Tube Kite structural analysis with FEM

The goal of this research (Tube Kite structural analysis) is to generate a model, which can predict the deformations of a kite in different load cases. These load cases are the aerodynamic loads, which are acting on the kite. To predict accurately the behavior of the kite in flight, the aerodynamic model is very important. Computational Fluid Dynamics (CFD) is a possibility to model the aerodynamic loads in different flight conditions.

The design of kites, especially when used in industrial environment where kites grow bigger and bigger (Skysails with 300 m² and more), becomes very important and costly. Nowadays, the kites are built based on the experience of kite-surf kites. These are a lot smaller in size compared to the kites that will be used for energy production (surf kite: maximum surface is 17 m² compared to 25 m² in the prototype of the 20 kW system at the Delft University of Technology (DUT)). The design of a kite consists of an iterative process in building a kite, testing a kite and giving recommendations to improve the kite. When an accurate model exists, the design can possibly be done with less iteration in building test kites. The first iterations can be done with simulations instead of building and testing a number of kites. There are a few advantages in using a FEM model:

• Reduced design cost for designing a new kite: to decrease the building, testing rebuilding iterative process, which can become costly and time consuming. Especially when kites become bigger (up scaling of the kite).

• Improved design: the design of the kite will be better. This can lead to less material usage, less mass, higher ultimate load.

• Research tool: FEM is a good way to analyze the structure of a kite and to gain knowledge and experience in this exciting, new and challenging research field. In FEM, you get a detailed data set (deformations, stresses, strains etc.) which can be analyzed and validated.

In the structural FEM analysis within the complete Kite Power system, the kite including the bridle is only component that is analyzed. However the kite component is a very complex system in which the Aerodynamic load, the inflatable tube structure and the bridle determine the desired- (e.g. steering) and undesired (e.g. collapse) deformations of the kite.

In the future, ultimately, three research fields must interact to ensure accuracy and predictability of the Finite Element Method (FEM) model.

• An accurate FEM model of the kite, which describes the deformation under a static and dynamic load with a certain accuracy.

• Accurate aerodynamic model (e.g. by using Computational Fluid Dynamics (CFD) analysis).

• Accurate interaction between the aerodynamic- and the FEM model.

The first step in this process is to make a FEM model for the kite including the bridle, which is loaded with static forces. The currently used FEM solver is Mathematical Dynamic Model (Madymo) developed by TASS. Madymo is used in the automotive industry for airbag simulation.