Flows around wind turbines generally fall under the high Reynolds number and low Mach number regime. The high Reynolds number means large regions of the flow will be inviscid except for the boundary layers and wakes. The low Mach numbers imply that the flow remains incompressible. This combination of conditions have been exploited to develop a wide variety of numerical tools based on simplified Navier-Stokes equations like blade element momentum theory, lifting line methods, panel methods, viscous inviscid interaction methods, among others. Many concepts to improve the efficiency of the turbines are continuously proposed. Traditional tools cannot be reliably used to evaluate such concepts and a higher fidelity tool becomes necessary. This, among other factors, motivated the development of a new open-source CFD tool for the wind energy community. We hope to leverage the excellent multi-physics capabilities of SU2 and make it available as an open source tool for the wider wind energy community.

In this paper we present a pressure based incompressible flow solver implemented within SU2. Collocated unstructured grids are used, while the equations are discretized using the second order finite volume method. The integration in time is carried out using Euler implicit and explicit methods. Two turbulence models - Spalart-Allamaras(S-A) and the Menter Shear Stress Transport (SST), are available for turbulence modeling. The Langtry-Menter and BC transition model and will be incorporated as the next step.

One of the main challenges of solving the incompressible flow equations is the pressure-velocity coupling. There is no explicit equation to compute the evolution of the pressure field. For compressible flow problems, the continuity equation acts as an evolution equation for density which can be used in conjunction with the energy equation and gas law to obtain the pressure field. However, the continuity equation reduces to a divergence condition on the mass flux for incompressible flows and the energy equation is decoupled. A new equation for the pressure is derived by combining the continuity and momentum equations (pressure projection method). The SIMPLE-like algorithms are very popular for this type and is also implemented in the current study.

Momentum interpolation methods to compute the mass flux is used to overcome the checkerboard pressure fields introduced due to the collocated grids. A multigrid method is used to accelerate convergence. The Full Approximation Scheme (FAS) is widely used for non-linear problems and is implemented for the pressure-correction approach. Smoothers used are Symmetric Gauss-Seidel and Jacobi methods. An investigation is also carried out on the effect of using Krylov solvers as a smoother.

The code will be validated against standard test cases and will then be applied to various test cases to analyze different wind energy applications like vortex generators, surface roughness, flow over turbine blades among others. The resulting tool will be made available on Github under the SU2 repository (LGPL 2.1 license).