Towards hybrid modeling of hybrid VAWT

Abstract

Vertical axis wind turbines (VAWTs) are popular design solutions for omnidirectional wind and limited installation area, particularly in urban areas, due to their advantages of low noise, low energy cost, and lower sensitivity to turbulence. They are often found in either of two distinct designs: the lift-driven Darrieus, with high aerodynamic power, and the drag-lift-driven Savonius, which excels in its start-up performance. The Darrieus- Savonius combined vertical axis wind turbine (hybrid VAWT) emerges from combining the advantages of Darrieus and Savonius. However, the hybrid VAWT has complex fluid dynamics, with multiple scales and interactions. Therefore, a comprehensive analysis and modeling of hybrid VAWTs can benefit from a multi-fidelity approach.
The choice of numerical methods, in either Eulerian or Lagrangian reference frames, holds paramount importance in simulating VAWTs, each method offering distinct ad- vantages and limitations. Through high-fidelity Eulerian unsteady Reynolds averaged Navier Stokes (URANS) simulations, insights into airflow patterns, and turbulence phe- nomena across varied operating conditions are gained. Conversely, Lagrangian models such as the vortex particle method (VPM) can enable efficient analyses of vortical struc- tures and wake interactions. While URANS simulation offers high-fidelity representa- tions of complex flow phenomena and allows for precise optimization of turbine design parameters, it demands significant computational resources and expertise. In contrast, VPM excels in capturing flow features efficiently but may struggle to accurately repre- sent boundary layer effects and near-wall flows. Consequently, the integration of both the Eulerian URANS and the Lagrangian VPM (hybrid method) is crucial for achieving comprehensive, reliable, and cost-effective simulations of VAWT performance. In this thesis, the hybrid VAWT is investigated using multi-fidelity numerical tools to estimate rotor/blade aerodynamics, computational efficiency and accuracy. Eulerian (U)RANS simulations in OpenFOAM and Lagrangian VPM are first applied to different types of VAWTs, followed by the application of the hybrid method in the hybrid VAWT case.
The goal of this thesis is also to investigate the flow features and power performance of the hybrid VAWT with multi-fidelity methods. In the context of the Eulerian reference frame, this thesis advances the knowledge of hybrid VAWT aerodynamics in several as- pects. The Darrieus and Savonius parts in a hybrid VAWT are modeled as uniform force fields to exclude the effects of structural and operational parameters on the power losses of the wind turbines. The results show that the hybrid configuration cannot show a sig- nificant power increase, and it is only beneficial for the startup performance. The vor- tex dynamics behind the hybrid VAWT are analyzed in different attachment angles and tip speed ratios. The blade-vortex interaction is characterized and correlated with the torque generation of the Darrieus blade. Results show that the Darrieus blade torque in- crease is dependent on the interaction with the shed vortex from the advanced Savonius blade.
In the context of Lagrangian reference frame, both Savonius and hybrid VAWT con- ix
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figurations are employed to assess the computational efficiency and accuracy of the vor- tex particle method. In the case of Savonius rotor simulations using the vortex method, the Savonius is defined as a rotor with two trailing edges because it has no clear lead- ing/trailing edge like an airfoil, named double-trailing-edge-wake-modeling vortex par- ticle method (DTVPM). Results show that a maximum power coefficient is achieved at a tip speed ratio of approximately 0.8, consistent with experimental findings. Further- more, the process of trailing-edge vortex generation and detachment is effectively cap- tured. A comparative analysis between Eulerian URANS simulations and Lagrangian DTVPM reveals that DTVPM offers a more efficient simulation of Savonius rotors with- out the need for empirical parameters. Notably, DTVPM demonstrates remarkable com- putational speed, with simulations being approximately 20 to 104 times faster than par- allel URANS simulations over five revolutions. This significant reduction in computa- tional time underscores the potential of DTVPM to enhance existing engineering mod- els for wind energy applications. In the case of hybrid VAWT simulations, this thesis extends the application of VPM to hybrid VAWTs and introduces a viscous correction to improve simulation accuracy. By incorporating the airfoil polar, the proposed La- grangian DTVPM effectively predicts comparable force variations to Eulerian URANS simulations. Importantly, the computational efficiency of URANS and DTVPM for hy- brid VAWTs is compared, revealing that serial DTVPM simulations are approximately 20 times faster than parallel URANS simulations over ten revolutions. This notable increase in computational speed highlights the potential of DTVPM to provide efficient and ac- curate simulations for hybrid VAWTs, facilitating further advancements in wind energy technology.
In the context of the hybrid Eulerian-Lagrangian reference frame, this study aims to enhance accuracy and efficiency in analyzing complex flow phenomena. Through con- ducting various scenarios of hybrid VAWT using the hybrid method, this study concludes with the demonstration of the hybrid solver’s capability in simulating hybrid VAWT aero- dynamics. The final goal of this thesis is to ascertain an efficient model for hybrid VAWT and determine the limits of individual Eulerian and Lagrangian methods while consid- ering specific flow features of VAWTs. Overall, this study contributes to comprehensive insights into the correlation of blade-vortex interaction and torque variation. The mod- eling challenge in the hybrid VAWT simulation is studied and a hybrid model is suggested to understand the performance and flow features of hybrid VAWTs.