Journal of Fluids and Structures最新文献

Torsional vibration of the propulsion shaft system has a significant influence on the safety and stability of marine navigation. Additionally, the resulting instantaneous fluctuation of rotational speed affects the hydrodynamic loading of propeller. To investigate this influence, a numerical model of propeller hydrodynamics influenced by hull wake and torsional vibration is established using delayed detached eddy simulation. First, the modeling method is described, and the model is verified and validated. Second, simulations are carried out for different amplitudes and frequencies of torsional vibration, and the hydrodynamic excitation, pressure pulsations and flow field features are analyzed detailly. The results show that torsional vibration significantly affects the hydrodynamic excitation of propeller, due to the fluctuations in blade section velocity, angle of attack and loading induced by instantaneous rotational speed, which can be equivalent to non-negligible added mass and damping. Through statistical analysis of the temporal-spatial pressure distribution, the complex modulation of torsional vibrations with different frequencies on the flow field from macroscopic hydrodynamic excitation to microscopic flow features is revealed. The effect of fluctuating small-amplitude loading on the dynamics and stability of propeller wake is also studied. This study provides theoretical support for designing and optimizing marine propellers and propulsion shaft systems.

Simulations of blade flutter are highly sensitive to undesired wave reflections at inlet and outlet boundaries. A careful treatment of boundary conditions is required to prevent the generation of perturbations. This study is motivated by the need to perform flutter analysis of low-pressure steam turbine blades, for which supersonic inflow conditions may occur in the near-tip region. The exact steady non-reflecting boundary condition (NRBC), the spectral NRBC and a simple isentropic boundary condition are implemented in a time-marching flow solver and applied to turbomachinery flutter simulations covering a wide range of operating conditions. For the first time, the spectral NRBC is applied to a blade flutter simulation with a supersonic inlet and its performance is analysed and compared with other boundary condition formulations. It is shown that an effective non-reflective treatment in the design of the boundary condition is essential for an accurate aeroelastic prediction at all operating conditions, including the subsonic flow regime. The limitation of the exact steady NRBC to spatial modes causes it to perform poorly in some unsteady flow simulations, whereas the spectral NRBC achieves a satisfactory suppression of undesired wave reflections in all investigated cases.

Traditional hull fabrication relies on labor- and time-intensive methods to generate smooth, curved surfaces. These conventional methods often lead to hull surface topologies that are static in design with hydrodynamics aimed at handling a broad range of sea conditions but not optimized for any specific scenario. In this paper, we introduce a method of rapidly fabricating planing hulls using the principles of curved-crease origami. Starting from a flat-folded state, the curved-crease origami hulls can be deployed to match traditional planing hull shapes like the VPS (deep-V, Planing hull with Straight face) and the GPPH (General Purpose Planing Hull). By extension of the ability to conform to a desired shape, we show that the curved-crease origami hulls can emulate desired hydrodynamic characteristics in still as well as wavy water conditions. Furthermore, we demonstrate the shape-morphing ability of curved-crease origami hulls, enabling them to switch between low and high deadrise configurations. This ability allows for on-demand tuning of the hull hydrodynamic performance. We present proof-of-concept origami hulls to demonstrate the practical feasibility of our method. Hulls fabricated using the curved-crease origami principles can adapt to different sea states, and their flat foldability and deployability facilitate easy transport and deployment for rapid response naval operations such as rescue missions and the launch of crewless aquatic vehicles.

In this work, a numerical framework for global stability analysis of rigid-body-motion fluid–structure-interaction problems is presented. The Jacobian matrices which arise in the linearization procedure are derived numerically via the first-order finite difference scheme. The linearized fluid–structure coupled equations are solved using an immersed boundary method. The linear stability solver is first tested on two canonical cases, i.e., the flow past a stationary cylinder and the flow past an isolated elastically mounted cylinder. An excellent agreement between the results obtained here and those from available published research is achieved. The solver is then used to study the linear stability of the flow past two elastically mounted cylinders in tandem arrangement. The variations in growth rate and frequency of two leading modes with reduced velocity are examined. The mechanisms of lock-in and galloping phenomena observed in nonlinear simulation are elucidated from the perspective of linear instabilities in the leading modes.

