Aerodynamics and aeroacoustics of turbulent flows over and past permeable rough surfaces

A variety of turbulent flows are either over porous surfaces or in the wake of bodies with surface porosity. Evolution, engineering design, manufacturing constraints and natural phenomena lead to rough and permeable boundaries in these porous surfaces (e.g. flow in heat exchangers, forest and urban canopies, bird feathers and river beds). The permeability and the roughness of a porous surface alters the turbulent boundary layer that develops over it and consequently the wake past an object with surface porosity. This entirely depends on the interaction between the external flow in the boundary layer over the roughness or the wake and the flow field within the porous media. Despite the wide-ranging impact and relevance, there is a clear lack of fundamental understanding and the scaling laws that relate the properties of porous media to the features of external flow. This points to the clear need for a systematic fundamental study aimed at understanding the flow mechanisms and the relationship between the properties of porous substrates, the flow field within the porous media and the structure of turbulent flow over and past them.

In this ambitious collaborative project we combine the experimental expertise at Southampton and Bristol with the computational and modelling expertise at Cambridge and Southampton, to gain fundamental understanding of the turbulent boundary-layer and wake flow over and past permeable rough surfaces. This will be used to develop a fully-validated modelling framework that can predict the aerodynamics and aeroacoustics of turbulent flows interacting with realistic porous surfaces. Specifically, we will (1) examine the aerodynamic and aeroacoustic characteristics of wall-turbulence that develop over porous surfaces using high-fidelity experiments, (2) we will perform detailed DNS and LES of flow over porous surfaces using new in-house tools to understand the interaction between internal flow within the porous media and external flows, (3) use the experimental and numerical data to develop new models to represent the interaction between internal and external flow of porous surfaces in other lower fidelity simulations, (4) Utilise the new models in LES to predict the aerodynamics and aeroacoustics and (5) carry out measurements on the aerodynamics and aeroacoustics of flow over and past permeable rough surfaces over a large range of Reynolds numbers to further our understanding as well as to validate our new models. Ultimately, this will enable us and our industry partner to examine the utility of different realistic porous surfaces in flow/noise control.

This research will benefit the transportation and the energy supply sector in the UK and around the world. This project is relevant to various applications including in the aerospace and maritime shipping industries as well oil and natural-gas production industries where flow and noise control is essential to meet the challenges of the 21st century. In addition to the commercial private sector (Economy) and the associated environmental impacts (Society), this research will deliver benefits via the scientific advances (Knowledge) and training (People).

The proposed work is of high relevance to aviation, automotive, maritime and energy Industries. To meet their environmental targets, the aviation and wind energy sectors need to gradually improve their products and reduce their noise signature. Therefore, the ability to reduce the noise generation and accurately predict the noise level is of great importance. Part of this project is funded by Industry to ensure a proper knowledge transfer during the course the research and development of an industrially viable noise reduction method based on the findings of this research. Throughout the course of this project, we will invite members from different companies relevant to the transport and energy sectors to the University and brief them on the progress of this project and its importance to strategies that they may be already pursuing. Ultimately, this project aims to be game changer in terms of options available for flow and noise control in high Reynolds number flows.

Exposure to high levels of noise over long periods of time is a critical problem. This is particularly the case for the residential areas near large airports and wind farms. The significant increase in the number of short- and mid-range flights in the near future and also construction of new large wind-farms will mean that more people will be exposed to high levels of noise. A major part of this noise is due to the flow interaction within turbulent boundary layers. This project is to tackle the noise problem at a fundamental level, enhancing our understanding of the noise generation mechanism and ultimately helping the development of more robust noise reduction methods. Similarly, improving the aerodynamic performance of different systems across different applications (aerospace, shipping, automotive) is vitally important to reduction of harmful emissions. For example, the aircraft industry has committed to reducing NOx emissions by 80% and halving carbon emissions by 2020. Although the proposed work is not going to make an impact now, it will certainly be critical in the medium to longer term. This is clearly recognised by our project partner (Embraer) who are supporting this work with a PhD student.

Apart from developing a new flow control technology as well a fully-validated modelling framework, the additional benefits of the current project to the general public is two-fold. First, the fact that this is first study to systematically examine flow over porous media in the context of flow control and ensure UK as a leading player in this area of research. Second, the training of researchers in this critical research area will prevent the erosion of expertise in the area of experimental and computational fluid mechanics in the UK, which is an area of priority for EPSRC.