Young J. Moon

Linearized perturbed compressible equations for low Mach number aeroacoustics

For efficient aeroacoustic computation at low Mach numbers, the linearized perturbed compressible equations (LPCE) are proposed. The derivation is based on investigation of the perturbed vorticity transport equations. In the original hydrodynamic/acoustic splitting method, perturbed vorticity is generated by a coupling effect between the hydrodynamic vorticity and the perturbed velocities. At low Mach numbers, the effect of perturbed vorticity on sound generation is not significant. However, the perturbed vorticity easily becomes unstable, and causes inconsistent acoustic solutions, based on grid dependence. The present LPCE ensures grid-independent acoustic solutions by suppressing the generation of perturbed vorticity in the formulation. The present method is validated for various dipole and quadruple vortex-sound problems at low Mach numbers: (i) laminar dipole tone from a circular cylinder at Reynolds number based on the cylinder diameter, ReD = 150 and free stream Mach number, M∞ = 0.1, (ii) quadruple sound of Kirchhoff vortex at Mach number based on the rotating speed, MΘ = 0.1, and (iii) temporal mixing layer noise at Reynolds number based on the shear layer thickness, Reδ = 10000 and Mach number based on the shear rate, Ms = 0.1.

Computation of unsteady viscous flow and aeroacoustic noise of cross flow fans

Abstract Time-accurate viscous flow solutions are sought for the prediction of unsteady flow characteristics and associated aeroacoustic blade tonal noise of a cross flow fan. The two-dimensional incompressible Navier–Stokes equations in a moving coordinate are time-accurately solved by an unstructured finite-volume method on triangular meshes, and a sliding mesh technique is utilized at the interface between the domain rotating with blades and the stationary one for allowing the unsteady interactions. An accuracy assessment of the present method is made by comparing the fan performances with experimental data for a rotational speed at 1000 rpm and the Reynolds number 5300 based on blade tip speed and chord length. With the computed unsteady viscous flow solutions, sound pressure is predicted using Curle’s equation and narrow-band noise characteristics of three impellers with a uniform and two random pitch (type-A and -B) blades are compared by their sound pressure level spectra. Also, the frequency modulations of the blade passing frequency noise by random pitch fans are discussed.

Discrete noise prediction of variable pitch cross-flow fans by unsteady Navier-Stokes computations

The unsteady viscous flow fields of a cross-flow fan are computed by time-accurately solving the two-dimensional incompressible Navier-Stokes equations with the unstructured triangular mesh solver algorithms. Based on pressure fluctuation data acquired at the surfaces of 35 rotating blades and stabilizer, acoustic pressures are predicted by the Ffowcs Williams-Hawkings equation. The aerodynamic noise sources of the cross-flow fan are also identified by correlating the acoustic pressure fluctuations with the unsteady flow characteristics during one revolution of the impeller. The present method is applied to the uniform and random pitch fans to investigate their performance and aeroacoustic noise characteristics, especially the frequency modulation of the tonal noise at the blade passing frequency (BPF).

Prediction of cavitating flow noise by direct numerical simulation

In this study, a direct numerical simulation procedure for the cavitating flow noise is presented. The compressible Navier-Stokes equations are written for the two-phase fluid, employing a density-based homogeneous equilibrium model with a linearly-combined equation of state. To resolve the linear and non-linear waves in the cavitating flow, a sixth-order compact central scheme is utilized with the selective spatial filtering technique. The present cavitation model and numerical methods are validated for two benchmark problems: linear wave convection and acoustic saturation in a bubbly flow. The cavitating flow noise is then computed for a 2D circular cylinder flow at Reynolds number based on a cylinder diameter, 200 and cavitation numbers, @s=0.7-2. It is observed that, at cavitation numbers @s=1 and 0.7, the cavitating flow and noise characteristics are significantly changed by the shock waves due to the coherent collapse of the cloud cavitation in the wake. To verify the present direct simulation and further analyze the sources of cavitation noise, an acoustic analogy based on a classical theory of Fitzpatrik and Strasberg is derived. The far-field noise predicted by direct simulation is well compared with that of acoustic analogy, and it also confirms the f^-^2 decaying rate in the spectrum, as predicted by the model of Fitzpatrik and Strasberg with the Rayleigh-Plesset equation.

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.

Assessment Of CFD Broadband Noise Predictions On A Rod-Airfoil Benchmark Computation

This paper presents a selection of acoustic far field, unsteady aerodynamic and mean flow results obtained from a variety of unsteady codes on a low Mach rod-airfoil test case. These codes are all applicable to complex geometries such as encountered in turbo engines. Therefore the present computations can be regarded as an evaluation of their capability to predict turbomachinery broadband noise. This quite unique benchmarking work of turbomachinery noise prediction tools has mainly been achieved within the framework of the European Project PROBAND, but some of the results where also obtained by an academic cooperation with a Korean partner.

Counter-rotating streamwise vortex formation in the turbine cascade with endwall fence

Abstract The three-dimensional turbulent flows in the turbine cascade with and without endwall fences are numerically investigated by solving the incompressible Navier–Stokes equations with a high-Reynolds number k – ϵ turbulence closure model. The limiting streamline patterns and the static pressure contours at the suction surface of the blade as well as on the cascade endwall are employed to visualize the effectiveness of the endwall fence for the secondary flow control. The analysis on the streamwise vorticity contour maps along the cascade with the three-dimensional representation of their iso-surfaces reveals the complicated structures of the vortical flows in the turbine cascade with endwall fence, and also leads to an understanding on the formation of a counter-rotating streamwise vortex over the fence. The mechanisms of controlling the secondary flow and the proper selection of an optimal fence height are also explained.

Prediction of flat plate self-noise

In this study, the accuracy of the computational methodology developed for prediction of turbulent flow noise at low Mach numbers is assessed for the flat plate self-noise. The far-field self-noise and the wall-pressure field over the flat plate (chord=10cm, thickness=3mm, and span=30cm) are measured at Ecole Centrale de Lyon for a flow speed UO=20m/s at zero angle of attack. The three-dimensional turbulent flow over the 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. Comparisons are made for the wall pressure PSD spectra near the trailing-edge, the spanwise coherence function of the surface pressure, and the farfield sound pressure level spectrum. The computations agree well with the experiment. The tonal and broadband noise characteristics of the flat plate are also discussed.

Prediction of Low Mach Number Turbulent Flow Noise Using the Splitting Method

A new methodology is proposed for the low Mach number turbulent flow noise prediction. Based on the hydrodynamic/acoustic splitting method, turbulent flow field is computed by an incompressible large eddy simulation(LES), while its acoustic field is predicted by the filtered perturbed-compressible-equations(FPCE). The FPCE which is derived from the incompressible and compressible LES equations does not require any particular modeling of the source term for turbulence. The present method is applied to turbulent noise prediction of flow past a circular cylinder at ReD=46000 and M=0.21. The predicted sound pressure level spectrum agrees fairly well with the experimental data of Boudet et al(AIAA Paper 2003-3217).