Flow structures, loadıng and mıtıgatıon for a large-scale dıscrete vortex gust encounter

Abstract

Large-scale gusts have significant implications for both military and civil aviation sectors, with their effects extending from operational efficiency to safety and aircraft design. In military aviation, large gust encounters are critical for low-speed, low-altitude missions such as surveillance, search-and-rescue, or tactical operations, where aircraft, including helicopters and drones, must navigate in adverse atmospheric conditions. This thesis is a synopsis of the ongoing experimental investigations on the discrete vortex gust encounters started in 2018 (Engin et al., 2018) aiming to simplify the problem of the mitigation of continuous vortex gust encounters and to gain insight on the flow physics of vortex gusts. Experiments were conducted in the free-surface water channel of the ITU Faculty of Aeronautics and Astronautics Trisonic Laboratories. The freestream velocity used throughout the investigations is kept constant to be 0.1 m/s which corresponds to the Reynolds number (Re) of 10,000. A flat plate of aspect ratio, AR = 4, is used as a test model. A vortex gust was created by clockwise half-rotation of an upstream gust generator plate. The major parameters of the study are the duration of the half rotation (T) of the gust generator plate and the transverse distance (∆y) with respect to the free-stream between the gust generator and the wing. The former affects the propagation speed of the gust and the latter controls the trajectory of the generated negative vortex gust. Three values for the half rotation speed and two transverse distances are considered. T = 4 s and ∆y = −36 mm constitutes the base case. Quantitative flow field images are obtained using a 2D-DPIV (2D Digital Particle Image Velocimetry) system in conjunction with simultaneous force/torque measurements. A vortex gust characterization algorithm is developed; it detects the vortex core, outputs the vortex trajectory, consequently the vortex gust propagation speed; and using vortex models it also determines the vortex size and strength. Therefore, the characterization yields the gust ratio and the gust encounter width. The vortex is found to be characterized as a Lamb-Oseen vortex. The gust ratio of the base case is approximately equal to 1 and the encounter width approximately 0.4 chord (c). The vortex gust propagation speed is also determined in average to be equal to half of the freestream irrespective of gust generator rotation speed and the transverse distance between the gust generator and the model. The gust generator rotation speed does not actually control neither the size nor the strength of the vortex gust. It only affects the propagation speed of the vortex. This is actually equivalent to the velocity profile gradient distance of the transverse gusts which is used in the civilian requirements for certification or in the military standards. A MatLab code is also developed to assess the correlation between the effective angle of attack and the lift coefficient variation. There exists a phase difference in time between the effective angle of attack results and the lift coefficient variation. This difference arises because the effective angle of attack is calculated at a pre-determined area located in front of the leading edge of the wing, before the flow reaches the wing model. The shifted effective angle of attack variation with the lift coefficient time traces shows a highly correlated match for all cases. The effective angle of attack measurements has a high potential to be used in gust mitigation as the effective angle of attack variation leads and dictates the lift variation. Since it is challenging to perform real-time angle of attack measurements in water, several experiments were conducted to show the applicability of the effective angle of attack for gust mitigation. Force data was used to generate a flap detection profile to mitigate the effects of the gust which takes the virtual camber and added mass effects into account with an ordinary differential equation. This thesis is a synopsis of the ongoing research on the discrete vortex gust encounters started in 2018 (Engin et al., 2018) aiming to simplify the problem of the mitigation of continuous vortex gust encounters and to gain insight on the flow physics of vortex gusts.