Journal of Fluids and Structures最新文献

This study investigates Active Structural Acoustic Control (ASAC) applied to sandwich plates featuring Functionally Grade (FG) porous Graphene-Platelet-Reinforced Piezoelectric (GPLRP) materials. These plates incorporate a core layer with internal pores and GPLs dispersed in a metal matrix, along with piezoelectric sensors and actuators. Using a spatial state-space formulation based on linear 3D piezoelasticity theory, a semi-analytical solution is derived for the vibroacoustic response of these sandwich plates. The mechanical properties of the porous core are modeled using a closed-cell metal foam. The effects of various parameters, including porosity distributions, porosity coefficient, weight fractions of nanofiller, and geometric parameters on the radiation efficiency and radiated sound power have been investigated. Then, the validation of the proposed model is examined by comparing the natural frequencies calculated in the present study to those from the literature. This comparison allows us to assess the accuracy and reliability of our model against established findings. Radiated sound power reduction from mechanically excited structures is achieved by employing a dual-stage Proportional Integral–Proportional Derivative with Filter (PI-PDF) controller, optimized using Grey Wolf Optimization (GWO). Finally, several numerical simulations are conducted to validate the effectiveness and accuracy of the proposed active control strategy.

The paper presents a monolithic finite element model for the hydro-visco-elastic analysis of floating membranes interacting with ocean waves. The formulation couples linearised potential flow and viscoelastic membrane equations, offering a versatile tool for modelling arbitrarily shaped floating membranes in varying sea-bed topography. The paper also presents a wet modal analysis for the coupled problem, accounting for the added mass and stiffness of the surrounding fluid. This model is used to study the dependence of the wet natural frequencies of floating membranes on the material properties. It is also used to analyse the reflection, transmission, scattering and absorption of ocean wave energy by 1D and 2D floating membranes. Notably, the paper underscores the impact of proportional material damping on these observed phenomena. The results highlight local peaks in the viscoelastic behaviour at the calculated wet natural frequencies, and demonstrate the outward dispersion of incoming wave around finite 2D membranes. Furthermore, the model is employed to examine the interaction of viscoelastic membranes with other structures, such as a monopile, under the influence of ocean waves. This comprehensive investigation contributes to a deeper understanding of the fluid–structure interaction inherent to certain floating solar, wave-energy converter and floating breakwater technologies.

Understanding the impact of bathymetry features on the wake and loading of a tidal stream turbine is crucial to inform deployment of tidal turbine farms. This study investigates the influence of a Gaussian ridge on a single turbine of diameter ($D$) using high-fidelity large-eddy simulations. The ridge height is 0.33$D$ and turbine locations at ridge centre and at six upstream and six downstream distances are analysed. The analysis elucidates the important role of bathymetry on wake recovery and fatigue design providing valuable insight for real-world planning of turbine arrays. The rate of wake recovery is increased both for turbine locations beyond 1.5$D$ upstream of the ridge due to the favourable pressure gradient over the upslope, and for locations beyond 3$D$ downstream of the ridge due to elevated turbulence intensity. For locations, close to and atop the ridge, the higher flow-speed and adverse pressure gradient of the downslope of the ridge were found to reduce the rate of wake recovery. When unaffected by the ridge wake meandering is similar to the flat bed case and characterised by Strouhal number but modulated by the frequency of ridge shedding downstream of the ridge. Damage equivalent loads are slightly increased at upstream locations due to flow speed-up and further increased at downstream locations due to a combination of increased turbulence intensity and variation over the rotor plane of both onset flow and turbulence.

The presence of complex buildings introduces interference effects and complicates the wind field around the long-span roof structure. The complexity of the problem makes it challenging to predict wind load of long-span roof structures, resulting in a scarcity of design data available to engineers and designers. Thus, the influence of interference effect on mean and peak pressure coefficients of a long-span tri-centered cylindrical roof structure was investigated through wind tunnel tests conducted in an atmospheric boundary layer wind tunnel. The study focused on the interference effects caused by two cooling towers, chimney, and some low-rise buildings, considering all wind directions. The results show that the interference effects are the result of a combination of amplification effects from the adjacent cooling towers and chimney and shielding effects from the surrounding low-rise buildings. Due to the shielding effect of low-rise buildings, the wind suction on the roof is mitigated compared to that of a single building, while at the end of the roof, wind suction is amplified by cooling tower effects. The interference effect of the building clusters, however, amplifies the fluctuating wind pressure coefficient and the peak pressure coefficient on the roof. To accurately estimate the wind load of building components and envelopes, furthermore, this study investigates the influence mechanism of interference effect on non-Gaussian characteristics of wind pressure combining with fluctuating wind pressure spectrum, spatial correlation of fluctuating wind pressure, and probability distribution characteristics of standardized wind pressure coefficient. The spatial correlation of wind pressure in the roof interference area exhibits a strong association, and the probability density distribution characteristics of the standardized wind pressure coefficient significantly deviate from the Gaussian curve, with a peak factor exceeding 3.5. Thus, it can be concluded that the wind pressure within this region demonstrates significant non-Gaussian characteristics. Hence, the zone values of peak pressure coefficients are determined using the peak factor approach to inform the wind resistance design of the envelope structure.

