Flow Visualization around a Flapping-Wing Micro Air Vehicle in Free Flight Using Large-Scale PIV

Abstract

Flow visualizations have been performed on a free flying, flapping-wing micro air vehicle (MAV), using a large-scale particle image velocimetry (PIV) approach. The PIV method involves the use of helium-filled soap bubbles (HFSB) as tracer particles. HFSB scatter light with much higher intensity than regular seeding particles, comparable to that reflected off the flexible flapping wings. This enables flow field visualization to be achieved close to the flapping wings, in contrast to previous PIV experiments with regular seeding. Unlike previous tethered wind tunnel measurements, in which the vehicle is fixed relative to the measurement setup, the MAV is now flown through the measurement area. In this way, the experiment captures the flow field of the MAV in free flight, allowing the true nature of the flow representative of actual flight to be appreciated. Measurements were performed for two different orientations of the light sheet with respect to the flight direction. In the first configuration, the light sheet is parallel to the flight direction, and visualizes a streamwise plane that intersects the MAV wings at a specific spanwise position. In the second configuration, the illumination plane is normal to the flight direction, and visualizes the flow as the MAV passes through the light sheet.Aerospace 2018, 5, 99 2 of 15 Quite a variety of visualization PIV studies on animals in free flight have been reported, see e.g., [11][12][13]; however, the direct correspondence to the current MAV configuration is limited. Firstly, most of these studies consider single-wing configurations at relatively fast forward flight, for which the flow dynamics are less complex than for hovering or slow forward flight, especially in the case of counter-flapping wing configurations with large-amplitude and high-frequency stroke behavior, as considered here. Furthermore, flow visualizations reported in literature have predominantly been limited to wake visualizations, while near-wing visualizations suffer in large regions around the wing from reflections or the obstruction of the illumination [13].The MAV used in the current tests is the DelFly II (henceforth called DelFly for simplicity) [3], for which tethered experiments have been previously performed in hovering [14,15] and symmetric forward flight (at zero pitch angle) [9] configurations. These configurations both result in symmetrical flow patterns around the wings, which is not representative of true forward flight, where an appreciable forward velocity is combined with a high pitch angle. This will result in upper and lower wings experiencing distinctly different aerodynamic conditions. Tethered flow visualization experiments have been carried out on a different DelFly version at relatively small pitch angles [16], whereas [17] reports on preliminary results of a setup intend to enable free-flight wake visualization by controlling the MAV at a fixed position in the exit of a large wind tunnel. The current investigation takes a different and novel approach by aiming to visualiz...

Large-scale volumetric flow visualization of the unsteady wake of a flapping-wing micro air vehicle

“…However, the use of Taylor's hypothesis fails to represent the wake at more realistic flight conditions, where the angle of attack is quite high and the interaction between the velocities induced by the wings and the free stream is too complex to be accounted for by a simple uniform convection model. Free-flight visualizations of the DelFly (del Estal Herrero et al 2018;Karasek et al 2019) give, furthermore, clear evidence that in slow forward flight, the wake is predominantly aligned along the body axis direction, making the assumption of convection in the free stream direction unrealistic.…”

“…The above considerations motivate the (large-scale) volumetric imaging to properly capture the longitudinal development of the wake, under realistic conditions that are representative of actual flight. With the wing span of the Delfly being ~ 30 cm and typical wake structures ~ 20-25 cm in length (del Estal Herrero et al 2018, see also Sect. 3.3), the minimum volume requirements would amount to around 20,000 cm 3 .…”

“…Large-scale experiments using HFSB have been reported, amongst others, by Kühn et al (2011) using tomo-PIV in a convection cell (56,000 cm 3 ) and Huhn et al (2018) using Lagrangian particle tracking of a jet flow at 4 m/s (54,000 cm 3 ). Earlier experiments have shown that the use of HFSB is not detrimental to the operation of the DelFly (del Estal Herrero et al 2018), although occasional cleaning of the wings is required, which, therefore, makes it a feasible option for the current purpose as well. In the present investigation, the particular strategy to achieve the required large-scale measurement volume to visualize the entire wake extent of the DelFly is combining the HFSB seeding with a robotic PIV system based on coaxial illumination and imaging Jux et al 2018); more details are given in Sect.…”

“…Different strategies may be distinguished regarding the experimental procedure followed in the aerodynamic investigation of flapping-wing propulsion. The most direct approach captures the animal (or MAV), as it passes in its natural flying mode through the observation area while performing measurement of the footprint of the vortex structures generated during the flapping flight (Langley et al 2014;Gutierrez et al 2016;del Estal Herrero et al 2018). Although this puts relatively low demands on the test setup (no wind tunnel is needed, for example), it may require extensive training (animal) or accurate control (MAV) of the test object, to properly align the flight path with the observation area.…”

“…Although this puts relatively low demands on the test setup (no wind tunnel is needed, for example), it may require extensive training (animal) or accurate control (MAV) of the test object, to properly align the flight path with the observation area. Even then, the repeatability of the measurements is likely to be negatively affected by the inherent unpredictability of animal behaviour or the limited control authority on the MAV (del Estal Herrero et al 2018). For these reasons, like in general aerospace research, the common practice to investigate flapping-wing flight has been to keep the test object at a fixed position relative to the laboratory frame (and hence the measurement setup) while representing the forward flight effect using a wind tunnel facility.…”