Anastasios S. Lyrintzis

Surface integral methods in computational aeroacoustics—From the (CFD) near-field to the (Acoustic) far-field

A review of recent advances in the use of surface integral methods in Computational AeroAcoustics (CAA) for the extension of near-field CFD results to the acoustic far-field is given. These integral formulations (i.e. Kirchhoffs method, permeable (porous) surface Ffowcs-Williams Hawkings (FW-H) equation) allow the radiating sound to be evaluated based on quantities on an arbitrary control surface if the wave equation is assumed outside. Thus only surface integrals are needed for the calculation of the far-field sound, instead of the volume integrals required by the traditional acoustic analogy method (i.e. Lighthill, rigid body FW-H equation). A numerical CFD method is used for the evaluation of the flow-field solution in the near field and thus on the control surface. Diffusion and dispersion errors associated with wave propagation in the far-field are avoided. The surface integrals and the first derivatives needed can be easily evaluated from the near-field CFD data. Both methods can be extended in order to include refraction effects outside the control surface. The methods have been applied to helicopter noise, jet noise, propeller noise, ducted fan noise, etc. A simple set of portable Kirchhoff/FW-H subroutines can be developed to calculate the far-field noise from inputs supplied by any aerodynamic near/mid-field CFD code.

Review: The Use of Kirchhoff’s Method in Computational Aeroacoustics

A comprehensive review of the use of Kirchhoffs method in computational aeroacoustics is given. Kirchhoffs integral formulation allows radiating sound to be evaluated based on quantities on an arbitrary control surface S if the wave equation is assumed outside. The control surface S is assumed to include all the nonlinear flow effects and noise sources. Thus only surface integrals are needed for the calculation of the far-field sound. A numerical CFD method can be used for the evaluation of the flow-field solution in the near-field and thus on surface S. Kirchhoffs integral formulation has been intended to an arbitrary, moving, deformable piecewise-continuous surface. The available Kirchhoff formulations are reviewed and various aeroacoustic applications are given. The relative merits of Kirchhoffs method are also discussed

Coupling of integral acoustics methods with LES for jet noise prediction

This study is focused on developing a Computational Aeroacoustics (CAA) methodology that couples the near field unsteady flow field data computed by a 3-D Large Eddy Simulation (LES) code with various integral acoustic formulations for the far field noise prediction of turbulent jets. The LES code employs state-of-the-art numerical schemes and a localized version of the dynamic Smagorinsky subgrid-scale (SGS) model. The code also has the capability to turn off the SGS model and treat the spatial filter that is needed for numerical stability as an implicit SGS model. Noise computations performed for a Mach 0.9, Reynolds number 400,000 jet using various integral acoustic results are presented and the results are compared against each other as well with those from experiments at similar flow conditions. Our results show that the surface integral acoustics methods (Kirchhoff and Ffowcs Williams – Hawkings) give similar results to the volume integral method (Lighthills acoustic analogy) at a much lower cost. To the best of our knowledge, Lighthills acoustic analogy is applied to a Reynolds number 400,000 jet at Mach 0.9 for the first time in this study. The distribution of Lighthill sources that radiate noise in the direction of various observer locations is evaluated. A source decomposition shows significant cancellations among the individual components of the Lighthill source.

3-D large eddy simulation for jet aeroacoustics

of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in excellent agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by LES is coupled with the Ffowcs Williams-Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with another computational study.

Aerodynamic and Aeroacoustic Optimization of Rotorcraft Airfoils via a Parallel Genetic Algorithm

A parallel genetic algorithm (GA) methodology was developed to generate a family of two-dimensional airfoil designs that address rotorcraft aerodynamic and aeroacoustic concerns. The GA operated on 20 design variables, whichconstitutedthecontrolpointsforasplinerepresentingtheairfoilsurface.TheGAtookadvantageofavailable computer resources by operating in either serial mode, where the GA and function evaluations were run on the same processor or “ manager/worker” parallel mode, where the GA runs on the manager processor and function evaluations areconducted independently on separate workerprocessors. The multiple objectives of this work were to minimizethedrag and overall noiseof the airfoil. Constraintswereplaced on liftcoefe cient, moment coefe cient, andboundary-layerconvergence.TheaerodynamicanalysiscodeXFOILprovidedpressureandsheardistributions in addition to liftand drag predictions. Theaeroacousticanalysis code, WOPWOP, provided thicknessand loading noise predictions. The airfoils comprising the resulting Pareto-optimal set exhibited favorable performance when compared with typical rotorcraft airfoils under identical design conditions using the same analysis routines. The relationship between the quality of results and the analyses used in the optimization is also discussed. The new airfoil shapes could provide starting points for further investigation.

