**Commenced**in January 2007

**Frequency:**Monthly

**Edition:**International

**Paper Count:**31107

##### Numerical Investigation of Dynamic Stall over a Wind Turbine Pitching Airfoil by Using OpenFOAM

**Authors:**
Mahbod Seyednia,
Shidvash Vakilipour,
Mehran Masdari

**Abstract:**

Computations for two-dimensional flow past a stationary and harmonically pitching wind turbine airfoil at a moderate value of Reynolds number (400000) are carried out by progressively increasing the angle of attack for stationary airfoil and at fixed pitching frequencies for rotary one. The incompressible Navier-Stokes equations in conjunction with Unsteady Reynolds Average Navier-Stokes (URANS) equations for turbulence modeling are solved by OpenFOAM package to investigate the aerodynamic phenomena occurred at stationary and pitching conditions on a NACA 6-series wind turbine airfoil. The aim of this study is to enhance the accuracy of numerical simulation in predicting the aerodynamic behavior of an oscillating airfoil in OpenFOAM. Hence, for turbulence modelling, *k-ω-SST* with low-Reynolds correction is employed to capture the unsteady phenomena occurred in stationary and oscillating motion of the airfoil. Using aerodynamic and pressure coefficients along with flow patterns, the unsteady aerodynamics at pre-, near-, and post-static stall regions are analyzed in harmonically pitching airfoil, and the results are validated with the corresponding experimental data possessed by the authors. The results indicate that implementing the mentioned turbulence model leads to accurate prediction of the angle of static stall for stationary airfoil and flow separation, dynamic stall phenomenon, and reattachment of the flow on the surface of airfoil for pitching one. Due to the geometry of the studied 6-series airfoil, the vortex on the upper surface of the airfoil during upstrokes is formed at the trailing edge. Therefore, the pattern flow obtained by our numerical simulations represents the formation and change of the trailing-edge vortex at near- and post-stall regions where this process determines the dynamic stall phenomenon.

**Keywords:**
CFD,
Unsteady Aerodynamics,
Wind turbine,
openFOAM,
moderate Reynolds number,
pitching oscillation

**Digital Object Identifier (DOI):**
doi.org/10.5281/zenodo.1132248

**References:**

[1] D. Shipley, M. Miller, M. Robinson, “Dynamic Stall Occurrence on a Horizontal Axis Wind Turbine Blade,” Technical Report, NREL/TP-442-6912, National Renewable Energy Laboratory, 1995.

[2] C. P. Butterfield, “Aerodynamic pressure and flow visualisation measurement from a rotating wind turbine blade,” in Proc. 8th ASME Wind Energy Sumposium, Houston, Texas, 1989.

[3] K. W. McAlister, L. W. Carr, W. J. McCroskey, “Dynamic stall experiments on the NACA0012 airfoils,” Technical Report, NASA, paper 1100, 1978.

[4] W. J. McCroskey, L. W. Carr, K. W. McAlister, S. L. Pucci, “An experiment study of dynamic stall on advanced airfoil sections,” Technical Report, NASA TM-84245, 1982.

[5] W. J. McCroskey, “The Phenomenon of Dynamic Stall,” Technical Report, NASA TM-81264, 1981.

[6] W. J. McCroskey, “Unsteady Airfoils”, Annual Review of Fluid Mechanics, vol. 14, pp. 285-311, 1981.

[7] L. W. Carr, “Progress in Analysis and Prediction of Dynamic Stall,” Journal of Aircraft, vol. 25, No. 9, pp. 6-17, 1988.

[8] R.A. Piziali, “2-D and 3-D oscillating wing aerodynamics for a range of angles of attack including stall.” Technical Report TR 94-A001. NASA; 1994.

[9] J. A. Ekaterinaris, F. R. Menter. “Computation of oscillating airfoil ﬂows with one-and two-equation turbulence models.” AIAA J. vol. 32(12), pp.2359–65, 1994.

[10] G. N. Barakos, D. Drikakis, “Unsteady separated flows over manoeuvering lifting surfaces,” Philosophical Transactions of The Royal Society, vol. 358, pp. 3279-3291, 2000.

[11] T. N. Nandi, J. Brasseur, and G. Vijayakumar, “Prediction and Analysis of the Nonsteady Transitional Boundary Layer Dynamics for flow over an Oscillating Wind Turbine Airfoil using the γ-Reθ ransition Model,” 34th Wind Energy Symposium. San Diego, California, USA.

