Abstract

Airborne wind energy (AWE) is an emerging technology for the conversion of wind energy into electricity by flying crosswind patterns with a tethered aircraft connected to a generator either on board or on the ground. Having a proper understanding of the unsteady interaction of the air with the flexible and dynamic system during operation is key to developing viable AWE systems. The research goal is to simulate the time-varying fluid-structure interaction (FSI) of an AWE system in a crosswind flight maneuver using high fidelity simulation tools. In this work a framework is presented that serves as a proof of concept to perform high fidelity simulations of airborne wind energy systems. This is done using a partitioned and explicit approach in the open-source coupling tool CoCoNuT. An existing finite element method (FEM) model of the wing structure is coupled with a newly developed computational fluid dynamics (CFD) model of the wing aerodynamics including rigid body motion. It has been found that the mesh deformation is quite sensitive to dynamic mesh parameters. On the other hand, the overset/Chimera technique has been proven to be a robust approach to simulate the motion of an AWE system in CFD simulations.

Abstract

The Chimera method [1] is an established method for simulation of overlapping grids. Meshing parts independently has made this method popular for complex geometries as well as moving bodies like propellers and rotors (e.g. [2]) or control surfaces (e.g. [3]). It is thus a promising method to simulate deflecting high-lift systems. The motion of the Krueger flap – as the most promising leading edge high-lift system device for laminar wing technology – is characterized by a relatively large movement (about 140 deg deflection) at a relatively high deflection speed (up to 200 deg/s) compared to classical leading edge devices. In terms of simulation, the grid properties of the overlapping mesh regions vary throughout the motion from a nearly sealed retracted position to a gapped flow in fully deflected position comparable to a slat device. This expects dynamic effects may get dominant and a valid simulation of this flow is needed for proper design and analysis. In the frame of the UHURA project1 , several partners applied their CFD capabilities based on Chimera in order to validate the method for this specific application in comparison to wind tunnel tests. The presentation outlines the different Chimera approaches ranging from structured/2D to hybrid/3D in steady and unsteady simulations for the different type of setups investigated, namely straight and swept wing with full-span and part-span Krueger flap. It summarizes common challenges and best practice for application of the Chimera approach for such a device.