Improved high-order model for freeway traffic flow

An improved high-order continuum model is developed based on hyperbolic conservation laws with relaxation, linearized stability analysis, and more realistic considerations of traffic flow. The improved high-order model allows smooth traveling wave solutions as well as contact shocks (different densities moving at the same speed), is able to describe the amplification of small disturbances on heavy traffic, and allows fluctuations of speed around the equilibrium values. Furthermore, unlike existing high-order models, it does not result in negative speeds at the tail of congested regions and disturbance propagation speeds greater than the traffic flow velocity because the improved model has a zero characteristic speed and a nonnegative characteristic speed that is equal to the traffic flow velocity. The relaxation time is a function of density and, in the equilibrium limit, the improved high-order model is consistent with the simple continuum model. The improved high-order model is compared with the simple continuum model. Exemplary test results suggest that the improved high-order model is intuitively correct. Comparison of numerical results with field data suggests that the improved high-order model yields lower error levels than the simple continuum model.

Application of Compact Schemes to Large Eddy Simulation of Turbulent Jets

We present 3-D large eddy simulation (LES) results for a turbulent Mach 0.9 isothermal round jet at a Reynolds number of 100,000 (based on jet nozzle exit conditions and nozzle diameter). Our LES code is part of a Computational Aeroacoustics (CAA) methodology that couples surface integral acoustics techniques such as Kirchhoffs method and the Ffowcs Williams– Hawkings method with LES for the far field noise estimation of turbulent jets. The LES code employs high-order accurate compact differencing together with implicit spatial filtering and state-of-the-art non-reflecting boundary conditions. A localized dynamic Smagorinsky subgrid-scale (SGS) model is used for representing the effects of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in very good agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by the LES is coupled with the Ffowcs Williams–Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with experimental measurements of jets at similar flow conditions. The aeroacoustics results are encouraging and suggest further investigation of the effects of inflow conditions on the jet acoustic field.

Development of an Improved Kirchhoff Method for Jet Aeroacoustics

Research in the development of an improved Kirchhoff method is described. The Kirchhoff method is a means of evaluating radiated sound from flow acoustic quantities on a computational surface. The linear, homogeneous wave equation is assumed to be valid in the propagation region, outside this surface (the Kirchhoff surface). The surface quantities are generally obtained from a computational fluid dynamics (CFD) calculation of the acoustic near field. We outline the development of a Kirchhoff method for use when the linear, homogeneous wave equation is not valid in a portion of the region outside of the Kirchhoff surface. The new method is derived through the use of a permeable-surface formulation of the Ffowcs Williams-Hawkings equation. This modified integral equation allows the Kirchhoff methodology to be employed in problems with large, noncompact source regions that extend beyond the limits of the CFD calculation. Test calculations are presented in an attempt to validate the method for use in jet aeroacoustics studies. However, the method is presented in a manner to make it easily applicable in cases where Kirchhoff methods have been used in the past.

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.

Numerical Investigation of 3-D Supersonic Jet Flows using Large Eddy Simulation

The farfield noise generated by supersonic jets is investigated by a computational aeroacoustics methodology that couples 3-D large-eddy simulation (LES) near field data with the Ffowcs Williams- Hawkings method for farfield noise prediction. In order to accurately simulate jets at off-design supersonic conditions, we employ LES with characteristic filters for shock-capturing. This approach limits the dissipation of noise-producing turbulent fluctuations, and is suitable for incorporation into existing solvers. To further limit dissipation, a shock detector is used to determine shock locations and characteristic filters are applied locally. In this study, both perfectly-expanded and under-expanded unheated jets are investigated, with and without using characteristic filters. Comparisons with similar numerical and experimental data show reasonable agreement of the jet mean flow, turbulent statistics, and acoustics results. Preliminary grid-refinement shows improvement in these results.