Aerospace Design and Structures

Designing the world’s largest wind turbine

The Structural Dynamics Technical Committee focuses on the interactions among a host of forces on aircraft, rocket and spacecraft structures.

This was another fruitful year for the structural dynamics discipline across industry, national laboratories and academia.

In the field of energy generation, researchers at the University of Texas at Dallas in April completed a design study for what would be the world’s largest wind turbine. The UT Dallas team is investigating rotor blade designs beyond 150 meters in length for SUMR, the Segmented Ultralight Morphing Rotor. The largest blades today are roughly 90 meters. Longer blades would increase energy capture and dramatically reduce the cost of generating electricity. The ultimate goal is to develop a 50-megawatt wind turbine with blades of lengths over 200 meters. The SUMR project team is developing and integrating technologies to enable extreme-scale wind turbine rotors, including advanced controls and aerodynamics, hinge morphing, and novel structural design. The UT Dallas team is focused on lightweight structural design with an emphasis on addressing critical requirements for extreme loads, rotor dynamics and aeroelastic stability, which are of growing importance and a significant challenge for these extreme-scale rotors.

Turning to aircraft wing design, in July and August the University of Bristol and Airbus UK performed low-speed wind tunnel tests on a very flexible 2.4-meter-long model wing. The tests were part of the three-year Agile Wing Integration project funded by the United Kingdom’s Aerospace Technology Institute. The objective of the tests was to validate nonlinear aeroelastic predictions of the static and dynamic behavior of high aspect ratio wings by making detailed simultaneous structural and aerodynamic measurements. Static tests included measurement of deflections and resulting lift, drag and pressure distributions for different speeds and root angle of attack. Further tests have characterized the dynamic behavior, including limit cycle oscillations occurring due to geometric nonlinearities and stall.

In the area of computational tools, researchers at Sandia National Laboratories performed a fatigue margin assessment on a Sandia system using their newly developed parallel python toolbox, SIESTA. The finite element model had 12 million degrees of freedom and 3.5 million elements, and had several structural aspects, including fasteners. A normal mechanical environment lifetime was simulated, which consisted of random vibration and shock events. The analyses required several terabytes of data storage for the full system assessment. SIESTA predicts high cycle fatigue in large system models in the time and frequency domains and has been verified against test data. Credibility was established by comparing predictions against experimental work, including a simple tension test, a narrow-band random vibration, and a wideband random vibration.

In 2018, Sierra Nevada Corp. advanced toward a final detailed design in developing its Dream Chaser spacecraft under NASA’s Commercial Resupply Services 2 contract to transport cargo to and from the International Space Station. Reusable spacecraft like the Dream Chaser are subject to significant cumulative thermal and structural loads over their design life. As such, the Sierra Nevada team needed to perform detailed structural fatigue assessments to ensure the vehicle’s safety. To overcome the computational challenges associated with performing time-domain fatigue analysis of Dream Chaser’s composite structures, Sierra Nevada partnered with ATA Engineering Inc. ATA delivered software tools in March capable of efficiently generating and combining time-domain loads for various stages of flight in a parallelized framework. ATA’s software tool provides properly phased dynamic stress results under combined loading for fatigue analysis of composite structures, as well as cycle-count histograms via rainflow counting of principal stress time histories for assessment of isotropic components.

Illustration: The Dream Chaser spacecraft is being developed for NASA’s Commercial Resupply Services program to take cargo to and from the International Space Station. Credit: Sierra Nevada Corp.

Designing the world’s largest wind turbine