CBET-EPSRC: Turbulent flows over multiscale heterogeneous surfaces

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A turbulent boundary layer is formed when a fluid flows past a surface. This boundary layer is primarily responsible for the skin-friction drag incurred by the surface. In almost all engineering and environmental flow applications, these boundary layers are formed over non-smooth or rough surfaces where the roughness of the surface plays a significant role in setting the drag and its repercussions on the flow. And yet, we are unable to truly predict the influence of these rough surfaces on the flow. This is primarily because the topography of surface roughness is usually "multiscale" in nature that contains a wide variety of roughness length scales. More importantly, the variation in the range of roughness length scales and the distribution of the roughness features is "heterogeneous" across the surface. Examples include edges of forests or wind-farms, urban canopies, crop boundaries, river-beds, land-water interfaces, rivets on aircraft, ablated turbine blades, macro bio-fouled ship hulls etc. The turbulent boundary layers that evolve over such heterogeneous multiscale roughness experience non-uniform surface conditions and as a result exhibit properties that are different from flows that develop over homogeneous roughness. Consequently, current modelling and prediction strategies (such as the Moody diagram) that were developed for surfaces with homogeneous roughness can neither accurately predict nor offer insights into the complex physics of flow over heterogeneous multiscale surfaces. Therefore, considerable advancements and benefits would result across a whole range of sectors if we are able to predict the effects of multiscale surface heterogeneity on turbulent boundary layers.In this collaborative research, we aim to apply a systematic approach to characterize drag and mechanisms of momentum transfers in flows over heterogeneous multiscale surfaces. A series of physical experiments - to be performed at Southampton in the UK - and Large Eddy Simulations - to be carried out at Johns Hopkins in the US - will generate unprecedented data of flows over heterogeneous multiscale surfaces. Examining flows over such surfaces will develop our fundamental understanding of the coupling between the non-linearities in the turbulent flow and its relationship with the multiscale heterogeneity of the surface. This understanding will bring about a paradigm shift in how we understand and predict non-equilibrium turbulent wall-flows and the multiscale interactions that are responsible for its development. Synthesizing the new insights obtained from the data, we will develop new analytical models to predict the drag and properties of momentum transfer based only on available information about the surface topography. The ultimate aim is to develop truly predictive models for engineering and environmental flow applications.