Chingwei M. Shieh

Linearized Navier-Stokes Analysis for Rotor-Stator Interaction Tone Noise Prediction

Tonal noise due to aerodynamic interaction between turbomachinery blade rows has been conventionally studied using time-linearized, inviscid analyses. While the perturbation equations can employ the inviscid flow approximation, the meanflow calculation has to be viscous to capture the loading distribution on the blades. In a previous study, the authors suggested calculating approximate inviscid meanflow with a “comparable” loading distribution. Experience has shown that this can not only be tedious but also can introduce large variations in predictions. Therefore, a linearized RANS calculation is proposed to replace the linearized Euler calculation. Validation of the approach is performed against semi-analytical solutions of canonical problems, and comparisons against NASA Source Diagnostic Test (SDT) data are presented. It is shown that the geometric trends are reasonably well captured with the approach although no clear improvement in prediction accuracy is observed in comparison to the inviscid approach. Detailed comparisons of predicted rotor wake harmonics against Laser Doppler Velocimetry (LDV) data are also presented.

Large Eddy Simulation for Jets from Chevron and Dual Flow Nozzles

Large eddy simulations based on high-order finite d ifference schemes have been carried out for jet flows from a NASA ARN2 circular and a SMC001 chevron nozzle at acoustic Mach 0.9. Simulations have also been carried out fo r a bypass ratio 5 dual-flow nozzle at a typical take-off condition. Implicit LES approach i s applied, wherein high-order numerical filters are used in place of SGS models. Far field noise spectra are computed from near field flow solutions with the use of the FWH method. Numerical experiments are conducted to explore the sensitivity of the predicted flow and a coustic field to grid size. Good agreement with experiment data is achieved for both flow and acoustic predictions at moderate grid sizes. It is shown that the first-principles based implicit LES approach can capture the effect of chevron geometry on the far-field acoustics. The predicted noise reduction trend is in good agreement with the experimental data. The LES approach has also been demonstrated for a dual flow realistic engine configuration

Multiple Pure Tone Noise Prediction and Comparison with Static Engine Test Measurements

This paper presents an integrated numerical procedure to predict the generation, in-duct propagation, and radiation of multiple pure tone (MPT) noise of aircraft engines. Reynoldsaveraged Navier-Stokes (RANS) computational fluid dynamics (CFD) simulations using part-annulus grids have been performed to resolve the non-uniform shock signature just upstream of the fan blades. A linear superposition method is then used to reconstruct the full-annulus pressure field using the part-annulus CFD results and the as-manufactured blade stagger angles measured by a coordinate-measuring machine. In order to account for the nonlinear propagation of the shock waves inside the nacelle, a one-dimensional model has been employed to simulate the propagation of MPT from upstream of the fan leading edge to the nacelle lip. For far-field propagation, a commercially available software ACTRAN/TM is used to linearly propagate the acoustic modes to far-field microphone locations. The entire analysis process has been applied to predict MPT noise of a typical high bypass ratio engine at operating conditions when the relative tip Mach number of the fan is transonic. Comparisons against static engine test measurement are made for in-duct, near-field, and far-field sound pressure levels. Good agreement has been observed between predictions and measured data.

High-Order Accurate Dual Time-Stepping Algorithm for Viscous Aeroacoustic Simulations

The field of computational aeroacoustics (CAA) has shown great promise in the solution of primarily inviscid noise generation and wave propagation problems that are governed by the Euler equations. The present research extends these ideas to the solution of the Navier-Stokes equations in multi-dimensions where the acoustic and flow fields may be influenced by viscous effects. In this paper, efficient acceleration techniques typical of explicit steady-state solvers are extended to time-accurate calculations. Stability restrictions based on the grid spacing are relaxed greatly with the implementation of a fully implicit time discretization. Some simplified problems that address the important issues of viscous calculations are investigated. A two-dimensiona l, Navier-Stokes code, written in Fortran 90 with the Message Passing Library (MPI) as a parallel implementation, is used to perform these calculations. As an example calculation, scattering of an acoustic source by a flat plate in the presence of a mean flow including a viscous boundary layer is presented.

Multiple Pure Tone Noise Prediction for Acoustically Treated Aircraft Engines

A multiple pure tone noise prediction approach for acoustically treated aero-engine inlets is described. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with lined walls inside the nacelle, and linear acoustic propagation outside the engine. The predictions are compared with static test measurements for a typical high bypass ratio engine at inlet nacelle unsteady pressure transducer, near-field microphone array, and far-field microphone array locations.

Numerical Prediction of Exhaust Fan Tone Noise from High Bypass Aircraft Engines

The ability to accurately predict fan noise is important in designing and optimizing aircraft engine turbofans for low noise emissions. In this paper, a prediction methodology for exhaust fan tone noise analysis is described and validated against various canonical test cases and NASA Source Diagnostic Test (SDT) data. The prediction process consists of solving Reynolds-Averaged Navier-Stokes (RANS) equations to compute the fan wake, and calculating the acoustic response of the outlet guide vanes (OGV) to the fan wake using linearized Euler equations. Very good agreement is observed between numerical predictions and semi-analytical results for canonical cases. Detailed comparisons against SDT data are presented for unsteady vane pressure and integrated in-duct exhaust noise power levels. Geometric trends for different OGV configurations at various operating conditions are also analyzed.

Unsteady Acoustic Forcing on an Impeller Due to Coupled Blade Row Interactions

This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.

Numerical simulation of minor losses in thermoacoustic devices

As thermoacoustic devices become more efficient, minor losses through sudden expansions or contractions in such acoustic resonators become more important. To further improve the efficiency of thermoacoustic devices it is necessary to understand in detail the fluid dynamic mechanisms involved in these minor losses. In the present study, a parallel numerical simulation of a three‐dimensional acoustic resonator, typical of a thermoacoustic device, is presented. A computational aeroacoustic (CAA) approach is used. The Navier–Stokes equations are discretized in space with the fourth‐order dispersion‐relation‐preserving (DRP) scheme of Tam and Webb and are integrated in time with a fourth‐order Runge–Kutta scheme. In the attached regions of the flow the Spalart–Allmaras one‐equation turbulence model is used. For separated flow regions this transitions automatically to a detached eddy simulation (DES). In the present study a high amplitude standing wave is generated in a resonator with a sudden change in cross‐s...

A numerical solution of acoustic scattering by an aircraft wake vortex

Monostatic and bistatic acoustic scattering has been used in studies of the characteristics and motions of the atmosphere. This technique has been successfully implemented in the Monostatic Acoustic Vortex Sensing System (MAVSS) for the measurement of the characteristics of aircraft wake vortices during landing and take‐off. In the present study, calculations are performed to simulate the MAVSS. The results of this study are giving information about the acoustic scattering by the vortex, as well as providing a controlled test bed for the instrument’s operation. Since the Doppler shifts, characteristic of the vortex velocity, are dependent on the incoherent scattering of the acoustic waves by turbulence advected by the vortex, a statistical model is proposed that describes the turbulence imbedded in the vortex. The simulations are performed using high‐order accuracy finite‐difference algorithms that have been developed for computational aero‐acoustic (CAA) and wave propagation problems.