Upcoming Release of the University of Minnesota’s Virtual Wind Simulator

Large-eddy simulation (LES) of wind farms with wind-turbine parameterization is emerging as a powerful tool for improving existing wind farm performance and lowering maintenance costs—and assessing the potential sites for installing wind farms.

Sandia is working with the University of Minnesota (UMN) St. Anthony Falls Laboratory to document and prepare UMN’s offshore version of the Virtual Wind Simulator (VWiS) code for release. VWiS is a state-of-the-art LES code that is capable of simulating atmospheric turbulence interacting with wind farms in complex terrain in both land¹ and offshore² environments. VWiS uses the Curvilinear Immersed Boundary method to simulate flow around geometrically complex moving bodies.

Figure 1. (Left) Water entry of a horizontal cylinder moving with prescribed velocity. (Right) A falling wedge showing the free-surface elevation.

For wind-farm applications, it can either resolve turbine geometrical details or use several turbine rotor parameterizations. It has a two-phase flow module based on the level set method that allows simulation of coupled free-surface flows with water waves, winds, and 6-degree-of-freedom (6DOF) fluid-structure interaction (FSI) of floating structures. The code can also incorporate the effects of broadband ocean waves via a multiple-scale coupling approach.

To advance CEC-simulation capabilities in an actively maintained modeling framework, the equivalent CEC module will be developed for Delft3D.

Figure 2. Overhead (left) and cross-sectional (right) contour plots of flow velocity around the Delft3D turbine model.

The code is planned for release in September 2015, and will include a detailed manual with several test cases. Sandia has been working with UMN to ensure the test cases are user-friendly and well documented, in addition to reviewing the manual. As an example, one test case is the free heave decay test of a horizontal cylinder, which validates the coupled FSI algorithm. A figure of the water entry of the cylinder moving with prescribed velocity is shown in Figure 1 (left). An example of a wedge impinging on the free surface² is shown in Figure 1 (right). A 6DOF FSI simulation of a floating turbine under real-life ocean waves is shown in Figure 2 (left), with the structural response of the turbine in heave and pitch in Figure 2 (right).

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1. Yang, X., F. Sotiropoulos, R.J. Conzemius, J.N. Wachtler, and M.B. Strong, “Large-eddy simulation of turbulent flow past wind turbines/farms: the Virtual Wind Simulator (VWiS),” Wind Energy, DOI: 10.1002/we.1802, (2014).
2. Calderer, A., S. Kang, and F. Sotiropoulos, “Level set Immersed Boundary Method for Coupled Simulation of Air/Water Interaction with Complex Floating Structures,” Journal of Computational Physics, 266, 201–227, (2014).
3. Calderer, A., X. Guo, L. Shen, and F. Sotiropoulos, “Coupled fluid-structure interaction simulation of floating offshore wind turbines and waves: A large eddy simulation approach,” Journal of Physics: Conference Series, 524(1), 012091, (2014).