The effect of flow-permeable material on the flow field and the aerodynamic
noise of cylinders
Thomas F. GEYER(1) , Richard PESCHEL
1 Technical Acoustics Group, Brandenburg University of Technology Cottbus - Senftenberg, Germany, thomas.geyer@b-tu.de

Abstract
The flow around circular cylinders is one of the major aeroacoustic noise source mechanisms. Such a cylinder
represents a simple model for technical applications like parts of the landing gear of airplanes, train pantographs,
antennas or other protruding parts of vehicles. A reduction of the flow noise is especially desirable since it
contains both a broadband component and tonal components due to the regular vortex shedding. One approachto reduce this noise is to cover the cylinder with flow-permeable materials, such as foams or porous rubbers.
In a recent experimental study in the aeroacoustic wind tunnel at the Brandenburg University of Technology,
both the aerodynamic noise radiation as well as the flow field around a set of porous covered cylinders were
investigated. The acoustic measurements were performed at Reynolds numbers (based on cylinder diameter)
between 16,000 and 100,000 using single microphones, while the detailed flow measurements were done using
hot-wire anemometry. The results show the influence of the material properties and of the thickness of theporous layer on the aeroacoustic noise and on the turbulence in the wake of the cylinders.
Keywords: Porous Cylinder, Vortex Shedding, Aeroacoustic Noise

INTRODUCTION
The noise generated by a two-dimensional cylinder in a cross flow is a fundamental problem in aeroacoustics. It
is composed of both broandband noise and a strong tonal noise contribution generated by the regular sheddingof vortices (the so-called aeolian tone). Thus, much research is done to study the effects of passive porous
coatings on the flow field and the associated generation of aeroacoustic noise [1–16].
The present paper is a continuation of a previous study [8] on the reduction of vortex shedding noise from twodimensional cylinders through the use of porous covers. However, the current paper focuses on the measurementof the flow in the vicinity of the porous covered cylinders. Only selected results of the acoustic measurements
will be shown to correlate the flow field results.

MEASUREMENT SETUP
2.1 Wind Tunnel
All experiments took place in the small aeroacoustic open jet wind tunnel at the Brandenburg University ofTechnology in Cottbus [17], using a nozzle with a rectangular exit area of 0.23 m × 0.28 m. With this nozzle,
the maximum flow speed is in the order of 60 m/s, with a low turbulence intensity in the order of 0.2 %. The
circular cylinders were fixed between two side plates, with a distance of 0.15 m from the nozzle exit area.
2.2 Porous Cylinders
As in [8], the cylinders examined in the present study consist of non-porous core cylinders made of polyurethane,
which are covered by different flow permeable materials. The resulting outer diameter D of these cylinders is
30 mm, with a length l of 0.28 m. This results in an aspect ratio l/D of 9.3. In most cases, the diameterof the core cylinder, d, was 10 mm, while for one porous cover this parameter was varied, taking values of
6 mm, 10 mm, 15 mm and 20 mm. This was done in order to examine the influence of the thickness of theporous layer on the noise generation and the flow field. The porous materials are characterized by their airflow

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Table 1. Overview of cylinders used for the present study (for the cases with the outer cylinder diameter printed
bold, only acoustic measurements were performed)
NameMaterial

Description

Reference
Porous 1
Porous 2
Porous 3
Porous 4

Regufoam Vibration 190 plus
Getzner CM GR 0525
Damtec Black Rubber
Packing Foam(non-porous) polyurethane
rubber-polyurethane-composite
polyurethane
rubber granulate
polyurethane

