Post on 12-Dec-2015
description
© 2014 ANSYS, Inc. September, 2014 1
Aero-acoustic assessment of turbomachinery using advanced turbulence modelling methods
Satish Patange
ANSYS UK Ltd
© 2014 ANSYS, Inc. September, 2014 2
Outline
Acoustics modeling
Applications of rotating machines
Sound propagation
Fan flow
SRS
© 2014 ANSYS, Inc. September, 2014 3
Acoustics – Key featuresMagnitude of acoustic waves is very
small compared to aerodynamic
pressure.
Acoustic radiation contains only a tiny
fraction of primary flow energy.
• Most unsteadiness in flow is ‘pseudo sound’
and does not radiate!
Acoustic problems are unsteady!
Frequency range of interest is quite
large:
� Frequency range 20Hz – 20kHz
• Temporal resolution for acoustics is orders of
magnitude larger than the interesting time
scales in the flow.
• Small eddies need to be captured, requires
spatial resolution.
© 2014 ANSYS, Inc. September, 2014 4
Aero acoustics – Source Classification
Monopole simple source
Quadrupoletwo dipoles
Unsteady mass
injection
Acoustic ~ U 3M
Power
Unsteady external
forces
Acoustic ~ U 3M 3
Power
Unsteady turbulent
shear stresses
Acoustic ~ U 3M 5
Power
Monopole and dipole sources dominant at low Mach numbers.
Scaling valid for acoustically compact sources, λ >> L!
psurface = psurface(t) τ = τ(t))(tmm && =
Dipole two mopoles
© 2014 ANSYS, Inc. September, 2014 5
Turbomachinery NoiseDiscrete + Broadband
Steady Rotating Forces
(Lowson/Gutin Models)
Discrete
Unsteady Rotating Forces
Discrete + Broadband
– Steady flow, discrete – Unsteady flow, discrete + broadband
– Secondary flow, discrete + broadband
– Vortex shedding, narrowband + broadband
– Turbulent BL, broadband
MonopoleBlade Thickness NoiseDiscrete
DipoleBlade Loading NoiseDiscrete + Broadband
QuadrupoleTurbulence NoiseBroadband
Aero acoustics – Approaches
© 2014 ANSYS, Inc. September, 2014 6
Aero acoustics – Simulation Basics
Aeroacoustics modeling involves simulation of
two aspects
• Sound source
– Provides source characteristics and rankings
• Sound propagation
– Propagation of sound from the source to the receiver
• Requires input of source characteristics
• Provides
– Sound spectrum and receiver
– Sound directivity
© 2014 ANSYS, Inc. September, 2014 7
Sound source simulation is done with detailed CFD analysis of
flow around the blades, hub, shroud, etc.
Can be done in two ways
• Steady State
• Transient
Advantages/Disadvantages
• Steady State
– Computationally cheap, fast, but not very accurate
• Transient
– Computationally expensive, slow, but more accurate
• After all, sound generation is a highly transient phenomenon
Aero acoustics – Simulation Basics
© 2014 ANSYS, Inc. September, 2014 8
Practical usage of Steady State and Transient methods
Simple hand
calculations
Steady StateTransient and
Experimentation
Design
Possibilities
Final
Design
Design
Screening
Methods
Increasing Accuracy and Expense
Aero acoustics – Simulation Basics
© 2014 ANSYS, Inc. September, 2014 9
Aero acoustics – Simulation Basics
Sound propagation can be calculated in different ways
CAA (Computational Aero Acoustics)
• Direct sound computation
• Uses the transient turbulence modeling capability in CFD
– LES, WMLES, DES, Detached DES and Scale Adaptive
simulation
SSPM (Segregated Source-Propagation Methods)
• Propagation is decoupled from source
– Source and propagation are treated as mutually independent
– Models many be used for computing propagation
• Lighthill-Curle Method
• Ffowcs-Williams-Hawkings Method
• FEM/BEM (Solution of Lighthill’s equation/Wave equation)
© 2014 ANSYS, Inc. September, 2014 10
Computational Aero Acoustics (CAA)
In fact, wave equation is a special case of Navier-Stokes equations.
� CFD solves the Navier-Stokes equations.
� In theory, sound generation as well as propagation can be simulated by:
• Transient, compressible CFD simulation
With computational domain spanning from sources to receivers!
• Monitor static pressure at the receiver locations as function of time
SRS (or URANS if tonal noise)
� No further models involved!
10 100 1000Frequency [Hz]
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
SPL [dB]
Freestream Velocity = 140 km/h
Experimental data
SAS model
Sensor 121
SAS of a side view mirror
(ReD = 520 000)
© 2014 ANSYS, Inc. September, 2014 11
Acoustic analogy
Acoustic analogy assumes acoustics can be decoupled from flow dynamics.
