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Reduction of Aerofoil Noise through Design Optimisation of the Trailing Edge

Reduction of aero acoustic pollution by introducing novel serrated trailing edges

Published: 19th December 2018
Reduction of Aerofoil Noise through Design Optimisation of the Trailing Edge
Please note, header image is purely illustrative. Source: Richard Unten, Wikimedia, CC BY 2.0

Background

The Problem

In the aviation industry, the broadband and tonal noise generated by the fan blades in the aircraft engine and the airframe’s high-lift devices have not been significantly reduced by the latest technologies, which were aimed primary at suppressing jet noise. It is clear that fan rotor noise and airframe noise surpass other components especially under landing conditions. Of all the possible fan engine and airframe noise mechanisms, the ‘self-noise’ radiated from the trailing edges of the fan rotor blade, the Outlet Guided Vane (OGV) and the leading edge slat remains one of the most important.

In the drone industry there is also a pressing need for ultra-quiet technology to enable applications such as large scale use for urban delivery or in conservation, filming and security/surveillance work where animals and people cannot be disturbed.

In the wind turbine industry, the proliferation of wind turbines as an environmentally more acceptable form of energy has important implications for their noise nuisance to communities living in close proximity. Wind turbines produce a characteristic ‘swishing’ noise that can be heard at considerable distance. This is particularly true at lower frequencies, which are not well attenuated by the atmosphere, and at night, when the downward refraction of sound can promote it being heard over larger distances. Similarly, trailing edge noise represents one of the major noise sources associated with the wind turbine blades.

Other industries which could also benefit from fan noise reduction are the Heating, Ventilating and Air Conditioning (HVAC) industry for both domestic and industrial uses, in-home fans (e.g.: on computers) and submarine propellers.

Previous Solutions

One of the effective approaches to reduce the trailing edge noise is to mimic the owl’s wing (known for quiet flight albeit the relatively large wing span) and to insert flat plates with “serration/saw-tooth” patterns to the trailing edges of an aerofoil blade. Various researchers have shown that trailing edge noise reductions at certain characteristic frequencies are possible. However, whilst noise reduction is achieved this method has FIVE major issues:

(1) This “add-on” method lacks the strength and structural integrity required in the wind turbines, propellers and rotor blades which are constantly subjected to high tensile stresses. This important safety issue becomes an obvious obstacle to widespread use.

(2) Only a moderate overall noise reduction (OASPL 2-3 dB) can be achieved due to the increased noise at high frequencies by the “leakage” of the transverse velocity component at the interstices between adjacent members of the saw-tooth;

(3) The majority of the serration is formed by inserting thin flat plates into the trailing edge, thereby altering the original aerofoil shape;

(4) The resulting aerodynamic performance deteriorates (increased drag and loss of lift)

(5) The overall level of noise reduction cannot exceed the limit prescribed by the serration geometry (i.e. serration amplitude and wavelength).

Technology Overview

Brunel University has recently developed a novel trailing edge serration device which can overcome the above issues. For the new technology, the saw-tooth patterns of the serration are filled with a porous material to suppress the vortex shedding tonal noise. Several new aerofoil prototypes have been tested, demonstrating significant broadband noise reduction, whilst the high frequency “leakage” noise and the vortex shedding tonal noise generated at the interstices between adjacent members of the saw-tooth is inhibited effectively. Most importantly, the addition of porous materials within the interstices provides an additional mechanism of self-noise reduction. In other words, the serration and porous material can work constructively with each other to significantly improve the aeroacoustics performance of aerofoil.

An optimal choice of the porous materials has been identified, where it can achieve broadband noise reduction of at least 4 dB higher in single frequency, or 1.5 dB higher in overall sound pressure level than the conventional serrated trailing edge whilst completely suppress the vortex shedding tone (http://dx.doi.org/10.1121/1.4961362)

Importantly, the new trailing edge serration device is more industrially-ready than the conventional “add-on” type serration due to its superior structural integrity.

In addition the new device produces an aerodynamic performance similar to that of the baseline aerofoil.

Benefits

Pursuing this innovative research can significantly reduce aerofoil self-noise radiation to levels much lower than possible by current technologies:

  • Achieves significant broadband noise reduction across a large velocity band
  • Completely suppresses the vortex shedding tonal noise
  • No noise increase at high frequencies
  • The noise performance is insensitive to the unsteady aero-elasticity forces acting on the turbine blade
  • Preserves the original aerofoil profile
  • Provides improved structural integrity
  • No degradation of the aerodynamic performance at pre-stall regime

Considering the current restrictions on expanding airport capacity resulting from the environmental issues and the associated financial implications, the ability to significantly reduce aircraft noise, particularly landing noise would be of particular benefit to the aviation industry.

In the drone market there is a significant need for ultra-quiet flight technology.

With the government’s target of acquiring energy from renewable sources, this new technology for fan noise reduction has application in the wind turbine industries.

Applications

The application of the noise control treatment is on any device or machine in which aerofoil blades rotate in a smooth (non-turbulent) air flow and hence where trailing edge noise is the dominant noise source.

As far as the aerospace industry is concerned, the reduction of rotor/ propeller noise, will allow aircraft manufacturers to meet future noise targets and it can be used by commercial helicopter operators, military operators, emergency services, turboprops etc. At the drone industry, they can be used for delivery (quieter automated delivery systems, so more acceptable in urban areas), film making (reduced video editing required due to less sound pollution) and surveillance, as well as security/ military purposes.

A wind turbine blade with this technology can operate faster, longer and in night time, hence increasing the amount of electric generation and consequently the onshore wind turbine farms.

However, there is scope for creating a family of products that can also be applied elsewhere. These include the Heating, Ventilating and Air Conditioning (HVAC) industry, open rotor, rotating wing aircraft, power generation gas turbines, cooling fans in home appliances including personal computers.

Poro‑serrated trailing edges at the HVAC (construction, industrial premises and domestic use (e.g kitchen, home air conditioning & appliances) industry will provide a quieter working environment with potential better psychological state implications. Moreover, it will be easier to reach regulatory standards for fan manufacturers, making increased sales and profitability possible for them.

The adoption of the trailing edge device for the above applications is straightforward and does not require frequent maintenance. Further avenues of improvement involve the creation of the porosity in situ using advanced manufacturing techniques or replacing the metal foams with other lightweight, permeable objects. Lastly, it has the poro‑serrated trailing edge to be created via 3D printing, an easy, fast method, with the ultimate goal of large scale manufacturing.

Opportunity

Whilst the proof of concept has been established, the new trailing edge device could be further developed by using other metal foams/porous materials. The porous medium can even be designed in-house using Computer Aided Design software to arrive with the desired shapes, and to tune the desired porosity, permeability and flow resistivity. Most importantly, advanced manufacturing technique should be developed to increase the Technological Readiness Level of the new trailing edge device.

Seeking fan blade manufacturers and 3D designers in respective above mentioned industries for the discussion, further research and or licensing of this invention.

Seeking co-development partners for improving the TRL, joint application of KTP etc.

Brunel University researchers have also developed the related technology: “Actively-Controlled Morphing Wing Adaptive Skin for Enhanced Aircraft Control and Performance

IP Status
  • Patent application submitted
Seeking
  • Development partner