Fundamental Study of Aero Acoustic Emission of Single and Tandem Cylinder Winson Lim Tan Chun Hern Voo Keng Soon Vince 05 Nov 12 Slide 1
Brief Review of Flow Over Cylinder Slide 2
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 3
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 4
Wake Shedding Frequency vs Re from Single Cylinder Simulated Re in current study Slide 5
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 6
Wake Shedding States (Cylinders in Tandem) Upstream & Downstream Cylinder CL vs Time (DES) 1.50 0.20 0.15 1.00 0.10 0.50 CL_Downstream 0.05 CL_Upstream 0.00 0.00 -0.05 -0.50 -0.10 -1.00 -0.15 0.02 0.07 0.12 0.17 -1.50 • Critical tandem cylinder spacing -0.20 Time t/s CL_Upstream CL_Downstream configuration of L/D = 3.7 chosen • Bistable flow state exists Constant shedding state Intermittent shedding state between cylinders Slide 7
CFD Simulations of Cylinder Flow at Re = 3900 (URANS, LES & DES) Slide 8
Meshing and Testing Conditions •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 Y Z Microphone Positions X Slide 9
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 Far Field Frequency of 333 Hz Cd and CL vs Time (DES simulation) 1.5 1.5 1.4 -> Strouhal no. ~0.196 1.4 1.3 1.3 1.2 1.2 1.1 1.1 1.0 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 Cd 0.2 0.1 CL 0 0.1 -0.1 0.0 -0.2 -0.1 -0.3 -0.2 -0.4 -0.3 -0.5 -0.4 -0.6 -0.5 -0.7 -0.6 -0.8 -0.7 -0.9 -1 -0.8 -1.1 -0.9 -1.2 -1.0 -1.3 -1.1 -1.4 -1.2 -1.5 60 160 260 Time t (U/D) Slide 10
Streamwise Reynolds Stresses DES • DES predicts <u'u'> vs z/d at x/d=1.54 plane comparable peak values as compared to exp 0.2250 results 0.2000 • Higher level of mixing predicted at the wake 0.1750 centre 0.1500 LES < u’u’ • LES under predicts 0.1250 peak values as compared to exp results 0.1000 0.0750 0.0500 URANS 0.0250 • URANS under predicts <u'u'> in the wake z/d 0.0000 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Experiment DES URANS LES Streamwise Reynolds Stress Slide 11
Shear & Spanwise Reynolds Stress <u’v'> vs z/d at x/d=1.54 plane <w‘w'> vs z/d at x/d=1.54 0.2000 0.0990 plane 0.1500 0.0890 0.0790 0.1000 Experiment 0.0690 0.0500 0.0590 URANS <u’v'> 0.0490 0.0000 <w’w'> 0.0390 LES -0.0500 0.0290 -0.1000 0.0190 DES 0.0090 -0.1500 -0.0010 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 -0.2000 -2.0 -1.0 0.0 1.0 2.0 z/d z/d Experiment URANS LES DES Slide 12
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 13
OSPL Comparison at 70 Diameter Cases Overall Peak Peak Strouhal Sound Frequency no. Pressure Level (OSPL) WT data (Journal of 83.8 308 0.193 Sound and Vibration) DES 83.6 313 0.196 Receiver position 70D D Slide 14
Far-Field Sound Directivity Pattern ΔP'rms vs Receiver Positions 90 115 75 0.0001 • Far field directivity 120 60 0.00008 pattern exemplify the 135 45 0.00006 characteristics of dipole 150 30 0.00004 sound propagation 165 15 0.00002 180 0 0 •Microphones are placed at 70D around the 195 345 cylinder monitoring total pressure •Compute the Δ P’ rms (pressure fluctuation 210 330 component) at r=70D, non-dimensionalised by free stream velocity and density 225 315 240 300 255 285 270 Slide 15
CFD Simulations of Cylinder Flow at Re = 46,000 (DES) Slide 16
Meshing and Testing Conditions •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 Y Z Microphone Positions X Slide 17
