Application of porous material to reduce aerodynamic sound from bluff bodies

Abstract

Aerodynamic sound derived from bluff bodies can be considerably reduced by flow control. In this paper, the authors propose a new method in which porous material covers a body surface as one of the flow control methods. From wind tunnel tests on flows around a bare cylinder and a cylinder with porous material, it has been clarified that the application of porous materials is effective in reducing aerodynamic sound. Correlation between aerodynamic sound and aerodynamic force fluctuation, and a surface pressure distribution of cylinders are measured to investigate a mechanism of aerodynamic sound reduction. As a result, the correlation between aerodynamic sound and aerodynamic force fluctuation exists in the flow around the bare cylinder and disappears in the flow around the cylinder with porous material. Moreover, the aerodynamic force fluctuation of the cylinder with porous material is less than that of the bare cylinder. The surface pressure distribution of the cylinder with porous material is quite different from that of the bare cylinder. These facts indicate that aerodynamic sound is reduced by suppressing the motion of vortices because aerodynamic sound is induced by the unstable motion of vortices. In addition, an instantaneous flow field in the wake of the cylinder is measured by application of the PIV technique. Vortices that are shed alternately from the bare cylinder disappear by application of porous material, and the region of zero velocity spreads widely behind the cylinder with porous material. Shear layers between the stationary region and the uniform flow become thin and stable. These results suggest that porous material mainly affects the flow field adjacent to bluff bodies and reduces aerodynamic sound by depriving momentum of the wake and suppressing the unsteady motion of vortices.

Vortex shedding and structural loading characteristics of finned cylinders

“…For this pitch ratio, the size of the formation region exceeds that for a uniform cylinder of fin diameter ( = c D T D / / ), whose formation region is approximately twice the size of that for ≥ c D / 0.361. A similar enlargement of the vortex formation region has been reported in previous studies of finned cylinders with low pitch ratios (Hamakawa et al, 2010;Khashehchi et al, 2014;Ryu et al, 2003) and porous cylinders (Naito and Fukagata, 2012;Sueki et al, 2010).…”

Tandem cylinder aerodynamic sound control using porous coating

“…Results will be presented for various coating thickness (h=R ¼ 0:2, 0.3 and 0.4) and porosities (ϕ ¼0.8 and 0.95). Earlier studies on the application of porous materials for isolated bluff bodies have shown that effective flow stabilization can be achieved using high porosity (ϕ Z0:8) materials, with normalized permeability (Darcy number Da ¼ K=D 2 ) ranging from 10 À 1 to 10 À 3 [27,30,32,47].…”

Aerodynamic Noise Reduction using Porous Materials and their Application to High-speed Pantographs

“…We assumed that the closed-cell type would not be effective because flow is not able to enter the porous material. Verification of this supposition and the mechanism of noise reduction are described in the references [6]. Figure 1 metal) have an identical form referred to as a three-dimensional frame (net-like) structure.…”