Download passive control of flow and noise around a circular cylinder covered

XXIVICTAM, 21-26 August 2016, Montreal, Canada
PASSIVE CONTROL OF FLOW AND NOISE AROUND A CIRCULAR CYLINDER COVERED
WITH POROUS MATERIAL
Chen Xu, YijunMaoa)
School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, China
Summary This paper presents a numerical study to investigate flow and noise around a cylinder surface covered by porous material. The large
eddy simulation and acoustic analogy are employed to predict the cylinder flow and noise, respectively. Three-dimensional instantaneous
flowfield inside the porous zone is simulated by using a volume-averaged method. The flow information on different permeable data surfaces is
successively employed to predict the aerodynamic noise. The result obtained from unsteady flow simulation reveals that the porous material
postpones the alternating vortex shedding from the cylinder, not only reducing the pressure drag but also suppressing the pressure fluctuation on
the cylinder surface. The aeroacoustics prediction results confirm that the porous material significantly reduces the aerodynamic noise of the
cylinder flow.
INTRODUCTION
Porous materials, such as open-cell metal foams, show an appealing application potential in control of flow and noise.
Experimental study of Sueki et al.[1] has shown that the noise radiated from flow past a cylinder can be significantly
reduced by covering the porous material. This paper conducts a numerical study to validate the flow and noise prediction
methods related to porous material and aims to reveal the mechanism of noise control.
NUMERICAL SIMULAITON
The numerical case corresponds to an experiment conducted by Sueki et al. [1].The length and the diameter of the bare
cylinder are l=600mm and d=25mm, respectively. In the present simulation, the cases of the bare cylinder and the cylinder
covered with open-cell porous material, which correspond to the types of A and D in Ref [1], are computed. The surface of
the bare cylinder is covered with the porous material with the uniform thickness of t=10mm, and the outside diameter of the
cylinder is 45mm. The porosity of the porous material is 97% and the pores per inch are approximately 13, corresponding to
the equivalent diameter of the pore 2mm. The uniform flow velocity at the inlet is 27.7m/s, it can be obtained that the flow
Mach number is 0.08, the inflow Reynolds number is 8.3h104 based on the outside diameter of the porous cylinder and
porous flow Reynolds number is 3.7h103 based on the pore diameter of the porous material.
At this flow condition, the flow outside as well as inside the porous material can be regarded as fully turbulent. The
porous material is assumed to be homogenous and the permeability and porosity are uniform in each direction. The flow
inside and outside the porous zone is computed using the continuum approach. The incompressible continuity and NavierStokes equations are employed to describe the flow outside the porous zone. For the flow inside the porous region, the
Ergun’s equation is employed to consider the viscous and inertial effects of the porous material, and the macroscopic flow
inside the porous region is solved by using the volume-averaged method.
Aeroacoustics prediction is also carried out by using Formulation 1A of Farassat [2], where two different permeable
cylindrical surfaces with different diameters (45mm and 65mm) are selected as the input data surfaces. The numerical
results obtained from these two permeable surfaces include both the contributions from the loading source on the bare
cylinder surface and the near-field quadrupole source inside the date surface.
NUMERICAL RESULTS
Figure 1 compares the time history of lift and drag coefficients between the bare cylinder and the porous cylinder. The
result shows that the amplitude of the lift fluctuation for the porous cylinder is much smaller than that for the bare cylinder,
and the averaged drag coefficient for the porous cylinder is also smaller than that for the bare cylinder. This feature implies
that porous material is beneficial to reduce the noise and the flow drag as well.
(a)
(b)
Figure 1 Time history of lift and drag coefficients: (a) bare cylinder; (b) porous cylinder
a)
Corresponding author. Email:[email protected].
Figure 2 displays instantaneous contours of the velocity magnitude for the bare and porous cylinders. Alternating vortex
shedding from the bare cylinder is observed, and this phenomenon causes a low-pressure region downstream the bare
cylinder, as shown in Figure 3(a). However, the alternating vortex shedding is postponed by the porous cylinder and there is
an obvious low-speed region downstream the porous cylinder (Figure 2(b)). The above flow feature causes that the pressure
drag of the porous cylinder is much smaller than that of the bare cylinder, as shown in Figure 3(b).
(a)
(b)
Figure 2 Instantaneous contours of the velocity magnitude: (a) bare cylinder; (b) porous cylinder
-1400 -1200 -1000 -800 -600 -400 -200
-1400 -1200 -1000 -800 -600 -400 -200
0
200
0
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400
0.05
400
y (m)
y (m)
0.05
0
-0.05
-0.1
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x (m)
0.1
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(a)
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x (m)
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(b)
Figure 3 Instantaneous contours of the static pressure:(a) bare cylinder; (b) porous cylinder
Figure 4 presents A-weighted sound pressure level spectra of the bare and porous cylinders. The numerical results
obtained from different data surfaces are in reasonable agreement with the experimental data. All the results validate that the
porous material significantly reduces the noise radiated from the cylinder.
(a)
Figure 4 A-weighted SPL spectra: (a) bare cylinder; (b) porous cylinder
(b)
CONCLUSIONS
A numerical study is carried out to analyze the effect of the porous material on the cylinder flow and noise. Result
shows that porous material postpones the alternating vortex shedding from the cylinder surface. This phenomenon brings
the following two benefits: (1) the lift fluctuation on the cylinder surface is suppressed and noise radiated from cylinder
surface is reduced as well; (2) the flow drag is also diminished because the obvious low-pressure region downstream the
cylinder disappears.
ACKNOWLEDGEMENTS
The research has been supported by the National Natural Science Foundation of China (No. 51476123 and
No.51206127).
References
[1] Sueki, T., Takaishi, T., Ikeda, M. & Arai, N. Application of porous material to reduce aerodynamic sound from bluff bodies. Fluid Dyn Res, 42, 015004,
2010.
[2] Farassat, F. Derivation of Formulations 1 and 1A of Farassat. NASA/TM 2007-214853, 2007.