Hydroelastic wave interaction with a circular crack of an ice-cover in a channel together with some related problems is considered, based on the linearized velocity potential theory and Kirchhoff plate theory. The domain decomposition method is adopted in the solution procedure. Two sub-domains are divided by the crack, one below the inner ice sheet and the other below the outer ice sheet. By using the Green function of an ice-covered channel, the velocity potential in the outer domain is established from the source distribution formula over an artificial vertical surface extended from the crack. The source distribution is expanded in both vertical and circumferential directions, which allows the velocity potential to be obtained in an explicit form with unknown coefficients. The velocity potential in the inner domain is expanded into a double series. An orthogonal inner product is used to impose continuity conditions on the artificial vertical surface and the edge conditions at the crack. The derived formulation is not just limited to the circular crack problem but can also be readily used in a variety of other problems, including wave diffraction by a surface-piercing vertical cylinder, polynya and circular disc floating on the free surface in a channel. Extensive results are provided for the forces on the inner ice sheet, the transmission and reflection coefficients. In particular, a detailed analysis is made on their behaviours near the natural frequencies of the channel, and the natural frequencies corresponding to the motion of the inner ice sheet.

Vibration and noise reduction of motional structures is a conventional challenge in a variety of industrial realms due to synchronous spatial motions present. In this case, optimizing structure design could provide a promising way for solution. Motivated by the idea of wave manipulation via phononic crystals (PCs), this paper aims to control three-dimensional (3D) vibration transmission of a rotating pipe by introducing an axial periodic design. The pipe is arranged as a composite structure comprised of alternate materials along the axial direction, and a constant fluid flows inside the pipe. Based on the Rayleigh beam theory, a set of 3D doubly-gyroscopic equations governing in-plane, out-of-plane flexural and axial motions of the pipe is established, which accounts for rotation gyroscopic force and fluid gyroscopic force. The spectral element technology is applied in such multi-dimensional system for solution. Following a validation by the finite element (FE) simulation, the band structure, frequency response function (FRF) and elastic wave shapes are presented to elucidate the 3D bandgap (BG) mechanism of the rotating PC pipe. The results obtained demonstrate the superior effectiveness of the proposed model for the 3D vibration suppression. Extensive parametric discussions reveal that the rotating motion, flowing fluid and geometry of the pipe all have significant impacts on the BG performance of the present rotating PC pipe system.

Numerical simulations of the flow around spanwise-flexible flapping wings in tandem are reported, focusing on a thrust-generating configuration. Wings of aspect ratio 2 and 4 in forward flight undergo heaving and pitching motion following optimal 2D kinematics. The Reynolds number of the simulations is $Re=1000$. The effect of flexibility is explored by varying the effective stiffness of the wings, while the effective inertia is kept constant. The aerodynamic performance of the tandem system results from a combination of unsteady aerodynamics mechanisms, fluid–structure resonance, vortex–wing interactions (denoted wake capture in this study) and aerodynamic tailoring. It is found that the aerodynamic performance and structural behavior of forewings are dominated by a fluid–structural resonance. The maximum mean thrust for the forewings is obtained when the driving frequency approaches the first natural frequency of the structure, ${\omega }_{n,f}/\omega \approx 1$, similarly to what is observed in isolated wings undergoing the same kinematics. On the other hand, hindwings show optimal performance in a broad region near ${\omega }_{n,f}/\omega \approx 2$, and their aerodynamic performance seems to be dominated by wake–capture and aerodynamic–tailoring effects. The aerodynamic performance of the hindwings is dependent on the flexibility of the forewing, which impacts the intensity of the vortices shed into the wake and the resulting effective angle of attack (i.e., wake capture). The timing between the effective angle of attack and the pitching motion of the hindwing controls the generation of thrust (or drag) of each spanwise section of the hindwing (i.e., aerodynamic tayloring). A proof of concept study on the aerodynamic performance of systems made of wings with different flexibility suggests that they could outperform tandem systems with equally flexible wings. Thus, the optimal mixed–flexibility tandem system is composed by a resonant forewing, which maximizes the thrust generation of the forewing and the intensity of the vortices shed into the wake, and a hindwing whose flexibility must be tuned to maximize wake capture effects.