fundamental study of aero acoustic emission of single … · fundamental study of aero acoustic...

Fundamental Study of Aero Acoustic Emission of Single

and Tandem Cylinder

Winson Lim Tan Chun Hern

Voo Keng Soon Vince 05 Nov 12

Brief Review of Flow Over Cylinder

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

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)

Wake Shedding Frequency vs Re from Single Cylinder

Simulated Re in current study

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

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

CFD Simulations of Cylinder Flow at Re = 3900

(URANS, LES & DES)

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

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

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

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

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

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

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

CFD Simulations of Cylinder Flow at Re = 46,000

(DES)



•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:


X

Y

Z

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

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

Flow Structures Visualized by the Q Criteria (DES simulation, Re=46,000)

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

CFD Simulations of Tandem Cylinder Flow at Re = 1.66E5

(DES)



•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:


X

Y

Z

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

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

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.

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


25

35

45

55

65

75

85

95

20 200 2000

PSD

(dB

/Hz)

Frequency

Microphone C


A

B

C

~30D

Peak SPL (dB)= 92.5 (exp 93.7)



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

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

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