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.
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 study, asymptotic analysis of the frequency-domain formulation to compute the tonal noise of the small rotors in the now ubiquitously multi-rotor powered drones is conducted. Simple scaling laws are proposed to evaluate the impacts of the influential parameters such as blade number, flow speed, rotation speed, unsteady motion, thrust and observer angle on the tonal noise. The rate of noise increment with thrust (or rotational speed) is determined by orders of blade passing frequency harmonics and the unsteady motion. The axial mean flow influence can be approximated by quadratic functions. At given thrust, the sound decreases rapidly with the radius and blade number as the surface pressure becomes less intensive. The higher tonal harmonics are significantly increased if unsteady motions, although of small-amplitude, are existed, as indicated by the defined sensitivity function, emphasizing that the unsteady motions should be avoided for quiet rotor designs. The scaling laws are examined by comparing with the full computations of the rotor noise using the frequency-domain method, the implementation of which has been validated by comparing with experiments. Good data collapse is obtained when the proposed scaling laws, which highlights the dominant influence of the design parameters, are incorporated.
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