  We present our approach to making direct numerical simulations of turbulence
with applications in sustainable shipping. We use modern Fortran and the
spectral element method to leverage and scale on supercomputers powered by the
Nvidia A100 and the recent AMD Instinct MI250X GPUs, while still providing
support for user software developed in Fortran. We demonstrate the efficiency
of our approach by performing the world's first direct numerical simulation of
the flow around a Flettner rotor at Re=30'000 and its interaction with a
turbulent boundary layer. We present one of the first performance comparisons
between the AMD Instinct MI250X and Nvidia A100 GPUs for scalable computational
fluid dynamics. Our results show that one MI250X offers performance on par with
two A100 GPUs and has a similar power efficiency.
