The Use of Kirchhoff's Method in Jet Aeroacoustics

Supersonic jet aeroacoustics will be studied using computational techniques. In the study, a Kirchhoff method is used to predict flow generated noise in the mid- and far-fields. This type of method shows promise because it is based on surface integrals and not the volume integrals found in traditional acoustic prediction methods. The Kirchhoff method is dependent on accurate prediction of flow variables in the near-field. Here, computational fluid dynamics (CFD) programs are used for these predictions. Specifically, an existing large eddy simulation (LES) code will be modified for aeroacoustic applications. Issues involved in the implementation of the Kirchhoff method as well as the coupling with the CFD code will be discussed. Important physical noise parameters will be identified and investigated in the study

Development of Improved Surface Integral Methods for Jet Aeroacoustic Predictions

The accurate prediction of aerodynamically generated noise has become an important goal over the past decade. Aeroacoustics must now be an integral part of the aircraft design process. The direct calculation of aerodynamically generated noise with CFD-like algorithms is plausible. However, large computer time and memory requirements often make these predictions impractical. It is therefore necessary to separate the aeroacoustics problem into two parts, one in which aerodynamic sound sources are determined, and another in which the propagating sound is calculated. This idea is applied in acoustic analogy methods. However, in the acoustic analogy, the determination of far-field sound requires the solution of a volume integral. This volume integration again leads to impractical computer requirements. An alternative to the volume integrations can be found in the Kirchhoff method. In this method, Green's theorem for the linear wave equation is used to determine sound propagation based on quantities on a surface surrounding the source region. The change from volume to surface integrals represents a tremendous savings in the computer resources required for an accurate prediction. This work is concerned with the development of enhancements of the Kirchhoff method for use in a wide variety of aeroacoustics problems. This enhanced method, the modified Kirchhoff method, is shown to be a Green's function solution of Lighthill's equation. It is also shown rigorously to be identical to the methods of Ffowcs Williams and Hawkings. This allows for development of versatile computer codes which can easily alternate between the different Kirchhoff and Ffowcs Williams-Hawkings formulations, using the most appropriate method for the problem at hand. The modified Kirchhoff method is developed primarily for use in jet aeroacoustics predictions. Applications of the method are shown for two dimensional and three dimensional jet flows. Additionally, the enhancements are generalized so that they may be used in any aeroacoustics problem

Efficient Helicopter Aerodynamic and Aeroacoustic Predictions on Parallel Computers

This paper presents parallel implementations of two codes used in a combined CFD/Kirchhoff methodology to predict the aerodynamics and aeroacoustics properties of helicopters. The rotorcraft Navier-Stokes code, TURNS, computes the aerodynamic flowfield near the helicopter blades and the Kirchhoff acoustics code computes the noise in the far field, using the TURNS solution as input. The overall parallel strategy adds MPI message passing calls to the existing serial codes to allow for communication between processors. As a result, the total code modifications required for parallel execution are relatively small. The biggest bottleneck in running the TURNS code in parallel comes from the LU-SGS algorithm that solves the implicit system of equations. We use a new hybrid domain decomposition implementation of LU-SGS to obtain good parallel performance on the SP-2. TURNS demonstrates excellent parallel speedups for quasi-steady and unsteady three-dimensional calculations of a helicopter blade in forward flight. The execution rate attained by the code on 114 processors is six times faster than the same cases run on one processor of the Cray C-90. The parallel Kirchhoff code also shows excellent parallel speedups and fast execution rates. As a performance demonstration, unsteady acoustic pressures are computed at 1886 far-field observer locations for a sample acoustics problem. The calculation requires over two hundred hours of CPU time on one C-90 processor but takes only a few hours on 80 processors of the SP2. The resultant far-field acoustic field is analyzed with state of-the-art audio and video rendering of the propagating acoustic signals

