Unsteady Thick Airfoil Aerodynamics: Experiments, Computation, and Theory

Abstract

An experimental, computational and theoretical investigation was carried out to study the aerodynamic loads acting on a relatively thick NACA 0018 airfoil when subjected to pitching and surging, individually and synchronously. Both pre-stall and post-stall angles of attack were considered. Experiments were carried out in a dedicated unsteady wind tunnel, with large surge amplitudes, and airfoil loads were estimated by means of unsteady surface mounted pressure measurements. Theoretical predictions were based on Theodorsen's and Isaacs' results as well as on the relatively recent generalizations of van der Wall. Both two-and three-dimensional computations were performed on structured grids employing unsteady Reynolds-averaged Navier-Stokes (URANS). For pure surging at pre-stall angles of attack, the correspondence between experiments and theory was satisfactory; this served as a validation of Isaacs theory. Discrepancies were traced to dynamic trailing-edge separation, even at low angles of attack. Excellent correspondence was found between experiments and theory for airfoil pitching as well as combined pitching and surging; the latter appears to be the first clear validation of van der Wall's theoretical results. Although qualitatively similar to experiment at low angles of attack, two-dimensional URANS computations yielded notable errors in the unsteady load effects of pitching, surging and their synchronous combination. The main reason is believed to be that the URANS equations do not resolve wake vorticity (explicitly modeled in the theory) or the resulting rolled-up unsteady flow structures because high values of eddy viscosity tend to "smear" the wake. At post-stall angles, three-dimensional computations illustrated the importance of modeling the tunnel side walls.

The unsteady lift of an oscillating airfoil encountering a sinusoidal streamwise gust

“…However, as far as we know, no experimental research has reported the existence of the sum frequency and difference frequency. A relevant study set the gust fluctuation frequency the same as the model oscillating frequency in experiments for consideration of engineering applications (Strangfeld et al 2015).…”

Response to Reviewers

“…The CFD solutions are based on unsteady Reynolds-averaged Navier-Stokes (URANS) equations. After conducting thorough verification and validations of multiple URANS solvers with multiple turbulence closure models, Rumsey showed that URANS-based models have a very limited ability to model cases with separation (Stangfeld et al, 2015;Rumsey, 2016). Furthermore, the tendency of URANS codes to smear vorticity will cause errors in the wake induction for cases in which the blades are modelled.…”

Large Eddy Simulation of Surging Airfoils at High Advance Ratio and Reynolds Number

“…Numerical simulations have also been used as an effective way to further understand these effects. Numerical investigations on a quasi-3D model include that of Visbal [3] and Gross and Wen [4], with a high-order finite difference based large eddy simulation (LES), and Strangfeld [5], with unsteady Reynolds-averaged Navier-Stokes (URANS) simulations. Hodara et al [6] investigated a pitching airfoil in a constant reverse flow (leading to a reserve dynamic stall) through a joint wind tunnel testing and numerical simulation based on a hybrid RANS-LES approach.…”