On LES Assessment in Massive Separated Flows: Flow Past a NACA Airfoil at Re = 50000
Mechanisms of quasi-periodic flow oscillation observed in airfoils near stall and stall behaviour, which affect airfoil efficiency, remain still not fully understood. In this sense, numerical simulations using subgrid-scale models can be useful in order t
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Abstract. Mechanisms of quasi-periodic flow oscillation observed in airfoils near stall and stall behaviour, which affect airfoil efficiency, remain still not fully understood. In this sense, numerical simulations using subgrid-scale models can be useful in order to shed some light in the underlying complex physics. However, the assessment of these models is an important aspect to consider for obtaining credible results. In this work, two LES models are compared with results from direct numerical simulation. The results have shown that LES models are capable of capturing the complex flow phenomena present at these AOA such as the low frequency fluctuations near stall and the vortex shedding produced at higher AOAs.
1 Introduction Stall on airfoils is caused by massive separation of the flow leading to a sharp drop in the lift and an increase in the drag over the airfoil surface. NACA0012 airfoil exhibits two types of stall. A trailing-edge stall at all Reynolds numbers and a combined leading-edge/trailing-edge stall at intermediate Reynolds number. The latter shows the combined presence at stall of a turbulent boundary layer separation moving forward from the trailing-edge and a small laminar bubble in the leading-edge region failing to reattach which complete the flow breakdown. An oscillating situation I. Rodr´ıguez · A. Baez · C.D. Perez-Segarra Heat and Mass Transfer Technological Center (CTTC), Technical University of Catalonia, Terrassa, Spain e-mail: [email protected] O. Lehmkuhl TermoFluids S.L. e-mail: [email protected] University of Catalonia e-mail: [email protected] (CTTC), Technical University of Catalonia e-mail: [email protected] M. H¨olling et al. (eds.), Wind Energy – Impact of Turbulence, Research Topics in Wind Energy 2, c Springer-Verlag Berlin Heidelberg 2014 DOI: 10.1007/978-3-642-54696-9_24,
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is often noticed near stall angles. However, mechanisms of quasi-periodic oscillation observed near stall and stall behaviour, which affect airfoil efficiency, remain still not fully understood. In this sense, Large-Eddy Simulations (LES) have been performed. Two different models have been considered, the Wall-adapting Local-eddy diffusivity model within a variational multi-scale framework (VMS-WALE)[1, 2] and the QR-model [3]. The results have been compared to Direct Numerical Simulations (DNS) and after that, LES models have been used to study the stall phenomenon. Thus, the objective of this work is two-fold: i) to assess the performance of these LES models for predicting the flow with massive separation and, ii) to study the flow behaviour at near stall and stall angles in order to shed some light into the physics of this complex flow. To do this, we have performed LES of the flow past a NACA0012 at Reynolds number of Re = 5 × 104 and at angles of attack (AOA) up to 12◦ .
2 Mathematical and Numerical Model The methodology for solving the flow past bluff bodies which exhibits massive separation has been described in detail in [4, 5]. The governing equations
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