Phoi-Tack Lew

Recent Progress of Hot Jet Aeroacoustics Using 3-D Large-Eddy Simulation

Improvements in computing speed over the past decade have made Large Eddy Simulation (LES) an attractive tool to study jet noise. In addition, the study of turbulent hot jets for noise prediction is desirable compared to cold/isothermal jets since all jet engines fltted on aircraft operate at hot exhaust conditions. In this regard, we present results for two heated jets with temperature ratios of Tj=T1 = 1:76 and Tj=T1 = 2:70, respectively. A computational grid with approximately 4.8 million grid points is used the simulation. Spatial flltering is used as an implicit subgrid scale SGS model in place of the classical Smagorinsky and Dynamic Smagorinsky models. To study the far-fleld noise, the porous Ffowcs Williams-Hawkings (FWH) surface integral acoustic formulation is employed. The jet development results obtained using our LES methodology are consistent with other LES data and experimental results. The predicted OASPL values for our heated jets follow the trend measured by experiments though our results over-predict by approximately 3dB. Overall, our LES methodology coupled with the Ffowcs Williams-Hawkings aeroacoustics methodology provide satisfactory results.

Fundamental Aeroacoustic Capabilities of the Lattice-Boltzmann Method

The Lattice Boltzmann Method (LBM) was used to model four canonical problems in acoustics. The goal was to show that the LBM, which recovers the transient, compressible, and viscous Navier-Stokes equations, allows the accurate capture of time-dependent acoustic phenomena. The first case was that of a planar propagating sound wave. The dispersion of an initial Gaussian pulse in a two-dimensional domain was then investigated. Grid resolution requirement for minimizing numerical dispersion were determined for these two cases. The influence of noise and that of the numerical bulk viscosity was investigated and is discussed. The case of a driven standing-wave tube was then used to investigate the possibility of implementing sound-absorbing boundaries. Finally the case of a Helmholtz resonator was investigated. Results that are consistent with basic acoustic theory were obtained for all cases. The results illustrate the capability of the LBM to model acoustic problems accurately. Since these numerical schemes are already utilized for the modeling of external fluid flows, they are useful for the modeling of strongly coupled fluid-acoustic interactions.

Effects of Inflow Forcing on Jet Noise Using Large Eddy Simulation

Recent discoveries have shown that by adjusting selected in∞ow forcing parameters, properties such as turbulent ∞ow development and most importantly jet noise are in∞uenced to some extent. To implement fully a nozzle structure in a high-end simulation like Large Eddy Simulation (LES) would require a prohibitive number of grid points to resolve the boundary layer for realistic Reynolds numbers. Thus, in∞ow forcing currently seems to be a reasonable substitute for a nozzle geometry. However, the drawback of this approach is that the ∞ow fleld results are sensitive to in∞ow forcing parameters used. With LES as an investigative tool, this paper studies the efiects of in∞ow forcing with particular emphasis on the number of azimuthal modes. We flnd that by removing the flrst few modes results in the jet developing slower, i.e. longer potential core. Furthermore, the peak turbulence intensities increase when we remove the flrst 6 and 8 modes of forcing. Due to this high peak turbulence intensities we found that the overall sound pressure level (OASPL) also increases at all observation angles for a closed control surface using the Ffowcs Williams-Hawkings method.

Towards Lattice-Boltzmann Prediction of Turbofan Engine Noise

The goal of the present paper is to report verification and validation studies carried out by Exa Corporation in the framework of turbofan engine noise prediction through the hybrid Lattice-Boltzmann/Ffowcs-Williams & Hawkings approach (LB)-(FW-H). The underlying noise generation and propagation mechanisms related to the jet flow field and the fan are addressed separately by considering a series of elementary numerical experiments. As far as fan and jet noise generation is concerned, validation studies are performed by comparing the LB solutions with literature experimental data, whereas, for the fan noise transmission through and radiation from the engine intake and bypass ducts, LB solutions are compared with finite element solutions of convected wave equations. In particular, for the fan noise propagation, specific verification analyses are carried out by considering tonal spinning duct modes in the presence of a liner, which is modelled as an equivalent acoustic porous medium. Finally, a capability overview is presented for a comprehensive turbofan engine noise prediction, by performing LB simulation for a generic but realistic turbofan engine configuration.

Computational Aeroacoustic Analysis of Flow Around a Complex Nose Landing Gear Configuration

A Lattice-Boltzmann Method (LBM) based Very Large-eddy Simulation (VLES) approach was used to study the near-field noise generation around a complex nose landing gear configuration. LBM describes a fluid flow in terms of discrete kinetic equation based on the particle density distribution function (the Lattice Boltzmann equation). The macroscopic flow properties are results of the moments of these particle density distribution functions. The e ects of turbulence are modeled using two transport equations based on a revised renormalization-group (RNG) theory, and realized through an e ective particle-relaxation-time scale in the extended kinetic equations. The flow solution is obtained on a Cartesian grid system that resolves the boundary geometry exactly. A three dimensional, unsteady, compressible flow simulation is conducted to capture the instantaneous flow-field that is responsible for the near-field noise generation around the gear assembly. It is found that the interaction between small details of the gear components and local flow tends to produce high level fluctuations in both the low and high frequency ranges, which, contribute significantly to the overall noise generation.

