As silent as an owl - Acoustic Metamaterials reduce energy consumption and noise of objects in air or water

A case study by Tim Starkey^a and Alastair Hibbins^a

^aUniversity of Exeter

The problem

The movement of any object through a fluid, such as air or water, will create a disturbance in the medium surrounding it.  Up to a characteristic speed, the fluid will move in a smooth path or layer around the moving object – it is said to be laminar.  However once a critical limit is exceeded, this disturbance becomes irregular – the speed of the fluid will continuously undergo changes to its magnitude and direction, i.e. it is chaotic.  In turbulent flow, unsteady vortices appear of many sizes which interact with each other, consequently drag due to friction effects increases. This increases the energy consumption of this system – the energy is wasted in the form of acoustic noise (e.g. an open window in a moving car) and/or in the “wake” behind the ship in the ocean.  While turbulence can be exploited, for example, by devices such as aerodynamic spoilers on aircraft that destroy the laminar flow to increase drag and reduce lift, turbulence is an indicator of inefficient energy use. In addition, aircraft noise can cause health problems, for both people living near to airports and marine life living near shipping channels.

Our solution

In our research we employ surfaces and structures that support trapped acoustic surface waves. In this way we can influence the disturbances in the fluid generated by the movement of the body, and in turn delay the onset of turbulence, reduce the radiated noise and the energy wasted in the wakes.

Our work is being carried out through a number of different projects, primarily via a collaboration with Prof William Devenport in the Department of Aerospace and Ocean Engineering at Virginia Tech (USA) as part of an ONR-funded program, making use of their Wind Tunnel facility. Prof Devenport and his team are world-leaders in this field, having studied the silent flight of many species of owls – a trait that they owe to the fine structure of their feathers and the downy canopy that surrounds them. 

Exeter research staff and PhD students are supported by Dstl, and there is a related EPSRC Industrial Case Award PhD studentship with Thales UK “Control of flow-induced near-field noise through the use of metasurfaces”.

Why use a metamaterial?

As is often the case, new opportunities are found at the intersection of different areas. Metamaterials research has raised new possibilities for the manipulation and control of sound fields, including the design of structures that allow air flow but block sound, and the cloaking of solid objects. However, there has been little, if any, work on applying such ideas to realistic fluid flow situations, despite the fact that this technology might provide new avenues to flow control, the control of flow noise, and the shielding of surface mounted instrumentation.

Our research in this area is a systematic study with the overall goal of revealing the fundamental physical mechanisms and issues that come into play when metamaterials are used with flow.

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