Seong Ryong Koh

Numerical Analysis of Sound Sources in High Reynolds Number Single Jets

Jet noise at high Reynolds number is simulated for an isothermal single jet flow at Mach number 0.9. The numerical approach is based on large-eddy simulations and solutions of the acoustic perturbation equations (APE). The sound field is computed by the seven-point stencil dispersion-relation preserving (DRP) scheme for the spatial discretization and by an alternating 5-6 stage low-dispersion and low-dissipation Runge-Kutta method for the temporal integration. The effects of various sources in the APE system are illustrated using differentgrid resolutions showing the overall acoustic features and the detailed soundspectrum to be predicted in good agreement with direct acoustic simulations in the near far field. Moreover, the power spectrum of the noise sources expressed in the frequency-wave number space is analyzed to explain the instability waves generating the acoustic radiation.

Airframe-Noise Reduction by Suppressing Near-Wall Turbulent Structures

To reduce trailing edge noise an investigation of a prospective active noise control technique is presented. The computational approach is based on large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE). These methods are used to investigate the trailing-edge noise of a flat plate at a freestream Mach number 0.6 and a Reynolds number of 12000 based on the boundary layer thickness 0 at the inflow boundary. Gas mixtures at different thermodynamic properties, i.e., carbon dioxide, helium, and hexafluoroethane gases, are injected as an anti-source fluid into the external flow field. The modified velocity field as well as the controlled density fields impact the corresponding acoustic sources. The presented results include the analysis of turbulent structures which are responsible for the noise generation. The sound spectra evidence a noise reduction by manipulating the turbulent flow structures. Especially the injection of a low-density helium-air mixture shows an effective noise reduction over a wide frequency band.

Sound Generation Control by Fluid Bleeding

To reduce trailing edge noise a preliminary investigation of a prospective active noise control technique is presented. The computational approach is based on large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE). The method is used to investigate the acoustic fields of trailing edge flow at a freestream Mach number 0.6 and a Reynolds number based on the reference length L of 12000. To modify the near-wall flow structures CO2 gas, the density of which is about 1.5 times higher than that of air, is injected into the turbulent shear layer. The detailed investigations include the analysis of turbulent structures which influence the noise generation. The sound spectra evidence the noise reduction by changing the turbulent flow structures. Especially the injections into the shear flow immediately adjacent to the wall and right into the wake of the trailing edge possess the most eective noise reduction over a wide frequency band. The overall sound pressure level determined in the near far-field shows a noise reduction of up to 4dB.

Noise Sources of Trailing-Edge Turbulence Controlled by Porous Media

To reduce trailing-edge noise an investigation of a noise reduction technique based on porous media is presented. Large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE) are used to investigate the trailing-edge noise of a flat plate at a freestream Mach number 0.06 and a Reynolds number of 135000 based on the chord length and the freestream velocity. The acoustic fields are determined in a three dimensional domain to include the impact of the spanwise coherence length on the noise generation. The porous surface at the trailing edge covers an area in the spanwise times streamwise direction of 512 times 800 in inner wall units. The two-point correlation of the velocity components shows that the modified velocity field by the porous surface has a smaller correlation length and a smooth variation of the turbulence length at the trailing edge. The porous surface reduces the overall sound pressure level from 3dB to 8dB. The sound spectra possess a strong tone at the Strouhal number of fh/U∞ = 0.2 and the broadband spectrum follows the −2 power slope of the frequency. Due to the uniform porous surface the peak of the tone was decreased by 10dB.

Reformulation of Acoustic Entropy Source Terms

The noise generation and propagation of a subsonic round helium-mixture coaxial jet is analyzed by a hybrid large-eddy simulation (LES) acoustic perturbation equations (APE) method. The Mach number of the primary Map and the secondary stream Mas is Map = 0:6 and Mas = 0:9 and the Reynolds number based on the velocity and the diameter of the secondary stream is ReD = 40000. The helium-mixture jet serves as a test problem to show the impact of the formulation of the entropy source terms on the acoustic eld. To be more

Analysis of acoustic source terms of a coaxial helium/air jet

Jet noise poses a significant problem, both ecologically and economically. Nevertheless, the related noise generating mechanisms are not fully understood and despite decades of research most noise reducing strategies are fully empirical. The most important noise generating jet flows are characterized by varying densities of the involved fluids. However, the density gradient alone seems to dominate the noise source mechanisms, so differing densities can be obtained by either using air at different temperatures or gas mixtures involving multiple species. In this paper a coaxial helium/air mixture jet is compared with a corresponding hot-air jet. The focus of this comparison is on a detailed analysis of the contributions of the various source terms of the coaxial helium/air jet and their mutual cancelation. The low frequency and the peak location is dominated by the linear parts of the perturbed Lamb vector, whereas the mid and higher frequencies are characterized by the nonlinear sources. In the sideline direction clear cancelation effects are identified. The results for the radiation angles dominated by instability waves are in very good agreement with experimental data, while there is a slight offset for the sideline direction, probably caused by the lower Reynolds number and unresolved turbulent scales and acoustic waves.