This study is inspired by the extraordinary suppression performance of the intrinsic vortex-induced vibration (VIV) of a harbor seal vibrissa, which is renowned for its excellent detection characteristics in sensing upstream flow information. A physical understanding of the suppression mechanism of vibrissal cylinder oscillation is mandatory for the design of a sensor detection system. In this study, the suppression performances are studied for a vibrissal cylinder at a reduced velocity U_r = U/(f_nw D) of 5, with the corresponding Reynolds number of Re = 3,750, according to the swimming velocity and real geometrical scale for a harbor seal vibrissa. In addition, a verified fluid and structure interaction (FSI) solver implemented with an improved dynamic mesh strategy is adopted. Different transverse spring-mounted oscillation responses are obtained by alternating the angle of attack (AOA). It is found that the attachment and secondary separation of vortices in the near wake significantly influence the shedding pattern and can lead to the distinct suppression of oscillating responses. A pair of attached vortices are observed for the vibrissal cylinder at AOA = 0°, where the oscillating response is almost fully suppressed. Forced vibration is applied to address the role of attached vortices in the suppression mechanism. In contrast, when the suppression mechanism diminishes at AOA = 90°, the vortex-shedding pattern is characterized by unstable discrete secondary vortices. These secondary vortices re-separate from the primary vortices, indicating high stability of the primary vortices in the near wake, which represents the primary mechanism deteriorating the suppression of vortex-shedding. Furthermore, the interaction of attaching and secondary discrete vortices results in a partially suppressed oscillating response for the harbor seal vibrissa at AOA = 45°. Of note, these two mechanisms cause the shedding mode to switch between the “2S” and “P + S” modes while partially suppressing the oscillation response.

This study delves into the aerodynamic behaviour of a highly flexible NACA 0012 aerofoil, drawing inspiration from avian feathers to handle a gust. We first examined unsteady flow on a rigid wing both experimentally and numerically and then explored the implications of introducing wing flexibility purely numerically. Our findings underscore the potential of composite materials in alleviating the oscillating aerodynamic forces on a wing under a streamwise gust. This behaviour is attributed to its capacity to destabilize the laminar separation bubble, fostering a more stable turbulent boundary layer. While direct avian evidence remains limited, it is postulated that in nature, such mechanisms could mitigate undesired wing flapping, optimizing energy consumption in perturbed environments.

This work assesses the validity of transfer learning the hydrodynamic coefficient database, consisting of the added mass and lift coefficients, applicable to flexible bodies undergoing vortex-induced vibrations. Specifically, the hydrodynamic coefficient database learned on data collected by Braaten and Lie (2005) are used to predict the motions observed during in house bare riser model experiments at the MIT Towing Tank. A fully immersed vertical flexible riser model with a length-to-diameter ratio of 145 is towed at different flow speeds and top tensions. Motion is tracked using underwater cameras and the motions are reconstructed using a machine-vision framework eliminating the need for expensive sensing hardware. The vibration amplitude, frequency, and mode shape are determined and the results are compared with those in the literature. Finally, blind predictions of the in-house observed experiments are made using the software VIVA informed with transfer learned hydrodynamic coefficients learned on the experiments by Braaten and Lie (2005).

The main goal of this work is to develop a data-driven Reduced Order Model (ROM) strategy from high-fidelity simulation result data of a Full Order Model (FOM). The goal is to predict at lower computational cost the time evolution of solutions of Fluid–Structure Interaction (FSI) problems. For some FSI applications, the elastic solid FOM (often chosen as quasi-static) can take far more computational time than the fluid one. In this context, for the sake of performance one could only derive a ROM for the structure and try to achieve a partitioned FOM fluid solver coupled with a ROM solid one. In this paper, we present a data-driven partitioned ROM on two study cases: (i) a simplified 1D-1D FSI problem representing an axisymmetric elastic model of an arterial vessel, coupled with an incompressible fluid flow; (ii) an incompressible $2D$ wake flow over a cylinder facing an elastic solid with two flaps. We evaluate the accuracy and performance of the proposed ROM-FOM strategy on these cases while investigating the effects of the model’s hyperparameters. We demonstrate a high prediction accuracy and significant speedup achievements using this strategy.

Flexible wingtip extensions matched to the Cauchy and Reynolds numbers of a peregrine falcon’s primary feather in flight have been tested in differing configurations and compared to rigid ones for reference. The wingtip configurations were attached to the end of a symmetric (NACA 0012) aerofoil and were tested at 5° and 10° angles of attack and Reynolds numbers of $70k$ and $90k$. Time resolved particle image velocimetry (TR-PIV) was used to study the dynamics of the individual vortices. The results show that, at increased angle of attack the configuration with C-type variation of the free length of the winglets is spreading the vorticity into spanwise and vertical directions, generating a circular multi-core vortex arrangement. In contrast, for the case of winglets of the same length (I-type configuration) a continuous vortex sheet is formed which rolls up into a single core, dislocated outboards and upwards from the original tip-vortex location. This remains the case even for larger angle of attack. It is concluded that – besides the known reduction of induced drag – the former configuration is also beneficial for a more rapid disintegration of the tip-vortex in the wake, while the latter shows less instability. This let us speculate that the latter could be relevant for reducing the tip-noise at higher angle of attack such as for Owls, who hunt during night and have adapted to fly silently.