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This paper focuses on the development of a three-dimensional panel method in time and frequency domains combined with the finite element method for analyzing the hydroelastic responses of rotating marine propellers in the wake of ships. A fully non-penetration boundary condition imposed on the deformed blade surface is conducted, in which the corrections of both the incoming flow velocities and the normal vectors imposed on the deformed and undeformed blade surface are taken into account. The added-mass and -damping matrices due to strongly coupled fluid-structure interaction are considered. Results of the present method are compared with experimental data available in the literature. It is observed that the fully non-penetration boundary condition applied on the deformed blade surface should be imposed to predict the unsteady performance of elastic propellers, which is due to the change of the added damping predicted by considering different non-penetration boundary conditions.

• Developing a vibro-acoustics model between a hyperelastic structure and fluids. • Structural-acoustic interface is of high numerical stability. • Illustrating the relevance between the in-plane vibrations and acoustic distribution. • Material nonlinearity mainly determines higher-order acoustic radiation responses. This work is concerned with numerical studies on nonlinear vibration and acoustic radiation behaviors of hyperelastic plates made of rubber material. Considering both the geometric and material nonlinearity of the rubber material, structural model of the hyperelastic plate is developed based on the nonlinear finite element method and the Mooney-Rivlin constitutive model. Acoustic waves in an inviscid and compressible fluid perturbed by the in-plane vibrations of the hyperelastic plate are governed by a set of first-order linearized partial differential equations. A fourth-order dispersion-relation-preserving (DRP) finite difference scheme is utilized to compute numerical solutions of the acoustic pressure responses. The structural-acoustic interface between the hyperelastic plate and exterior fluid is constructed through a robust ghost-cell sharp-interface immersed boundary method (IBM) of high numerical stability such that the compatibility conditions on the interface can be satisfied implicitly, although the structural Lagrangian meshes of the rubber plate and the fluid Eulerian grids are not matched. Several numerical examples are designed to check the effectiveness, convergence, and availability of the numerical structural-acoustic coupling model. Based on the numerical model, nonlinear vibro-acoustics response behaviors of the hyperelastic rubber plate subject to uniform dynamic loads are analyzed. Relevance between the in-plane vibrations of the plate and the spatial distribution of the acoustic pressure in exterior fluid is revealed. Effects of the excitation frequency and amplitude of the external loads on the nonlinear vibro-acoustics behaviors of the hyperelastic plate are discussed. Radiation directivity patterns of the fundamental and high-order components show significant differences, and may change dramatically against the excitation frequency and amplitude. In addition, the influences of the geometric and material nonlinearity of the hyperelastic plate on the higher-order vibro-acoustics responses are evaluated. [ABSTRACT FROM AUTHOR]

The intelligent morphing airfoil can adaptively change the shape of the airfoil according to different flight conditions to improve aerodynamic efficiency and aircraft performance. Thus, an intelligent airfoil structure is designed to achieve continuous and smooth deformation of the trailing edge of the airfoil, where a macro fiber composite is considered as an actuator, and a carbon fiber reinforced composite material is chosen as a load-bearing part. An experimental model is made based on the three-dimensional designed model, and the trailing edge deformability of the airfoil under direct current drive is analyzed through the static experiments. Then, a shear stress transfer turbulence model is used to analyze the aerodynamic characteristics of the designed airfoil by the computational fluid dynamics module in COMSOL Multiphysics. The effects of the trailing edge deflection angle, the attack angle of the airfoil, and the incoming velocity on the aerodynamic characteristics of the morphing airfoil are also studied. Finally, the dynamical characteristics of the variable camber airfoil are discussed experimentally, including the natural frequencies and the effect of the high-low coupling excitations on the vibration of the airfoil.

This paper is concerned with experimental analyses on the vibration behaviors of a horizontal pipe containing gas–liquid two-phase flow with different flow patterns. The effects of flow patterns and superficial velocities on pressure fluctuations and structural responses are evaluated in detail. Numerical simulations on the fluid–structure interactions within the pipe are carried out using the volume of fluid method and the finite element method. A strongly partitioned coupling method is adopted to ensure the compatibility and equilibrium interface conditions between the fluid and the elastic pipe. The accuracy of the numerical solutions is confirmed by comparing with experimental results. It is found that the fluctuation frequency of the pressure fluctuations of the two-phase flow ranges from 0 Hz to 5 Hz. The standard deviation (STD) value of the pressure fluctuation of the two-phase flow increases with an increase in the superficial liquid velocity, and the maximum magnitude appears in slug flows. The vibration responses of the pipe exhibit strong dependence on the momentum flux of the two-phase flow, which mainly excites the fundamental flexural vibration mode of the pipe. The magnitude of vertical vibration response of the pipe is equal to that of the lateral vibration response, and the vibration response measured at the middle of the pipe does not contain the second-order operating mode. Moreover, the STD value of the structural responses of the pipe increases proportionally with an increase in the gas flowrate, while the predominant vibration frequency of the pipe slightly increases.

