Collaborative Proposal 1066873 Xing 1066627 Carrica
The PIs will develop a high performance Computational Fluid Dynamics (CFD) tool capable of simulating the aero-hydroelastic problem of offshore wind turbines. The tool is based on the general-purpose CFD code CFDShip-Iowa, which uses state-of-the-art technology and has been validated for many applications in ship hydrodynamics and onshore wind turbines for a wide range of conditions. The PIs plan to develop the CFD tool by implementing an (incompressible) viscous flow Navier-Stokes equation solver (justified by low tip speeds of utility-scale wind turbines) and the use of full 3-D grids that will resolve the full geometry of the rotor (rotating blades), the nacelle, and the tower. Regular and random waves will be studied; tip vortices and other turbulent structures will be modeled using detached eddy simulation. The proposal builds on previous work performed by the PIs which includes fluid structure interaction. The work is potentially transformative in that the proposed project will provide the wind turbine community with a validated, high fidelity tool to guide the design of offshore wind turbines.
In terms of broader impacts, the simulation based design tool can be used by the wind turbine community to increase the efficiency of offshore wind turbines while minimizing turbine and tower vibrations. There is the potential to make substantial contributions to enhance the economic and energy security of the United States including reductions of imports and greenhouse gases. Although offshore wind farms have been developed in Europe, no offshore wind farms have been built in the US, and domestically developed design tools may be helpful in advancing American competitiveness in this area. The PIs will promote teaching, training and learning while conducting the research, particularly with respect to underrepresented groups for the lead PI at Tuskegee University.
Current design trends of wind turbine system favor larger turbines, which tend to be more cost-effective. However, this results in more complex turbine systems with more demanding structural constraints. These large turbines, with rotor areas equivalent to 3 to 4 football fields, expose the wind turbine to high wind shear and turbulence. The long and slender blades are subject to large amplitude changes in wind loads, causing reliability issues due to fatigue. Variable-speed, variable-pitch and yaw control are needed for the turbine system to achieve best performance. High tip speeds due to the long blades introduce noise and environmental impacts. Challenges increase when several wind turbines are operated as wind farms, with stronger velocity gradients and fluctuations caused by momentum deficits and wake turbulence of upwind turbines. Maintenance cost is another critical factor influencing wind turbine design, especially for offshore farms with expensive accessibility. Of all components in a turbine, gearbox, drive train and generator contribute most to downtime, while rotor hub and blades are the next critical factors. Development of methodologies and techniques capable of modeling the interaction between realistic wind loads and the structural components is the most promising way to improve designs that will better perform in complex operational environments. During the course of this awarded project, a high fidelity approach coupling the computational fluid dynamics method (CFD) and multibody dynamics method (MBD) is developed for aero-servo-elastic wind turbine simulations. The approach uses the incompressible CFD dynamic overset code CFDShip-Iowa v4.5 to compute the aerodynamics, coupled with the MBD code Virtual.Lab Motion to predict the motion responses to the aerodynamic loads. The wind turbine international standard IEC 61400-1 ed. 3 recommended wind turbulence model proposed and developed by Mann was implemented into the code CFDShip-Iowa v4.5 as boundary and initial conditions, and used as the explicit wind turbulence for CFD simulations. A drivetrain model with control systems was implemented in the CFD/MBD framework for investigation of drivetrain dynamics. The tool and methodology developed here are unique, being the first time a complete wind turbine simulation includes CFD of the rotor/tower aerodynamics, elastic blades, gearbox dynamics and feedback control systems in turbulent wind. A set of figures help visualize the methodology used to compute the wind turbine loads and response. Figure 1 shows the setup of the drivetrain of the NREL 5 MW turbine. Figure 2 (primary image) shows an instantaneous solution of the turbine operating in turbulent winds of approximately 8 m/s, using a torque controller attempting to maximize power production. Figure 3 shows the wake produced by the turbine at fixed pitch or using a pitch controller. The use of efficient controllers not only maximizes the power produced by the turbine, but also minimizes turbine stress and reduces the wake and fluctuations arriving to the turbines downstream. This grant made possible to support a PhD student to completion. Underrepresented minorities participated in the project.
