FlexWing - Fluid-Structure Interaction analysis of flexible membrane rotors

Project Support

The research project "Fluid-Structure-Characteristics of Flexible Wings for Wind Turbines" is supported by the German Research Foundation - project number 280772364

Motivation and Objectives

Membrane structures can efficiently carry eternal loading via in-plane pre-stresses and thus can achieve with minimum use of materials very large spans. The coupling between geometry and stress state is very important in design of membrane structures. They usually have high deformations under external loading, causing the structural response to by highly non-linear. To realize proper deformation and to avoid development of wrinkles, membrane structures should be stabilized via pre-stresses and surface curvature. The shape of the pre-stressed structure is calculated using a special non-linear method, called form finding. Diefferent methods are used for form finding analysis ranging from dynamic relaxation to force density method and the updated reference strategy. The main influencing parameters in form finding analysis are magnitude and distribution of pre-stresses in the membranes and the supporting edge cables. The concept of SailWing rotors is initialy proposed during the 1970s by researchers at Princeton university sailwing group. Membrane blades are lighter and easier to transport in segments, compared with a conventional composite blade made of glass- fibre-reinforced plastics (GRPs). The flexibility of a membrane wing enables it to adapt itself to the flow field to a certain extend. The advantages of this passive adaption to the surrounding flow are from aerodynamics point of view a higher lift slope, higher maximum lift coefficient and postponed stall to higher angles of attack compared with their rigid counterparts. The response of such a wing to aerodynamic loads depends on the membrane's stresses, so two-way coupled fluid-structure interaction simulations (FSI) are necessary to analyze its performance. In this project FSI analysis of the membrane wing should be performed using models with different level of complexity for the fluid side. Initially the vortex panel method would be used for design space explorations for different sets of pre-stresses in membranes and edge cables. The panel method is computationally less demanding and is a good alternative to numerical solution of the Navier-Stokes equations. Optimization would be done using the panel method with internal pre-stresses as design variables. Detailed FSI analysis of the membrane wing is later done for the chosen pre-stress candidates form the optimization step. The final goal is to apply the membrane wing concept to a wind turbine rotor and to assess the potential of the membrane wing in wind turbine industry by comparing its performance with the corresponding conventional blade.