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Numerical Investigation of Reynolds Number and Pitch Ratio Effect on
Lock-in Ability of an Aeroacoustic Field in Ducted Flows
Dept. of Mechanical and Manufacturing EngineeringTrinity College Dublin
Cristina Paduano
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Aeroacoustic Resonance of Bluff Bodies in Ducted Flows Noise intensification It can occur when a Gas Flow in a duct/cavity exhibits Periodic Vortices
Vortex shedding Duct acoustic mode
HYDRODYNAMIC
Vortex shedding at acoustic frequency
=
Tonal noise is emitted
Vorte
x she
ddin
g fre
quen
cy
LOCK-IN
Flow velocity
flow
đť’‡ đť’‚đť’…đť’–đť’„đť’•
Off resonance
Off resonance
NOISE SELF-SUSTAINS and
ENHANCES
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Aeroacoustic Resonance Behaviour of Tube Array
10 15 20 25 300
500
1000
1500
2000
V (m/s)
P a (Pa)
10 15 20 25 300
100
200
300
400
500
V (m/s)
Freq
uenc
y (H
z)
Pressure measurements (heat exchanger)
UNPREDICTABLE VELOCITYEXTENTS OF LOCK IN RANGE UNKNOWN
Velocity measurements (heat exchanger)
140 dB
(images from Finnegan -2011)
“Tube array resonance occurs when the energy available in the flow(dynamic head) overcomes the acoustic damping of the system” - (Feenstra et al.- 2006)
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Conditions for Resonance
(Hall, Ziada, Weaver data -2003)
Lock-in map (EXPERIMENTAL DATA)Co
nditi
ons f
or re
sona
nce
Amplitude of the acoustic wave
Frequency ratio
This research: Reynolds number and Pitch ratio
• To understand aeroacoustic resonance in tube array it is necessary to understand the strength of the sound sources formed around the tubes.
• Numerous experimental study for reduced array configuration (single -2- 4 cylinders) used a fixed width test section ( 1 fa) and varied fv.
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Research Motivations and Objectives• Mechanism of lock in is not yet clear
• Effect of turbulence increasing and variation of the vortices patterns were indicated as possible parameters contributing to resonance of tube array (Fitzpatrick -1980, Ziada-1989). However many experiments focused more on variation of frequency ratio.
Is there a flow characteristic which causes Lock in to occur ?
Does the aeroacoustic resonance of 2 and 4 cylinders configuration represent the aeroacustic resonance of tube array ?
Vorte
x she
ddin
g fre
quen
cy
Flow velocity
1Vortices incoherentstructure
Coherent acoustic sources Vortices
incoherentstructure
LOCK INFLOW
STRUCTURE
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CFD Simulation of Aeroacoustic Resonance
ACOUSTICS IS
“ COMPRESSIBLE”
INCOMPRESSIBLEFLOW
(uRANS, SST) += OSCILLATING VELOCITY (BOUNDARY CONDITION)
Hydrodynamic Analogy (Tan ,Thompson, Hourigan-2003)
TRASVERSAL ACOUSTIC WAVE replaced by the Flow OSCILLATION which it causesRESONANCE: chosen to be in LOCK-IN ratio with
=Asin(2t)
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ApplicationTwo cylinders in tandem
Four cylinders in square
In line multiple cylinder arrayVo
rtex s
hedd
ing
frequ
ency
Flow velocity
1
Pre-coinc. resonance
Coinc. resonance
IMPOSED LOCK IN CONDITION
FLO
W S
TRU
CTU
RE V
ARIA
TIO
NTURBULENCE EFFECT
Mean flow velocity variation applied (i.e. RE variation 10000-36000)
VORTICES CONVECTIVE VEL. VARIATION Variation of vortices convective velocity is
obtained by varying the pitch ratio L/D 2.5-3.
(Configuration analysed – Re and pitch as Finnegan-2011)
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Reynolds number Normalized frequency f/fv
Normalized frequency f/fvReynolds number
Pres
sure
, Pas
cals
Pres
sure
, Pas
cals
PreCoincidence /=1.2
Coincidence /=0.85
Two Cylinder Resonance- Reynolds number dependency
Lock in only occurring above Re 27000 –Reynolds number dependency
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LOCK-IN and Velocity contours
% V inlet
Normalized velocity WITHOUT EXCITATION
% V inlet
Normalized velocity case NOT LOCKED IN (Re=10000)
Normalized velocitycase LOCKED IN (Re=36000)
Normalized velocity WITHOUT EXCITATION
% V inlet% V inlet
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EXPERIMENTAL ACOUSTIC POWER
Acoustic PowerNUMERICAL ACOUSTIC POWER
(Finnegan, Meskell and Ziada data-2010)
PreCoincidence <
Coincidence >
Sinks (Flow takes energy from acoustics) Sources (Flow puts energy into acoustics)
PreCoincidence <
Coincidence >
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Four Cylinder Resonance - Summary of Results
Normalized frequency f/fv
Coincidence /=0.85 PICTH 2.5• Lock in only occurring at
Coincidence and for all Reynolds numbers
PICTH 3• Lock in only occurring at
Coincidence ONLY at the higher Reynolds number
Pres
sure
, Pas
cals
Reynolds number
Coincidence (Finnegan, Meskell and Ziada data-2010)
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Multiple Cylinder Array Resonance - Summary of Results
Coincidence /=0.85 –Pitch L/D 2.5 PICTH 2.5• Lock in only occurring at
Coincidence and for all Reynolds numbers
PICTH 3• Lock in NEVER OCCURRING
(Finnegan, Meskell and Ziada data-2010)Coincidence
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Conclusions
The cylinder configurations analysed have shown a different resonance response to the similar lock in excitation;
The onset of resonance appeared to be influenced by the Reynolds number (Two cylinders case) and influenced by the variation of the cylinders Pitch ratio
(Four cylinders case);
The frequency ratio could not be the only parameter instigating acoustic resonance, the flow condition (i.e. Turbulence and Vortices Convective Velocity) should be considered as well.
RE Pre-Coinc. Coinc.
Two Cylinders(L/D 2.5)
12000
36000
No resonance
Resonance
No resonance
Resonance
Four Cylinders(L/D 2.5)
12000
36000
No resonance
No resonance
Resonance
Resonance
Four Cylinders(L/D 3)
12000
36000
No resonance
No resonance
No resonance
Resonance
Array(L/D 2.5)
12000
36000
No resonance
No resonance
Resonance
Resonance
Array(L/D 3)
12000
36000
No resonance
No resonance
No resonance
No resonance