Ryu Fattah

Evaluation and development of non-reflective boundary conditions for aeroacoustic simulations

The performance of several non-reflective acoustic boundary conditions is quantitatively compared using a plane wave test configuration. The study uses a high-order linearized Euler equation solver to find a non-reflective boundary condition which gives good performance in a variety of cases. The performance for acoustic waves with varying frequency and incident angle is compared in flows with varying Mach number. The performance of buffer zone, far-field, and characteristic non-reflective methods is compared. Some non-reflective boundary methods, such as buffer zones, contain tuneable parameters that are optimized in the current work. This provides a more comprehensive evaluation than previous studies which used constant values of the tuneable parameters. A new generic non-reflective zonal characteristic boundary condition is proposed and is shown to give improved performance in comparison to other tested methods. The performance of the proposed boundary condition is also demonstrated in a two-dimensional airfoil turbulence-interaction case that includes vortical waves leaving the domain, and in a three-dimensional duct mode case.

High-Order Hybrid Cell-Centered Method for Computational Aeroacoustics

High-order cell-vertex finite difference schemes applied to multi-block structured grids are used widely in computational aeroacoustics for their low-dispersion and low-dissipation properties. Structured grids for complex geometries may contain discontinuous grid metrics at multi-block interfaces. In this work it is demonstrated that the grid-induced errors from such interfaces can be reduced by applying finite difference schemes in the cell-centered space. Further reduction of these grid-induced errors can be achieved by applying an additional finite volume method, which serves as an interface condition. In this paper, the development of a hybrid cell-centered finite difference and finite volume method is demonstrated. An interpolation scheme is derived from a high-order finite difference scheme to apply the finite volume method at interfaces. The order of accuracy of this hybrid method is demonstrated and the method is used to simulate the flow around a single cylinder, tandem cylinders, and a complex isolated wheel. Comparisons with experimental measurements and numerical predictions show that the hybrid method can provide accurate results at block interfaces and can be applied to high-order simulations of complex geometries.

Reflected wave manipulation by inhomogeneous impedance via varying-depth acoustic liners

Acoustic liners, consisting of a perforated panel affixed to a honeycomb core with a rigid back plate, are widely used for noise attenuation purpose. In this study, by exploiting inhomogeneous impedance properties, we report an experimental and numerical study on a liner-type acoustic metasurface, which possesses the functionality of both reflected wave manipulation and sound energy attenuation simultaneously. To realize the inhomogeneous acoustic impedance, an acoustic metasurface constructed by varying-depth acoustic liners is designed and fabricated. The reflected sound pressure fields induced by the metasurface are obtained in both experiments and simulations. A complete characterization of this metasurface is performed, including the effects of depth gradient, incident angle, and incident frequency. Anomalous reflection, apparent negative reflection, and conversion from an incident wave to a surface wave with strong energy dissipation are achieved by the structure. Moreover, our proposed structure ca...

Airfoil-Gust Interactions in Transonic Flow

Leading edge noise is a significant broadband noise source in aircraft engines, and is the primary broadband noise mechanism in outlet guide vane noise in turbofans, and broadband rotor wake interaction noise in contra-rotating open rotor engines. Previous authors have studied the effects of various aspects relating to this noise source, including airfoil geometry effects, cascade effects, and Mach number effects. However, previous literature has not addressed the effects on the noise due to locally supersonic regions that might be present in the mean flow around the rotor blades. The current work uses computational aeroacoustic methods to investigate the effects of locally supersonic regions on the noise due to airfoil- gust interactions. An established computational aeroacoustics code has been extended to give stable predictions in supersonic regions with a localized artificial diffusivity method. Initial results of a NACA 0012 airfoil in M = 0.8 flow interacting with oncoming vortical waves are shown, alongside results for a NACA 0006 airfoil in M = 0.5 flow at a 6 ? angle of attack. The changes to the noise and the underlying mechanisms are discussed for both cases, including additional noise sources caused by the supersonic region.

A priori grid quality estimation for high-order finite differencing

Structured grids using the finite differencing method contain two sources of grid-induced truncation errors. The first is dependent on the solution field. The second is related only to the metrics of the grid transformation. The accuracy of the grid transformation metrics is affected by the inverse metrics, which are spatial derivatives of the grid in the generalised coordinates. The truncation errors contained in the inverse metrics are generated by the spatial schemes. Fourier analysis shows that the dispersion errors, by spatial schemes, have similarities to the transfer function of spatial filters. This similarity is exploited to define a grid quality metric that can be used to identify areas in the mesh that are likely to generate significant grid-induced errors. An inviscid vortex convection benchmark case is used to quantify the correlation between the grid quality metric and the solution accuracy, for three common geometric features found in grids: abrupt changes in the grid metrics, skewness, and grid stretching. A strong correlation is obtained, provided that the grid transformation errors are the most significant sources of error.

