High fidelity modeling tools for engine liner design and screening of advanced concepts

With aircraft engines trending toward ultra-high bypass ratios, resulting in lower fan pressure ratios, lower fan RPM, and therefore lower blade pass frequency, the aircraft engine liner design space has been dramatically altered. This result is also due to the associated reduction in both the available acoustic treatment area (axial extent) as well as thickness (liner depth). As a consequence, there is current need for novel acoustic liner technologies that are able to meet multiple physical constraints and simultaneously provide enhanced noise attenuation capabilities. In addition, recent advances in additive manufacturing have enabled the consideration of complex liner backing structures that would traditionally be limited to honeycomb cores. This paper provides an overview of engine liner modeling and a description of the key physical mechanisms, with some emphasis on the use of low to high-fidelity tools such as empirical models and commercially available software such as COMSOL, Actran, and PowerFLOW. It is shown that the higher fidelity tools are a critical enabler for the evaluation and construction of future complex liner structures. A systematic study is conducted to predict the acoustic performance of traditional single degree of freedom liners and comparisons are made to experimental data. The effects of grazing flow and bias flow are briefly addressed. Finally, a more advanced structure, a metamaterial, is modeled and the acoustic performance is discussed.

Experimental Investigation of Acoustic Characteristic on Orifice Shaped With Bias Flow

Combustion dynamics has been a significant problem for a lean, premixed, prevaporized (LPP) combustor. Understanding the acoustic characteristics of combustor components is essential to modeling thermoacoustic behavior in a gas turbine combustion system. Acoustic characteristics such as impedance and scattering matrix elements are experimentally determined for different-shape orifices with an emphasis on the effect of the flow field on them. These orifices are used to represent premixed swirl cups in LP combustors. The validity and limitation of two different methodologies are evaluated by comparing measured results with those of others. Consistent with analytical predictions, the measured resistance through an orifice increases as the bias flow increases. Different types of orifices considered in this study behave similarly to a thin orifice at high bias flow even though the discharge coefficients vary as much as 30% between them. The conventional method produces impedance values independent of waves reflected from the end boundary condition only when the scattering elements at the orifice downstream are roughly equal to those upstream of the orifice. However, the scattering matrix method produces impedance values that are not affected by the source or reflected waves at the system’s boundary. The scattering matrix measurements show that the reflection and transmission elements increases and decreases, respectively, as the bias flow through an orifice increases.

Dense U-Net for Limited Angle Tomography of Sound Pressure Fields

Tomographic reconstruction allows for the recovery of 3D information from 2D projection data. This commonly requires a full angular scan of the specimen. Angular restrictions that exist, especially in technical processes, result in reconstruction artifacts and unknown systematic measurement errors. We investigate the use of neural networks for extrapolating the missing projection data from holographic sound pressure measurements. A bias flow liner was studied for active sound dampening in aviation. We employed a dense U-Net trained on synthetic data and compared reconstructions of simulated and measured data with and without extrapolation. In both cases, the neural network based approach decreases the mean and maximum measurement deviations by a factor of two. These findings can enable quantitative measurements in other applications suffering from limited angular access as well.