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.

Potential Impact:
The aim of this project is to advance our fundamental understanding of flows over heterogeneous multiscale surfaces in order to develop new predictive models for engineering and environmental flow applications. In addition to academic beneficiaries, the outcomes of this research will lay a foundation for future societal and economic benefits to be reaped through existing networks and industrial partnerships, as well as providing further impact by training new highly-skilled researchers, public engagement, and developing a major international collaboration between the UK and the US.

This research will be broadly relevant to a large number of industries where flows over rough surfaces are critical for performance. Most notably, in the transportation industry, the drag incurred by rough surfaces has important impact on transportation efficiency and its environmental footprint. The European Commission estimates that road vehicles, marine shipping, and aviation together represent almost a quarter of Europe's greenhouse gas emissions and are the main cause of air pollution in cities. Advances in our ability to understand and predict drag are therefore key for improving urban air quality and for tackling global climate change due to carbon pollution. In fact, the UK and the EU have committed to 20% reductions in greenhouse gas emissions by 2020 and 40% by 2030 (compared with 1990 levels); similarly, the US has announced its target to reduce emissions by 26-28% below 2005 levels by 2025. The impact of this research towards meeting these global targets will be realized by feeding the knowledge developed into the umbrella of ongoing boundary layer research across the community, which will ultimately lead to new design methods and operational regimes of flow systems in transport and energy sectors. Specifically, the UoS has existing ties with marine transport and the oil and gas industry through Shell and Lloyds Register, who are based locally in Southampton.

This research is also important for understanding and modelling of atmospheric flows and wind energy. The flows over complex terrain are currently poorly resolved in most atmospheric flow models and there is a need for improved predictive models. This is especially important for modeling flows in urban regions and understanding their associated heat island effects. This is also critical for planning the sites of wind turbine farms and predicting the wind patterns they experience. The new predictive models will enable us to accurately and truly predict the influence of surface topography on momentum fluxes and drag and could help mitigate the negative impact of heat islands, and develop quicker and more reliable identification of optimal sites for wind farms. The impacts to these applications will be realized by sharing the results of this project through existing networks: BG and CV already actively participate at the national Urban Working Group meetings, which bring together meteorological researchers from universities across the UK and the MET Office; CM and RM are a part of WINDINSPIRE, a NSF-supported international collaboration for computational modeling of wind farm turbulence.

Finally, this project will support ongoing outreach activities which are fundamental in making science relevant to society and inspiring the next generation of scientists and engineers. We will participate in outreach at local primary and secondary schools once a year. Specifically, we aim to bring a group of students from one (or all) school(s) in to our lab and allow them to experience the excitement of scientific research through personal involvement. Overall, we hope our efforts will raise aspirations and generate excitement amongst a potential next generation of researchers.