l
(mm)
280
280
280
280
280

D
(mm)
30
30
30
30
30

d
(mm)
10
10
6, 10, 15, 20
10r(Pa s/m2 )∞314,800
141,300
9,400
4,100

Figure 1. Photograph of the four cylinders (half-)covered by Damtec Black Rubber (from top to bottom:
d = 20 mm, 15 mm, 10 mm and 6 mm)
resistivity r, which was measured according to ISO 9053 [18]. In total, acoustic measurements were performedon cylinders covered with 19 different materials, while flow measurements were only performed on a subset ofthese cylinders. For brevity, results will only be shown in the present paper for seven different cylinders (fourporous covers plus different thicknesses) and a non-porous reference cylinder with the same outer diameter Dthat serves as a baseline. Table 1 lists these cylinders, while Figure 1 shows a photograph of the cylinders halfcovered by Damtec Black Rubber.
2.3 Acoustic Measurement Setup
For acoustic measurements, the wind tunnel test section is surrounded by absorbing side walls, leading to aquasi anechoic measurements environment for frequencies approximately above 125 Hz. The cylinders were
mounted vertically (along the z-direction). The measurements were conducted with 16 1/4th inch free fieldmeasurement microphones, located on a horizontal arc centered at the cylinder axis approximately at mid span(see Figure 2). The exact microphone positions are at azimuthal angles of 30◦ to 130◦ in 10◦ -steps on one sideand -30◦ to -110◦ in 20◦ -steps on the other side of the cylinder, with a distance of 0.66 m to the cylinder axis.
For brevity, only results obtained with the microphone located at +90◦ to the flow will be shown in the present
paper.
The data were recorded with a sampling frequency of 51.2 kHz and a duration of 90 s using a National
Instruments 24 Bit multichannel measurement system. In post processing, the time data were converted to thefrequency domain by a Fast Fourier Transformation (FFT) on Hanning-windowed blocks of 16,384 samples withan overlap of 75 % using Welchs method [19]. This lead to sound pressure level spectra with a frequency step
size of 3.125 Hz.

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Figure 2. Photograph of the setup for the acoustic measurements
2.4 Flow Measurement Setup
In order to quantify the flow field around the cylinders, Constant Temperature Anemometry (CTA) was utilized.
For these experiments, the cylinders were mounted horizontally (along the y-direction). The measurements were
performed using a Dantec single wire P11 probe, which was positioned using a 3D traverse system with aminimum step size of 0.1 mm, and a Dantec 8-channel CTA hardware system. The sampling frequency was set
to 25.6 kHz. A photograph of the measurement setup is shown in Figure 3(a).
In total, three different kinds of measurements were performed. First, profiles of the mean velocity and theturbulence intensity were obtained along the vertical direction at four downstream distances of 0.5D, 1D, 5D and
10D in the wake of the cylinders, approximately at mid span. Thereby, the turbulence intensity was calculated
as the ratio of the root-mean-square value of the turbulent velocity fluctuations to the “outer” flow speed U0 .
Figure 3(b) shows a schematic of the setup used for these measurements. Second, power spectral densities of
the turbulent velocity fluctuations were measured at a point one outer diameter downstream and one half outer
diameter off center, approximately at mid span. This was done for ten flow speeds approximately between 8 m/s
and 52 m/s. Finally, the coherence γ 2 was measured along a spanwise line one outer diameter downstream fromthe cylinders. This was done using two identical probes. One probe was fixed, with a distance of one outer
diameter from the side plate, while the other was traversed along the span in steps of 2.5 mm (D/12) awayfrom the first probe.

RESULTS
3.1 Acoustic Results
Figure 4 exemplarily shows sound pressure level spectra obtained for the different cylinders at a single flow
speed of 40.7 m/s. In agreement with previous results shown in [8], it is visible that the porous covers do notlead to a complete suppression of the vortex shedding tone, but to a notable narrowing of the peak. At the sametime, the peak level may even increase compared to the reference cylinder. The noise reducing effect is betterfor porous covers with low air flow resistivities (Figure 4(a)). Although the effect is rather small, in increase of
the thickness of the cover (or a decrease of the inner diameter d) is also beneficial, as visible in Figure 4(b).
3.2 Flow Field Results
The effect of the porous covers on the wake profiles of the mean velocity and the turbulence intensity is shownfor two different distances in Figures 5(a) and 5(b). It can be seen that the porous covers lead to a reduced
mean velocity and a reduced turbulence intensity directly downstream of the cylinder in its shadow zone. For

