Reduction of aero acoustic pollution by introducing novel serrated trailing edges.
In the drone industry there is a pressing need for ultra-quite technology to enable applications such as large scale urban use or in conservation, filming and security/surveillance work where animals and people cannot be disturbed. As development of electric urban aerial transport systems moves forward the silence of the electric engine makes the noise from the blades all the more noticeable.
In the aviation industry, the broadband and tonal noise generated by the fan blades in the aircraft engine and the air-frame’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 air-frame noise surpass other components especially under landing conditions. Of all the possible fan engine and air-frame 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 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.
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).
Brunel’s New Solution
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.
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
In the drone market there is a significant need for ultra-quiet flight technology.
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.
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.
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.
Bespoke modifications to blades can be tested on Brunel's testing ring as demonstrated here https://youtu.be/9200O2sB7QY
A family of patents:
Reduction of noise from drone (and other unmanned aerial vehicles), airplanes, wind turbines, fans and other applications of aerofoils
Aviation, Wind-power electricity generation, rotary blades