This paper is concerned with the development of an arbitrary Lagrangian–Eulerian (ALE) method for predicting nonlinear acoustic waves radiated from large-amplitude vibrational solid boundaries immersed in viscous fluids of infinite extent. The method is formulated based on a nonlinear acoustic model characterized by velocity potential and acoustic pressure. Unsplit implementations of three perfectly matched layer (PML) techniques are presented, namely conventional PML, convolution PML, and complex frequency-shifted PML (CFS-PML), to enable bounded-domain modeling of nonlinear acoustic waves propagating in an unbounded fluid medium. The long-time stability characteristics of the three distinct PML techniques are examined through theoretical analysis and numerical verification of their ability to absorb evanescent waves. Our results reveal that the CFS-PML technique exhibits superior long-time stability performance compared to the other two techniques. The CFS-PML is also compared to two different absorbing layer techniques: the Kosloff Absorbing Layer (KAL) technique and the Acoustic Black Hole (ABH) technique. The finite element method is adopted for spatial discretization of the finite-amplitude acoustic wave model, and an implicit predictor–corrector algorithm is proposed for time advancement. Several benchmark problems, including acoustic waves produced by a vibrational piston, and transversely oscillating and pulsating cylinders, are analyzed by the proposed method. It is found that the proposed method attains a second-order rate of convergence for spatial discretization, and the convergence rate in terms of the time step size is about first-order. The application of the method to acoustic levitation problems is also demonstrated, and the nonlinear phenomenon of acoustic waves in a single-axis acoustic levitator is discussed. • A finite-amplitude acoustic wave model is established in the ALE framework. • An implicit predictor–corrector integration scheme is developed to simulate the nonlinear acoustic response. • The long-time stability of three different PML techniques is analyzed theoretically and verified numerically. • The accuracy of the proposed method is verified with several benchmark problems. [ABSTRACT FROM AUTHOR]

Phononic crystals are periodically engineered structures with special acoustic properties that natural materials cannot have. One typical feature of phononic crystals is the emergence of band gaps wherein the wave propagation is prohibited due to the spatial periodicity of constituents. This article presents a generalized plane wave expansion method (GPWEM) and a voxel-based discretization technique to calculate the band structures of given three-dimensional phononic crystals. Integrated with the adaptive genetic algorithm (AGA), the proposed method is used to perform topological optimization of constituent distribution to achieve maximized band gap width. Numerical results yielded from the optimization of a three-dimensional cubic phononic crystal verify the effectiveness of the proposed method. Eigenmodes of the phononic crystal with the optimized topology are investigated for a better understanding of the mechanism of band gap broadening.

This paper is concerned with the nonlinear behaviors of acoustic waves produced by two degrees-of-freedom rigid oscillators containing nonlinearities and immersed in infinite fluid medium. The vibrations of the oscillators are computed by both the harmonic balance method (HBM) and the direct-time integration scheme, whereas the linearized Euler equations (LEEs) of the acoustic fluid are discretized by a fourth-order dispersion-relation-preserving (DRP) scheme in space and a four-level explicit time marching scheme in time. A constrained moving least-squares (CMLS) immersed boundary method (IBM) is used to enforce the boundary conditions on the common interfaces of the rigid oscillators and the Cartesian grid of the acoustic fluid. A serially staggered procedure is adopted to solve the governing equations of the oscillators and the acoustic fluid as a coupled system. The perfectly matched layer (PML) technique is utilized to damp out the out-going acoustic waves on the boundaries of the truncated computational domain to approximate the nonreflecting wave conditions. Physical insights into the mechanism of the nonlinear acoustic waves induced by super-harmonic resonances, principal resonances, internal resonances, and combination resonances of two degrees-of-freedom nonlinear oscillator systems are provided. The interference fringes of the acoustic waves due to the nonlinear vibration of the system are also discussed. Numerical results show that the sound fields radiated from the vibration system with the above nonlinear behaviors exhibit more complicated interference phenomena since the high-order harmonic components are introduced.

