Journal of Fluids and Structures最新文献

Due to the complexity of fluid–structure interactions (FSI), the majority of studies in the literature dealing with the sloshing problem are restricted to rigid tanks. This paper is devoted to a numerical investigation of the liquid sloshing behavior in a flexible tank subjected to external loading. A numerical methodology is proposed, taking into account the FSI problem by coupling two open-source codes: OpenFOAM for the fluid and FEniCS for the solid, using the preCICE library, a free library for fluid–structure interaction. The Arbitrary Lagrangian–Eulerian formulation is used for the two-phase flow system to solve the Navier–Stokes equations in the fluid domain using the finite volume method. Simultaneously, the linear-elastic equation of the structure is solved using the finite element method. An implicit coupling scheme is considered at the fluid–structure interface. The numerical methodology is validated by the results given in literature for harmonic excitation at different frequencies. Subsequently, an analysis of complex external loading, such as Gabor wavelets and earthquake ground motion, is conducted to highlight the significant impact of the wall flexibility on sloshing, as well as the influence of hydrodynamic forces on the structure’s deformation. The proposed coupling methodology is robust and effective, it can be applied to all types of liquids and materials. A dataset of one of the studied cases is given as a supplement to the paper (Kha et al., 2024).

Highly nonlinear near-breaking and spilling breaking wave groups are common extreme events in the ocean. Accurate force prediction on offshore and ocean structures in these extreme wave conditions based on numerical approaches remains a problem of great practical importance. Most previous numerical studies have concentrated on non-breaking wave forces on rigid structures. Taking advantage of the smoothed particle hydrodynamics (SPH) method, this paper addresses this problem and presents the development and validation of a numerical model for highly nonlinear hydrodynamics of near-breaking and spilling breaking waves interacting with a vertical cylindrical structure using the SPH-based DualSPHysics solver. Open boundaries are applied for the generation of extreme wave conditions. The free-surface elevation and flow kinematics pre-computed within another numerical model are used as boundary conditions at the inlet of a smaller 3-D SPH-based numerical model to replicate the near-breaking and spilling breaking waves generated in a physical wave flume. A damping zone used for wave absorption is arranged at the end of the domain before the outlet. Numerical results are validated against experimental measurements of surface elevation and horizontal force on the vertical cylinder, demonstrating an agreement. After validation using a fixed model for the cylinder, a dynamic model is used to study the response to extreme wave events. Numerical results have also shown that the spilling breaking wave forces are significantly larger compared with near-breaking wave forces, and the secondary load cycle phenomenon becomes larger with dynamic response included in the present study.

Branched structures are present in a diverse set of problems, from modeling branch pipe connections to simulating tree dynamics. Soft corals like the Bipinnate sea plume, have a branched geometry and are soft enough to bend under the waves. Due to their circular cross section, a vortex street forms in the coral’s wake inducing vibrations of its branches. Despite extensive studies on VIV in straight geometries, the three-dimensional (3D) dynamics of flexible branched structures remains uninvestigated. In this numerical and experimental study, we develop a novel formulation for the accurate computation of in-line and cross-flow VIV of frame structures undergoing large deformation. The finite element approach is used to model arbitrarily complex geometries of branched frame structures. Our formulation allows to model complex geometries with forks or sharp angles. The consistent 3D corotational formulation for frame elements computes the internal, inertial and hydrodynamic forces. A wake-oscillator approach models the near wake dynamics with fluctuating fluid forces on the structure in the in-line and cross-flow directions. The drag and lift coefficients follow distributed Van der Pol oscillators. Moreover, we implement the numerical resolution procedure in the open-source library ONSAS. The present formulation and numerical resolution procedure is validated by solving three examples, including comparisons with an analytical solution, a wake-oscillator, and experimental data from the literature. We also conduct experiments of a flexible and elastic cylinder clamped inside a water tunnel under a constant uniform flow. Amplitudes and power spectral density of the tip transverse displacements are compared with the model prediction. Finally, the proposed formulation is applied on a cylinder with two branches. The simulations demonstrate a multi-frequency response with higher amplitudes of displacements when additional branches are incorporated onto the cylinder, emphasizing the significance of considering VIV in nature and engineering applications for such geometries.

