Post on 31-Aug-2018
Slide 1
Fundamental Study of Aero Acoustic Emission of Single
and Tandem Cylinder
Winson Lim Tan Chun Hern
Voo Keng Soon Vince 05 Nov 12
Slide 2
Brief Review of Flow Over Cylinder
Slide 3
Rationale • Landing gear system – major contributor to airframe noise during
approach • Cylinder–like structure includes: Gear main strut, hydraulic lines,
brake pistons, wheels and axles • Unsteady wake interacts with downstream components to create
dominant noise sources • Tandem cylinder – simplified prototype • Models component level interactions • To understand noise generation mechanism and achieve reduction
of airframe noise
Slide 4
Wake Characteristics of a Wake Behind a Cylinder
• At very low Re number < 49, the wake is steady and the wake comprises of two symmetrically placed vortices on each side of the wake
• At higher Re number, the wake becomes unsteady forming a vortex street as seen in (b)
Slide 5
Wake Shedding Frequency vs Re from Single Cylinder
Simulated Re in current study
Slide 6
Flow Interference Flow Characteristics (Cylinders in Tandem)
• Flow interference imposes continuous and discontinuous changes in vortex shedding
• Cylinder oscillations modified by and strongly dependent on cylinder arrangement
Slide 7
Wake Shedding States (Cylinders in Tandem)
• Critical tandem cylinder spacing configuration of L/D = 3.7 chosen
• Bistable flow state exists between cylinders
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
0.02 0.07 0.12 0.17
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
CL_
Dow
nstr
eam
Time t/s C
L_U
pstr
eam
Upstream & Downstream Cylinder CL vs Time (DES)
CL_Upstream CL_Downstream
Intermittent shedding state Constant shedding state
Slide 8
CFD Simulations of Cylinder Flow at Re = 3900
(URANS, LES & DES)
Slide 9
Meshing and Testing Conditions
Microphone Positions
•Trimmer mesh of 3 million cells •URANS,LES,DES •Diameter of 20mm and span of 80mm (4D) •Periodic boundary conditions at cylinder ends •Time –Step: 1.0e-5 sec •Reynolds Number studied: 3900 •Aeroacoustics module:
Ffowcs Williams – Hawkings
X
Y
Z
Slide 10
Comparison of Time Averaged Results (Re=3900)
Cases Span Cd Strouhal no. Experimental 4D 1.01 0.205 URANS 4D 1.078 0.214 LES 4D 0.998 0.213 DES 4D 1.088 0.196
-1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
60 160 260
CL
Cd
Time t (U/D)
Cd and CL vs Time (DES simulation) Far Field Frequency of 333 Hz -> Strouhal no. ~0.196
Slide 11
Streamwise Reynolds Stresses
0.0000
0.0250
0.0500
0.0750
0.1000
0.1250
0.1500
0.1750
0.2000
0.2250
-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
<u’u
’
z/d
<u'u'> vs z/d at x/d=1.54 plane
Experiment DES URANS LES
Streamwise Reynolds Stress
DES
URANS
LES
• DES predicts comparable peak values as compared to exp results
• Higher level of mixing predicted at the wake centre
• LES under predicts peak values as compared to exp results
• URANS under predicts <u'u'> in the wake
Slide 12
Shear & Spanwise Reynolds Stress
-0.2000
-0.1500
-0.1000
-0.0500
0.0000
0.0500
0.1000
0.1500
0.2000
-2.0 -1.0 0.0 1.0 2.0
<u’v
'>
z/d
<u’v'> vs z/d at x/d=1.54 plane
Experiment
URANS
LES
DES
-0.0010
0.0090
0.0190
0.0290
0.0390
0.0490
0.0590
0.0690
0.0790
0.0890
0.0990
-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
<w’w
'>
z/d
<w‘w'> vs z/d at x/d=1.54
plane
Experiment URANS
LES DES
Slide 13
Flow Structures Visualized by the Q Criteria (DES simulation, Re=3900)
• Pairs of counter-rotating streamwise vortices -> Karman vortex shedding characteristics
• Longitudinal vortices interlaced between counter-rotating vortices
Slide 14
OSPL Comparison at 70 Diameter Cases Overall
Sound Pressure Level (OSPL)
Peak Frequency
Peak Strouhal no.
