The thicker NACA 0025 airfoil, more » where the thickness is 25 percent of the chord length, performed normally in the wind tunnel test. Two airfoil sections, NACA 0012 and NACA 0025, of 7.39 cm chord length (i.e., leading to trailing edge dimension) were shaped from curved laminated Lauan blanks which had the grain oriented along the blade arc. Although the tensile strengths of all candidate unidirectional laminates (Lauan, Monalava, poplar and maple) exceeded the induced blade stresses, Lauan was chosen for wind tunnel testing based on cost and dimensional stability. The manufacture, testing and tensile stress analysis of laminated wooden blades are described. The use of laminated Lauan plywood in a 2-meter-diameter, 3-bladed Darrieus wind turbine is described. Furthermore, the potential benefit of reducing the airfoil drag is clearly illustrated by the presentation of the individual contributions of lift and drag to power. The airfoil shapes considered are the conventional airfoils NACA 0018 and NACA 0021, and the SNLA 0018/50 airfoil designed at Sandia. To achieve this goal, the streamtube model of Paraschivoiu (1988) is used to predict the performance of VAWTs equipped with blades of various airfoil shapes. This technical brief illustrates the benefits and losses resulting from using NLF airfoils on VAWT blades. These features are similar to those of Natural Laminar Flow airfoils (NLF) and gave birth to the SNLA airfoil series. Objectives formulated for the blade profile were: modest values of maximum lift coefficient, low drag at low angle of attack, high drag at high angle of attack, sharp stall, and low thickness-to-chord more » ratio. As these blades were designed for aviation applications, Sandia National Laboratories developed a family of airfoils specifically designed for VAWTs in order to decrease the Cost of Energy (COE) of the VAWT (Berg, 1990).
Most VAWTs currently operating worldwide use blades of symmetrical NACA airfoil series. The successful design of an efficient Vertical Axis Wind Turbine (VAWT) can be obtained only when appropriate airfoil sections have been selected. This present simulations have shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke.= , The trend of the lift coefficient hysteresis plot with the experimental data for the Re = 1 million case is also similar. The net lift coefficient for the Re = 100,000 case during downstroke matches with the corresponding experimental data, the present study under-predicts the lift coefficient as compared to the experimental values at the start of downstroke and over-estimates for the remaining part of the downstroke. There is a fairly good match between the predicted and experimentally measured lift coefficient during the upstroke for both cases. The development of dynamic stall vortex, vortex shedding and reattachment as predicted by the present study are discussed in detail. The lift hysteresis plots of this simulation for both the configurations are compared with the corresponding experiments. The turbulence in the flow field is resolved using large eddy simulations with dynamic Smagorinsky model at the sub grid scale. and the amplitudes of pitching are 6 deg. The work presented in this thesis attempts to provide an understanding of the physics behind the dynamic stall process by simulating the flow past pitching NACA-0012 airfoil at 100,000 and 1 million Reynolds number based on the chord length of the airfoil and at different reduced frequencies of 0.188 and 0.25 respectively in a three dimensional flow field.