Time Averaged Results for Re = 46000 Cases C D,average C D,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 Far Field Frequency of 1455.8 Hz - 1.90 C D 1.70 > Strouhal no. ~0.20 1.50 1.30 1.10 C D,average 0.90 0.70 0.50 0.30 0.10 -0.10 C L,average -0.30 -0.50 -0.70 -0.90 -1.10 -1.30 C L C L Time t(U/D) -1.50 300 350 400 450 500 550 Slide 18
Reynolds Stresses Characteristics at x/d=1.54 (Re=46,000) <u'u'> vs z/d <w’w'> vs z/d 0.200 0.060 0.050 0.150 • Trends of the 0.040 Reynolds stresses <w’w'> 0.100 0.030 <u'u'> are typical to that of 0.020 0.050 a turbulent wake 0.010 0.000 0.000 z/d z/d -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 <v’v'> vs z/d 0.600 <u’v'> vs z/d 0.150 0.500 0.100 0.400 0.050 <v’v'> 0.300 0.000 <u’v'> 0.200 -0.050 0.100 -0.100 0.000 z/d -0.150 z/d -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 Slide 19
Flow Structures Visualized by the Q Criteria (DES simulation, Re=46,000) Slide 20
Sound Directivity Pattern in terms of P rms & OSPL ΔP'rms vs Receiver Positions SPL vs Receiver Positions •SPL of Experiment 91dB 90 90 115 75 1.40E-04 100 •SPL of DES~ 92dB 120 60 120 60 1.20E-04 Occurring at 1455.8 Hz 80 135 45 1.00E-04 which corresponds to St 8.00E-05 60 150 30 150 30 6.00E-05 number of 0.20 40 4.00E-05 165 15 •SPL at other positions 2.00E-05 20 match quite closely between 180 0 0.00E+00 180 0 0 experimental and CFD 195 345 results 210 330 225 315 210 330 240 300 255 285 270 240 300 270 DES Experiment Slide 21
CFD Simulations of Tandem Cylinder Flow at Re = 1.66E5 (DES) Slide 22
Meshing and Testing Conditions •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 Y Z X Microphone Positions Slide 23
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 Far field Frequency of 166 Hz -> Near field Frequency of 158 Hz DES Strouhal no. ~0.213 -> Strouhal no. ~0.203 140 120 100 PSD (dB/Hz) 80 60 40 20 0 10 100 1000 Frequency DES_Far field DES_Near field Slide 24
Flow Structures Visualized by the Q Criteria (DES simulation, Re=1.66E5) •Counter-rotating •Wake interference streamwise vortices amplifies downstream observed in inter- shedding and cylinder flow region oscillation •Constant shedding flow state upstream • Only shear layer roll up is observed in inter-cylinder flow •Wake interference region damp out downstream shedding and •Intermittent oscillation shedding state upstream Slide 25
Near Field Pressure Spectra Upstream Cylinder 90 deg Spectra Downstream Cylinder 90 deg Spectra 130 140 130 120 120 110 PSD (dB/Hz) PSD (dB/Hz) 110 100 100 90 90 80 80 70 70 10 100 1000 10 100 1000 Frequency Frequency DES Literature_CFD Literature_Experimental 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 26
Microphone Spectra at 30D B Microphone B 95 A Peak SPL (dB)= 91.6 C 85 (exp 95.6) PSD (dB/Hz) 75 ~30D 65 55 45 35 25 20 200 2000 Frequency Receiver / Microphone position DES Literature_CFD Literature_Experiment Microphone A Microphone C 95 Peak SPL (dB)= 92.5 95 85 Peak SPL (dB)= 88.5 (exp 93.7) 85 (exp 92.7) PSD (dB/Hz) 75 PSD (dB/Hz) 75 65 65 55 55 45 45 35 35 25 25 20 200 2000 Frequency 20 200 2000 Frequency DES Literature_CFD Literature_Experiment DES Literature_CFD Literature_Experiment Slide 27
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