WT data (Journal of Sound and Vibration)
83.8 308 0.193
DES 83.6 313 0.196
70D
Receiver position
D
Slide 15
Far-Field Sound Directivity Pattern
•Microphones are placed at 70D around the cylinder monitoring total pressure •Compute the ΔP’rms (pressure fluctuation component) at r=70D, non-dimensionalised by free stream velocity and density
0
0.00002
0.00004
0.00006
0.00008
0.0001 90
75 60
45
30
15
0
345
330
315
300 285
270 255
240
225
210
195
180
165
150
135
120 115
ΔP'rms vs Receiver Positions
• Far field directivity pattern exemplify the characteristics of dipole sound propagation
Slide 16
CFD Simulations of Cylinder Flow at Re = 46,000
(DES)
Slide 17
Meshing and Testing Conditions
Microphone Positions
•Trimmer mesh of 3.6 million cells •DES •Diameter of 9.8mm and span of 29.4mm (3D) •Periodic boundary conditions at cylinder ends •Time –Step: 1.0e-5 sec •Reynolds Number studied: 46000 •Aeroacoustics module:
Ffowcs Williams – Hawkings
X
Y
Z
Slide 18
Time Averaged Results for Re = 46000 Cases CD,average CD,rms
Strouhal no.
Experimental 1.35 0.16 0.19
LES_OpenLit 1.24 0.10 0.19
DES_current 1.235 0.141 0.20
CL -1.50 -1.30 -1.10 -0.90 -0.70 -0.50 -0.30 -0.10 0.10 0.30 0.50 0.70 0.90 1.10 1.30 1.50 1.70 1.90
300 350 400 450 500 550
Time t(U/D)
CD
CD,average
CL,average
Far Field Frequency of 1455.8 Hz -> Strouhal no. ~0.20
CL
Slide 19
Reynolds Stresses Characteristics at x/d=1.54 (Re=46,000)
0.000
0.010
0.020
0.030
0.040
0.050
0.060
-3 -2 -1 0 1 2 3
<w’w
'>
z/d
<w’w'> vs z/d
0.000
0.100
0.200
0.300
0.400
0.500
0.600
-3 -2 -1 0 1 2 3
<v’v
'>
z/d
<v’v'> vs z/d
-0.150
-0.100
-0.050
0.000
0.050
0.100
0.150
-3 -2 -1 0 1 2 3
<u’v
'>
z/d
<u’v'> vs z/d
0.000
0.050
0.100
0.150
0.200
-3 -2 -1 0 1 2 3
<u'u
'>
z/d
<u'u'> vs z/d
• Trends of the Reynolds stresses are typical to that of a turbulent wake
Slide 20
Flow Structures Visualized by the Q Criteria (DES simulation, Re=46,000)
Slide 21
0
20
40
60
80
100 90
60
30
0
330
300
270
240
210
180
150
120
SPL vs Receiver Positions
DES Experiment
Sound Directivity Pattern in terms of Prms & OSPL
•SPL of Experiment 91dB •SPL of DES~92dB Occurring at 1455.8Hz which corresponds to St number of 0.20 •SPL at other positions match quite closely between experimental and CFD results
0.00E+00 2.00E-05 4.00E-05 6.00E-05 8.00E-05 1.00E-04 1.20E-04 1.40E-04
90 75
60 45
30
15
0
345
330
315 300
285 270
255 240
225
210
195
180
165
150
135 120
115
ΔP'rms vs Receiver Positions
Slide 22
CFD Simulations of Tandem Cylinder Flow at Re = 1.66E5
(DES)
Slide 23
Meshing and Testing Conditions
Microphone Positions
•Trimmer mesh of 6.2 million cells •DES (K-w sst) •Diameter of 57.15mm and span of 171.45mm (3D) •Periodic boundary conditions at cylinder ends •Time –Step: 2.0e-5 sec •Physical Time Simulated: 0.216 sec •Reynolds Number studied: 1.66E5 •Aeroacoustics module:
Ffowcs Williams – Hawkings
X
Y
Z
Slide 24
0
20
40
60
80
100
120
140
10 100 1000
PSD
(dB
/Hz)
Frequency
DES
DES_Far field DES_Near field
Comparison of Time Averaged Results Upstream 90 Deg Experiment CFD(Literature) DES_Near field DES_Far field
Span 18D 18D 3D 3D
Frequency 178Hz 166Hz 158Hz 166Hz
Strouhal no. 0.232 0.219 0.203 0.213