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D

d
1/2D

D

z

5D

y

10D
x

(a) Photographtraverse system(b) Schematic of the wake measurements (not to scale)Figure 3. Setup for the constant temperature anemometry measurements
90
70
60
50

70
60
50

40

40

30

30

20
0.1

0.2

0.5

f in kHz12(a) Influence of porous material (d = 10 mm)

porous 3, d = 6 mm
porous 3, d = 10 mm
porous 3, d = 15 mm
porous 3, d = 20 mm
reference
background80
Lp in dB80
Lp in dB90

porous 1, d = 10 mm
porous 2, d = 10 mm
porous 3, d = 10 mm
porous 4, d = 10 mm
reference
background

5

20
0.1

0.2

0.5

f in kHz125(b) Influence of thickness (Porous 3)

Figure 4. Sound pressure levels measured at a flow speed of 40.7 m/s (Re = 80,700)
x/D = 0.5, the mean velocity in the side region (the shear layer) is increased due to the porous covers, but
their effect on the turbulence intensity in this region is very small only. The width of the shear layer seems to
be increased for the porous cylinders, since the region of high turbulence has an increased extent in z-direction.
Power spectral densities of the velocity fluctuations measured at different flow speeds in the wake of the cylinders are shown in Figure 6. Basically, the same trend is visible as for the acoustic spectra shown in Figure 4:
The porous covers lead to a narrowing of the vortex shedding peaks and to an increase of the peak amplitude.
The latter effect is again presented in Figure 7(a), where the magnitude of the vortex shedding peak of the
turbulence spectra is shown as a function of the Reynolds number Re = U · D/ν (with ν being the kinematic
viscosity of the fluid). Basically, the peak magnitude of the porous covered cylinders is always higher than thatof the reference cylinder. Materials having a low airflow resisitivity lead to higher peaks than materials withlow air flow resistivities, while there is no clear trend regarding the influence of the thickness of the porous
cover. The trend regarding the corresponding Strouhal number based on vortex shedding, Sr = fpeak · D/U0 , ismore complex, which is shown in Figure 7(b). At Reynolds numbers approximately below 40,000, the Strouhal
numbers of the porous covered cylinders are slightly below that of the non-porous reference cylinder. At higher
Reynolds numbers, the Strouhal numbers of the porous covered cylinders increase notably to values well abovethat of the reference cylinder. Thereby, materials with low air flow resistivities lead to a higher Strouhal number

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3
2

x/D = 0.5

porous 1, d = 10 mm
porous 2, d = 10 mm
porous 3, d = 10 mm
porous 3, d = 20 mm
porous 4, d = 10 mm
referencex/D = 10

z/D

1
0
-1
-2
-3

0.4

0.6

0.8

1.0

U/U0

1.2 0.3

0.4

0.5

0.6

0.7

U/U0

0.8

0.9

1.0

1.1

(a) Mean flow velocity

3x/D = 0.5

2porous 1, d = 10 mm
porous 2, d = 10 mm
porous 3, d = 10 mm
porous 3, d = 20 mm
porous 4, d = 10 mm
referencex/D = 10

z/D

1
0
-1
-2
-3
0.00

0.05

0.10

0.15

Tu

0.20

0.25

0.30

0.35 0.00

0.05

0.10

0.15

Tu

0.20

0.25

0.30

0.35

(b) Turbulence intensity

Figure 5. Profiles of mean velocity (top) and turbulence intensity (bottom) measured at two downstream distances at a flow speed of 34 m/s (left: close to the cylinder, right: far downstream of the cylinder)
than materials with low air flow resistivities. Such a behavior was also observed in previous acoustic measurements [8].
Finally, the spanwise coherence γ 2 between data from one hot-wire probe at a fixed position and one hot-wireprobe that was traversed along a spanwise line in the wake of the cylinders is shown in Figure 8. Interestingly,
the coherence measured for the porous covered cylinders is much stronger along the spanwise direction than for
the reference cylinder. This means that turbulent structures, which are shed from the porous covered cylinders,have a greater spanwise length than for the baseline case. Thus, the flow noise reduction that can be achievedby the porous covers is not caused by any disturbance of the spanwise coherence. Still, it can be observed
that, as is the case for the turbulence spectra measured in the wake (Figure 6), the maxima of the spanwise
coherence appear narrower along the frequency axis.