Abstract: The process of DTH (down-the-hole) hammer drilling has been characterized as a very complex phenomenon due to its high nonlinearity, large deformation and damage behaviors. Taking brittle materials (concrete, granite and sandstone) as impact specimens, the explicit time integration nonlinear finite element code LS-DYNA was employed to analyze the impact process and the penetration boundary conditions of DTH hammer percussive drilling system. Compared with previous studies, the present model contains several new features. One is that the 3D effects of DTH hammer drilling system were considered. Another important feature is that it took the coupling effects of brittle materials into account to the bit-specimen boundary of the drilling system. This distinguishes it from the traditional approaches to the bit-rock intersection, in which nonlinear spring models are usually imposed. The impact forces, bit insert penetrations and force-penetration curves of concrete, granite and sandstone under DTH hammer impact have been recorded; the formation of craters and fractures has been also investigated. The impact loads of piston-bit interaction appear to be relatively sensitive to piston impact velocity. The impact between piston-bit interaction occurs at two times larger forces, whereas the duration of the first impact doesn’t change with respect to the piston velocity. The material properties of impact specimen do not affect the first impact process between the piston and bit. However, the period between the two impacts and the magnitudes of the second impact forces greatly depend on the specimen material properties. It is found that the penetration depth of specimen is dependent on the impact force magnitude and the macro-mechanical properties of the brittle materials.

• Developing a sharp-interface Cartesian grid method for sound scattering from complex geometries. • Interpolation instabilities are effectively suppressed in the present method. • The method can deal with the complex geometry with concave corners. • The method is confirmed by several aeroacoustics benchmark problems. • The convergence rate of the numerical method is second-order. This paper is concerned with the development of a Cartesian grid method for predicting the acoustic field scattered by complex geometries immersed in an infinite fluid. The sound wave in the fluid is formulated by the linearized Euler equations (LEEs), which are computed on a fixed Cartesian grid by using a fourth-order dispersion-relation-preserving (DRP) scheme. The solid object naturally cuts the underlying Cartesian grid with its geometrical boundary, which is modeled by the Lagrangian method. A sharp-interface Cartesian grid model based on the ghost-cell immersed boundary method is established to impose the non-penetrating conditions on the complex boundary of the solid. The values of the acoustic variables at the ghost points are constructed by a linear extrapolation in conjunction with a constrained moving least-squares (CMLS) interpolation method. The interpolation method is capable of eliminating the numerical instability encountered in the conventional moving least-squares (MLS) formulation. The perfectly matched layers are adopted to absorb the out-going sound waves without any reflections from the computational boundary to the interior domain. The proposed method is verified by several benchmark problems in computational aeroacoustics, including the initial value problem of a pressure perturbation scattered by a rigid cylinder, and Gaussian distributed acoustic sources scattered by a single cylinder or two cylinders. Application of the proposed method to the problem of acoustic waves scattered by solids with complex geometries is also presented to demonstrate the effectiveness and robustness of the method. [ABSTRACT FROM AUTHOR]

The vibro-acoustic characteristics of a submerged shaft-hull system are investigated by numerical and experimental methods. A numerical model for the structure-fluid interaction of the system is formulated by the coupled finite element/boundary element methods. With this model, the influence of the shaft vibration on the dynamic and acoustic responses of the submerged shaft-hull system is analyzed via the modal decomposition technology. It is found that reduction of the stiffness of the stern bearing and symmetrization of the foundation can reduce the sound radiation from the submerged shaft-hull system subjected to transversal and axial force excitations, respectively. The numerical solutions are validated by the experimental results, and reasonable agreement is observed. [ABSTRACT FROM AUTHOR]

In this paper, the problem of two-dimensional wing flutter control with sensor and actuator failures is studied. Firstly, the aeroelastic equations of motion for two-dimensional wing with plunge and pitch are established; then the wing flutter model with failure modes is established considering the failure modes of sensor and actuator. Secondly, by designing a novel adaptive fault-tolerant controller, the wing flutter can be suppressed effectively and the external disturbance can be dealt with successfully. The stability of the system is proved by the constructed Lyapunov function. Finally, numerical simulation results show that the adaptive flutter fault-tolerant controller can effectively suppress wing flutter when sensors and actuators fail, and the controller has good robustness to the changes of external disturbances. [ABSTRACT FROM AUTHOR]