The lagRST Model: A Turbulence Model for Non-Equilibrium Flows

This study presents a new class of turbulence model designed for wall bounded, high Reynolds number flows with separation. The model addresses deficiencies seen in the modeling of nonequilibrium turbulent flows. These flows generally have variable adverse pressure gradients which cause the turbulent quantities to react at a finite rate to changes in the mean flow quantities. This "lag" in the response of the turbulent quantities can t be modeled by most standard turbulence models, which are designed to model equilibrium turbulent boundary layers. The model presented uses a standard 2-equation model as the baseline for turbulent equilibrium calculations, but adds transport equations to account directly for non-equilibrium effects in the Reynolds Stress Tensor (RST) that are seen in large pressure gradients involving shock waves and separation. Comparisons are made to several standard turbulence modeling validation cases, including an incompressible boundary layer (both neutral and adverse pressure gradients), an incompressible mixing layer and a transonic bump flow. In addition, a hypersonic Shock Wave Turbulent Boundary Layer Interaction with separation is assessed along with a transonic capsule flow. Results show a substantial improvement over the baseline models for transonic separated flows. The results are mixed for the SWTBLI flows assessed. Separation predictions are not as good as the baseline models, but the over prediction of the peak heat flux downstream of the reattachment shock that plagues many models is reduced

Development of Advanced Traffic Flow Models and Implementation in Parallel Processing, Phase II (9/15/92-9/15/93)

In this report, five high-order continuum traffic flow models are compared: Payne's model; Papageorgiou's model; the semi-viscous model and the viscous model as well as a proposed high-order model, and the simple continuum model. The stability of the high-order models is analyzed and the shock structure investigated in all models. In addition, the importance of the proper choice of finite-difference method is addressed. For this reason, three explicit finite-difference methods for numerical implementation, namely, the Lax method, the explicit Euler method and the upwind scheme with flux vector splitting, are discussed. The test with hypothetical data and the comparison of numerical results with field data suggest that high-order models implemented through the upwind method are better than the simple continuum model. The proposed high-order model appears to be more accurate than the other high-order models

Review of Control Technologies for Quiet Operations of Advanced Air-Mobility

The current technologies for developing quiet rotor noise in urban canyons are reviewed. Several passive noise control approaches are discussed with their limitations in reducing both tonal and broadband noise. Blade tip modifications are seen to be one of the more successful in reducing tonal noise, with serrations at the trailing edge useful in reducing trailing edge broadband noise. Due to the adverse performance limitations of passive control, several optimization approaches are reviewed to discuss the possible improvements in performance of rotors. Additionally, a few legacy control technologies for helicopters are discussed. Active control technologies are investigated. The overall outlook and challenges to these methods are discussed with an eye on Advanced Air Mobility Vehicles (AAM)

A COMPARISON OF COMPUTATIONAL AEROACOUSTIC PREDICTION METHODS FOR

This paper compares two methods for predicting transonic rotor noise for helicopters in hover and forward ight. Both methods rely on a computational uid dynamics (CFD) solution as input to predict the acoustic near and far elds. For this work, the same full-potential rotor code has been used to compute the CFD solution for both acoustic methods. The rst method employs the acoustic analogy as embodied in the Ffowcs Williams{Hawkings (FW{H) equation, including the quadrupole term. The second method uses a rotating Kirchho formulation. Computed results from both methods are compared with one other and with experimental data for both hover and advancing rotor cases. The results are quite good for all cases tested. The sensitivity of both methods to CFD grid resolution and to the choice of the integration surface/volume is investigated. The computational requirements of both methods are comparable; in both cases these requirements are much less than the requirements for the CFD solution

Continuum modelling of traffic dynamics for congested freeways

This paper investigates advanced continuum models for describing traffic dynamics at congested flows. First, the existing high-order models are reviwed and discussed, and a new formulation is proposed which does not require the use of an equilibrium speed-density relationship. (A model is of high-order when it includes both mass and momentum conservation.) Traffic friction at interrupted flows and changing geometries is also addressed through the use of a viscosity term. The model is then discretized through two alternative numerical schemes to examine the effectiveness of the implementation. Stability analysis is performed to set up numerical limits of the model parameters. Finally, in two test cases with congested and uncongested field data, the new model is compared with the existing models; this comparison suggests that the proposed mode is more accurate and computationally more efficient.