Impact of transversal traveling surface waves in a non-zero pressure gradient turbulent boundary layer flow

The impact of non-zero pressure gradient flow in a turbulent boundary layer flow over a surface undergoing spanwise transversal traveling waves is investigated via large-eddy simulations. While it is known that in zero-pressure gradient flow spanwise surface waves can lead to drag reduction, this question still remains open for non-zero pressure gradient flows. In the present analysis, the effect of a linear pressure gradient is investigated and compared to the zero-pressure gradient flow at a momentum thickness based Reynolds number R e ? = 2000 for a constant surface wave, i.e., the wave length is λ + = 500 , the amplitude A + = 50 , and the wave speed is c + = 6.25 . The results show a drag reduction of about 10% for the zero-pressure gradient flow, 6% for the adverse-pressure gradient, and a drag increase of 4% for the favorable-pressure gradient flow. The analysis of the velocity profiles shows a reduced gradient at the trough region for all actuated setups. At the crest, an increased gradient is obtained. Furthermore, the viscous sublayer is extended. The streamwise turbulent intensity is reduced for all configurations compared to the non-actuated reference case at the crest. At the trough, the shift off the wall is only present for the zero-pressure gradient flow and the adverse pressure gradient flow. The hypothesis based on numerous zero-pressure gradient flow investigations of a reduced wall-normal vorticity component at the crest and trough indicating drag reduction is corroborated. That is, for the adverse-pressure gradient flow the wall-normal component distribution is lowered and for the favorable pressure gradient flow, which possesses a drag increase, the distribution at the trough is similar to that of the reference non-actuated case.

On the Adjoint-based Control of Trailing-Edge Turbulence and Noise Minimization via Porous Material

In this paper, we present a discrete adjoint-based optimization framework to obtain the optimal distribution of the porous material over the trailing edge of a 3-D flat plate. The near-body strength of the noise source generated by the unsteady turbulent flow field is computed using a high-fidelity large-eddy simulation (LES). The acoustic signal thus generated is then propagated to the far-field using the acoustic perturbation equations (APE). The design gradients are computed using the forward and reverse modes of algorithmic differentiation (AD). The increase of memory requirement in the reverse mode AD is alleviated by checkpointing. By optimally controlling the material porosity and permeability, it is possible to minimize the turbulence intensity responsible for noise generation at the trailing edge and thus significantly reduce the radiated noise. The optimal porous trailing-edge design achieves a noise reduction of up to 18dB.

A Discrete Adjoint Approach for Trailing-Edge Noise Minimization Using Porous Material

In this paper, we present a discrete adjoint-based optimization framework to obtain the optimal distribution of the porous material over the trailing edge of a 3-D flat plate. The near-body strength of the noise source generated by the unsteady turbulent flow field is computed using a high-fidelity large-eddy simulation (LES). By optimally controlling the material porosity and permeability, it is possible to minimize the turbulence intensity responsible for noise generation at the trailing edge and thus significantly reduce the radiated noise. We demonstrate, using a simple geometry as a first step, the efficacy of the discrete adjoint method in achieving minimum-noise design via optimal distribution of porous media, with future applications to aircraft high-lift devices.

Adjoint-based Trailing-Edge Noise Minimization using Porous Material

In this paper, we present a discrete adjoint-based optimization framework to obtain the optimal distribution of the porous material over the trailing edge of a 3-D flat plate. The near-body strength of the noise source generated by the unsteady turbulent flow field is computed using a high-fidelity large-eddy simulation (LES). The acoustic signal thus generated is then propagated to the far-field using the acoustic perturbation equations (APE). The design gradients are computed using the forward and reverse modes of automatic differentiation (AD). The increase of memory requirement in the reverse mode AD is alleviated by checkpointing. By optimally controlling the material porosity and permeability, it is possible to minimize the turbulence intensity responsible for noise generation at the trailing edge and thus significantly reduce the radiated noise. We demonstrate, using a simple geometry as a first step, the efficacy of the discrete adjoint method in achieving minimum-noise design via optimal distribution of porous media, with future applications to aircraft high-lift devices.