Journal of Fluids and Structures最新文献

In this paper, we introduce a new nonlocal modal hydrodynamic theory for fluid–structure interactions (FSI) of light, flexible cantilever beams and plates undergoing small amplitude vibrations in Newtonian, incompressible, viscous, heavy fluids otherwise at rest. For low aspect ratio flexible structures and high mode numbers, three dimensional (3D) and nonlocal fluid effects become prominent drivers of the coupled dynamics, to the point that existing local hydrodynamic theories based on two dimensional (2D) fluid approximations become inadequate to predict the system response. On the other hand, our approach is based on a rigorous, yet efficient, 3D treatment of the hydrodynamic loading on cantilevered thin structures. The off-line solution of the FSI problem results in the so-called nonlocal modal hydrodynamic function matrix, that is, the representation of the nonlocal hydrodynamic load operator on a basis formed by the structural modes. Our theory then integrates the nonlocal hydrodynamics within a fully coupled structural modal model in the frequency domain. We compare and discuss our theory predictions in terms of frequency response functions, mode shapes, hydrodynamic loads, quality factors, added mass ratios with the predictions of the classical local approaches, for different actuation scenarios, identifying the limitations of the hypotheses underlying existing treatments. Importantly, we also validate our new model with experiments conducted on flexible square plates. While computationally efficient, our fully coupled theory is exact up to numerical truncation and can bridge knowledge gaps in the design and analysis of FSI systems based on low aspect ratio flexible beams and plates.

The grooved and the wavy leading edge structures have been designed to reduce the interaction noise generated by the cylinder-airfoil model. The wind tunnel tests conducted at different incoming velocities ranging from 40 to 60 m/s, revealing that the wavy leading edge structure only exhibits a noise reduction effect within the mid-frequency band (800∼4000 Hz). However, the combination of the two structures compensates for the insensitivity to low-frequency peak noise. At the velocity of 60 m/s, there are reductions of 14.7 dB for peak noise and 5.4 dB for average noise within the mid-frequency band. Numerical simulations based on large eddy simulation and the Ffowcs Williams–Hawkings acoustic analogy are performed to further explore the mechanisms of noise reduction. The results indicate that integrating the two structures has a substantial impact on reducing the pulsation pressure and enhancing the decorrelation and decoherence effects among the noise sources. The strong phase interference effect leads to a decrease in the radiation efficiency of the interaction noise.

To estimate nonlinear flutter response of long-span bridges, this study established a method for identifying full set of amplitude-dependent flutter derivatives (FDs) from free vibration wind tunnel tests. Taking a typical double-deck truss bridge as a Case study, the amplitude-dependent FDs of the bridge deck at the whole wind speed regime are identified and cross-validated based on large-amplitude free vibration wind tunnel tests of its single degree of freedom (SDOF) torsional and 2DOF vertical-torsional section models. The influential mechanism of vertical DOF on nonlinear flutter was revealed by quantitatively comparing the nonlinear aerodynamic damping of the SDOF and 2DOF systems. The amplitude-dependent FDs are then used to calculate the nonlinear flutter responses of the 2D bridge section and a prototype long-span suspension bridge (1650m) with four main cables based on developed 2D and 3D nonlinear flutter analysis methods. Finally, the influence of structural parameters and 3D effects on nonlinear flutter are quantified and discussed. The results show that the 2DOF system has a lower critical wind speed and higher torsional stable amplitudes compared with the SDOF system since the participation of vertical DOF introduces the negative coupled aerodynamic damping to the system. The aerodynamic nonlinearity becomes stronger and stronger as the wind speed increases and it mainly leads to the significant amplitude dependence of the uncoupled aerodynamic damping, which is the key factor to cause the limit cycle oscillation (LCO)-type of flutter. While the coupled aerodynamic damping appears to be a relatively linear damping with weak amplitude-dependence within the studied wind speed and it mainly plays the role of reducing the stability of the system. The 3D effects of the vibrating bridge deck will reduce the system stability mainly by increasing the negative uncoupled aerodynamic damping. Therefore, the amplitudes of nonlinear flutter will be seriously underestimated if the 3D effects are ignored.

