The efficiency and performance of an aerodynamic surface (wind turbine blade or a ship rudder) is a trade-off between the lift generated and the drag incurred by it. The required lift force is attained by changing the shape and orientation of the geometry, which generates a pressure and load distribution along its length. The drag incurred depends on the characteristics of the turbulent boundary layer flow that develops over the surface. Unfortunately, these two design requirements are not independent of each other as the turbulent boundary layer depends on the overlying non-zero pressure gradient due to the pressure distribution. Additionally, the boundary layer flow also depends on the topographical features (or roughness) of the surface where features as small as 10 microns are considered to be hydrodynamically "rough". This situation where the boundary layer flow is under the influence of both non-zero pressure gradient and surface roughness is pervasive in engineering and environmental applications. Examples include flow over airfoils/turbine blades, ducts, flow over/around buildings, hills/valleys etc. Despite its prevalence, the effects of pressure gradient on flow over rough surfaces remains largely unexplored. As we strive towards net zero carbon emissions by 2050, it is timely to develop new understanding and modelling strategies that capture the influence of pressure gradients on the performance of flow over rough surfaces.

In this project, we aim to characterise the evolution of non-zero pressure gradient (PG) turbulent boundary layers (TBL) over rough surfaces and thereby identify the parameters that dictate the response of boundary layers to streamwise pressure gradients. A comprehensive series of wind-tunnel experiments and numerical simulations will be performed to generate unprecedented data on flow over rough-walls subject to favourable and adverse pressure gradients (FPG and APG). The data will underpin identification and validation of potential universalities (and differences) in mechanisms of momentum/energy transfer compared to zero-pressure-gradient (ZPG) flows. The data will then be used to develop and validate both integral models and new Large-Eddy Simulation (LES) models that can be used to predict the performance of flow over arbitrary rough surfaces under the influence of varied pressure gradients. The data will be made available in the public domain through our roughness database (www.roughnessdatabase.org). The overall aim is to establish the interrelationship between roughness and pressure gradients over a broad range of parameters (for pressure gradient and roughness properties), understand the limitations of current models and develop new modelling methods that can be used for predictions in a wide range of applications.