Optimization of implementation of Chimera technique for large scale FSI simulations with wind turbine FSI as an application

Rishith Ellath Meethal

Rishith Ellath Meethal did his honours’ project at the Chair for Computation in Engineering (TUM) and
was supervised by Philipp Bucher M.Sc (Hon), Aditya Ghantasala M.Sc and PD Dr.-Ing habil. Roland W├╝chner.

The field of Computational Fluid Dynamics (CFD) has always been interested in solving problems involving relative motion of solid geometries in fluid. Different methods like sliding mesh, re-meshing, embedded approach and Chimera are available for such problems. Among them, Chimera method has proved to be one of the best with a combination of high performance and accuracy. The Chimera approach consist of many overlapping sub-domains/grids covering the computational domain. The flow variables are exchanged on the boundaries of these grids to ensure continuity of flow variables. So the grid containing the moving body can be given movement and other grids can be kept stationary. Along with this, it gives advantage of simplified and easy mesh generation due to different component meshes.

In this work, implementation of Chimera method for a multi-step(fractional step) Finite Element (FE) based CFD solver in Kratos multiphysics, for running simulations like wind turbine is discussed. Existing Chimera implementation on a FE based monolithic fluid solver was extended to the multi-step solver. Multistep solver solves pressure and velocity separately. This results in linear systems of smaller size and allow us to run simulations with less computational cost. The Chimera implementation also extended to Fluid Structure Interaction (FSI) problems. FSI problems involving large structure movements are complicated to perform as re-meshing may be required at some stage. Application of Chimera for such FSI simulation cases enable us to run them without the need of re-meshing.

Figure 1 shows the simulation of a drifting flexible structure in fluid. A flexible structure attached to a spring drifts and deforms due to the force of fluid until the spring force matches with fluid force. This example shows the flexibility and advantage of the method to simulate a FSI problem involving large motion of a structure. The figure shows the velocity profile around the structure at different time instances. It can be observed that structure moves as well as deforms due to the force from fluid.

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Fig.1: Velocity distribution plot in drifting flexible structure in fluid simulation
This implementation was tested to run simulations of real life engineering examples like wind turbine. The figure 2 shows the results of a 10MW DTU reference wind turbine using Chimera approach. A rotating sub domain was created around the blades and acts as patch mesh. The whole computational domain acts as the background numerical wind tunnel with patch mesh rotating in it. Figure 2 shows the streamlines around the wind turbine simulated using Chimera method. And figure 3 shows the contour plot of the pressure in the domain to show the continuity of physical variables across the sub-domains.

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Fig.2: Streamline plot around the 10MW DTU reference simulation using Chimera method

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Fig.3: Isosurface plot around the 10MW DTU reference simulation using Chimera method

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