This numerical investigation studied the effect of Reynolds number and pitch ratio on the lock-in ability of an aeroacoustic field in ducted flows. The study found that:
- The onset of aeroacoustic resonance depended on both the Reynolds number and the pitch ratio of the cylinders. Higher Reynolds numbers and smaller pitch ratios increased the likelihood of resonance.
- For a two cylinder configuration, resonance only occurred above a Reynolds number of 27,000.
- For a four cylinder configuration, resonance was more likely at smaller pitch ratios and higher Reynolds numbers.
- A multiple cylinder array only exhibited resonance under conditions of high Reynolds number and small pitch ratio that produced coherent vortex shedding matching the acoustic frequency
Semelhante a Numerical Investigation of the Reynolds Number and Pitch Ratio Effect on the Lock-In Ability of an Aero-Acoustic Field in Ducted flows". (20)
Numerical Investigation of the Reynolds Number and Pitch Ratio Effect on the Lock-In Ability of an Aero-Acoustic Field in Ducted flows".
1. 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 Engineering
Trinity College Dublin
Cristina Paduano
2. 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
𝒇 𝒗 𝒇 𝒂
HYDRODYNAMIC
Vortex shedding at
acoustic frequency
𝒇 𝒗 = 𝒇 𝒂
Tonal noise
is emitted
Vortexsheddingfrequency
LOCK-IN
Flow velocity
flow
𝒇 𝒂 𝒅𝒖𝒄𝒕
Off
resonance
Off
resonance
NOISE
SELF-SUSTAINS and
ENHANCES
3. Aeroacoustic Resonance Behaviour of Tube Array
10 15 20 25 30
0
500
1000
1500
2000
V
(m/s)
Pa(Pa)
10 15 20 25 30
0
100
200
300
400
500
V
(m/s)
Frequency(Hz)
Pressure measurements (heat exchanger)
UNPREDICTABLE VELOCITY
EXTENTS 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)
4. Conditions for Resonance
(Hall, Ziada, Weaver data -2003)
Lock-in map (EXPERIMENTAL DATA)
Conditionsforresonance
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.
5. 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 ?
Vortexsheddingfrequency
Flow velocity
𝒇 𝒂
𝒇 𝒗
=1
Vortices
incoherent
structure
Coherent
acoustic
sources Vortices
incoherent
structure
LOCK IN
FLOW
STRUCTURE
6. CFD Simulation of Aeroacoustic Resonance
ACOUSTICS
IS
“ COMPRESSIBLE”
INCOMPRESSIBLE
FLOW
(uRANS, SST) += OSCILLATING VELOCITY
(BOUNDARY CONDITION)
Hydrodynamic Analogy (Tan ,Thompson, Hourigan-2003)
TRASVERSAL ACOUSTIC WAVE replaced by the Flow OSCILLATION which it causes
RESONANCE: 𝑓𝑎 chosen to be in LOCK-IN ratio with 𝑓𝑣
𝑼 𝒂𝒄𝒔=Asin(2 𝑓𝑎t)
7. Application
Two cylinders in tandem
Four cylinders in square
In line multiple cylinder array
Vortexsheddingfrequency
Flow velocity
𝒇 𝒂
𝒇 𝒗
=1
Pre-coinc.
resonance
Coinc.
resonance
IMPOSED LOCK IN CONDITION
FLOWSTRUCTUREVARIATION TURBULENCE 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)
9. LOCK-IN and Velocity contours
% V inlet
Normalized velocity
WITHOUT EXCITATION
% V inlet
Normalized velocity
case NOT LOCKED IN (Re=10000)
Normalized velocity
case LOCKED IN (Re=36000)
Normalized velocity
WITHOUT EXCITATION
% V inlet% V inlet
10. EXPERIMENTAL ACOUSTIC POWER
Acoustic Power
NUMERICAL ACOUSTIC POWER
(Finnegan, Meskell and Ziada data-2010)
PreCoincidence 𝑓𝑣 < 𝑓𝑎
Coincidence 𝑓𝑣 > 𝑓𝑎
Sinks (Flow takes energy from acoustics)
Sources (Flow puts energy into acoustics)
PreCoincidence 𝑓𝑣 < 𝑓𝑎
Coincidence 𝑓𝑣 > 𝑓𝑎
11. Four Cylinder Resonance - Summary of Results
Coincidence 𝑓𝑎 /𝑓𝑣 =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
Pressure,Pascals
Coincidence 𝑅𝑒 36000 (𝑃𝑖𝑐𝑡ℎ 2.5) (Finnegan, Meskell and Ziada data-2010)
12. Multiple Cylinder Array Resonance - Summary of Results
Coincidence 𝑓𝑎 /𝑓𝑣 =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 𝑅𝑒 36000 (𝑃𝑖𝑐𝑡ℎ 2.5)
13. 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