ACKNOWLEDGEMENTS
The authors thank Julia Genßler for her help with the acoustic measurements.

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20

10

10

10·log10 (Φ/Φ0 ) in dB

10·log10 (Φ/Φ0 ) in dB

20

0

-10

-10

-20
-30
0.03

0

-20

0.05

0.1

0.2

f in kHz

0.3

0.5

1

-30
0.03

0.05

20

20

10

10

0

-10

1

0.5

1

0.5

1

-20

0.05

0.1

0.2

f in kHz

0.3

0.5

1

-30
0.03

0.05

0.1

0.2

f in kHz

0.3(d) Porous 3, d = 10 mm

20

20

10

10

10·log10 (Φ/Φ0 ) in dB

10·log10 (Φ/Φ0 ) in dB

0.50(c) Porous 2, d = 10 mm

0

-10

0

-10

-20
-30
0.03

0.3

-10

-20
-30
0.03

0.2

f in kHz(b) Porous 1, d = 10 mm

10·log10 (Φ/Φ0 ) in dB

10·log10 (Φ/Φ0 ) in dB(a) Reference

0.1

-20

0.05

0.1

0.2
f in kHz

0.3(e) Porous 3, d = 20 mm

0.5

1

-30
0.03

0.05

0.1

0.2

f in kHz

0.3(f) Porous 4, d = 10 mm

Figure 6. Power spectral densities of the velocity fluctuations measured at velocities between 8 m/s and 52 m/s(the opacity of the lines decreases with increasing flow speed)

REFERENCES[1] Ikeda, M., Takaishi, T., Perforated pantograph horn aeolian tone suppression mechanism. Quarterly Reportof RTRI, 45(3), 2004, 169-174[2] Yahathugoda, I., Akishita, S., Experimental investigation on surface impedance effect of sound radiationfrom low Mach number flow around a circular cylinder. JSME International Journal, Series B Fluids andThermal Engineering, 48(2), 2005, 342-349[3] Bruneau, C. H., Mortazavi, I., Numerical modelling and passive flow control using porous media. Computers& Fluids, 37(5), 2008, 488-4985340

max(10·log10 (Φ/Φ0 )) in dB

18
16
14
12
10
8
6
4
2
0
10000

porous 1, d = 10 mm
porous 2, d = 10 mm
porous 3, d = 10 mm
porous 3, d = 20 mm
porous 4, d = 10 mm
reference

20000

30000

40000

50000

60000
Re

70000

80000

90000

100000110000Sr(a) Dependence of peak magnitude of turbulence spectral density on Reynolds number based on outer cylinder diameter

0.200
0.195
0.190
0.185
0.180
0.175
0.170
0.165
0.160
10000

porous 1, d = 10 mm
porous 2, d = 10 mm
porous 3, di = 10 mm
porous 3, d = 20 mm
porous 4, d = 10 mm
reference

20000

30000

40000

50000

60000
Re

70000

80000

90000

100000110000(b) Dependence of peak Strouhal number on Reynolds number based on outer cylinder diameter