On the basis of Lighthill’s analogy:
• Noise Sources are assumed in a uniform fluid at rest
• Acoustic field at observer is described by wave equation
• Resolution of acoustic and dynamic flow field are decoupled
Based on two steps:
• Simulate transient flow field accurately using CFD to get the acoustic sources
location and intensity
• Propagate noise from sources to receiver by solving wave equation
analytically
CFD domain
Wave EquationAcoustic sources
Acoustic
receiver
© 2014 ANSYS, Inc. September, 2014 12
Acoustic Analogy – Integral FW-H
Williams, J. E. F. and Hawkings, D. L. (1969): Sound generation by turbulence and surfaces in
arbitrary motion. Philosophical Transactions of the Royal Society, Vol. A264, pp. 321-342.
The solution contains surface integrals over source surfaces and a
volume integral
� Less sensitive to proper placement of permeable source surfaces
than other integral methods (e.g. Kirchhoff)
Volume integral not directly solved – too time consuming
� Collect all sources inside permeable surface
Noise generated in the fluid volume
(Quadrupole)
Loading noise
(Dipole)
Thickness noise
(Monopole)
where
© 2014 ANSYS, Inc. September, 2014 13
FW-H – Example: Canon Loudspeaker
Bass-reflex loudspeaker to increase
efficiency of the system at low
frequencies
� Low frequencies sound radiated
through the port and added in phase
with the driver front wave
3 million cells, Δt = 8*10-6 s
deforming zone
moving zone
Courtesy of Canon
M.Younsi, G.Kergourlay, V. Morgenthaler, Near Field and
Far Field Prediction of Noise in and around a
Loudspeaker: A Numerical and Experimental
Investigation EURONOISE 2012, 10–13 June, Prague
© 2014 ANSYS, Inc. September, 2014 14
Lowson/Gutin ModelLowson
• Noise level @ specific location
• Unsteady load replaced by steady load
multiplied by exponential decay function
(semi-empirical):
− Wave propagation from rotational
symmetric geometries (Bessel
function)
Gutin
• Steady loading noise of blades
• Solver rotates signal, not geometry
• Considers thickness noise (monopoles)
and loading noise (dipoles)
( )( )
h
0y
0x
y
x
F
F
F
F−λ
=
λ
λDirectivity plot of
1st harmonic
Steady-state vs. transient for 2-bladed fan
© 2014 ANSYS, Inc. September, 2014 15
Aero acoustics – Approaches
Features &
LimitationsCAA
Acoustic
analogy
(FWH)
Lowson/
Gutin
Modal-
Analysis
Broadband
noise
modeling
Computation
costMost Fair Moderate Moderate Least
Account for
reflectionYes No No No No
Account for
effect of
sound on
flow
Yes No No No No
Solution
schemeTransient Transient Steady State Steady State Steady State
AccuracyVery
GoodGood Limited Limited Limited
3rd party
coupling
(1-way)
Fair
Yes
No
Transient
Good
Decreasing computational effort
Increasing accuracy
© 2014 ANSYS, Inc. September, 2014 16
Applications in rotating machines
© 2014 ANSYS, Inc. September, 2014 17
Fan Noise Macro in CFD-Post
Specific to ANSYS CFX
Based on the Lowson Noise Model
• Low speed machines : Tip Mach Number < 0.35
• High Mach number : Less accurate
• Forces acting as punctual force on gravity center
• Small blade span
• Usage of semi-empirical coefficient to define loading decay
Acoustics Pressure at mth Harmonic
( ) ( ) ( ) ( )ϕ
λλ−−λϕ−
πω
= λ−
+∞=λ
−∞=λ
λ−∑ sinmzMJM
F.
mz
mzFcosi
r.c..2
imzp mz
y
X
mz
1o
2
m
Where, λ = Harmonic Mode; M =Mach Number; z = Blade Number; ω = Rotational Speed (rad/s)
h = Loading Coefficient (2.0 ~ 2.5)
( )( )
h
0y
0x
y
x
F
F
F
F−λ
=
λ
λUnsteady Force Components Steady State Force Components : Fx0 & Fy0
© 2014 ANSYS, Inc. September, 2014 18
Fan / Turbo Noise Macro [1]
Typical Fan Noise Output Results
Fan / Turbo Noise Calculation Examples
© 2014 ANSYS, Inc. September, 2014 19
Broadband Noise Models
Two kinds of models are available in ANSYS Fluent
• Broadband models based on averaged quantities
– Proudman’s formula for turbulence noise
– Turbulent boundary layer noise model
– Jet noise model (2D axisymmetric only)
• Broadband models based on reconstruction of flow field fluctuations
– Source terms in Linearized Euler equations (LEE)
– Source terms in Lilley’s equation
ANSYS CFX
• Estimate of noise source strength
– Monopole sources
– Dipole or rotating dipole sources
