Computational Fluid Dynamics (CFD) is a prime contender for reaching exascale performance: In fluid dynamics there is virtually no limit to the size of the systems to be studied via numerical simulations, which can be exploited for extreme parallel scaling. Moreover, fluid flows are an essential element of many industrial and academic problems: A crude estimate shows that 10% of the energy use in the world is spent overcoming turbulent friction. As such, collaboration between software creators and European industries within automotive, manufacturing, aerospace, energy and health care is crucial. The goals of ExaFLOW comprise four key innovation areas, including aspects of:

• Accurate error control, mesh adaptivity
• Strategies to ensure fault tolerance and resilience
• Strong scaling at exascale through novel CG-HDG discretisations
• I/O techniques for extreme scale data

 In all these areas, significant progress has been made since the project start. For instance, we have developed fault tolerance mechanisms, such that the new algorithm “survives” >90% of the errors that would otherwise have resulted in an execution failure; this all with very little overhead both in fault-free execution and in recovery. New I/O techniques and compression algorithms allowing very high compression ratios (>1:100) with very small error rates due to lossy compression. Scalable numerical methods, including efficient AMG based pre-conditioners, CG-HDG discretisations to reduce communication requirements and h- p-type mesh refinement. Finally, development of new algorithmic techniques for error control based on spectral error indicators or adjoint error estimators to steer mesh refinement in an adaptive simulation. In the final year of the project, ExaFLOW conducted three flagship runs with high academic and industrial relevance to assess the outcome of the project.