Marine Current Energy Extraction Using Torsional Galloping Based Turbine
Authors: Fernandes Antonio Carlos , Armandei Mohammadmehdi 
Published: May 2013

Abstract
One of the resources for alternative energy extraction in the area of offshore engineering is the marine current. Traditionally, the marine current energy is extracted through rotation based turbines. However, it became clear that to use of oscillation-based devices rather than the rotation-based ones has some advantages. Based on this idea, an innovative concept in energy extraction from the marine current is presented here.
The research introduces an oscillation-based turbine built and qualified through experimental studies. The turbine consists of a flat plate connected to a torsion spring and located vertically in the water current. The flat plate has only one degree of freedom that is yawing about its axis, located in its mid-chord length. The phenomenon based on which the device works is Torsional Galloping, which is a category of Flow-induced Oscillation of instability type. As the current speed exceeds a critical limit, the plate becomes dynamically unstable due to the Fluid-Structure Interaction, and begins to yaw due to the restoration provided by the torsion spring. Although Torsional Galloping is usually suppressed because of its destructive effects on the structures, the turbine here is based on the idea of maximizing it.
The efficiency and feasibility of extracting energy are discussed here. The preliminary results show that the maximum efficiency around 10% is achievable through this turbine, whereas this efficiency can be increased even more through further improvements such as non-linear springs and the foil profile.
In fact, one such turbine, if improved and designed well, can be more efficient than the common rotation-based turbines. The main advantage about this turbine is that the characteristic length of its oscillating part is not limited by the depth of the water. That is a new approach which possibly makes this turbine to operate better than a rotation-based turbine.

1- Introduction
In the last recent years, an international commitment has started between the countries to reduce the dependency on the fossil fuels. This commitment aims to decrease the environmental problems caused by this fuels and the limitations of their supplies. As a result, some trials have been performed to utilize the alternative energy resources which are clean and renewable. Brazil, as the fifth biggest country of the world, possesses a large number of alternative energy resources too. One of these resources is the current energy. This energy can be found either in rivers spread all over the country, or in tides which sometimes are seven meter high in the northeastern coasts of Brazil.
Traditionally, the current energy is extracted via rotation-based turbines that work in the high-Reynolds number currents. This fact necessitates building dams to get higher hydraulic heads. Naturally, to build dams imposes an extra cost and also can have destructive effects on the natural environment. On the other hand, the structural weakness associated with the centrifugal force in these devices necessitates the use of high performance materials, and consequently imposes another cost.
The new family of turbine that has been introduced and developed, following the researches in the last recent decades, is the oscillation-based turbines, which are inspired from the birds and aquatic animals. The principal advantage of this type of turbines is their ability to operate in low-Reynolds number currents. Therefore, these turbines do not need dams to be able to operate and they can work in the natural currents. Consequently, the costs of installation and operation will lower significantly, together with the environmental impacts.