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In many aeronautics and marine applications, the unsteady forces generated by propulsion rotors due to turbulence ingestion are a significant source of noise and create serious concerns. The understanding of rotor turbulence ingestion and the rotor noise generation mechanisms is vital to achieve an optimal design or apply noise control strategy. The current study is the first attempt to numerically investigate an underwater rotor ingesting grid-generated turbulence by large eddy simulation combined with the Ffowcs-Williams and Hawkings equation. The flow characteristics of two directly simulated turbulence grids with a mesh spacing of 4 and 6 in. are investigated and proved to be adequate for the rotor turbulence ingestion study. The simulated unsteady force spectra of the rotor show better agreement with the experimental results in terms of amplitudes and frequency ranges of the first haystack and broadband components compared to previous theoretical and numerical studies. The unsteady forces and noise of the rotor downstream of the 4-in. grid are slightly higher than those downstream of the 6-in. grid. A far-field noise prediction approach based on the time derivatives of the rotor axial and radial unsteady forces are investigated and validated. The far-field noise spectra and the rotor noise directivities predicted by the time derivative approach agree well with the Ffowcs-Williams and Hawkings equation in the main frequency range of underwater rotor noise. The noise directivities are found to be dominated by axial force related dipole sources. [ABSTRACT FROM AUTHOR]

Unmanned aerial vehicles (UAVs) have become an important research topic due to their wide range of applications and potential to meet the future air transportation demands. In many configurations of UAVs, the rotors interact with the wake of upstream bodies, which is of pivotal importance to the aerodynamic and aeroacoustic performance. The studies of flow structures and noise sources for rotors interacting with adjacent upstream bodies at Reynolds numbers lower than 105, which is the operating condition of most small UAVs, are limited. In the current study, numerical simulations are conducted to investigate the interaction between a rotor and the wake of an airfoil at low Reynolds numbers around 104 using the Spalart–Allmaras delayed detached eddy simulation method and acoustic analogy based on Farassat's formulation 1 A. The results show that the axial forces and the surface pressure perturbations of the airfoil are closely coupled with the rotor rotation. The effect of interaction on the rotor blades depends on the relative angle of attack. Furthermore, an analysis of the surface pressure fluctuations corresponding to the noise sources shows that the tonal noise at the blade passing frequency and its harmonics is mainly caused by periodical interaction, and the high-frequency noise is produced by the shedding vortex at the trailing edge of the rotor. [ABSTRACT FROM AUTHOR]