Fluid-Structure Interaction of Inflatable Wing Section

Abstract

Airborne wind energy (AWE) is a concept for harvesting wind energy using tethered flying devices. Compared toconventional wind turbines AWE systems require substantially less support structures like a tower. The replacementof towers by tethers also allow accessing higher altitudes where winds blow stronger and more persistent. Severalconfigurations are currently pursued. Electricity can be generated with on-board propellers which are driven by the airflow and the power is transmitted to the ground through a conducting tether. Another configuration is the pumping cyclein which case the kite flies crosswind to pull a tether that is unreeled as it moves a ground-based electrical generator, anda retraction phase when the kite is reeled in. Next to electricity generation AWE can also be used for ship propulsion. We consider a single cell of a ram-air wing in our study which is based on an inflatable double skin design. Ram-air wings are inflated by the stagnation pressure entering through inlets at the leading edge. The internalpressure provides structural stability and stiffness. The fully or partially inflated structure is flexible and can thereforeexhibit large deformations during flight. This introduces a strong coupling between the structure and the air flow sincethe internal pressure in dependent on the wind speed, and a deformed kite will inevitably have a different pressure fieldcaused by the flow compared to an un-deformed kite. Also, bridle system induces a significant additional drag to the wingdrag and therefore the kites fly with high angle of attack to obtain high lift and to ultimately maximise power output. Highangles of attack causes the flow to separate which cannot be simulated with fast inviscid methods and therefore a CFDanalysis tool such as OpenFOAM is required.The main challenge in analysis and design of these kites is the governing fluid-structure interaction (FSI) mechanism,which leads to a drastic increase in model complexity. On the other hand utilising FSI is crucial to obtain reliable results. on performance measures and structural integrity. We follow the partitioned coupling approach, where the fluid andstructure domains are solved individually and coupled at their interface. In this work we couple OpenFOAM with ourfinite element (FE) solver mem4pyby using the coupling tool preCICE. The coupled solver is then used to simulate asingle ram-air wing section (cell) and its change in aerodynamic performance due to deformation.