Experimental investigation of gust response in flapping wing aerodynamics

Abstract

This PhD thesis explores the generation and characterization of strong periodic transverse gusts for Micro Air Vehicle applications and investigates their effects on fixed, pitching-up, and plunging wings. The study aims to enhance our understanding of gust impacts on flapping wing aerodynamics. The literature is reviewed focusing on the theory of unsteady aerodynamics and the historical development of experimental gust generation methods, as well as the aerodynamic mechanisms of pitching-up and plunging wing. The experiments are conducted in the closed-circuit, large-scale, free-surface water channel located in the Trisonic Laboratories at the Department of Aeronautics and Astronautics Engineering of Istanbul Technical University. A flat plate made of Plexiglas is used as the gust generator. A connection rod connects the flat plate to a servo motor, which provides the pitching motion with a servo controller. This pitching motor is placed on a linear table, which provides the plunging motion with a servo motor as well. The motion system is controlled by a Labview VI code providing the motion signal and synchronization with the measurement systems. For the gust response experiments, a NACA0012 airfoil is mounted on an identical motion system from its quarter chord at the downstream of the gust generator. There are two chords of distance between the mounting axes of the gust generator and the airfoil. A rectangular end plate with an outward bevel is suspended from the top of the channel to reduce the free-surface effects and increase the aspect ratio of the wings acting as a symmetry plane. A Digital Particle Image Velocimetry system is used to acquire the vector fields in the wake of the gust generator. The flow is illuminated by a dual cavity Nd:Yag laser, and the water is seeded with silver-coated glass spheres of 10μm diameter. Two cameras are positioned under the water channel. The two double-frame images from the two cameras are stitched using an in-house code. The resulting PIV images are interrogated using a double frame, cross-correlation technique with a window size of 64×64 pixels and 50% overlapping in each direction. Force and moments acting on the NACA0012 airfoil are measured using a six-component Force-Torque sensor. The sensor is attached to the vertical cantilevered mounting beam of the test model, oriented with its cylindrical z-axis normal to the pitch-plunge plane. The data acquisition is accomplished by a separate Labview VI. All measurements are obtained with a 10kHz sampling rate, then the data is down-sampled to 100Hz with block averaging to smooth the signal. The flat-plate gust generator that moves with periodic functions in pitch and plunge axes produces distinct vortices to simulate transverse quasi-sinusoidal gusts. The motion parameters are targeted for the generation of strong gusts. The gust characterization methodology by in-depth spectral analysis of the wake is the pivotal point of this study to produce gusts with the desired intensity and characteristics. Instead of a point or a line in the wake, this approach examines the whole flowfield in the wake of the gust generator, where the airfoil will be present at the next phase of the study. The spectral analysis consists of an FFT analysis of the velocity field to obtain the predominant frequency of the gust, followed by auto-spectral and cross-spectral analysis. The three gust types selected after this process are validated by vorticity plots and velocity profiles. Quantitative gust characterization parameters are calculated for these gusts, which are named as A, B, and C. It has been observed that Gust A and C exhibit approximately twice the amplitude compared to Gust B, while Gust A has a frequency that is twice as high as that of Gust B and C. Additionally, it has been noted that Gust A and B slightly increase the average velocity in the streamwise direction. Gust encounter investigation begins with a steady flow analysis of a NACA0012 airfoil at a Reynolds number of 10000 to establish a baseline for gust cases, which reveals continuous lift generation even at high angles of attack. Fixed-wing experiments are conducted at angle of attack values from zero to 45 degrees under gust conditions. The results confirm the quasi-sinusoidal nature of the selected gusts, with Gusts A and B enhancing lift at higher angles of attack due to LEV attachment during positive transverse velocity perturbation peaks. In contrast, Gust C, with its distributed smaller vorticities, does not replicate this effect. Analytical lift predictions using the Sears formula, compared with experimental data, show a good correlation up to 30 degrees of AoA, although some overprediction in lift delay occurs due to complex flow interactions. Gust encounter of a pitching-up wing with a ramp motion of two different angular velocities (7.5 deg/s and 45 deg/s) is explored. The encounter timing significantly influences the forces on the wing, with faster maneuvers causing more drastic changes due to gust timing. Gusts A and B exhibit more pronounced effects during fast maneuvers, consistent with the RMS plots from fixed-wing experiments. For plunging wing experiments, steady flow conditions are evaluated at four flapping frequencies, generating drag-producing, zero-drag, and thrust-producing wakes. Especially resonant frequencies of plunge motion with gust frequencies result in significant thrust variations, influenced by gust timing. Although high plunge frequencies mitigate gust effects, due to producing large forces of lift and thrust, gusts still produce notable variations compared to lower frequencies. In conclusion, this study contributes valuable insights into gust generation and characterization for MAVs. The frequency and timing of the gust, as well as the characteristics of its vortices are found to be important parameters of the gust encounters with flapping wings. Further studies are recommended to refine analytical models and explore additional experimental gust encounter scenarios for an enhanced understanding of MAV aerodynamics under gusty conditions.