Experimental study on discrete vortex gust encountered by a flat plate at 45˚ wing sweep

Abstract

ircraft operating in atmospheric environments are routinely subjected to unsteady aerodynamic disturbances caused by gusts, vortices, and turbulent flow structures. These disturbances can severely affect flight stability, maneuverability, and structural integrity, particularly for small-scale and low-inertia aerial vehicles such as Micro Air Vehicles (MAVs) and Unmanned Aerial Vehicles (UAVs). Despite the importance of gust response, the vast majority of existing research has concentrated on two-dimensional wings. While a considerable body of work exists on gust response for flat plates at no wing sweep, studies incorporating with wing sweep, especially at moderate to high sweep angles, are still rare. The influence of wing sweep introduces additional complexities such as spanwise flow development, asymmetric vortex breakdown at high sweep angles, and directional sensitivity to the incoming gust, which cannot be captured by purely two-dimensional models. Given the increasing demand for agile and gust-resilient aerial platforms, there is a strong need to experimentally study these interactions under controlled laboratory conditions. The primary objective of this study is to experimentally investigate the aerodynamic response of a flat plate wing model with a 45° sweep angle when subjected to discrete vortex gusts. The scope of this work encompasses both quantitative flow visualization and aerodynamic force assessment, enabling a unified view of the fluid-structure interaction under unsteady gust environments. The results are expected to clarify the influence of sweep geometry, symmetry plane mirror effect, and tip geometry, thereby contributing to gust modeling strategies for swept-wing designs. To explore the unsteady aerodynamic response under controlled gust conditions, a series of water channel experiments are conducted using a gust generation mechanism. The gusts are generated via controlled rotational motion of a flat-plate gust generator, simulate localized, transient disturbances that resemble vortex gust phenomena encountered in real-world scenarios. By varying key parameters such as the vertical distance between the gust generator and the wing (Δy) and the half-rotation duration of the generator (T), the study aims to systematically characterize how the gust encounter, trajectory, and the sweep angle affect the resulting force dynamics and flow structures. Three distinct wing configurations are examined: a no-swept flat plate, a 45° swept flat plate, and a 45° swept plate with a sharp tip modification. For each configuration, multiple test cases are executed across two vortex trajectories (h/c), which are adjusted by varying Δy, and three half-rotation durations (T), resulting in a comprehensive matrix of 6 cases. High-resolution force measurements and Digital Particle Image Velocimetry (DPIV) analyses are conducted to quantify vorticity fields in conjuction with lift coefficients. Reynolds number of 10,000 is maintained throughout all experiments. This experimental setup enables both high temporal resolution of aerodynamic loads and direct visualization of vortex-wing interaction mechanisms, forming a strong foundation for interpreting the complex, transient flow phenomena associated with discrete gust encounters. In this study, discrete vortex gust encounter experiments are conducted on several wing configurations, including a no-swept flat plate wing and two swept wing models with differing tip geometries: a sharp tip and a tip aligned with the freestream. Additionally, two submergence levels are tested for the swept wing models to examine their effect on gust response. Comparison between standard submergence level, where the water level coincides with the wing's symmetry axis (root section), and a deeper submergence including 15% of the opposite half-span (creating a partially V-shaped model), revealed no significant influence on the aerodynamic response. Similarly, the tip geometry difference demonstrated negligible differences in aerodynamic response. While the variations in submergence level and tip geometry showed minimal impact under the specific test conditions, the influence of sweep angle emerged as a critical factor. The sweep angle introduced a notable time delay in the lift response to vortex gust encounters and a reduction of lift amplitude compared to the no-swept configuration. Additionally, a time shift in lift response observed along the wing span indicates that different sections of the swept wing experienced the gust at slightly different times. This phase lag was confirmed by comparing instantaneous flow visualizations along the span. These observations support the value of simplified modeling techniques, such as time-shifted lift profiles, to approximate complex gust interactions on swept configurations.