Manipulating reflected acoustic wave via Helmholtz resonators with varying-length extended necks

In this work, we develop a deep subwavelength metasurface which is capable of reflected wave manipulation arbitrarily. Each unit cell of the metasurface is constructed of a Helmholtz resonator with an extended neck. The possibility of creating a phase shift offered by different unit cells is analytically explored based on characteristic mode analysis and demonstrated by the finite element method. It is found that the phase shift of the reflected wave ranging from 0 to 2π in a supercell (consists of eight inhomogeneous unit cells) can be engineered by tuning the length of the extended neck. A periodical array of the supercell is used to construct the designed metasurface. The reflection performance of the proposed metasurface is investigated both numerically and experimentally, and good agreement is achieved. Anomalous phenomena such as converting an incident wave to a surface wave and negative reflection are demonstrated using the designed metasurface. The key features of the proposed metasurface are the thin thickness ≈λ/30 (λ is the operation wavelength), simple configuration, and easy fabrication, making it possess a promising potential in miniaturization and integration in acoustic devices.In this work, we develop a deep subwavelength metasurface which is capable of reflected wave manipulation arbitrarily. Each unit cell of the metasurface is constructed of a Helmholtz resonator with an extended neck. The possibility of creating a phase shift offered by different unit cells is analytically explored based on characteristic mode analysis and demonstrated by the finite element method. It is found that the phase shift of the reflected wave ranging from 0 to 2π in a supercell (consists of eight inhomogeneous unit cells) can be engineered by tuning the length of the extended neck. A periodical array of the supercell is used to construct the designed metasurface. The reflection performance of the proposed metasurface is investigated both numerically and experimentally, and good agreement is achieved. Anomalous phenomena such as converting an incident wave to a surface wave and negative reflection are demonstrated using the designed metasurface. The key features of the proposed metasurface are the ...

Towards a generic non-reflective characteristic boundary condition for aeroacoustic simulations

A blended zonal characteristic boundary condition is proposed following a quantitative investigation of the performance of several non-reflective boundary conditions. Two test cases are considered that investigate the effects of acoustic and vortical plane waves impinging on the domain outflow region. A third test case investigates the effects of broadband turbulent flow impinging on a non-reflective outflow boundary condition. From these studies, two non-reflective boundary conditions based on a zonal characteristic method are found to provide a minimal acoustic response for impinging acoustic and vortical disturbances, respectively. These methods both make use of the transverse characteristic terms to improve performance, although each method uses a different inclusion of these terms. A final boundary condition is proposed that blends the performance of the two zonal characteristic methods. A blending function is used that switches the characteristic boundary condition smoothly between regions dominated by acoustic, or vortical, disturbances. The feasibility of this novel method is demonstrated on a test case where broadband turbulence impinges on a small section of an outflow region.

Aeroacoustics of a Landing Gear Door

ow between an aircraft main landing gear leg-door and the main strut. Two parameters are considered for the interaction: the proximity between the two elements, and the leg-door angle relative to the ow. A two-dimensional parametric study, conducted by solving the incompressible Reynolds Averaged Navier-Stokes equations, shows that both parameters can signicantly aect ow characteristics and its radiated sound to the far eld, which is calculated through the Ffowcs-Williams and Hawkings acoustic analogy. Three-dimensional compressible Delayed Detached-Eddy simulations, on similar cases are performed. It is shown that as the leg-door angle increases, the wake width increases and the noise spectra peak tends towards lower frequencies and at higher levels. It is also shown that the leg-door element becomes a more signicant noise source, as the interaction at high leg-door angle can cause the turbulent intensity in the wake from the cylinder to decreases. All numerical works were performed at a Mach number of 0.2 and a Reynolds number based on the cylinder diameter of 1:7 10 6 . The experimental data was conducted at a Mach number of 0.09 and Reynolds number of 0:202 10 6 and consists of time averaged velocity eld statistics and an on surface microphone spectra. The experiment was conducted for three leg-door angles. The velocity and turbulent intensity along the wake is compared to the 3-D simulations to show agreements.