| dc.description.abstract | In the applications of computational aeroacoustics (CAA) involving far-field noise predictions, the most common solution strategy is the \textit{hybrid method} which combines a computational fluid dynamics (CFD) solver for the sound source field with an acoustic solver for the acoustic far-field. Hybrid CAA methods provide flexibility to select the most appropriate methods to compute the sound source and the acoustic fields, respectively, to suit various aeroacoustic problems. The present study reports the development of a hybrid large-eddy simulation (LES)-acoustic analogy method to effectively predict the noise of viscous flow over complex geometries.
With complex geometries, difficulties arise with numerical methods based on body fitted grids. Generating good quality body fitted grids around complex geometries is challenging and time-consuming. Alternatively, numerical methods based on non-body conformal grids can deal with bodies of almost any arbitrary shape. Although the present research was initially motivated for CAA applications, most of the contributions and the novelty of the work is in the development of efficient, easy-to-implement and more accurate non-body conformal methods that can be used for flow over complex geometries. To date, most of the listed work on non-body conformal methods is applied to incompressible flows. The use of non-body conformal methods for compressible turbulent flows is still rare and immature.
Two non-body conformal grid methods are developed and assessed in this work: the ghost-cell based immersed boundary method (GC-IBM) and the ghost-cell based cut-cell method (GC-CCM). In both methods, the boundary conditions on the immersed boundary are enforced through the use of ``ghost cells'' located inside the solid body. Variables on these ghost cells are computed using linear interpolation schemes. The implementation using GC-IBM is simpler; however, the exact shape of the fluid cells in the vicinity of the solid boundary is not detailed, which results in the loss or gain of mass and momentum. As such, sufficiently refined meshing is required in the vicinity of the solid boundary to mitigate the error on mass conservation. The implementation using GC-CCM requires more work; however, the underlying conservation laws is guaranteed by introducing ``cut cells''. A cell-merging approach is used to address the \textit{small-cell problem} associated with a Cartesian cut-cell method, which, if untreated, results in the numerical instability and stiffness of the system of equations.
The applicability of the developed non-body conformal methods is investigated in the compressible LES framework. Turbulent flows in various complex geometric settings are simulated using these non-body conformal methods for a wide range of Reynolds numbers and Mach numbers. For high Reynolds number flows, the developed non-body conformal methods employ a wall model to approximate the wall-shear stress, thus avoiding a requirement for severe grid resolution near the wall. No previously published work involves LES of high Reynolds number compressible flows using a wall model and a non-body conformal method. This research uses a simple wall model based on a wall function to approximate the near wall behaviour, but this approach can be extended to other wall models if necessary. Better wall modelling strategies should be investigated in the future. The numerical results demonstrate that the GC-CCM is capable of capturing near-wall flows relatively well despite the simple wall model used. GC-CCM also provides relatively accurate results compared to other non-body conformal methods.
Returning to the original research efforts for aeroacoustic applications, the GC-CCM is finally benchmarked for the prediction of far-field radiated noise from a flow over a circular cylinder. Of many hybrid approaches available in CAA, Ffowcs-Williams and Hawkings (FW-H) approach is selected to explore the far-field acoustic calculation. Comparison of the results to the experimental data shows that the developed hybrid LES-acoustic analogy method is capable of accurately predicting the sound spectrum for this case of three-dimensional flow over a cylinder in the sub-critical regime. Large-eddy simulations with more complex geometries, such as wings or high-lift systems, have not been performed as a part of this research. Further work is encouraged in order to conclude the research direction originally envisioned by the author. | en |