Investigation of Noise Sources in Turbulent Hot Jets using Large Eddy Simulation Data

Through the use of Lighthill’s acoustic analogy, the aim of this paper is to investigate the noise sources of turbulent heated round jets using previously simulated Large Eddy Simulation (LES) data. Two heated and one unheated jet are considered to study the e ects of heating on the noise source contributions to the far-field. Firstly, the computed overall sound pressure level (OASPL) and spectra are in good agreement with the prediction obtained from the porous Ffowcs Williams-Hawkings (FWH) surface integral method. Like the FWH prediction, however, the computed OASPL over-predicts the experiments by approximately 3dB but the trends agree reasonably well with the experimental results. Through decomposition of the Lighthill source term we obtain such sources as shear, self and entropy noise. An important finding is that when a high speed subsonic compressible jet is heated while keeping the ambient jet Mach number constant, significant cancellations occur in the far-field between the shear and entropy noise. In addition, heating a jet reduces the intensity of the nonlinear self noise terms compared to an unheated jet. For a low speed heated jet, the main contributing source is the entropy noise source while the shear and self noise sources hardly contribute to the far-field noise.

Unsteady numerical simulation of a round jet with impinging microjets for noise suppression

The objective of this study was to determine the feasibility of a lattice-Boltzmann method (LBM)-Large Eddy Simulation methodology for the prediction of sound radiation from a round jet-microjet combination. The distinct advantage of LBM over traditional computational fluid dynamics methods is its ease of handling problems with complex geometries. Numerical simulations of an isothermal Mach 0.5, Re(D) = 1 × 10(5) circular jet (D(j) = 0.0508 m) with and without the presence of 18 microjets (D(mj) = 1 mm) were performed. The presence of microjets resulted in a decrease in the axial turbulence intensity and turbulent kinetic energy. The associated decrease in radiated sound pressure level was around 1 dB. The far-field sound was computed using the porous Ffowcs Williams-Hawkings surface integral acoustic method. The trend obtained is in qualitative agreement with experimental observations. The results of this study support the accuracy of LBM based numerical simulations for predictions of the effects of noise suppression devices on the radiated sound power.

An Extended Lattice Boltzmann Methodology for High Subsonic Jet Noise Prediction

The feasibility of an extended Lattice Boltzmann Methodology (LBM) for high subsonic jets is studied. The distinct advantage of LBM over traditional computational fluid dynamics (CFD) methods is in its ease of handling problems with complex geometries. Simulations of three subsonic round circular jets using the SMC000 nozzle were performed. The conditions are, two isothermal jets with ambient Mach numbers of, M1 D 0:5, and M1 D 0:9, and one heated jet with a Mach number of M1 D 0:9, with a jet temperature ratio to ambient of, Tj=T1 D 2:7, respectively. The trends obtained for the mean centerline streamwise velocity, turbulence intensity and far-field sound are in good agreement with experimental results. It is believed that simulations using LBM for high subsonic jets is the first of its kind.

Simulation of Sound Radiated from Turbulent Heated Jets Using the Lattice-Boltzmann Method

The aim of this study was to investigate the near and far-field characteristics of an axisymetric subsonic heated jet using the Lattice Boltzmann Method (LBM). The LBM-LES method with no subgrid model was used. The results were compared to experimental data and other LES results obtained using a Navier-Stokes LES code. Heat transfer was modeled using a supplemental energy transport equation and solved using a second-order Lax Wendroff finite difference scheme. Both mean flow and turbulence data were in good agreement with experiment. The far-field noise was computed using the porous Ffwocs William-Hawkings (FWH) surface integral acoustic method. Qualitative comparison were made between acoustic results and available experimental data. The results support the viability of the LBM for heated jet applications at low Mach numbers. Nomenclature aJ = speed of sound in jet a∞ = ambient speed of sound ci = particle velocity f = particle distribution function fi = force coefficient in the i- direction fi eq = equilibrium distribution function

Large Eddy Simulation of Jet Noise Suppression by Impinging Microjets

Sound suppression by impinging microjets was modeled using Large Eddy Simulation (LES). A Mj = 0:9, unheated jet, at ReDj = 400; 000 was considered. A higher-order, inhouse code was used to solve the compressible Navier-Stokes equations in the neareld. The eects of a circumferential array of microjets were modeled through source terms added to the Navier-Stokes equations. It was observed that the penetration of microjets in the core jet plume induced secondary instabilities in the shear layer which trigger a transition to turbulence close to the nozzle. The fareld sound was calculated using the Ffowcs Williams-Hawkings surface integral methodology. The microjet injection resulted in a reduction of about 4 dB in overall sound pressure levels in almost all observer locations. The power spectral density of fareld sound pressure was reduced by approximately 4 to 6 dB in very low frequency regions compared to those of the base round jet. The dierence between the spectra of the base round jet and those of the microjet setup decreased with increasing frequency. It was also observed that the microjet spectra show higher energy content beyond the cross-over frequencies corresponding to St 0:8, and St 3, for = 30 , and 90 , respectively. These trends are in very good agreement with those observed in experiments.