Yusuf Özyörük

Computation of sound radiating from engine inlets

A hybrid method has been developed to calculate sound radiation from turbofan engine inlets. Given the acoustic source at an interface near the fan, the method solves the full three-dimensional, time-dependent Euler equations in the near field and passes the solution to a moving-surface Kirchhoff method for far-field predictions. The Kirchhoff method enables one to limit the size of the computational domain where the essential acoustic signals are captured accurately by a high-order accurate flow solver and then to extrapolate these signals to the far field exactly only within the discretization error on the Kirchhoff surface. The steady flowfield is required by the hybrid approach and a high-order accurate multigrid method has been implemented to enhance convergence of steady-state calculations. The computations are all carried out on parallel processors using essentially high-performance Fortran language. Results indicate very good agreement with available numerical and analytical solutions.

A Time-Domain Implementation of Surface Acoustic Impedance Condition with and Without Flow

The impedance condition in computational aeroacoustic applications is required in order to model acoustically treated walls. The application of this condition in time-domain methods, however, is extremely difficult because of the convolutions involved. In this paper, a time-domain method is developed which overcomes the computational difficulties associated with these convolutions. This method builds on the z-transform from control and signal processing theory and the z-domain model of the impedance. The idea of using the z-domain operations originates from the computational electromagnetics community. When the impedance is expressed in the z-domain with a rational function, the inverse z-transform of the impedance condition results in only infinite impulse response type, digital, recursive filter operations. These operations, unlike convolutions, require only limited past-time knowledge of the acoustic pressures and velocities on the surface. Examples of one- and two-dimensional problems with and without flow indicate that the method promises success in aeroacoustic applications.

Frequency-domain prediction of turbofan noise radiation

This paper describes a frequency-domain numerical method for predicting noise radiation from ducted fans, including acoustic treatment and non-uniform background flow effects. The method solves the Euler equations linearized about a mean flow in the frequency domain. A pseudo-time derivative term is added to the frequency-domain equations so that a time marching technique can be employed to drive the acoustic field to steady state explicitly. This approach makes distributed parallel computing more viable for equations of this type and will allow for future use of well-known convergence acceleration techniques, such as multigrid, to obtain the solutions efficiently. Simulations of the JT15D static test inlet are performed including the effects of liners, and the results are compared with experimental data. A generic engine geometry is used for demonstrating further the prediction capability of the code, calculating the attenuation effects of different liner impedances and liner installation locations on the radiated sound fields.

COMPUTATIONAL SIMULATIONS OF FORE AND AFT RADIATION FROM DUCTED FANS

In this paper, we extend the work of Ozyoruk and Long, for predicting farfield noise from ducted fans using a higher order coupled Euler/Navier-Stokes solver along with a Kirchhoff formulation, to full engine configurations on distributed parallel computers. Incorporating the exhaust along with the inlet raises many issues relating to appropriate grid topologies, multigrid acceleration and the unsteady numerics. These issues are addressed in this paper and simulations pertaining to a realistic engine geometry are presented. Qualitative comparisons are in good agreement with results from a frequency domain code, pertaining to radiation from a dipole in a cylinder. Simulations relating to radiation from an engine geometry, and with and without the centerbody, with and without mass flow are also shown.

Influence of mean flow gradients on fan exhaust noise predictions

Aft fan noise is becoming a more dominant source as engine bypass ratio is increased n this paper an assessment of the effect of the mean flow gradients on fan exhaust noise propagation is carried out using both analytical models for simplified problems and numerical methods for realistic configurations. Fan exhaust noise can be significantly refracted by the mean flow gradients in the jet mixing layer, especially at high operating conditions (i.e. during take off). The refraction effect is predicted using either Lilley’s equation or the linearized Euler equations. For parallel base flows, an issue with these linear models is the presence of Kelvin-Helmholtz instabilities whose unlimited exponential growth is unphysical and problematic for computational methods. This problem is less critical for developing mixing layer for instance where the growth of the vorticity thickness reduces the growth of the instability waves [1]. Various techniques have been used for suppressing the instability; these include adding non-linear terms to saturate the growth of the instability [2], using frequency domain analysis [3], or removing the mean flow gradient terms [4]. It is the last approach, termed Gradient Term Suppression (GTS), which is investigated in the present work.

