EPSRC-CBET:Turbulent flows over heterogeneous multiscale surfaces

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In almost all engineering and environmental flows, turbulent boundary layers (the part of the flow closest to a given surface) are in the rough-wall regime. Typical examples include boundary layers developing over surface irregularities on aircraft and wind turbine blades, macro bio-fouled ship hulls, edges of forests or wind-farms, urban canopies, crop boundaries, river-beds, and wind over rough seas. Despite decades of sustained research, accurate predictions of momentum transfers and/or skin-friction drag based on geometric information about the surface alone are difficult. This is primarily because in most cases, the topography of surface roughness is multi-scale, that is to say, it contains a wide variety of roughness length scales. Moreover, the variation in the range of roughness length scales and the distribution of the roughness features is heterogeneous across the surface. Current predictive approaches, designed mostly for homogeneous and single-scale roughness element distributions, can neither accurately predict nor offer insights into the complex physics of flow over multi-scale heterogeneous surfaces. In this collaborative research,a systematic approach to characterize drag and the mechanisms of momentum transfers in flows over heterogeneous multi-scale surfaces will be applied. This research will be broadly relevant to a large number of industries where flows over rough surfaces are critical for performance. In the transportation industry for example, the drag incurred by rough surfaces has important impact on transportation efficiency and its environmental footprint. This research is also important for understanding and modeling atmospheric flows, of relevance to weather prediction. The flows over complex terrain are currently poorly resolved in most atmospheric flow models and there is a need for improved predictive models. Better predictive models are also important for understanding flows in urban regions and wind farms.In this project, a series of high-fidelity computer simulations - to be carried out at Johns Hopkins in the US - and of physical experiments - to be performed at Southampton in the UK - will generate unprecedented data of flows over heterogeneous, multi-scale surfaces. Numerical modeling will be based on Large Eddy Simulation that uses a novel integral wall model implemented in a high-accuracy finite difference solver that uses sharp immersed boundary method to resolve larger-scale roughness elements. Three different cases will be considered both numerically and experimentally: (i) an abrupt change in nature of multi-scale roughness, (ii) finite patch of multi-scale roughness, and (iii) repeated changes in multi-scale roughness. The data will be analyzed and simulations and experiments compared. The experimental and numerical data as well as the physical insights obtained will be used to test existing, and develop new, analytical models that enable accurate prediction of drag and momentum transfers based only on available information about the topography of multi-scale heterogeneous surfaces. The project will strengthen graduate education, since the PhD student who will be a part of this project will gain substantial expertise in computational methods, modeling strategies and collaborating internationally with experimentalists. This training will be invaluable as these methodologies are widely recognized as areas of substantial growth in the coming decades, where experienced researchers will be most needed.