پيش بيني تاثيرات ديواره‌ي تونل باد بر جريان حول ايرفويل نوساني به‌كمك حل عددي

NUMERICAL ESTIMATION OF WALL EFFECTS ON FLOW CHARACTERISTICS OF PITCHING AIRFOILS

هدف اين تحقيق ارايه‌ي روشي براي اصلاح نتايج تونل باد از طريق تخمين عددي خطاي ناشي از حضور ديواره در آزمون‌ هاي ايرفويل نوساني است. براي حل جريان از يك شبكه‌ي بي سازمان با سلول‌هاي منظم در لايه‌ي مرزي استفاده شده است. شبكه متناسب با حركت جسم حركت مي‌كند. معادلات حاكم بر جريان دوبعدي لزج مغشوش با استفاده از يك روش حجم كنترل با الگوريتم ضمني دو‌زمانه حل شده است. براي گسسته‌سازي مكاني معادلات از روش مبتني بر اختلاف مركزي استفاده شده كه براي جلوگيري از نوسانات ناخواسته ناشي از روش فوق از ترم هاي استهلاك مصنوعي بهره‌برداري شده است. ترم‌هاي اغتشاشي نيز با روش دومعادله‌يي مدل‌سازي مي‌شود. نتايج حاصله با داده‌هاي تجربي مقايسه شده است. همچنين تاثيرات ناشي از وجود ديواره بر مقادير ضريب نيرو و توزيع فشار روي سطح ايرفويل در حالت‌هاي جريان‌هاي پايا و ناپايا بررسي شده است

Implementing wind tunnel tests is still required for better understanding of the flow characteristics of aerodynamic shapes. The finite scales of wind tunnels usually lead to models smaller than their prototypes. Thus, in order to match the Reynolds Number, wind tunnel velocity should be increased. However, engineers prefer large models because increasing speed is not always possible. Enlarging models, compared to test section scales, may introduce higher experimental error. Different approaches for estimating these errors are given. But, in the range of oscillatory airfoils, there is not any well known method. The purpose of this study is to provide a numerical method for estimation of wind tunnel wall errors in the case of oscillating airfoil tests. In this study, wall effects on the aerodynamic characteristics of a pitching airfoil have been numerically investigated. The unstructured mesh is used to discretize the flow field. In the vicinity of the airfoil surface, and up/down walls, boundary layer grids are used. By maintaining the quality of the grid, the grid moves with the movement of the airfoil. An edge-base data structure is used, in which fluxes are also calculated in an edge-by-edge manner. The governing equations are two-dimensional compressible Navier-Stokes equations. Finite-volume spatial discretization based on central differences is used. To avoid unwanted spurious oscillations, artificial dissipation terms are applied. A dual-time implicit method is applied for time discretization of equations. In order to model the turbulent terms, a two-equation turbulence model is used. The wall function approach is applied to investigate near wall treatment. To optimize and gain the best grid, a grid sensitivity test is applied. Several test cases are applied and the results are compared with experimental data. Results show that the presence of wind tunnel walls not only increases the lift and drag coefficients of the airfoil, but also affects flow separation on the surface of the airfoil. Flow inside the wind tunnel, in both steady and unsteady regimes, has been numerically simulated. Also, the sensitivity of the numerical results to the distance of wind tunnel walls, for a spatial case, is obtained. Wall effects on lift, drag and pressure coefficients are investigated separately. Finally, the corrections are presented and the results are compared with free-stream results.