Numerical simulations of massive separated flows: flow over a NACA airfoil at moderate Reynolds numbers

This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 5º, 8º, 9.25º and  12º have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán-like vortex street in the airfoil wake.
All computed flows are around a NACA-0012 airfoil extended to include sharp trailing edge. Solutions are obtained in a computational domain of dimensions 40Cx40Cx0.2C with the leading edge of the airfoil placed at (0;0;0).  Computational meshes up to 50 million of control volumes have been used. Depending of the AOA, the flow separates forming a laminar separation bubble (AOA = 5º; 8º ) with further reattachment to the airfoil surface or fails to reattach forming a large separated zone (AOA = 9.25º; 12º). It has been observed that the separated flow is slightly different depending on the AOA. Indeed, coherent structures identified have shown that, in agreement with the experimental observations, at AOA = 5º the flow is in the subcritical mode, at AOA = 8º and 9.25º wake mode should correspond with the transitional one, whereas at AOA = 12º   supercritical mode is detected.

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Projects related: FI-2011-2-0016 and FI-2011-3-0003

Total hours awarded: 250 000 core hours at Marenostrum supercomputer and 156 000 core hours at Magerit CesVima supercomputer through RES – Spanish Network of Supercomputing

References
I. Rodríguez, O. Lehmkuhl, R. Borrell and A. Oliva. (2013). Direct numerical simulation of a NACA0012 in full stall. International J. of Heat and Fluid Flow. 2013. http://dx.doi.org/10.1016/j.ijheatfluidflow.2013.05.002

O. Lehmkuhl, I. Rodríguez, A. Baez, A. Oliva, C.D. Pérez-Segarra. (2013). On the Large-Eddy Simulations for the flow around aerodynamic profiles using unstructured grids. (2013) Computers&Fluids. http://dx.doi.org/10.1016/j.compfluid.2013.06.002

G. Colomer, R. Borrell, F.X. Trias, I. Rodríguez. (2013) Parallel algorithms for Sn transport sweeps on unstructured meshes. Journal of Computational Physics 232 118–135.  http://dx.doi.org/10.1016/j.jcp.2012.07.009

I. Rodríguez, O. Lehmkuhl, R. Borrell, A.Oliva. Direct numerical simulation of a NACA0012 airfoil with massive separation. In Direct and Large Eddy Simulation (DLES9) Workshop. Dresden. 2013.

O Lehmkuhl, I. Rodriguez, A. Baez, A. Oliva and C.D. Perez-Segarra. On the Large-Eddy Simulations for the flow around aerodynamic profiles using unstructured grids. Computers&Fluids. 2013.

O Lehmkuhl, I. Rodríguez, R. Borrell, A. Oliva. High-Performance computing of flows with massive separation: flow past a NACA 0012. ParCFD 2012.

I.Rodríguez, J. Calafell, O. Lehmkuhl, R. Borrell, Direct numerical simulation of a NACA0012 in full stall. Conference on Modelling Fluid Flow (CMFF’12). 2012

O. Lehmkuhl, J. Calafell, I. Rodríguez and A. Oliva. Large-Eddy Simulations of wind turbine dedicated airfoils at high Reynolds numbers. EUROMECH Colloquium 528. 2012. 

I. Rodríguez, O. Lehmkuhl, A. Baez and C.D. Pérez-Segarra. On LES assessment in massive separated flows: flow past a NACA airfoil at Re = 50000. EUROMECH Colloquium 528. 2012. 

A. Baez, O. Lehmkuhl, I. Rodríguez and C. D. Pérez-Segarra (2011) Direct Numerical Simulation of the turbulent flow around a NACA 0012 airfoil at different angles of attack. In Parallel CFD 2011. Barcelona.

O. Lehmkuhl, A. Baez, I. Rodríguez and C.D. Pérez-Segarra (2011). Direct numerical simulation and large-eddy simulations of the turbulent flowaround a NACA-0012 airfoil. In ICCHMT 7. Turkey.