Numerical Investigation of the Reynolds Number and Pitch Ratio Effect on the Lock-In Ability of an...
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Transcript of Numerical Investigation of the Reynolds Number and Pitch Ratio Effect on the Lock-In Ability of an...
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
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
𝒇 𝒂𝒅𝒖𝒄𝒕
Off resonance
Off resonance
NOISE SELF-SUSTAINS and
ENHANCES
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)
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.
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
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)
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)
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
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
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 >
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)
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
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