Question 3 Figure Q3.1 below (reproduced from "RANS Simulations of Aerodynamic Performance of NACA 0015 Flapped Airfoil", Obeid, Jha and Ahmadi, Fluids 2(1), 2 (2017)) shows lift co- efficient plotted against angle of attack for a NACA0015 airfoil measured/calculated using various experimental and computational techniques. Lift Coefficient (CI) 1.4 1.3 1.2 1.1 1- 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Potential. Flow From Panel Method RANS Simulations at Re = 10 (Current Study) RANS Simulations at Re = 10 (Joslin et al., [29]) Experimental Data at Re = 10 (Rethmel [27]) Experimental Data at Re = 1.6x10 (Lasse & Niels, [30]) LES Simulations at Re = 10 (You & Moin, [2]) 13 14 15 16 17 18 19 20 4 5 6 7 8 9 10 11 12 Angle of Attack (Deg) Figure Q3.1: Comparison of lift coefficient C values for airfoil NACA0015 at 0 flap deflection as a function of angle of attack a at chord Re 106 with experimental and numerical values. = (a) Explain how a panel code operates. Why is the black line (potential flow from panel method) a straight line? (b) What is meant by a RANS simulation? Name a typical RANS model. (c) Suggest how the experimental lift coefficients might have been measured. (d) What physical effect causes the experimental and RANS/LES values to decrease above about a > 12? (e) This airfoil section is to be used for a wind turbine blade. Estimate the lift force on the blade if the chord is 1.5 m, blade length is 26 m, wind speed of 15 m/s, and the blade is set at a constant angle of attack of a = 7.5. You will need to justify which set of data you use from the graph. (f) A real wind turbine blade is typically tapered, and the blade twists from root to tip. Explain why this is the case. (g) A HAWT turbine with blades of length 26 m develops 1700 kW in a wind of 15 m/s with a blade tip speed ratio of 4.5. Calculate the rotational speed of the blades, and the power coefficient of the turbine. How does this compare with the Betz limit?