from wind turbines
TRANSCRIPT
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1 / 04/20/2011
Aerodynamic Noise from Wind Turbines
Anupam Sharma Assistant Professor, Aerospace Engineering Iowa State University [email protected] http://aere.iastate.edu/sharma
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Agenda
> Basics of sound waves – What is sound? – Measurement units, energy in sound etc.
> Noise – what’s all the fuss about?
> Wind Turbine Noise (Sources) – Generated by vibration à Mechanical (Gearbox) – Generated by flow à Aerodynamic
> Noise Prediction
> Noise Reduction – Some Methods
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Sound Waves
> What is a sound wave?
> “Longitudinal” – particle motion in the direction of wave motion > Require a medium (air/water/solids) to propagate – unlike
electromagnetic waves
> “Perturbation” of density/pressure that propagates with the speed of sound
compression
rarefaction
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Sound Waves - Characteristics
Amplitude, frequency (pitch), speed, wavelength
Wave speed can be a function of frequency depending on the medium à dispersive media
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Sound Waves - Characteristics
> Frequency (cycles per second or Hz) – log scale
> Amplitude / energy measured in decibels – logarithmic scale
> Why logarithmic scale?
SPL = 10 log10
�p2rms/p
2ref
�
pref = 20 µPa
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Sound Waves - Characteristics
Time
domain
Frequency domain
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Sound Waves - Energy
Jet Engine: – 130 dB ~ 10 watts/sq. m of power density – 10 KW of net acoustic power – Contrast with ~ 100 MW of power generated by the engine (factor of
104) – “Needle in a haystack”
– Noise generation in jets is very inefficient … thank God for that! – I often tell my daughter “screaming does not work!”
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“Displeasing” Sound Waves - Noise
> Generate quite a bit of it when I “play” guitar! > Engineering machines make noise:
– Aircraft, aircraft engines, energy turbomachines, wind turbines, fans, your laptops … you name it
> Vibration generated noise – tuning fork, vibrating plate, etc.
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Sound generated by Aerodynamics
> Siren: (unsteady mass)
> Propeller: (unsteady force)
> Jet: (unsteady stress)
Heat (combustion): (unsteady heat) – http://www.youtube.com/watch?v=WBblqoVKO4w
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Noise – What’s All the Fuss About?
> Annoyance: sleep disturbance, “psycho”acoustics, deafness, etc
> $$ impact à Dept of Veteran Affairs spends ~ USD 1B / year on hearing loss!
> Regulations: can’t operate if exceed limits
> Limits use of Green Technologies – noise & efficiency trade
> In a wind turbine, noise limits rotor diameter, speed and number of turbines in a farm
Understanding / prediction of noise generation is a must before designing for “low” noise
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11 / 04/20/2011
GE 4.0
WIND TURBINE NOISE
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Wind Turbine Noise - Mechanical
Sources: Gearbox, generator, yaw/pitch drives, & cooling system > Gearbox: Teeth do not match exactly – periodic impact à vibration à
noise > Generator: vibration due to coil flexure > Drives: noise from hydraulic compressors > Cooling system: fan Mechanical Sources
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Aerodynamic Noise – Wind Turbines
13
Blade “self” noise
Inflow turbulence
noise
> Airflow over the turbine blades à aerodynamic noise > Primarily broadband – caused by turbulence:
1. Inflow turbulence noise 2. “Self” generated turbulence noise
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Wind Turbine Aerodynamic Noise
1. Inflow turbulence noise – Atmospheric / other turbine wake turbulence interacting with blade – Broadband at low frequencies (< 1KHz)
2. “Self” noise: turbulence generated by blade it“self” 1. Trailing edge noise 2. Separation / stall noise 3. Tip vortex noise 4. Other: laminar vortex shedding / trailing edge bluntness
Unsteady lift à noise
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Wind Turbine Aerodynamic Noise
Trailing edge noise – dominant
> Eddies in turbulent boundary layer (BL) – noise source in itself
> However, eddies radiate more efficiently (as dipoles) in the vicinity of the trailing edge
> “Scattering” from TE à alter TE geometry to reduce noise
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Wind Turbine Aerodynamic Noise
Separation / Stall noise:
> HAWTs often operate in a very transient, high angle-of-attack (Aoa) environment
– Finite time for pitch change to adapt to AoA change à BL separation and stall commonly observed
> Noise generation mechanism is the same – turbulence interacting with the blade (not just the trailing edge)
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Wind Turbine Aerodynamic Noise
M5 noise scaling limits rotor size … energy capture
Aerodynamic Sources
Tip Vortex Formation
Source ‘Assessment & Prediction of Wind Turbine Noise’ M.V. Lowson
Separated vortex flow interacts with blade surface
Broadband ~ 2 to 3 kHz
Component of blade swish
Minimised through design of the tip shape
Tip Vortex Noise
> Prandtl’s lifting line theory can be applied to study HAWT aero – Large vorticity shed near blade tip – Tip shape significantly affects noise
> Turbulence in blade vortex interacting with the tip (edge)
Tip vortices from aircraft wings
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HAWT Aerodynamic Noise – “Swishing”
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> “Picture” developed using an “acoustic telescope” > Noise from a turbine appears as “Swishing” noise (once per blade
passing)
Noise “sources” as perceived by an observer on ground
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What’s with the “Swish”?
Stand right below a HAWT (when running) You will hear a “swishing” noise It is not a blade passing tone It is due to “amplitude modulation” Approaching train … Doppler “shift” & “amplification” Now imagine a Ferris wheel where there is a siren on each car … stand right below it … you will hear a “swish” as each car goes by!
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Amplitude Modulation - AM
• Amplitude modulation (AM) for radio waves … now frequency modulation (FM) is more commonly used
• HAWT: 1. Blade passing time signal 2. Turbulence generated noise 3. Observer perceived signal
Signal to be transmitted
Carrier signal
Amplitude modulated signal
1
2
3
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Amplitude Modulation in a HAWT
Frequency
Sou
nd P
ress
ure
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Noise Prediction Approaches
> Empirical: a/f correlations, strip theory – Discretize blades into small span airfoils à sum contribution from each airfoil
> Modeling: RANS TKE à model sources; scattering from TE using analytical (Green’s functions) and numerical (BEM/LEE) methods
> Direct computation :: compute turbulence, scattering – all in one! EXPENSIVE!!
Source: Stanford flow gallery
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Noise Reduction – Some Concepts
> Trailing edge noise: – “sawteeth”, “brushes”, porous trailing edges
> Tip noise: – Tip shapes that reduce turbulence close to blade tip
> Inflow turbulence noise: – Ideas? … tubercles leading edge
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Farm Aero/Acoustics à Future
> Single turbine aero/noise reasonably well understood
> How turbines work in a farm? – Persistent wakes/vortices – Unsteady loads, aero, & inflow turbulence
noise
> Noise – Noise regulations on a farm level – Long range noise propagation: ray
acoustics, LEE, etc.
> Optimize: – layout (micrositing) & operations
X
Y
Z
SPL(TNO)5654525048464442403836343230
Farm noise
Farm aerodynamics
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Hope You Now Feel Familiar with …
> Basics of acoustics … aerodynamic noise
> Wind turbine aerodynamic noise sources
> Why the turbine produces a “swishing” noise
> General idea of how one can predict HAWT noise
> Concepts being tried to reduce noise
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Thank you for your attention
Questions?