Design optimisation of an open-source reference rotor library for multi-rotor development and innovation

Abstract. The multi-rotor wind turbine (MRWT) is an emerging concept that promises benefits in aerodynamic, structural, economic and environmental performance over the existing single-rotor paradigm. However, research into MRWTs generally lacks detailed aero-servo-elastic design studies, and modelling efforts undertaken in recent years have mainly focussed on rotor interaction, wake mechanics and support structure optimisation, without detailed modelling of the rotor blades themselves. Partly due to the lack of small (< 750 kW) reference rotor models, in the limited cases where rotors are modelled, blade designs are typically derived by geometrically scaling existing reference rotors. However, the choice of reference models and the underlying assumptions used for scaling can result in rough and non-optimised aerodynamic and structural blade designs, and significant variations in rotor performance – making fair and transparent comparisons between multi-rotor configurations, and against single-rotor configurations, very challenging. A roadblock in the comprehensive design of MRWTs therefore lies in the lack of bespoke blade and rotor models suitable for the MRWT use case. To address this limitation, we have developed an open-source library of reference blade and rotor designs, covering a rated power range of 100 kW – 1 MW, which have been generated using an in-house aeroelastic optimisation software, ATOM. While the generation of these novel rotor designs has been motivated by their application to multi-rotor research, their use is not strictly limited to this context. The newly designed rotors are compared against existing reference models, showing good agreement in mass and aerodynamic properties. Moreover, we propose and demonstrate an interpolation procedure to generate intermediate rotors within the rotor library for custom multi-rotor models, enabling the generation of MRWT models of any overall power rating, using any number of rotors. This allows for comparative studies between MRWTs and greater exploration into multi-rotor scaling laws. In this paper, the design and optimisation process is discussed in detail, with the resulting designs shared as open-source models in the OpenFAST format, alongside the comparison of the aeroelastic responses predicted by ATOM and OpenFAST. The repository of rotor designs can be found at https://github.com/Abdi-SH/MRRL_OpenFAST_Files.