Turbulent flows over rough-walls under the influence of streamwise pressure gradients

空气动力表面（风力涡轮机叶片或船舵）的效率和性能是产生的升力和阻力之间的权衡。通过改变几何形状和方向来获得所需的升力，从而产生沿其长度沿着的压力和载荷。所产生的阻力取决于在表面上发展的湍流边界层流的特性。不幸的是，这两个设计要求不是彼此独立的，因为湍流边界层取决于由于压力分布而导致的上覆非零压力梯度。此外，边界层流还取决于表面的形貌特征（或粗糙度），其中小至10微米的特征被认为是流体动力学上的“粗糙”。这种边界层流动同时受到非零压力梯度和表面粗糙度影响的情况在工程和环境应用中是普遍存在的。示例包括流过翼型/涡轮机叶片、管道、流过/绕过建筑物、山丘/山谷等的气流。尽管其普遍存在，但压力梯度对流过粗糙表面的气流的影响在很大程度上仍未得到探索。随着我们努力到2050年实现净零碳排放，现在是时候开发新的理解和建模策略，以捕捉压力梯度对粗糙表面流动性能的影响。在这个项目中，我们的目标是研究非零压力梯度（PG）湍流边界层（TBL）的演化从而确定决定边界层对流向压力梯度的响应的参数。将进行一系列全面的风洞实验和数值模拟，以生成有关在有利和不利压力梯度（FPG和APG）下粗糙壁流动的前所未有的数据。这些数据将支持识别和验证与零压力梯度（ZPG）流相比，动量/能量传递机制的潜在普遍性（和差异）。然后，这些数据将用于开发和验证积分模型和新的大涡模拟（LES）模型，可用于预测在不同压力梯度的影响下，任意粗糙表面上的流动性能。这些数据将通过我们的粗糙度数据库（www.roughnessdatabase.org）在公共领域提供。总体目标是建立粗糙度和压力梯度之间的相互关系，在广泛的参数范围内（压力梯度和粗糙度属性），了解当前模型的局限性，并开发新的建模方法，可用于预测在广泛的应用。

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.