This work investigates the effects of in-contact Underwater Explosion (UNDEX) on flat plates of various thicknesses. The interaction between generated bubbles and the plates is also studied. High-speed photography paired with digital image correlation (DIC) was used to capture full-field displacements, velocities, and strains on the plates during loading. Shockwave pressure was also recorded using pressure transducers strategically positioned in the water. The results show that, in the absence of rupture, thicker plates experience less deformation and allow the bubble to grow to a larger volume than the thinner plates, albeit smaller than that of a free field bubble. Bubbles generated in the vicinity of thicker plates also retain more energy. Contrary to cases of free field or near explosions which feature spherical and drifting bubbles, here the bubble assumes an ellipsoidal shape and attaches itself to the plate where it is confined to a more rapid cycle of collapse and regrowth before fully dissipating. When plate rupture does occur, it is immediate and is due to the initial shock. This structural failure drastically alters the behavior of the bubble.

Cylindrical shells filled with fluid are structures widely used in different engineering installations, such as oil platforms, nuclear power plants and storage tanks. In offshore structures, the action of ocean waves must be carefully evaluated, because the influence of the external fluid to the external walls of this structure can modify its dynamic behavior. This article presents an analytical-numerical approach to analyze the linear vibrations of a clamped cylindrical shell, considering the presence of external and internal fluid with the inclusion of the effects of the free surface motion of the internal fluid. The strain fields and curvature changes of the middle surface of the cylindrical shell are described by the Sanders–Koiter linear theory. The modal expansions, that describe the displacement fields of the clamped cylindrical shell, are obtained from the Chebyshev polynomials. Finally, the Rayleigh–Ritz method is used to derive the linear equations of motion of the system which, in their turn, are solved to obtain the natural frequencies and vibration modes of the clamped cylindrical shell with external and internal fluid interaction. The obtained results are compared, when it is possible, with others works found in the literature. Also, a finite element analysis in a commercial software is conducted to evaluate the reliability and the convergence of the obtained analytical-numerical results. They indicate that the fluid level alters the free vibrations of the cylindrical shell. The consideration of the external and internal fluid for free vibration analysis with application of a hydrodynamic pressures is important, since it incorporates additional mass and reduces the values of natural frequencies. The influence of the free surface also alters the free vibrations of the cylindrical shell, and for large structures, a considerable effect is observed that cannot be disregarded.

The red blood cell (RBC) membrane is often modeled by Skalak strain energy and Helfrich bending energy functions, for which high-order representation of the membrane surface is required. We develop a numerical model of RBCs using an isogeometric discretization with T-splines. A variational formulation is applied to compute the external load on the membrane with a direct discretization of second-order parametric derivatives. For fluid–structure interaction, the isogeometric analysis is coupled with the lattice Boltzmann method via the immersed boundary method. An oblate spheroid with a reduced volume of 0.95 and zero spontaneous curvature is used for the reference configuration of RBCs. The surface shear elastic modulus is estimated to be $G_s=4.0\times 10^{-6}$ N/m, and the bending modulus is estimated to be $E_B=4.5\times 10^{-19}$ J by numerical tests. We demonstrate that for physiological viscosity ratio, the typical motions of the RBC in shear flow are rolling and complex swinging, but simple swinging or tank-treading appears at very high shear rates. We also show that the computed apparent viscosity of the RBC channel flow is a reasonable agreement with an empirical equation. We finally show that the maximum membrane strain of RBCs for a large channel (twice of the RBC diameter) can be larger than that for a small channel (three-quarters of the RBC diameter). This is caused by a difference in the strain distribution between the slipper and parachute shapes of RBCs in the channel flows.

In this study, we examined the aerodynamics around the hindwing of a faithfully reproduced Pantala Flavescens (globe wanderer) under gliding conditions. The dragonfly wing is corrugated, with numerous veins running through the entire wing. This convexoconcave geometry improves the lift-to-drag ratio under low Reynolds number conditions. However, until now, aerodynamic analyses have only been performed on 2D chordwise cross-sections of the wing and pseudo-3D shapes extending the profiles spanwise. The aerodynamic performance of a 3D geometry that faithfully replicates all wing veins has yet to be investigated. Therefore, we prepared a faithful analytical model by 3D scanning the hindwing of a Pantala Flavescens specimen; as a migratory dragonfly, it is capable of long-duration and long-distance flight. In our simulation results, the V-shaped groove formed by the large wing veins was covered by separation vortices, resulting in a pseudo-smooth wing surface. The role of the differently-sized wing veins is supposedly to inhibit separation. The faithful reproduction of the wings provides a better understanding of the 3D flow structure and directly leads to a precise estimation of the underlying aerodynamic characteristics. Accurate performance must be evaluated by simulating a faithful geometry in low angle of attacks, where aerodynamic efficiency is required for long-distance flight.

