Journal of Fluids and Structures最新文献

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.

As a continuous and smooth morphing concept for aerofoils, the Fish Bone Active Camber (FishBAC) concept has demonstrated significant aerodynamic efficiency improvements over traditional hinged flaps. In this paper, to investigate the static and dynamic aeroelasticity of FishBAC aerofoils, an unsteady two dimensional coupled fluid-structure interaction model is developed, which includes the structural response of the FishBAC spine, skin, stringers, tendons and actuator, coupled to an unsteady aerodynamics model. The structural dynamic model is Timoshenko beam-theory-based, while the aerodynamic model is based on Peters’ unsteady model. The static and dynamic aeroelasticity is studied after the model is validated. Results show that the increase in pulley rotational angle reduces the zero-lift angle of attack, while keeping the slope between the lift coefficient and angle of attack the same. A shorter morphing region closer to the trailing edge is beneficial for generating larger lift coefficient with the same tendon moment and angle of attack. Flutter occurs with the increase of the air speed. When the morphing end position is fixed at 0.9 chord, increasing the morphing length reduces the critical flutter speed significantly, with the second bending mode tending to drive instability. With the same morphing length, moving the morphing region closer to the leading edge increases the critical flutter speed, and the unstable mode changes from the second mode into the first mode.

The unidirectional flow of nanofluids holds paramount importance in numerous nanofluidic applications. However, there remains a void in the practical implementation of a nano check valve specifically designed for ensuring such unidirectional flow. In this work, a nano check valve consisting of a diaphragm and a valve seat is designed and its forward opening and reverse shutoff processes are investigated using molecular dynamic simulations. Additionally, the effects of the modified groups of the diaphragm of the nano check valve are studied. The results demonstrate that the nano check valve can be opened efficiently under a certain forward differential pressure. Besides the differential pressure, the interaction between diaphragm and water molecules contributes to the opening process. Conversely, the non-bonding interaction between diaphragm and the valve seat prevents the opening. The reverse shutoff simulations reveal that the reverse shutoff function can be achieved under the backward pressure even when the nano check valve is firstly opened. It is observed that the ratio of hydroxyl to hydrogen groups on the edge of diaphragm has a significantly effect on the opening pressure due to the non-bond interaction of diaphragm and water molecules. This suggests that the opening pressure of the nano check valve could be regulated by changing the modifier groups.

This paper presents a fully explicit coupled wave–vegetation interaction model capable of efficiently solving the coupled wave dynamics and flexible vegetation motion with large deflections. The flow model is formulated using the continuity equation and linearized momentum equations of an incompressible fluid, with additional terms within the canopy region accounting for the presence of vegetation. This linearized flow solver is unconditionally stable and second-order accurate. The flow model is validated and verified against experimental measurements and analytical solutions for waves over a rigid canopy, demonstrating its capability to accurately capture the wave dissipation and flow velocity profiles, even with a relatively coarse grid. A truss-spring model is proposed to capture vegetation motion with substantial deflections, and is proven to be mathematically consistent with the governing equation for the flexible vegetation motion. It allows for explicit time integration with large time steps when dealing with highly compliant vegetation. The truss-spring model is validated and verified by experimental and numerical results for large-amplitude motions of a single elastic blade subjected to waves and sinusoidal oscillatory flows. The coupled model, combining the linearized flow solver and the truss-spring model, is applied to investigate wave propagating over a heterogeneous, suspended, and flexible canopy, showing high efficiency and good agreement with the experiments concerning wave attenuation and the hydrodynamic loads on the vegetation.

Entrapment of waves is a hydrodynamic phenomenon that occurs under certain incident wave circumstances. The present article, based on potential flow theory, deals with the phenomena of waves interacting with a proposed submerged structure. Focus of this study is on the frequencies for which the wave trapping occurs in the region of the proposed structure. The frequencies presented correspond to significantly higher values of velocity potential in the region. We refer this region as a wave-trapping zone. In scattering problem the reflection coefficient is also investigated with respect to submergence depth, water depth, length and rigidity of the structure and other physical parameters. The optimum values of the structural parameters are also proposed for zero transmission and trapping frequencies. The investigation establishes that for a suitable configuration we can achieve a range of frequencies for which such a geometry can be used as breakwater with total reflection and at certain frequencies also for trapping of waves.

The endothelial glycocalyx layer (EGL), with its inherent fibrous architecture enveloping the interior surfaces of blood vessels, paradoxically increases resistance to blood flow. This phenomenon poses a significant question: how do physiological systems overcome the enhanced resistance imparted by the EGL? Addressing this knowledge gap, this study proposes a new theoretical framework to analyze the dynamic behavior of the EGL in the setting of pulsatile blood flow. Central to our investigation is the novel concept of pulsatile soft lubrication, a potential mechanism for mitigating flow resistance. Utilizing a theoretical model that mimics fluid dynamics across parallel fibrous boundaries, we explore the intricate interplay between fluid motion and EGL fibers under pulsatile pressure gradients. The results indicate that the EGL's natural elasticity engenders a dynamic interface that notably lessens flow resistance, thereby enhancing flow rates. Beyond advancing our understanding of the EGL's critical function in hemodynamics, this research also highlights its broader implications, suggesting relevance in engineering and design principles. Insights into fluid dynamics and surface interactions garnered from this study could inform innovative strategies for reducing friction and optimizing flow across a variety of systems.