Neko: A new spectral element code applied to the simulation of a Flettner rotor

Ways to reduce the fuel consumption of ships have been gaining increased interest. One intereting approach is adding a Flettner rotor, i.e. a rotating cylinder that uses the Magnus effect to generate lift. While Flettner rotors have been considered in physical experiments and RANS-type simulations, no direct numerical simulation (DNS) of a Flettner rotor, focusing on the interaction with the surrounding turbulent boundary layer has been carried out. Our simulation demonstrates our new CFD code Neko, a solver that owes it homage to Nek5000. Neko is based on similar numerical methods, but differs in that it also accommodates modern computer architectures such as GPUs and has an object-oriented codebase written in modern Fortran. Special emphasis has been put on portability, scalability and the possibility to extend to code easily with new features; thereby keeping the complexity for the domain scientist to a minimum. The flow case under consideration is a Flettner rotor submerged in a turbulent boundary layer, consisting of 1M spectral elements, which turns into 0.5B unique grid points. We discuss the strong scaling efficiency with comparing several architectures, including Nvidia A100 GPU, AMD INSTINCT GPU and AMD EPYC CPU. We observe excellent parallel scaling, up to hundreds of nodes. Our initial findings for the lift are in excellent agreement with experimental data. The drag was found to be highly dependent on the Re. We observe that there is a strong interaction between the rotor and the turbulent boundary layer, in terms of modified coherent structures.