Youngmin Bae

Effect of passive porous surface on the trailing-edge noise

This study numerically investigates the effect of porous surfaces on the turbulent noise generated by a blunt trailing-edge of a flat plate. The three-dimensional turbulent flow over the flat plate (Rec = 1.3 × 105 and M = 0.06) is computed by incompressible large eddy simulation (LES) based on the volume-averaged Navier-Stokes equations, while the acoustic field is calculated by the linearized perturbed compressible equations (LPCEs) coupled with LES. The porous surface is applied to a small, selected area near the trailing-edge where vortex shedding and edge-scattering of convecting eddies generate dipole noise. The computed results show that the trailing-edge with porosity e=0.25 and permeability (normalized) K* = 0.01 yields a reduction of the tonal peak by 13 dB for the zero angle of attack (α = 0°) case, via breaking not only in the streamwise direction but also in the spanwise direction, the spatial correlation of the wall pressure fluctuations near the trailing-edge. For the separated flow case (α...

Aerodynamic sound generation of flapping wing

The unsteady flow and acoustic characteristics of the flapping wing are numerically investigated for a two-dimensional model of Bombus terrestris bumblebee at hovering and forward flight conditions. The Reynolds number Re, based on the maximum translational velocity of the wing and the chord length, is 8800 and the Mach number M is 0.0485. The computational results show that the flapping wing sound is generated by two different sound generation mechanisms. A primary dipole tone is generated at wing beat frequency by the transverse motion of the wing, while other higher frequency dipole tones are produced via vortex edge scattering during a tangential motion. It is also found that the primary tone is directional because of the torsional angle in wing motion. These features are only distinct for hovering, while in forward flight condition, the wing-vortex interaction becomes more prominent due to the free stream effect. Thereby, the sound pressure level spectrum is more broadband at higher frequencies and the frequency compositions become similar in all directions.

Effect of Porous Surface on the Flat Plate Self-Noise

In this study, 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=1.3×10 5 and M=0.06) is computed by incompressible large eddy simulation (LES) based on the volume-averaged Navier-Stokes equations, while the acoustic field is calculated by the linearized perturbed compressible equations (LPCE) coupled with LES solutions. The porous surface is applied to a small, selected area near the trailing-edge, where the Karman vortex shedding and eddy scattering produce dipole sounds. The computed results show that the trailing-edge with porosity e=0.25 and permeability (normalized) K * =0.01 yields a reduction of tonal peak by 13dB for zero angle of attack (α=0o), 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. For the separated flow case (α=5o), the same configuration of the porous surface is found to weaken the pressure fluctuations at the trailing edge. It results in 3-10dB noise reduction over a wide range of frequency, via reducing the separated flow region over the upper surface of the plate.

Brinkman Penalization Method for Computation of Acoustic Scattering from Complex Geometry

In this study, Brinkman penalization method (BPM) is extended for prediction of acoustic scattering from complex geometries. The main idea of the BPM is to model the solid obstacle as a porous material with zero porosity and permeability. With the aim of increasing the spatial accuracy at the immersed boundaries, computation is carried out on the boundary-fitted Cartesian-like grid with a high-order compact scheme combined with one-side differencing/filtering technique at the boundaries, while a slip boundary condition at the wall is imposed by introducing the ‘anisotropic’ penalization terms to the momentum equations. Several test cases are considered to demonstrate the accuracy, robustness and feasibility of the BPM. Numerical results are in excellent agreement with the analytic solutions for single and two cylinder scattering problems. The present BPM is then used to solve the acoustic scattering from a three-element high-lift wing (30P30N model). I. Introduction UMERICAL simulation of acoustic scattering from complex geometries has received attention in a wide range of aeroacoustic problems, such as slat noise and flap side-edge noise from a multi-element airfoil in high-lift configuration, rotor-stator interaction noise in turbo-machinery, etc. There are two main strategies in direct simulation of acoustic scattering from solid boundaries of complexity, i.e. structured/unstructured body-fitted grid methods [1-3] and immersed boundary methods [4-6]. In the former, implementation of wall boundary condition is straightforward, attaining a desired degree of accuracy at the boundaries. However, when complex geometries are concerned, the structured body-fitted grid method or even the overset grid method [7-9] often meets difficulties associated with the grid generation as well as with the quality of the grids. A discontinuous Galerkin (DG) method [10-12] based on the unstructured grids promises success for real complex geometries but computational cost has always been an issue. In this regard, an immersed boundary technique can be considered as an alternative because of its simple and efficient implementation for arbitrarily shaped surfaces with reasonable computational cost. Following the pioneering work of Peskin [13,14], a number of immersed boundary methods have been proposed to handle the complex geometries [4,15,16]. Among them, a Brinkman penalization method (BPM) [17], which was originally developed to model the fluid flow in porous media, appears attractive because of its easiness to handle the solid obstacle by simply treating as a porous medium of high impedance. In BPM, porosity and permeability in the penalty terms which are added in the compressible Navier-Stokes equations are set to zero in the solid region to impose the immersed boundary effect on the fluids. A no-slip boundary condition is therefore enforced naturally at the solid boundary. There are, however, two inherent limitations with this penalization technique. First, it requires a large number of grid points in solid region to retain the order of accuracy at the wall, thus making the method impractical at highly sophisticated geometries. Another drawback is that only the no-slip boundary condition is satisfied at the solid wall, whereas a slip boundary condition has to be met with the full or linearized Euler equations. In the present study, we address these numerical issues. With aim of increasing the spatial accuracy at the embedded boundary, we conform the immersed boundary grids to the actual shape of the surface following the idea of reshaped cell approach [18,19]. The slip boundary condition at the solid surface is imposed by introducing the ‘anisotropic’ penalization terms in the momentum equations. The validity of the present method is then assessed by considering the acoustic scattering from i) a single cylinder, ii) two circular cylinders, and iii) three element high-lift wing with the deployed slat and flap. We also discuss numerical issues related to the implementation of the reshaped cell approach and to the stiffness due to the penalty terms.

