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Numerical Investigation of Reynolds number effect on Lock-in
Ability of an Aeroacoustic Field in Ducted
Flows
Dept. of Mechanical and Manufacturing EngineeringTrinity College Dublin
Cristina Paduano Dr.Craig Meskell
Aeroacoustic Resonance Overview 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
Vo
rte
x sh
ed
din
g f
req
ue
ncy
LOCK-IN
Flow velocity
flow
𝒇 𝒂𝒅𝒖𝒄𝒕
Off resonance
Off resonance
NOISE SELF-SUSTAINS and
ENHANCES
Physics behind the phenomenonAeroacoustic resonance
Flow induces an Acoustic field
Flow-Acoustics exchange energy
Howe’s reformulation of Lightill’s analogy
Vorticity as a source of sound
Homogeneous Wave Equation Source of Sound
1𝑐2
𝜕2𝑝 ′
𝜕𝑡 2 −𝛻2𝑝 ′=𝜌𝛻 ∙ (�⃗�×𝑉 )
Vortices need to deform(interact-impact body)
Unbalanced Sound: flow compresses and
decompresses
Physics behind the phenomenonAeroacoustic resonance
Flow induces an Acoustic field
Flow-Acoustics exchange energy
Vorticity
Acoustic particles velocity
Velocity
Howe’s Integral Acoustic power, localisation of sources
Not organized vortices Organized
vortices
Flow induces an Acoustic field
Flow-Acoustics exchange energy
Vo
rte
x sh
ed
din
g f
req
ue
ncy
LOCK- IN
Flow velocity
Not organized vortices
Aeroacoustic resonance
Motivation
HOW THE ACOUSTIC FIELD
INITIATES RESONANCE ?
Low pressure vortices
Low energy induced acoustics
Acoustic pressures << flow pressures
Aeroacoustic Resonance behavior
10 15 20 25 300
100
200
300
400
500
V (m/s)
Freq
uenc
y (H
z)
10 15 20 25 300
500
1000
1500
2000
V (m/s)
Pa (
Pa)
PROBLEM OF NOISE ,VIBRATIONS (FATIGUE FAILURE)
UPPER LIMIT TO THE PRACTICABLE FLOW VELOCITIES ACROSS A SYSTEM
(REDUCED EFFICIENCY)
Pressure measurements (heat exchanger)
UNPREDICTABLE VELOCITYEXTENTS OF LOCK IN RANGE UNKNOWN
NO TOOLS AVAILABLE TO DESIGN AGAINST
AEROACOUSTIC RESONANCE
Velocity measurements (heat exchanger)
140 dB
An industrial design concern Heat exchangers Corrugated pipes Heat Ventilation Air Conditioning systems (HVAC) Aircrafts Environmental Control Systems (ECS) Aircrafts cavities
flow
Sound wavepropagation
Sound wavepropagation
Our research: Two cylinders in cross flow
Test cases for resonanceCylinders in cross flow : good model for aeroacoustic resonance of ducted flows
Flow separationFlow instabilitiesVortex shedding (well known)
• Minimal 3D effects if cylinder is long (Length > >Diameter)
TYPICAL OF NOISE
GENERATING FLOWS
flow
Cylinders Pitch L/D
D
L
Conditions for resonance
(Hall, Ziada, Weaver data -2003)
Lock-in map (EXPERIMENTAL DATA)CO
ND
ITIO
NS
for R
ESO
NAN
CE
Amplitude of the acoustic wave
Frequency of vortex shedding approaches natural
frequency of duct/cavity
OUR RESEARCH:REYNOLDS NUMBER
HAS AN EFFECT ON LOCK-IN!
CFD simulation of 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: fa chosen to be in LOCK-IN ratio with fv
Uacs=Asin(2fat)
Hydrodynamic analogy response: Lock-in maps
EXPERIMENTAL LOCK-IN MAP
(Mohany and Ziada data-2009)-Single cylinder
(Reyes,Finnegan, Meskell data -2010)-Two cylinders L/D=2.5
(Hall, Ziada, Weaver data -2003)
NUMERICAL LOCK-IN MAP
Simulations parameters
𝑓 𝑎𝑓 𝑣
=𝟎 .𝟖𝟓𝑓 𝑎𝑓 𝑣
=𝟏 .𝟐
12 flow velocities(from 12m/s to 40m/s)
Reynolds n.10000-36000
Vo
rte
x sh
ed
din
g f
req
ue
ncy
Flow velocity
1
Flows excitation:Uacs=Asin(2fat)
fa=1.2 fv
fa=0.85 fv
Pre-coinc. resonance
Coinc. resonance
fv
A=10% Vinlet
Vortex shedding frequencies
Oscillating Lift (verse changes at each shed )
FFT of oscillating of the downstream cylinder
Each simulation has shown a clear vortex shedding (example v=12 m/s )
Strouhal simulated against experimental
CONSIDERATIONS:
Simulated Strouhal higher (20%)
Model is 2D
No boundaries
Experiments in wind tunnel (side walls effects)
STROUHAL n. of 0.18 similar to experimental one (Finnegan, Ziada,Meskell-2010)
Normalized frequencies, f/fv Reynolds numbers
Pre
ssu
re,
Pasc
als
Excitation frequency 2
Acoustics has LOCKED the frequency of vortex shedding JUST for simulations run at Reynolds above 27000
Vortex shedding
frequencies
Results pre-coincidence 2 Acoustic
frequencies
Results coincidence
Normalized frequencies, f/fv Reynolds numbers
Pre
ssu
re,
Pasc
als
Acousticfrequencies Vortex
shedding frequencies
Excitation frequency 85
Acoustics has LOCKED the frequency of vortex shedding JUST for simulations run at Reynolds above 27000
Reynolds number Normalized frequency f/fv
Normalized frequency f/fvReynolds number
Pre
ssure
, Pa
scals
Pre
ssure
, Pa
scals
PreCoincidence /=1.2
Reynolds number dependency of Lock-in
Coincidence /=0.85
Lock-in map at Reynolds 27000
EXPERIMENTAL LOCK-IN MAP
Lock-in map of tandem cylinders obtained numerically well compare with the experimental one(L/D=2.5)
(Hall, Ziada, Weaver data -2003)
NUMERICAL LOCK-IN MAP
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
ConclusionsHOW THE ACOUSTIC FIELD INITIATES RESONANCE ?
Condition for resonance : Frequency ratioAmplitude
• Lock-in only occurred at the higher Reynolds n.(above 27000) for both pre-coincidence and coincidence situations
• Vorticity appears to be reduced in the 2D model at lock-in
Higher Reynolds n. (turbulence higher, flow stronger) flow more prone to be controlled by acoustics ?
Could Turbulence be a source of energy for the Acoustic field?
Lock-in
Pre
ssure
, Pa
scals
Reynolds number Normalized frequency f/fv