code based on OpenFOAM to predict the unsteady ... is represented by the code[2]: ... April 2, 2013. http://www.openfoam.com/features/creating-solvers.php.
Flow simulation over 2D airfoil using OpenFOAM Background With the growing size and cost of modern wind turbines it is important that the aerodynamic tools are improved in accuracy and are capable of predicting well the time dependent loads in yaw, wind shear and dynamic inflow. In the IEA-MEXNext project, a large validation study was carried out where different codes of increasing complexity were compared with an extensive of experimental wind tunnel results from the MEXICO (Model Experiments in Controlled environment) model rotor.
PhD Candidate: Ye Zhang Department: AWEP Section: Wind Energy Supervisor: Gerard van Bussel Promoter: Hester Bijl Start date: 7-9-2012 Funding: China Scholarship Council
The methodology and procedures for the project are described below: a. Verification and validation of flow simulation over 2D airfoil used for wind turbine blades. b. 3D blades for Mexico wind turbine blades, compared with different turbulence models and transitional models. c. Rotating wind turbine without tower and nacelle. d. Set up full 3D rotating OpenFOAM code. e. Validation calculation cases for the MEXICO rotor and fine-tune the code further.
Project Objectives The project is aim to develop a fully three dimensional CFD code based on OpenFOAM to predict the unsteady aerodynamic loads on wind turbine rotors, in conditions of axial flow, yawed flow and dynamic inflow. Meanwhile, the code will be developed such that CFD/FSI (Fluid Structure Interaction) will be possible in a later stage. After some verification and validation exercise, the code will be used in MEXNext II project, where present MEXICO experiment data are used for further validation.
2D airfoil flow simulation:
The flow over NACA 63618 airfoil which is used in wind turbine blade is investigated using OpenFOAM, the steady incompressible solver simpleFoam and k-Omega SST turbulence model from Menter[3] are chosen for this simulation. Pressure and velocity coupling for Navier-Stokes equation is solved by SIMPLE algorithm. The convective term is discrete with upwind scheme. Central scheme is used to discrete diffusion term and gradient term with second order.
Aerospace Engineering
Two types of grids are generated regarding of two cases simulation: low Reynolds number and high Reynolds number simulation. The previous case set y+ less than 1 in order to simulate sub layer flow in boundary layer. The later one used wall function for reducing computation time.
Mesh independency study
The result focus on at different angle of attack, the lift coefficient and drag coefficient compared with experimental data, which are defined as:
Cl
L 1 U 2 A 2
Cd
D 1 U 2 A 2
Lift coefficient comparison
MEXICO wind turbine rotor[1] Research Methodology:
An open source computational software package, OpenFOAM is used for the flow simulation over wind turbine. Many solvers and models are compiled in it and high-level syntax written by C++ language, which means the code can easily be modified further.
Sketch of computational domain
Drag coefficient comparison
The momentum equation:
U UU U p t
is represented by the code[2]: solve ( fvm:: ddt (rho, U) +fvm:: div (phi, U) - fvm:: laplacian (mu, U) == - fvc:: grad (p) );
Mesh around airfoil surface Boundary conditions: Inlet : fixedValue Outlet: Zerogradient Airfoil : no slip wall
The lift coefficient agrees very well with experimental results before a.o.a (angle of attach) 10 degree, but deviation become larger with a.o.a increases. The flow becomes separate and stall in that region.
The drag coefficient obtained in OpenFOAM is quite higher than the experiment data, even at 0 a.o.a. That is because for this kind of airfoil, the flow is mostly dominated Result and discussion: by laminar flow, but the k-omega SST turbulence model is Mesh independency is studied by different nodes of mesh, assumed fully turbulent flow, which results in high drag.
and the result is listed as followed:
Further Work: A .Do simulation with transitional model, and compared with ‘local flow’ pressure distribution with proper airfoil. B. Try different turbulence model and do 3D simulation.
Reference [1]-“ECN: Aerodynamics Wind Turbines.” Accessed April 2, 2013. http://www.ecn.nl/news/newsletter-en/2009/december-2009/aerodynamics-wind-turbines/. [2]-“Creating Solvers.” Accessed April 2, 2013. http://www.openfoam.com/features/creating-solvers.php. [3] Menter, F R. “Two-equation Eddy-viscosity Turbulence Models for Engineering Applications.” AIAA Journal 32, no. 8 (1994): 1598–1605. doi:10.2514/3.12149.