Study of the Aerodynamics/Aeroacoustics of an Axial-Flow Fan: Experimental Validation of a LES/LPCE/Brinkman Penalization Method

Department of Mechanical Engineering, Korea University, Seoul, 136-713, KoreaThe seek for an efficient aerodynamic and aeroacoustic design of axial-flow fans is animportant field of investigation for both academic and applied research. Improvementscan only be made with a better understanding of the physical mechanisms arising in thesemachines that combine strong interactions between rotating and non-rotating parts ofhighly complex geometries. One way is to couple well-suited experimental investigationsand innovative computational methods, that overtake the weaknesses of methods based forinstance on aeroacoustic analogy. In this paper we study an axial fan using a new numericalmethod based on LES/LPCE Brinkman Penalization Method. This method is developedin the Department of Mechanical Engineering at Korea University. The experimental testsand validations are performed in the Laboratory of Fluid Dynamics at Arts & M´etiersParisTech in Paris. Detailed analysis of numerical and experimental results are in progresswithin the two partner teams. In this paper we present preliminary encouraging results.

Computation of slat noise by a LES/LPCE hybrid method with Brinkman penalization

The slat noise of three-element 30P30N airfoil in high-lift configuration is computed by a LES/LPCE hybrid method with Brinkman penalization. The three-dimensional turbulent flow at Rec=1.7x10 6 and M=0.17 is computed by incompressible large eddy simulation (LES), while acoustic field is calculated by the linearized perturbe d compressible equations (LPCE) coupled with Brinkman penalization. An anisotropic permeability tensor is introduced in the momentum equations to impose a slip boundary condition at the impermeable boundaries. The result of present method with body-fitted Cartesian-like grids is validated with that of the conformal multi-block structured grids. Besides, flow structures in the slat cove region are investigated in relation to the noise generation, and finally the effect of porous trailingedge is examined for reduction of slat noise.

Computation of bio-fluid sounds

A hybrid method is proposed for prediction of bio-fluid sounds at very low Mach numbers. The unsteady hydrodynamic flow field is computed by the incompressible Navier-Stokes equations (INS), while the sound field is obtained by solving the linearized perturbed compressible equations (LPCE) with sound sources represented by a total derivative of the hydrodyanmic pressure, DP/Dt. With the present INS/LPCE hybrid method, the vocal sound in human larynx and the buzz sound of bumblebee are computed with more clear understanding on the sound generation processes associated with their characteristic motions such as the self-sustained oscillatory motions of the vocal folds and the figure-eight motion of the flapping wings.

Prediction of axial fan noise by linearized perturbed compressible equations with large eddy simulation.

The seek for an efficient aerodynamic and aeroacoustic design of axial‐flow fans is an important field of investigation for both academic and applied research. Improvements can only be made with a better understanding of the physical mechanisms arising in these machines that combine strong interactions between rotating and non‐rotating parts of highly complex geometries. One way is to couple well‐suited experimental investigations and innovative computational methods that overtake the weaknesses of methods based, for instance, on aeroacoustic analogy. In this paper we study an axial fan using a new numerical method based on large eddy simulation/linearized perturbed compressible equations with Brinkman penalization. This method is developed in the Department of Mechanical Engineering at Korea University. The experimental tests and validations are performed in the Laboratory of Fluid Dynamics at Arts & Metiers ParisTech in Paris. Detailed analysis of numerical and experimental results is in progress within...

Computation of Flapping Wing Sound

In the present study, unsteady flow and acoustic characteristics of the flapping wing are numerically investigated. The Reynolds number based on a maximum translational velocity of the wing and the wing chord length is Re=8800 and Mach number is M=0.0485. The flow around the flapping wing is predicted by solving the two-dimensional incompressible NavierStokes equations (INS), while the acoustic field is calculated by the linearized perturbed compressible equations (LPCE), both solved on the moving coordinates. The computational results show that the flapping wing sound is generated by the transverse and tangential motions of the wing with different sound generation mechanisms. A primary dipole tone at wing beat frequency is generated by the transverse motion, while other dipole tones at higher frequencies are produced by the vortex scattering at the trailing-edge of the wing during tangential motion. It is also found that the frequency composition of the primary tone changes with angle because of the torsional angle of the wing motion. This feature is only distinct for hovering, while at forward flight condition, the dipole tone at wing beat frequency is generated not only by the transverse motion but also by the wing-vortex interactions during upstroke. This wing-vortex interaction at forward flight also makes the far-field SPL spectrum more broadband.