Abstract
A symmetrical NACA 0018 airfoil is often used in such applications as small-to-medium scale vertical-axis wind turbines and aerial vehicles. A review of the literature indicates a large gap in experimental studies of this airfoil at low and moderate Reynolds numbers in the previous century. This gap has limited the potential development of classical turbulence models, which in this range of Reynolds numbers predict the lift coefficients with insufficiently accurate results in comparison to contemporary experimental studies. Therefore, this paper validates the aerodynamic performance of the NACA 0018 airfoil and the characteristics of the laminar separation bubble formed on its suction side using the standard uncalibrated four-equation Transition SST turbulence model and the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. A numerical study was conducted for the chord Reynolds number of 160,000, angles of attack between 0 and 11 degrees, as well as for the free-stream turbulence intensity of 0.05%. The calculated lift and drag coefficients, aerodynamic derivatives, as well as the location and length of the laminar bubble quite well agree with the results of experimental measurements taken from the literature for validation. A sensitivity study of the numerical model was performed in this paper to examine the effects of the time-step size, geometrical parameters and mesh distribution around the airfoil on the simulation results. The airfoil data sets obtained in this work using the Transition SST and the k-ω SST turbulence models were used in the improved double multiple streamtube (IDMS) to calculate aerodynamic blade loads of a vertical-axis wind turbine. The characteristics of the normal component of the aerodynamic blade load obtained by the Transition SST approach are much better suited to the experimental data compared to the k-ω SST turbulence model.
Highlights
The aerodynamic performance of almost all micro-air-aircraft, high-altitude unmannedair-vehicles, compressor blades in turbomachines, and wind turbines is strongly influenced by laminar separation bubbles, which may form at low Reynolds numbers [1,2,3]
This subsection presents the results of the aerodynamic force coefficients for a series of angles of attack from 0 to 11 degrees
The results for the Transition Stress Transport (SST) turbulence model are presented in graphs (Figures 7 and 8) and in Table 3, so that they can be used effectively by every reader
Summary
The aerodynamic performance of almost all micro-air-aircraft, high-altitude unmannedair-vehicles, compressor blades in turbomachines, and wind turbines is strongly influenced by laminar separation bubbles, which may form at low Reynolds numbers [1,2,3]. The lift, moment and stall behavior of airfoils can be affected by a laminar separation bubble [4,5]. The presence of bubbles, which are an attribute of low Reynolds numbers, can cause problems during wind tunnel measurements due to undesirable scale effects [6,7]. Simplified momentum methods and vortex models are still widely used engineering tools for analyzing the aerodynamic performance of vertical axis wind turbines [16].
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