Use of lesp (Leading edge suction parameter) and effective angle of attack measurements for gust mitigation

Abstract

This master thesis aims to investigate the initiation of flow separation at the leading-edge during gust encounters by analyzing parameters such as, lift and drag forces, leading-edge suction parameter (LESP), and effective angle of attack, in order to determine the most accurate and practical indicator for the initiation. Experimental method is used for this investigation, which includes measuring forces acting on wing models and using Digital Particle Image Velocimetry (DPIV) technique for flow visualization and vector field analysis. Additionally, numerical method is used to prove applicability and accuracy of the total force measurements from experimental method conducted in this study. For that, sectional lift and drag forces, which are calculated from computationally obtained pressure and shear stress distributions, are compared with total forces obtained from both Computational Fluid Dynamic (CFD) and experimental studies. The experiments are conducted in the closed circuit, large scaled, free-surface water channel located in the Trisonic Laboratory of the Faculty of Aeronautics and Astronautics of Istanbul Technical University (ITU). For all experiments, free-stream velocity is set to 0.1 m/s, therefore the Reynolds number corresponding to the wing models' chord length is determined as 10,000. The wing models, also known as test models, used for this study are a rectangular sharp-edged flat plate and a NACA0012 airfoil, both with AR=4. A gust generator, which is a rectangular flat plate with wedge shape edges, is positioned upstream of the wing model. During the experiments, force measurements are obtained from a force/torque sensor connected to the leading-edge of the wing model, with 1 kHz sample rate. Simultaneously with the force measurements, quantitative flow field images are recorded using DPIV system. CFD simulations are performed using the commercial software package of ANSYS Fluent® and k-ω SST-Delayed Detached Eddy Simulation (DDES) hybrid Reynolds Averaged Navier-Stokes (RANS)/Large Eddy Simulation (LES) method had been employed. The wing models and the gust generator are modelled with identical geometrical characteristics and dimensions with the experimental setup. The computational domain is discretized with a combination of tetrahedral and five-sided triangular prismatic mesh. From the CFD analysis, pressure and shear stress distributions around the cross sections located on quarter and mid span are obtained, along with total lift and drag forces acting on the wings. Additionally, for flow visualization, two-dimensional streamline and vorticity images and three-dimensional velocity images are obtained. During the experiments, a negative vortex gust is generated with the half rotation of the gust generator in the clockwise direction. While both of the wing models are stationary with zero angle of attack, the gust generator is subjected to three different half rotation periods (T=3, 4, and 6 s/(½rev)) and two different cross-flow distances (∆y= –36 & –111 mm) with respect to the wing models. Each case is repeated five times and the results are averaged to reduce the noise and increase the accuracy. The CFD simulations are only conducted to base case (T=4 s/(½rev) & ∆y= –36 mm) of the research for both wing models. Initially, the sectional lift and drag forces for mid-span and quarter-span cross sections are compared with total lift and drag acting on the wing models, from experimental and CFD analysis. To better interpret the results, the three-dimensionality effect of the flow is studied using CFD data. Additionally, Cf distributions from the upper and lower surfaces of the wing models are compared with the vorticity images from CFD and experimental. This allowed for the determination of detecting vortex formation through Cf and the observation of similarity with experimental data. After discovering a strong correlation between the sectional results and the forces acting on the wing, the study proceeds to a comprehensive investigation of flow separation prediction parameters. In this comprehensive investigation, parameters such as CL, CD, LESP and effective angle of attack are compared to determine the most accurate and applicable indicator for the initiation of flow separation at the leading-edge. Critical instances from DPIV are studied alongside these parameters. It is observed the location of the stagnation point and the formation of a leading-edge vortex are crucial steps in the development of flow separation at the leading-edge. The location of the stagnation point is linked to the effective angle of attack and CL. Since LESP is calculated from the axial force, a mostly symmetrical behavior is observed between LESP and CD results. When the instantaneous LESP reached to the critical LESP value, LESP generally indicated the initiation of flow separation at the leading-edge.