Ab initio hydrodynamic rough surface characterisation with applications

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Turbulent flow over rough surfaces is common in nature and in many technological applications, yet the methods used to predict it are based on a limited experimental database and on correlations that are known to give contradictory predictions. There is much still to learn about how particular surface features lead to certain drag increases and it is widely accepted that the standard measure of equivalent sand grain roughness is no longer sufficient, since surfaces with the same roughness on this scale have different behaviour in the transitionally rough flow regime. We propose a research programme based on numerical simulation to study rough surface flow, particularly in the high speed flight regime where we have an immediate requirement from our project partners in government and industry. With the proposed development of high-order implementations of immersed boundary conditions, numerical simulation of flow over regular or random rough surfaces will be feasible, resolving the scales of roughness that interact with turbulent flow near a wall. A programme of work is proposed to develop such a capability, initially based on parameteric studies and high resolution studies requiring the use of national supercomputer facilities. However, with the rapidly decreasing cost of computing power, the technique we will use for this work is believed to be more widely useful, and by the end of the project we propose to develop a rough surface characterisation workflow, whereby samples can be scanned, using for example a confocal microscope, surface data interpolated into a boundary condition for numerical simulation and then simulations run for a range of scales (surface scale relative to flow scale) to build a hydrodynamic characterisation map of the surface.