[12] M. R. Amiralaei, H. Alighanbari, S. M. Hashemi, “An investigation into the effects of unsteady parameters on the aerodynamics of a low Reynolds number pitching airfoil,” Journal of Fluids and Structures, vol. 26, pp. 979-993, 2010.

[13] K. Lu, Y.H. Xie, D. Zhang, “Numerical study of large amplitude, nonsinusoidal motion and camber effects on pitching airfoil propulsion.” Journal of Fluids and Structures, vol. 36, pp. 184–194, 2013a.

[14] D. Poirel, V. Me´tivier, G. Dumas, “Computational aeroelastic simulations of self-sustained pitch oscillations of a NACA0012 at transitional Reynolds numbers.” Journal of Fluids and Structures, vol. 27, pp. 1262–1277, 2011.

[15] T. Lee, P. Gerontakos. “Investigation of flow over an oscillating airfoil.” Journal of Fluid Mechanics, vol. 512, pp. 313–341, 2004.

[16] G. Martinat, M. Braza, Y. Hoarau, G. Harran, “Turbulence modelling of the flow past a pitching NACA0012 airfoil at 105 and 106 Reynolds numbers,” J. Fluids Struct. vol. 24, pp. 1294–1303, 2008.

[17] S. Wang, D.B. Ingham, L. Ma, M. Pourkashanian, Z. Tao. “Numerical investigations on dynamic stall of low Reynolds number ﬂow around oscillating airfoils.” Comput Fluids vol. 39, pp. 1529–41, 2010.

[18] S. Wang, D.B. Ingham, L. Ma, M. Pourkashanian, Z. Tao. “Turbulence modeling of deep dynamic stall at relatively low Reynolds number.” J Fluids Struct, vol. 33, pp. 191–209, 2012.

[19] K. Lu, Y. H. Xie, D. Zhang, J. B. Lan. “Numerical investigations into the asymmetric effects on the aerodynamic response of a pitching airfoil.” Journal of Fluids and Structures, vol. 39, pp. 76–86, 2013b.

[20] K. Gharali K, D. A. Johnson, “Dynamic stall simulation of a pitching airfoil under unsteady freestream velocity.” J Fluids Struct, vol. 42, pp. 228–44, 2013.

[21] G. H. Yu, X. C. Zhu, Z. H. Du, “Numerical simulation of a wind turbine airfoil: dynamic stall and comparison with experiments”, Journal of Power and Energy, Vol. 224, No. 5, pp. 657-677, 2010.

[22] Z. Zhou, C. Li, J. B. Nie, Y. Chen, “Effect of oscillation frequency on wind turbine airfoil dynamic stall,” IOP Conf. Series: Materials Science and Engineering, Vol. 52, 2013.

[23] S. Nagarajan, S. Hahn, S. Lele, “Prediction of sound generated by a pitching airfoil: a comparison of RANS and LES.” In Proc AIAA/CEAS aeroacoustics conference, Cambridge, Massachusetts, 2006.

[24] Y. Kim, Z. T. Xie, “Modelling the effect of freestream turbulence on dynamic stall of wind turbine blades,” Computers and Fluids, Vol. 129, pp. 53-66, 2016.

[25] OpenFOAM. User guide 2.3.1. Technical Report. OpenFOAM ®, 2014; http://www.openfoam.com/documentation/user-guide/.

[26] E. Dumlupinar, V. R. Murthy, “Investigation of dynamic stall of airfoils and wings by CFD,” 29th AIAA Applied Aerodynamics Conference, Honolulu, Hawaii, 2011.

[27] A. F. M. Correa, T. P. Sales, D. A. Rade, F. J. Souza, “Study of the flow over an oscillating NACA0012 airfoil,” National congress of mechanical engineering (CONEM 2014), Uberlandia, Brazil, Aug 10–15, 2014.

[28] P. R. Spalart, S. R. Allmaras, “A One-Equation Turbulence Model for Aerodynamic Flows,” Recherche Aerospatiale, No. 1, pp. 5-21, 1994.

[29] F. R. Menter, “Two-equation eddy-viscosity turbulence models for engineering applications,” AIAA Journal, Vol. 32, No. 8, pp. 1598-1605, 1994.

[30] H.R. Karbasian, J.A. Esfahani, E. Barati, “Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions.” Wind Energy 2014.

[31] H. Jasak, Z. Tukovic, “Automatic Mesh Motion for the Unstructured Finite Volume Method,” Transactions of FAMENA, Vol. 30, pp. 1-20, 2006.

[32] M. Lin, H. Sarlak, “A comparative study on the flow over an airfoil using transitional turbulence models,” AIP Conference Proceedings 1738 (1), 030050, 2016.