Figure 7. Properties of the vortex shedding peak derived from the turbulence spectra shown in Figure 6[4] Sueki, T., Ikeda, M., Takaishi, T., Kurita, T., Yamada, H., Reduction of aerodynamic noise from high-speed
pantograph using porous materials. Journal of Environment and Engineering, 5(3), 2010, 469-484[5] Sueki, T., Takaishi, T., Ikeda, M., Arai, N., Application of porous material to reduce aerodynamic soundfrom bluff bodies. Fluid dynamics research, 42(1), 015004, 2010
[6] Liu, H., Wei, J., Qu, Z., Prediction of aerodynamic noise reduction by using open-cell metal foam. Journalof Sound and Vibration, 331(7), 2012, 1483-1497[7] Naito, H., Fukagata, K., Numerical simulation of flow around a circular cylinder having porous surface.
Physics of Fluids, 24(11), 117102, 2012[8] Geyer, T. F., Sarradj, E., Circular cylinders with soft porous cover for flow noise reduction, Experiments in
Fluids, Vol 57 (3), 2016[9] Aguiar, J., Yao, H., Liu, Y., Passive flow/noise control of a cylinder using metal foam. In: Proceedings ofthe 23rd International Congress on Sound and Vibration, 2016[10] Yuan, H., Xia, C., Chen, Y., Yang, Z., Flow around a finite circular cylinder coated with porous media.
In: Proceedings of the 8th International Colloquium on Bluff Body Aerodynamics and Applications, 2016[11] Klausmann, K., Ruck, B., Drag reduction of circular cylinders by porous coating on the leeward side.
Journal of Fluid Mechanics, 813, 2017, 382-411[12] Aguiar, J., Birch, D. M., Pelacci, M., Wake vortex mechanisms behind semi-porous cylinders. In: 2018
Applied Aerodynamics Conference, AIAA paper 2018-3002, 2018

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4
3
2
1
0
0.125

0.2

0.315 0.5
f in kHz

0.8

5
4
3
2
1
0
0.125

(a) Reference

6

y/D

5
4
3
2
1
0.315 0.5
f in kHz(d) Porous 3, d = 10 mm

4
3
2
1
0
0.125

0.8

0.8

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

6
5
4
3
2
1
0
0.125

0.2

0.315 0.5
f in kHz(e) Porous 3, d = 20 mm

0.2

0.315

f in kHz0.5

0.8(c) Porous 2, d = 10 mm

7

y/D

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.2

0.315 0.5
f in kHz5(b) Porous 1, d = 10 mm

7

0
0.125

0.2

6

0.8

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

7
6
5
y/D

y/D

5

6

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

7

y/D

6

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

7

y/D

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

7

4
3
2
1
0
0.125

0.2

0.315

f in kHz

0.5

0.8(f) Porous 4, d = 10 mm

Figure 8. Spanwise coherence γ 2 measured downstream of the cylinders at a flow speed of 34 m/s[13] Arcondoulis, E., Liu, Y., The Effect of Porosity on the Porous Coated Cylinder Diameter. In: Acoustics
2018: Hear to Listen, Adelaide, 2018[14] Geyer, T. F., Sharma, S., Sarradj, E., Detached Eddy Simulation of the Flow Noise Generation of Cylinders
with Porous Cover. In: 2018 AIAA/CEAS Aeroacoustics Conference, AIAA paper 2018-3472, 2018[15] Xia, C., Wei, Z., Yuan, H., Li, Q., Yang, Z., POD analysis of the wake behind a circular cylinder coatedwith porous media. Journal of Visualization, Vol 21 (6), 2018, 965-985.
[16] Geyer, T. F., Al-Shuga, N. H. S., Reduction of vortex shedding noise from finite, wall-mounted, circularcylinders using porous material. In: 2019 AIAA/CEAS Aeroacoustics Conference, AIAA paper 2019-2695,
2019[17] Sarradj, E., Fritzsche, C., Geyer, T. F., Giesler, J., Acoustic and aerodynamic design and characterizationof a small-scale aeroacoustic wind tunnel. Applied Acoustics, Vol 70, 2009, 1073-1080[18] ISO 9053 (1993) Acoustics – Materials for acoustical applications – Determination of airflow resistance.
International Organization for Standardization[19] Welch, P. D. The use of fast Fourier transform for the estimation of power spectra: a method based ontime averaging over short, modified periodograms. IEEE Trans. Audio and Electroacoustics, 15(2), 1967,
70-73

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