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Oscillating Foil Technology

Introduction to Oscillating Foil Technology

Principle

Instead of a traditional rotary-type turbine, the Leading Edge technology harvests hydrokinetic energy via an oscillating hydrofoil.   Just as a wind turbine’s motion is driven by the constant flow of air, an oncoming  flow of water drives the motion of the hydrofoil which in turns powers a generator that converts it to electricity.   The motion of the hydrofoil is shown below: rather than a rotation motion commonly seen in wind turbines the foil oscillates in heave (up/down motion) and pitch (rotation about the mid-chord).

The unique geometry and motion of the hydrofoil lends itself to shallow locations where long slender foils can be deployed, including many riverine and tidal flows close to metropolitan areas.  The oscillation foil technology is also very modular and scalable, in which various sized foils can be easily manufactured to fit in a wide range of locations.   Additionally, the foils can be placed close to one another, or even share the same supporting structure, which saves in maintenance and installation costs.   The motion of the foils is also slow relative to a large rotary turbines, which means lower relative velocities and less of an environmental footprint on the local marine life.

How is Power Generated?

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As the hydrofoil moves through the oncoming flow it generates both lifting forces in the vertical direction, as well as a pitching moment about its axis of rotation.  The hydrodynamic power can be split into two portions, a linear component which is the product of the force and linear velocity, and an angular component which is the product of the moment and the angular velocity.  As the foil moves upward it has a positive angle of attack relative to the freestream flow, and thus a positive lift force, creating positive power.  At the top of the stroke, the foil reverses its pitch to a negative angle of attack which provides a downward force on the foil, coupled with a downwards velocity vector also yielding positive power.

The particular kinematics of the foil, such as the flapping frequency, heaving and pitching amplitudes are all critical factors in terms of how much energy can be extracted from a given flow.  An optimal set of kinematics yields mechanical efficiencies of approximately 30% and power coefficients of 0.9 for a single hydrofoil

What Makes the Leading Edge Technology Unique?

The optimal kinematics for the Leading Edge foils have a high pitching amplitude, or high maximum angle of attack (70-80 degrees).  At these high angles, the boundary layer, or the fluid on the surface of the foil, separates from the surface and forms a vortex at the leading edge of the foil.   This leading edge vortex persists over the top of the foil during the upstroke, increasing the lift force with its low pressure core for a large portion of the stroke.  When the foil reserves at the top of the stroke the vortex sheds, and another is formed on the lower side of the foil for the same effect.

 

Roll of the Leading Edge Vortex

The high amplitude pitching and heaving generates coherent vortices from both edges of the hydrofoil, which contribute significantly to the forces, moments, and power generation throughout a cycle.  The figure shows the kinematics, the total power P, and the power from the linear and angular contributions throughout one pitch/heave cycle with three time-shots indicated by the red, purple and cyan frames on the right.

Starting at the top of the downstroke, the foil is at zero pitch angle, coincident with the freestream flow (from left to right).

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  • Red Frame, 20% cycle: The foil moves downward and pitches clockwise about its center at a high negative angle of attack. The negative vorticity from the previous upstroke has recently been shed, and the positive vorticity on the lower surface is beginning to form into a leading edge vortex (LEV) shown by the lower pressure region at the leading edge.  The LEV enhances both linear and angular power at this point in the cycle, resulting in the maximum power.
  • Purple Frame, 35% cycle: In the second half of the downstroke the LEV grows in strength and size, and convects towards the trailing edge still contributing to the high linear power. However the reversal in pitching direction generates negative angular power.
  • Cyan Frame, 50% cycle: At the heave stroke reversal the linear power is close to zero due to low linear velocity. However the high angular velocity and high moment induced by the LEV produce maximum angular power.  At this point the LEV is pinched off and convected downstream, and the process repeats on the opposite side of the foil.

 

 

 

For more information contact Dr. Jennifer Franck.