ALGOR SOFTWARE AIDS U.K. SCIENTISTS DESIGNING "SMART" WINDMILL BLADES

Dr. George Jeronimidis, and Dr. Ramin Rezakhanlou, with the University of Reading Department of Engineering developed the innovative new "smart" turbine blade design.

It is often said that too much of a good thing can be bad. Nowhere is this more obvious than in the design of wind turbines. The idea of a wind turbine is, of course, to turn the power of the wind into inexpensive electricity. Since wind is an ongoing resource and turbine design is improving, the sight of "windmill farms" is becoming familiar.

One of the most challenging problems faced by engineers who design wind turbines is what to do when there is too much wind. This can cause excessive centrifugal forces and lead to structural failure.

In the past, designers have attempted to solve the problem with variable-pitch blades which change the angle of attack to reduce lift, stall or feather the blades at high wind speeds. This increases manufacturing and maintenance costs because the design requires special blade attachments, instruments for measuring wind speeds and power to alter the blade's pitch.

An Innovative Solution

Engineers at The University of Reading in the UK, under the direction of Dr. George Jeronimidis, Senior Lecturer in the Department of Engineering, have developed an ingenious, simpler method of limiting overspeed. The solution involves the use of "smart" blades which are fabricated from composite materials. As wind speed (and, therefore, centrifugal force) increases, the blades' construction causes them to "turn into the wind" without the use of mechanical devices.

"Our work has shown that one can couple the axial deformations of the composite blade, due to centrifugal loads, to twisting deformations which vary the angle of attack," said Dr. Jeronimidis. "The magnitude of the induced twist is a function of the composite materials used, their orientation and the centrifugal loads. Also, bending and torsion vibration modes of the blades need to be checked to prevent undesirable coupling between them and resonance with the rotor frequencies." "

Algor software enabled us to predict the magnitude of twist, composite stress levels and resonant frequencies for these 'smart' composite blades," said Dr. Ramin Rezakhanlou, Research Officer with the University of Reading Department of Engineering and another key researcher on the project. "In addition, Algor software allowed us to do more design work in less time." The blade dimensions and geometry are typical for medium-sized wind turbine applications.

The blade consists of a glass-fiber epoxy composite laminated skin bonded to a polyurethane foam core. The fiber orientation was chosen for maximum coupling between stretching and twisting. To enhance the effect of the centrifugal load, additional masses were included at the tip of the blade design.

This turbine wind generator is typical of the type used to study the new "smart" blade design.

Stretching and Twisting

"To analyze the stretching-twisting coupling, the blade was modeled as a hollow composite section without the core, because the core contribution was negligible," said Dr. Rezakhanlou, "This allowed us to model the skin with composite plate elements. Added tip masses were simulated by assigning a mass density to the last group of elements near the tip."

"The Algor analysis used the centrifugal loading option of the composite stress analysis processor," continued Dr. Rezakhanlou. "A rotational speed of 400 rpm was used. This is typical of the speed produced by high wind velocities. The Algor predictions of tip twist and composite stress levels were found to be in good agreement with experimental results."

Two Natural Frequency Analyses

To analyze the natural frequencies, it was necessary to include the core in the blade model. "We came up with two ways to perform this analysis," said Dr. Jeronimidis. "One was to use standard plate/shell elements for the skin and brick elements for the core. This called for a little work on our part since plate elements are normally formulated as orthotropic, that is with four elastic constants, and we needed six anisotropic elastic constants to account for the fiber orientation. Thanks to the versatility of Algor software, we were able to edit the processor input file and insert the constants. This required us to calculate the constants using theory. The other drawback was that, because the plate elements are not layered laminates, an exact stress analysis of the skin is not possible."

A Faster, Easier Approach

"Our second approach was to model the core of the blade with bricks and the outer skin with composite plate/shell elements. We then used Algor to combine the two element types into a single input file," said Dr. Jeronimidis. "The resulting file was analyzed with the composite element processor which handled both the composite and brick elements correctly."

Powerful Visualization

"The visualization was interesting and informative," continued Dr. Jeronimidis. "Using the Composite option, we visualized the composite stress components, S11, S22 and S12, in the skins without the core being visible. Selecting Tresca, von Mises or Stress Tensor allowed us to see core stresses without interference from the composite stresses. This is extremely powerful since it allows simple and accurate modeling and analysis of complex composite sandwich structures.

"The stresses and displacements obtained using this approach are in agreement with those from the other model," said Dr. Jeronimidis. "More importantly, they are confirmed by analytical results, providing the level of confidence needed for the ongoing use of Algor in the design and analysis of composite components."

Algor deflection analysis contours, such as the one shown above, helped the University of Reading researchers finalize the composite material design of the innovative "smart" wind turbine blade.