Lift enhancement mechanism of an airfoil with the dielectric elastic membrane skin is studied numerically for smart flow control. The flexible membrane is made of dielectric highly elastic polymer material. Such kind of material can deform and oscillate under the prescribed electric potential difference. The dynamic modeling of the dielectric elastic structure is established to describe the electromechanical behaviors. A high-fidelity aero-electromagnetic-structural coupling model is proposed and verified based on CFD/CSD coupling technique. The aerodynamic characteristics of the airfoil with the dielectric elastic membrane skin is analyzed at various angles of attack. The results show that the lift coefficient of the airfoil is 12.33% higher than that of the rigid airfoil at AOA=14°. The effects of coupling oscillation and applied voltages on the aerodynamic performance of the airfoil are emphasized. In the nonlinear coupling, the high-order lock-in frequency plays a significant role in lift enhancement. The lift coefficient is greatly improved when the second-order frequency lock-in occurs and the second-order lock-in frequency is no less than the second-order fundamental frequency of the flow past the rigid airfoil. The corresponding flow pattern is characterized with the formation and maintain of the vortices with similar scale.

The dynamics of slender cantilevered cylinders subjected to internal or external axial flow has been studied extensively from the 1960s onwards. In the early studies, the flow was directed from the clamped end towards the free end of the cantilever. The same is true for cantilevered plates (or “flags”) in axial flow. More recently, however, the dynamics in reverse axial flow, i.e., flow directed from the free towards the clamped end, has received attention, partly as curiosity-driven research, but also because of engineering applications. For example, cantilevered pipes aspirating fluid are used in ocean mining, cantilevered cylinders in reverse axial flow may be found as control rods in nuclear reactors, and cantilevered plates in reverse axial flow are a candidate system for energy harvesting.

The present paper provides a summary of the dynamics of these systems in conventional and reverse axial flow and compares their dynamical behaviour. For example, cantilevered cylinders in conventional axial flow are subject to a weak static divergence (buckling) at sufficiently high flow velocities, and to vigorous flutter at higher flow velocities. Cylinders in reverse axial flow, on the other hand, are subject to weak flutter at low flow velocities and to a large amplitude static divergence at higher flows. In the first case the dynamics is sensitive to the shape of the free end, and in the second hardly at all.

The differences in the dynamical behaviour in reverse flow vis-à-vis conventional flow for pipes and plates, not so neatly reversed as for cylinders, are also discussed, and some general conclusions drawn for all three systems, regarding similarities and differences in the dynamics and sensitivity to free-end shape arising from the direction of fluid flow.

This paper explores the conditions for hydroelastic trailing edge vibrations generating tonal noise on a NACA0015 aluminium hydrofoil clamped in a hydrodynamic tunnel. Tests were performed for Reynolds numbers $Re$, ranging from $2\times 10^5$ up to $12\times 10^5$ and various angles of attack $\alpha$, from 0 up to 10°. A laser vibrometer was used to characterize the hydrofoil vibratory response. Time Resolved Particle Image Velocimetry (TR-PIV) was used to scrutinize the origin of the hydrodynamic excitation mechanism. Hydroelastic trailing edge vibrations of significant amplitude were observed at moderate angles of attack $4\le \alpha \le 8.5$°, for Reynolds number such that the pressure side boundary layer transition was located close to the trailing edge, with a frequency signature allowing a lock-in with the hydrofoil trailing edge structural mode. Two passive solutions were tested to mitigate this hydroelastic flow-induced vibration: a truncated hydrofoil and a triggered one. The truncated configuration slightly impacts the vibration while triggering the pressure side boundary layer transition ahead of the trailing edge eliminates the trailing edge vibrations with negligible impact on the hydrofoil hydrodynamics performances.