SUMMARYThis contribution proposes a methodology for the numerical analysis and for the improvement of the design of free-form membrane structures subjected to flow-induced effects. Typical applications in such context are tents exposed to wind. Different physical factors connected to thin and flexible structures, highly turbulent air flows, as well as their interaction have to be taken into account. This necessitates the appropriate combination of different numerical disciplines which is done in the simulation of fluidstructure interaction. The over-all complexity of the problem favours a modular and flexible software environment with a partitioned coupling strategy. Within such an environment, the solution of each physical and algorithmic field is applicable with the most suited method. In the proposed framework, the structural field is solved with the in-house finite element program CARAT, which uses several finite element types and advanced solution strategies for form finding, nonlinear, and dynamical problems. The fluid field is solved with the CFD software package CFX-5. The interaction between both physical fields is realized by the exchange of boundary conditions. Beyond the mere exchange of data, the utilization of stabilized as well as efficient coupling strategies is mandatory. This contribution illuminates the scope of numerical simulation theory and presents implementations followed by illustrative examples.

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Modern wind turbines with high hub heights enable an economical energy production for locations with lower wind speed, complex terrain or in forests. High hub heights result in high requirements for the performance of turbine towers. For power production, the towers have to feature distinct static and dynamic properties. The feasibility of conventional, welded tubular steel towers faces technical and economical limits and alternative tower concepts are applied. For alternative steel towers concepts, friction connections with preloaded bolts have several advantages, but also raises new questions. The load bearing capacity of a friction connection depends on the coefficient of static friction and the applied pretension. During the turbine lifetime, the friction connection will be subject to a loss of pretension that needs to be considered. According to current design standards for steel constructions, the loss of pretension cannot be included in the design phase, but must be accounted for by means of maintenance. This article presents an advanced design method for friction connection in steel constructions that is based on the VDI guideline 2230 for bolted connections in machinery structures and includes a possibility to consider loss of pretension. In addition, friction connections with drilled and punched holes are considered. It can be shown, that for preloaded friction connections, the punching of holes leads to no adverse effects for fatigue resistance. On the basis of this design method, it was possible to issue a type approval for a maintenance free steel shell tower that consists of steel plates, which are joined by friction connections

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