– Quadrupole sources
© 2014 ANSYS, Inc. September, 2014 20
ANSYS CFX-Pre setupANSYS CFD Post
Monopole Terms
Sound Source Strength Prediction [1]
© 2014 ANSYS, Inc. September, 2014 21
ANSYS CFX-Pre setupANSYS CFD Post
Dipole Terms
Sound Source Strength Prediction [2]
© 2014 ANSYS, Inc. September, 2014 22
ANSYS CFX-Pre setupANSYS CFD Post
Quadrupole Terms
Sound Source Strength Prediction [3]
© 2014 ANSYS, Inc. September, 2014 23
Ori
gin
al
De
sig
n
Radial
Forward
Op
tim
ize
d
De
sig
n
Forward
radial
Design point
Measurements
Forw
ard
: Lo
w N
ois
e a
t D
esi
gn
Po
int
Design Comparison
Sound Source Strength Prediction [4]
© 2014 ANSYS, Inc. September, 2014 24
Case Study #1
• Aeroacoustics Modeling of a Centrifugal Fan Using ANSYS CFX
– N = 3000 rpm Z = 39 Blades
– Near Field & Far Field Noise Prediction
– Steady Flow Simulation using SST
Turbulence Model
– Unsteady Flow Simulation using Scale
Adaptive Simulation (SAS) Turbulence
Model
– Node Count = 2.177 Million
– 6 Near Field Microphone (Two used to
Capture Noise Spectra)
– 1 Far Field Microphone
– Far Field Noise Modeling using ANSYS
CFX Turbo Noise Macro Based on
Lowson Model
© 2014 ANSYS, Inc. September, 2014 25
Near Field Microphones
Far Field Microphones
Case Study #1
• Aeroacoustics Modeling of a Centrifugal Fan Using ANSYS CFX
© 2014 ANSYS, Inc. September, 2014 26
Near Field Noise Prediction
Microphone #1 Microphone #4
Case Study #1
• Aeroacoustics Modeling of a Centrifugal Fan Using ANSYS CFX
© 2014 ANSYS, Inc. September, 2014 27
0 2000 4000 6000 8000
Frequency, Hz
0
10
20
30
40
50
60
70
Sound Pressure levels, dB
Experimental data
TurboNoise macro
At BPF SPL [dB]
TurboNoise 56.8
Experiments 55.9
Near Field Noise Prediction
ANSYS CFX Turbo Noise (Based
on Lowson Model)
Case Study #1
• Aeroacoustics Modeling of a Centrifugal Fan Using ANSYS CFX
© 2014 ANSYS, Inc. September, 2014 28
Case Study #2
• Aeroacoustics Modeling of an Automotive Electric Cooling Fan
Using ANSYS Fluent
– Free-standing fan (open to
atmosphere on all sides)
– Nine, evenly spaced blades Fan Speed
= 2000 rpm
– Single Reference Frame
– Single Blade Modeling
– Cell Count ~ 10 Million
– LES Turbulence Model
– FW-H Model For Far-Field Sound
Propagation
© 2014 ANSYS, Inc. September, 2014 29
Low values
occur at
higher radius
due to higher
flow velocity
• Grid & Temporal Resolution Verification:
– Height of First Cell Center on the blade
is roughly equal to the Taylor length
scale (λλλλ)
– Grid is fine enough to capture eddies in
the inertial sub-range
– Therefore the grid is good for
conducting a “true” LES computation
– Time step required for LES ≈ λλλλ/U
– Steady state results indicate that the
timestep for transient LES solution
should be roughly 1E-6 second.
Taylor Length Scale λ:λ:λ:λ: Blade Pressure Side
• Aeroacoustics Modeling of an Automotive Electric Cooling Fan
Using ANSYS Fluent
Case Study #2
© 2014 ANSYS, Inc. September, 2014 30
• Aeroacoustics Modeling of an Automotive Electric Cooling Fan
Using ANSYS Fluent
Source Pressure Spectra
0
20
40
60
80
100
120
140
160
0 500 1000 1500 2000 2500
Frequency (Hz)
SPL (dB)
pt01 pt02
pt03 pt04
pt05 pt06
Near Field Sound Spectra
Case Study #2
© 2014 ANSYS, Inc. September, 2014 31
• Aeroacoustics Modeling of an Automotive Electric Cooling Fan
Using ANSYS Fluent
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400 1600 1800 2000
SPL (dBA)
Frequency (Hz)
2000 RPM, 0 degrees, 1 meter
Far Field Sound Spectra
Flow Structure
Instantaneous Iso-
Surface of 2nd Invariant
of Velocity Gradient
Rotation
Rotation
Vortices in Near Wall
Region
Case Study #2
© 2014 ANSYS, Inc. September, 2014 32
Conclusion
• Unsteady simulations are the future for many CFD applications.
• A wide spectrum of Scale Resolving models are available in ANSYS CFD :
o LES,
o WMLES,
o (D-)DES,
o EMBEDDED LES,
o SAS.
• Such models can be combined with different acoustic approaches,
particularly:
o Direct CAA,
o Acoustic analogy (FW-H).
• Question is: Which approach is best suited for which type of flows?
� Best ratio of cost vs. performance.
� Safest environment for user (limited sensitivity to mesh, time step, …).
• User feed-back is always welcome and appreciated!