Near field Frequency of 158 Hz -> Strouhal no. ~0.203
Far field Frequency of 166 Hz -> Strouhal no. ~0.213
Slide 25
Flow Structures Visualized by the Q Criteria (DES simulation, Re=1.66E5)
•Wake interference amplifies downstream shedding and oscillation
•Wake interference damp out downstream shedding and oscillation
•Counter-rotating streamwise vortices observed in inter-cylinder flow region
•Constant shedding flow state upstream
• Only shear layer roll up is observed in inter-cylinder flow region
•Intermittent shedding state upstream
Slide 26
Near Field Pressure Spectra
70
80
90
100
110
120
130
10 100 1000
PSD
(dB
/Hz)
Frequency
Upstream Cylinder 90 deg Spectra
DES Literature_CFD Literature_Experimental
70
80
90
100
110
120
130
140
10 100 1000
PSD
(dB
/Hz)
Frequency
Downstream Cylinder 90 deg Spectra
DES Literature_CFD Literature_Experimental
•Shedding frequency of near field spectra in good agreement with literature CFD results. •Deviation in peak values could be attributed to insufficient simulation time, where the bi-stable flow behaviour have yet to be statistically converged.
Slide 27
Microphone Spectra at 30D
Receiver / Microphone position
25
35
45
55
65
75
85
95
20 200 2000
PSD
(dB
/Hz)
Frequency
Microphone A
DES Literature_CFD Literature_Experiment
25
35
45
55
65
75
85
95
20 200 2000
PSD
(dB
/Hz)
Frequency
Microphone B
DES Literature_CFD Literature_Experiment
25
35
45
55
65
75
85
95
20 200 2000
PSD
(dB
/Hz)
Frequency
Microphone C
DES Literature_CFD Literature_Experiment
A
B
C
~30D
Peak SPL (dB)= 92.5 (exp 93.7)
Peak SPL (dB)= 91.6 (exp 95.6)
Peak SPL (dB)= 88.5 (exp 92.7)
Slide 28
Summary • Flow over cylinder at Re 3900 using URANS, DES and LES
– Strouhal number at vertical height of 70D away from the cylinder= 0.196 (exp 0.193); OSPL = 83.6 dB (exp83.8 dB)
– Directivity pattern of Δp’rms of receivers around the cylinder exemplify dipole sound propagation characteristics
– Trends of DES predicted Reynolds stresses were in relative good agreement with experimental results
– URANS & LES had tendency to under predict the magnitude of the principal components of the Reynolds stresses
• Flow over cylinder at Re 46,000 using DES – Fundamental Strouhal number = 0.2 (exp 0.19) at frequency of 1455.8 Hz – SPL at vertical height of 70D away from = 92 dB (exp 91dB) – Directivity pattern of Δp’rms of receivers around the cylinder exemplify dipole
sound propagation characteristics – Trends of the Reynolds stresses are typical to that of a turbulent wake
Slide 29
Summary • Flow over tandem cylinder at Re 1.66E5 DES
– Bi-stable flow states at critical cylinder spacing captured – Flow between cylinder switches between constant shedding and intermittent
shedding modes – Near field strouhal number at cylinder surface = 0.203 and far field = 0.213(exp
0.232) – Trends of DES predicted surface pressure distribution were in relative good
agreement with experimental results – Good farfield OASPL agreement with experiment at respective microphone
locations. Microphone A = 92.5 (exp 93.7) Microphone B = 91.6 (exp 95.6) Microphone C = 88.5 (exp 92.7)
– Directivity pattern of Δp’rms of receivers around the cylinder exemplify dipole sound propagation characteristics