In implicit large eddy simulations, a porous material was used to suppress the noise produced by a flow past an open cavity. The base case has a Mach number and Reynolds number based on the cavity depth ReD = 105. Strong pressure fluctuations were produced. The solid cavity floor was replaced by a porous material to suppress high-pressure oscillations. With a porous floor, both the pressure fluctuations inside the cavity and noise in the near field were substantially suppressed. Four controlled cases with different porosities were considered. For low porosities, the control was better with increasing porosity. The control was best when the porosity was about 11.2%, and the maximum noise reduction was more than 10 dB. As the porosity was further increased from 11.2% to 19.27%, the control effect was decreased slightly. A porous floor can produce effects of suction and injection, which alter the structures of the recirculation and the shear layer. The control is mainly influenced by the strength of the suction effect. With control, the shear layer has less energetic smaller structures, and the interactions between the shear layer and recirculation inside the cavity are weakened. The vortex–edge impingements are mitigated, and thus, the acoustic feedback is lower, which decreases the self-sustained oscillations and the noise. Basically, the noise reduction mechanisms of the four controlled cases are similar. Our results suggest that a porous cavity floor is an effective method of noise control. However, an appropriate porosity must be chosen.

Rectangular supersonic jets exist widely in propulsion systems of aircrafts. When they are imperfectly expanded under certain conditions, the upstream traveling waves referred to as screech tones will be produced, which may cause structural fatigue failure. In this work, high fidelity simulations are employed to investigate the bevelled effects due to the asymmetric lips of nozzles on shock structures and screech noise in planar supersonic jets. The present results are in agreement with previous experimental and numerical data for the symmetric case. For asymmetric cases, it is found that the bevelled effects will affect the shear layer transition, noise radiation, and shock cell oscillations. The level of screech noise generally decreases with increasing the length difference of two lips. The maximum 7.9 dB drop is identified, and the deflection angle of the mainstream of 9.35° is achieved when this length difference reaches the height of the nozzle. Moreover, dynamic mode decomposition (DMD) is specifically utilized to analyze shock cell oscillations. The results show that the bevelled effects suppress the most energetic DMD mode, corresponding to the dominant frequency of shock screech. The phenomenon of shock leakage is detected in the symmetric case, which is assumed to be an important screech noise source, while it seems to be weakened when the nozzle is bevelled. The longitudinal flapping motion of shock cells is substantially weakened due to the bevelled effects, which might be responsible for the suppression of shock leakage and the screech noise reduction.

Spatial instability waves associated with lowfrequency noise radiation at shallow polar angles in the chevron jet are investigated and are compared to the round counterpart. The Reynolds-averaged Navier-Stokes equations are solved to obtain the mean flow fields, which serve as the baseflow for linear stability analysis. The chevron jet has more complicated instability waves than the round jet, where three types of instability modes are identified in the vicinity of the nozzle, corresponding to radial shear, azimuthal shear, and their integrated effect of the baseflow, respectively. The most unstable frequency of all chevron modes and round modes in both jets decrease as the axial location moves downstream. Besides, the azimuthal shear effect related modes are more unstable than radial shear effect related modes at low frequencies. Compared to a round jet, a chevron jet reduces the growth rate of the most unstable modes at downstream locations. Moreover, linearized Euler equations are employed to obtain the beam pattern of pressure generated by spatially evolving instability waves at a dominant low frequency St = 0.3, and the acoustic efficiencies of these linear wavepackets are evaluated for both jets. It is found that the acoustic efficiency of linear wavepacket is able to be reduced greatly in the chevron jet, compared to the round jet.

A high fidelity flow simulation for complex geometries for high Reynolds number (Re) flow is still very challenging, requiring a more powerful HPC system. However, the development of HPC with traditional CPU architecture suffers bottlenecks due to its high power consumption and technical difficulties. Heterogeneous architecture computation is raised to be a promising solution to the challenges of HPC development. GPU accelerating technology has been utilized in low order scheme CFD solvers on the structured grid and high order scheme solvers on unstructured meshes. The high-order finite difference methods on structured grids possess many advantages, e.g., high efficiency, robustness, and low storage. However, the strong dependence among points for a high-order finite difference scheme still limits its application on the GPU platform. In the present work, we propose a set of hardware-aware technology to optimize data transfer efficiency between CPU and GPU, as well as communication efficiency among GPUs. An in-house multi-block structured CFD solver with high order finite difference methods on curvilinear coordinates is ported onto the GPU platform and obtains satisfying performance with a speedup maximum of around 2000x over a single CPU core. This work provides an efficient solution to apply GPU computing in CFD simulation with specific high order finite difference methods on current GPU heterogeneous computers. The test shows that significant accelerating effects can be achieved for different GPUs.

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Trailing-edge serrations inspired by owls are capable of reducing broadband noise. In this study, the wall-resolved large-eddy simulations (LES) are carried out on the flow over NACA-0012 airfoil with additional serrated trailing edges. The computations are conducted with the high-order flux reconstruction method on unstructured meshes. Three kinds of serrations with different lengths are studied and compared with the straight trailing-edge case, and all three types of serration achieved a certain degree of noise reduction. Presently, the medium-length serration achieves the best noise reduction effect. The maximum decrease of overall sound pressure level is approximately 2.4 dB, implying that the length of serration has a substantial impact on the noise reduction effect. The serration has no significant effect on the upstream turbulence statistics, but it changes the flow structure near the serration, such as inducing side vortex pairs attached to the serration edges. Moreover, dynamic mode decomposition shows that the pressure structures vary with the serration length. For the most unstable hydrodynamic wave, the spanwise coherence of the mode structure of pressure in the upstream boundary layer is weakened. In addition, serrations can redistribute the dipole sources on the surfaces of airfoil and serrations. The destructive interference is enhanced to some extent, which is favourable for noise reduction. In contrast with LES simulations, the pure dipole analysis shows that the longest serration case seems to be the best. Furthermore, a recently developed noise theory is used to evaluate the influence of serrations on the flow noise sources qualitatively and quantitatively. It is found that the serrations can mitigate noise source intensity near the serration edges but increase the source intensity in the near wake. The combined effect of serration on the dipole source and flow noise source determines the overall noise reduction effect. To conclude, destructive interference plays a primary role in suppressing noise radiation by serrated trailing edges, and the dual effect of flow noise sources should be considered in future serration designs. As the influence of turbulence structure will make it more difficult to find the optimal serration parameters, the position of high-fidelity simulation will become increasingly important.

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases.