the streamline profile of the wing is altered by ice-accretion on the leading-edge. To enhance our understanding of the impacts of ice-accretion on small UAV’s, an experimental and numerical analysis of the low Reynolds number aerodynamics of an ice-accreted RG-15 aerofoil was undertaken. The RG-15 aerofoil, which is a typical form used for UAV wing profiles, was tested in a wind tunnel at three Reynolds numbers, i.e., 5×10⁴,1×10⁵ and 2×10⁵, which are based on the chord of the clean aerofoil. The aerofoil was tested with four configurations, which are clean (without ice), with an original ice shape generated in an icing wind tunnel (Ice 1), and two further ice shapes obtained by enlarging the original ice shape (Ice 2 and Ice 3). Fluctuating pressure measurements were made using a multi-channel pressure acquisition system, to help reveal the time-averaged and unsteady aerodynamic behaviour. To compensate for the frequency limitations of the pressure system, a series of microphones were embedded to measure the boundary layer instabilities for both clean and ice-accreted aerofoils. In addition to that, a hotwire anemometer was utilized for only Ice 1 configurations, to obtain the level of turbulence generated from the ice. Moreover, Large Eddy Simulations were conducted for specific cases, to achieve an in-depth understanding and visualization of the flow field around the aerofoil. From the experimental investigations, the original ice shape (Ice 1) has only a minimal effect, whereas, the largest ice shape (Ice 3) caused a decrease in maximum lift of 14.2%, and increase in drag of up to 182%. These suggest that a small amount of streamwise ice at the leading-edge is relatively safe for UAV flight. However, with larger ice accretions there is an obvious increase in drag, which is hazardous for the operation of the UAV. A detailed study of the surface pressure measurements reveals the presence of separation bubbles on both clean and iced aerofoils. On the clean aerofoil, the laminar separation bubble (LSB) moves upstream and reduces in its extent with an increase in angle of attack. Also, the presence of freestream turbulence or increase in flow Reynolds number reduces the size of the LSB. Unlike the LSB, an ice-induced separation bubble (ISB) always remains anchored around the leading-edge regardless of the Reynolds number and angle of attack, in addition, the extent of the ISB is dependent on both Reynolds number and angle of attack. Moreover, the instabilities from the ISB are similar to those observed for the conventional LSB, where the dominant frequencies are typically higher than those of vortex shedding, with a Strouhal number higher than 0.64. Furthermore, the shear layer instability frequencies were observed to have a power law dependency on Reynolds number, with an exponent between 1.24 and 1.64 on the clean aerofoil, and 1.11 to 1.14 on the ice-accreted aerofoil. While comparing the convective velocities of the shear layer instabilities, the ISB on the original ice shape (Ice 1) was found to have a faster velocity than that of the LSB. The numerical simulations, based on Large Eddy Simulations, show good agreement with the experimental results, such as in the locations and the extent of both LSB and ISB. The numerical simulation also shows the ice on the leading-edge behaves like a turbulence generator, which increases the lift coefficient on the ice-accreted RG-15 aerofoil at 0° angle of attack. Normally, an iced aerofoil only shows the formation of an ISB, but at one of the lowest Reynolds numbers,5×10⁴, and at certain angles of attack, both an ISB and LSB were observed in the present study. While the ISB exists and remains anchored around the leading-edge of the aerofoil at all times, the level of turbulence generated by the ISB has a significant effect on the development of the secondary separation bubble (i.e. LSB), which was only observed under certain conditions.