Multigrid Acceleration of a High-Resolution

A multigrid method is applied to a fourth-order accurate (spatially and temporally) ® nite difference Runge± Kutta time-marching scheme. This is used to solve the nonlinearEuler equations for the ef® cient prediction of noise radiation from turbofan engine inlets. This noise prediction approach computes a steady-state solution ® rst, and thena source is turnedonand theunsteadysolution is computed.Thishas the advantageofusingthe samenumerical scheme to evaluate the residuals of the governing equations in both the steady and unsteady calculations, which means that the steady-state solution is smoothand nonumerical errors contaminate the acoustic solution.The highresolution steady-state ow® elds are calculated ef® ciently using a full approximation storage multigrid method. This makes it to possible to attain steady-state solutions on extremely ® ne meshes designed for high-frequency turbofan noise problems. It is also shown that acoustic results for a JT15D inlet are accurately represented by the present approach.

Multigrid Acceleration of a High-Resolution Computational Aeroacoustics Scheme

A multigrid method is applied to a fourth-order accurate (spatially and temporally) ® nite difference Runge± Kutta time-marching scheme.Thisisusedto solvethenonlinearEulerequationsfortheef® cientprediction ofnoise radiation from turbofan engine inlets. This noise prediction approach computes a steady-state solution ® rst, and thenasourceisturnedonandtheunsteadysolutioniscomputed.Thishastheadvantageofusingthesamenumerical scheme to evaluate the residuals of the governing equations in both the steady and unsteady calculations, which meansthatthesteady-statesolutionissmoothand nonumericalerrorscontaminatetheacousticsolution.Thehigh

TIME DOMAIN SIMULATIONS OF RADIATION FROM DUCTED FANS WITH LINERS

Over the last few decades, noise related concerns have played a major role in the development of aircraft engines. The previously dominant jet noise mechanisms are now being replaced by tonal and broadband noise from the fan and interactions from the fan wakes and the downstream stator. Alternately, engine inlet and exhaust ducts are being fitted with sophisticated liner materials that aid in damping fan related noise. In this paper, the authors investigate the radiation problem from the engine inlets with the aid of numerical simulations of the Euler/Navier-Stokes equations coupled with a time-domain methodology that analyzes the impedance characteristics of liner materials. In doing so, the authors present a simulation capability that can be used to identify and analyze tonal noise from high bypass ratio engines with acoustically treated nacelles. In this paper, we carry out numerical experiments and present results of radiation from two different engine inlet geometries with lined ducts.

Impedance boundary conditions for time-domain computational aeroacoustics methods

A time-domain impedance condition method has been developed for computational aeroacoustics applications. The basis for this method is the standard impedance condition stated in the frequency domain as the particle displacement continuity equation. The development of the time-domain impedance condition follows the relations among the frequency, z-, and discrete-time domains and a rational function representation of the impedance in the z-domain. The resultant impedance condition is finite, infinite-impulse-response type, digital filter operations in the time-domain, which is suitable to CAA methods. This paper describes the present approach and discusses the time-domain numerical simulations of the NASA Langley flow-impedance tube with a constant depth ceramic tubular liner. Excellent agreement is shown with experimental data at various frequencies and flow conditions.

Progress in time-domain calculations of ducted fan noise - Multigrid acceleration of a high-resolution CAA scheme

The numerical simulation of an aeroacoustics problem using the full, time-dependent Euler or Navier-Stokes equations requires both the mean and unsteady flow fields. Sometimes the mean flow is equivalent to the steady state flow and used to start the unsteady solution process. In this case, both the steady and unsteady flow fields have to be obtained through the same numerical method, so that the restart process is smooth and no spurious waves are generated. However, due to its low dissipation and consequently slow convergence, a high-resolution computational aeroacoustics scheme is not suitable for computing the steady flow field. In this paper, such a scheme is accelerated to convergence, without altering its residual, by using a full approximation storage multigrid method for the prediction of ducted fan noise. It is demonstrated that significant convergence improvements axe obtained using the multigrid method, making it possible to attain steady state solutions on extremely fine meshes designed for highfrequency radiation problems. Far-field noise results for a JT15D inlet are presented and compared with data at realistic flight configurations.