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2 nd eureca 2014 – Evaluation of The Measurement Accuracy of the Lift of NACA 0012 as a Function of Angle of Attack at Subsonic Speed Evaluation of The Measurement Accuracy of the Lift of NACA 0012 as a Function of Angle of Attack at Subsonic Speed Lim Wei Kit 1 , Abdulkareem Sh. Mahdi Al-Obaidi 2* Department of Mechanical Engineering, School of Engineering, Taylor’s University Malaysia *Corresponding email: [email protected] Abstract— This paper presents the evaluation of the measurement accuracy of the lift of NACA 0012 in a subsonic wind tunnel. Validation of the data acquisition system of Taylor’s subsonic wind tunnel was done by comparing the lift coefficient of NACA 0012 with theoretical results as well as computational fluid dynamics model to the lift coefficient obtained from the wind tunnel. Lift coefficient obtained from the CFD supports the theoretical results whereas the lift coefficient obtained from wind tunnel experiments has a larger percentage error. Keywords— Wind-tunnel, airfoil, lift coefficient, computational fluid dynamics. 1. Introduction Airfoil performance at subsonic speeds leads to a great impact to flying bodies ranging from manned aerial vehicles to unmanned aerial vehicles. In the wind tunnel laboratory of Taylor’s University in Malaysia, particularly in the area of fluid mechanics and aerodynamics, a wind tunnel measurement system to obtain aerodynamic data needs to be verified. Based on the methodology by Barlow et al., this paper presents the evaluation of the measurement of lift on an airfoil as a function of angle of attack at subsonic speeds using Taylor’s University subsonic wind tunnel [1]. Verification was performed with a NACA 0012 airfoil and data obtained was compared with published as well as numerical data for verification [2]. The wind tunnel data acquisition equipment is verified to determine if it is suitable for measuring aerodynamic performance of airfoils for education purposes. 2. Experimental Approach using Wind Tunnel NACA 0012 airfoil performance measurements described below were conducted in the subsonic Taylor’s wind tunnel, which is an open-type wind tunnel with a contraction ratio on 3.4:1. The rectangular rest sections measures 0.3 m by 0.3 m in cross section and 0.885 m in length. Test section speeds can be varied between 3.33 m/s up to 32.3 m/s by a 0.63 m in diameter fan, which is powered by a 3HP. 415V/50Hz motor. For the case of Taylor’s wind tunnel experiments only three types of dimensionless parameters are considered for matching of dynamic similarity, which are Reynolds number, Mach number and Froude number. [3] Matching of Mach and Froude number is not an important parameter in this experiment as it is a subsonic experiment and model motion is not involved therefore dynamic similarity can be achieved by equating the Reynolds number of both model and prototype [4]. ! = ! ! ! ! = ! = ! ! ! ! where is the density of the air, is the velocity of model or prototype, is the characteristic length of model or prototype and is the dynamic viscosity of air. Through this equation, the required velocity for the wind tunnel can then be obtained. All obtained data is collected manually and then processed using a software where the general equations used to calculate the coefficient of lift is as follows; ! = ! 0.5 ! ! ! ! 2.1 Experimental Set-up In this experiment, the NACA0012 airfoil was tested at a Reynolds number of 200,000, which corresponds to a wind tunnel velocity of 26.278 m/s where ! ! ! !"#"!"$%" = 1.15. Figure 1 shows the mounting used for the airfoil in the test section of the wind tunnel and experiments were conducted starting from an angle of attack of -20° up to 20° with an interval of 5° Fig 1 Mounting of the NACA0012 airfoil in the wind tunnel. 3. Numerical Approach using Computational Fluid Dynamics (CFD) ANSYS FLUENT 14.0 software was used to simulate the aerodynamics of the NACA 0012 airfoil to obtain the lift coefficient and result obtained is used to cross check the theoretical results. The turbulence model chosen for this numerical approach is the Spallart- Allmaras turbulence model and this is due to the simplicity of the equation thus leading to a shorter simulation time. The Spallart- Allmaras turbulence model consists of a single equation, which is modified specifically for the use airfoil simulation. [5] Figure 2 shows the full scale drawing of the cross section of the NACA 0012 airfoil and the meshing conditions surrounding the airfoil. Based on FLUENT, the mesh consists of 160,740 nodes and edge sizing with a bias factor of 6000 and 170 divisions was selected at the far end of the far-field domain to increase numerical accuracy of the results as mesh nodes becomes finer as it shifts nearer to the wall of the airfoil. For most aeronautical applications where the magnitudes of viscous and turbulent effects are similar it is preferable to keep the range of y + , smallest as possible between 5 and 30. [6] This is to ensure that the simulation runs as close as possible to the airfoil. For this simulation, the y + value denoted by FLUENT is 12. Fig. 2 Domain and meshing of the NACA 0012 airfoil.
