본 연구에서는 날개 끝단과 같이 Karman 와류나 난류와가 산란되면서 쌍극자 소음을 발생시키는 영역에 다공성 표면을 적용하여 소음을 저감시키는 다공성 표면 효과에 대하여 연구를 수행하였다. 특히 레이놀즈수가 13만5천이고 마하수가 0.06인 유동조건에서 무딘 끝단 ...

본 연구에서는 날개 끝단과 같이 Karman 와류나 난류와가 산란되면서 쌍극자 소음을 발생시키는 영역에 다공성 표면을 적용하여 소음을 저감시키는 다공성 표면 효과에 대하여 연구를 수행하였다. 특히 레이놀즈수가 13만5천이고 마하수가 0.06인 유동조건에서 무딘 끝단을 갖는 평판에 대해 연구를 수행하였다. 3차원 난류 유동은 비압축성 대와류모사기법을 사용하였고 소음의 근접장 및 원음장은 비선형 교란 압축성 방정식을 LES해와 병합하여 사용하였다. 또한 Brinkman penalization 기법을 이용하여 고체영역, 다공성 매질 유체영역, 유체 역에 각각 다른 매질의 투과성 값을 부여하여 해석하였다. 이와같은 해석기법을 이용하여 다공성 매질의 다공성과 투과성 값의 영향이 난류의 통계학적 특성과 소음 발생 경로에 어떻게 영향을 주는가에 대하여 분석하였다. 또한 공력학적 효율을 최소화하면서 끝단 소음을 감소시킬수 있는 다공성 매질의 최적값을 찾는 매개 변수 연구도 수행하였다. 분석 결과 25% 다공성과 무차원화된 값으로 투과성이 0.01인 경우의 다공성 매질이 광대역화 된 톤 소음을 13 dB 감소시키는 계산결과가 보여주었다. 이러한 유동소음 감소를 유발시키는 물리적 이유는 끝단 표면 압력 요동의 공간적 상관성이 주유동방향 뿐만 아니라 스팬방향으로도 감소시키기 때문인 것으로 확인되었다.

In this research, the effect of passive porous surface is investigated by applying a porous surface to a small, selected area of the trailing-edge (with a plenum inside), where the Karman vortex shedding and eddy scattering produce a dipole sound. A b ...

In this research, the effect of passive porous surface is investigated by applying a porous surface to a small, selected area of the trailing-edge (with a plenum inside), where the Karman vortex shedding and eddy scattering produce a dipole sound. A blunt trailing-edge of a flat plate is investigated at Rec=1.3x10^5 and M=0.06. The three-dimensional turbulent flow over the flat plate is computed by incompressible large eddy simulation (LES), while the near- and far-field acoustics are calculated by the linearized perturbed compressible equations (LPCE), coupled with the LES solutions. A Brinkman penalization method (BPM) is also employed, in which we can account for the solidity of the matter, i.e. solids, fluids, or homogeneous porous media by controlling the permeability coefficients. The analysis will be focused on investigating the effects of porous material properties such as porosity and permeability on the turbulence statistics as well as the subsequent noise generation processes. A parametric study is also conducted to find an optimal condition of the porous surface to reduce the trailing-edge noise with minor degradation in aerodynamic efficiency. The computed results show that the trailing-edge with porosity of 25% and permeability (normalized) of 0.01 yields a reduction of the broadened tonal peak by 13dB, via breaking not only in the streamwise direction but also in the spanwise direction the spatial correlation of the wall pressure fluctuations (Rpp) near the trailing-edge.

Turbulence trailing-edge noise continues to be one of the important concerns in airframe noise. It is often associated with the broadband noise generated by three-dimensional, spatio-temporal interactions of eddies in the boundary layer with the cut-o ...

Turbulence trailing-edge noise continues to be one of the important concerns in airframe noise. It is often associated with the broadband noise generated by three-dimensional, spatio-temporal interactions of eddies in the boundary layer with the cut-off edge and a tone associated with von Karrman vortex shedding for the edge-bluntness. In this research, the effect of passive porous surface is investigated by applying a porous surface to a small, selected area of the trailing-edge (with a plenum inside), where the Karman vortex shedding and eddy scattering produce a dipole sound. Based on the fact that the porous edge provides a mechanism of flow communication among the lower, upper, and back-end surfaces, keeping a finite value of surface impedance near the trailing-edge will reduce the trailing-edge noise. The effect of passive porous surface on the turbulent noise generated by a blunt trailing-edge of the flat plate is investigated at Rec=1.3x10^5 and M=0.06. The analysis will be focused on investigating the effects of porous material properties such as porosity and permeability on the turbulence statistics as well as the subsequent noise generation processes. A parametric study is also conducted to find an optimal condition of the porous surface to reduce the trailing-edge noise with minor degradation in aerodynamic efficiency.

The effect of porous surface on the turbulent noise generated by a blunt trailing-edge of a flat plate is investigated. The three-dimensional turbulent flow over the flat plate (Rec=1.3x10^5 and M=0.06) is computed by incompressible large eddy simulat ...

The effect of porous surface on the turbulent noise generated by a blunt trailing-edge of a flat plate is investigated. The three-dimensional turbulent flow over the flat plate (Rec=1.3x10^5 and M=0.06) is computed by incompressible large eddy simulation (LES), while the near- and far-field acoustics are calculated by the linearized perturbed compressible equations (LPCE), coupled with the LES solutions. The porous surface is applied to a small, selected area of the trailing-edge (with a plenum inside), where the Karman vortex shedding and eddy scattering produce a dipole sound.
A computational modeling of the porous medium is an important issue in this study. Because the extremely small length scales associated with the pores might pose a very stringent grid size as well as the time step, a direct flow simulation becomes very difficult and expensive. To resolve this matter, a Brinkman penalization method (BPM) is employed, in which we can account for the solidity of the matter, i.e. solids, fluids, or homogeneous porous media by controlling the permeability coefficients. The computed results show that the trailing-edge with porosity of 25% and permeability (normalized) of 0.01 yields a reduction of the broadened tonal peak by 13dB, via breaking not only in the streamwise direction but also in the spanwise direction the spatial correlation of the wall pressure fluctuations (Rpp) near the trailing-edge. In the future, the porous surface can be employed to any local area of aerodynamic blades where pressure fluctuations periodically occur via transient changes of coherent flow structures such as eddies or separated shear layers. This porous surface technique can be utilized for reduction of airfranme noise from airfoil blade trailing-edges or side-edges, or blades in turbomachineries, and etc.