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Aerodynamic Performance Prediction of a Tetrahedral Kite
B. Ashari1, R. Schmehl1, and L. Veldhuis2
1 Kite Power Research Group, Faculty of Aerospace Engineering, Delft University of Technology 2 Applied Aerodynamics and Design, Faculty of Aerospace Engineering, Delft University of Technology
In collaboration with Argentinian artist Tomas Saraceno an airborne platform is developed that is held in the air by wind energy, essentially being a large kite. A prototype is planned to fly in August 2013 on the Maasvlakte 2 in Rotterdam, The Netherlands. In contrast to conventional applications, wind energy is not used to generate electricity but to elevate a platform from the ground. The artist intends to have people aboard of the platform, resembling a flying park. However, the current prototype design is an artwork to show the art-ists idea and to express his vision, not to actually lift heavy loads with it.
A tetrahedral kite type, an idea founded by Alexander Gra-ham Bell in 1903, is chosen as the base concept. This kite is build up of tetrahedral cells with two sides closed by a canopy which function as a lifting surface. The individual tet-rahedral cells are used in a modular fashion increasing the lifting surface and improving the stability. The tetrahedral concept is chosen as, due to the presence of a rigid struc-ture, it mitigates the launch and inflating problem that soft kites encounter.
In the presentation a method is shown that, based on the ge-ometry of the tetrahedral kite, can predict the aerodynam-ic performance and provide the structural engineer with an aerodynamic force distribution per tetrahedral cell in a mul-ti-celled Bell kite. This knowledge enables an accurate pre-diction of the load in the structure which in turn allows for a weight- optimised design for different multi-celled configurations.
The method is developed by taking a close look at the
aerodynamic characteristics of a single tetrahedral cell through small scale in-house wind tunnel experiments, invis-cid CFD simulations and by using flat plate approximations of the canopy. The lifting surfaces of the tetrahedron will op-erate at high angles of attack, hence nonlinear aerodynam-ic models are employed for the calculation of the flat plate approximation.
Dimensional analysis is used to determine the general rela-tionship between the geometry and aerodynamic perform-ance indicators as lift and drag. This data combined yields an aerodynamic performance prediction model for a single tet-rahedral cell.
Additionally the flow stability is investigated. The frequen-cy and size of the vortices shed by a tetrahedral cell have a large impact on the flight stability of the whole platform. A large number of small tetrahedral cells that shed small vor-tices have less impact on the flight stability than big tetra-hedral cells.
The single cell model is augmented with wake models based on existing sail and flat plate wake literature. It is found that solar panel arrays resemble the 2D flow situation in a tetra-hedral kite, hence wake studies concerning these arrays are also incorporated in the method development.
With the collected knowledge different stagger configura-tions are modelled and verified through wind tunnel tests. This ultimately resulted in a general method that can predict the aerodynamic forces in each tetrahedral cell of a multi-celled Bell kite.
Bachtijar Ashari Delft University of Technology Faculty of Aerospace Engineering Wind Energy Section Kite Power Research Group Kluyverweg 1 2629 HS Delft The Netherlands b.ashari@student.tudelft.nl www.kitepower.eu