eX-Poro-HydroDynamic (XPHD) lubrication presents a different scientific approach to dealing with tribological problems. It is an innovative inter- and multidisciplinary research topic which offers a promising sliding solution for various applications, such as thrust bearings, various guide components and in terms of load capacity and damping. XPHD lubrication is a lubrication mechanism of biomimetic inspiration which features an additional parameter to the system “the porous media”. It consists of self-sustained fluid films generated within compressible porous layers imbibed with liquids in replacement for using the fluid film only as in the classic lubrication system. Soft and porous structures imbibed with liquids generate a high load support under compression. The load support is generated through the resistance to flow inside the porous material. During compression, the resistance to flow and load support increases the greater the compression rate. The main objective of this work is then to understand the behavior of the fluid flow inside the porous structures when subjected to axial compression stress. In the scientific literature, the works studying the flow in compressible materials are essentially experimental because of their very complex geometrical shape, the CFD (Computational Fluid Dynamics) simulations offer an economical solution to study the performance of this new concept of lubrication. To create the geometry, the morphological structures of foam samples are reconstructed at different levels of compression rates from 3D (Three Dimensional) X-ray microtomography. This is achieved by using the commercial software Avizo that allows to process 3D images and create 3D meshes suitable for numerical simulations. The numerical simulations of flows will be performed with the solver IcoFoam of the toolbox OpenFOAM for incompressible laminar flows, making it possible to study the pressure drop in these porous structures. The performed simulations were made with a polyurethane foam of 96% porosity using five compression rates for creating the different structures. The analysis of the numerical simulations shows the impact of the polyurethane foam compression on different key parameters such as the decrease in the permeability as function of the compression rate, the anisotropy of the flow within the compressible structure and the actual increase in the tortuosity generated by the compression of the foam and the variation of the porosity.

Design standards for drag loading on offshore jacket structures do not presently account for the reduction in forces arising from flow blockage effects in the event of combined waves and current. This force reduction is believed to originate in reduced mean flow velocity through the jacket, but this has never been directly measured. To address this, we conducted physical-model tests which measured the flow adjacent to a jacket structure in combined waves and in-line currents using acoustic Doppler velocimeters. Results confirm a dramatic reduction in the mean flow velocity up-wave and down-wave of a model jacket in waves and current, far greater than the flow reduction observed in current alone. These results unambiguously confirm the significant additional blockage (and hence reduction in structural loads) not captured in current offshore design standards.

Multiphase flow inside of pipes occurs in a wide variety of engineering applications, including offshore deep-water oil and gas transport. Vibrations induced by the flow inside of the pipe can lead to its mechanical failure and thus lead to uncontrolled release of the fluids being transported. In subsea applications, flexible J-risers are often employed to deliver the produced fluids from the seafloor to the host platform. Despite the potentially significant liabilities associated with subsea hydrocarbon leaks, there has been a distinct lack of investigations into how flow induced vibrations in large scale, pressurised flexible J-risers can lead to system integrity loss. Previous investigations have generally focused on the response of rigid pipes or small scale, unpressurised flexible risers. This study presents an investigation into the response of a 10 m long, 50.8 mm internal diameter composite riser containing a tensile armour helical structure to a variety of two-phase, water-nitrogen flows at 10.8 barg of pressure and ambient temperature. High speed cameras were used to investigate the structure of the flow at either end of the flexible riser, whilst synchronised surface mounted strain gauges and accelerometers were used to investigate the response of the pipe. Time-averaged data were acquired to assess the general response of the pipe, whilst a statistical analysis of the fluctuations highlighted the movement of the pipe. One-dimensional and computational fluid dynamics simulations were used to define the experimental test matrix and provide further insight into the structure of the flow inside the J-riser. Single-phase gas flow was found not to cause the J-riser to move significantly, whilst multiphase flow led to significant in-plane movement of the pipe. Increasing the liquid flow rate (or decreasing the gas flow rate) increased the mean strain experienced by the pipe. At low gas flow rates, the pipe oscillated smoothly about its mean position, but at higher gas flow rates a violent intermittent whipping motion was observed. The latter produced large in-plane and out-of-plane movement of the pipe which could pose a threat to system integrity. This work offers new insights into fluid-structure interactions in large scale engineering applications, contributing to improved system design and control.

The effects of unsteady motions of flapping flat plates and corrugated structures in different parameters are studied using fluid–solid coupling and overlapping grid methods. Based on the dragonfly’s right forewing and right hindwing model, these actions include sweeping and pitching, take-off acceleration, and tandem wings cruising in the reverse phase at $180°$. The results show that when the advance ratio $J=0.36$, the “inflow deflection” improves the aerodynamic force in two degrees of freedom compared to simple flapping. When considering only the impact of flexibility, the aerodynamic forces of flexible flat plates and corrugated structures are better than those of the rigid wing models. Considering the effect of corrugated structures, the lift of flexible corrugated wings diminishes, but more thrust is generated. From the perspective of vortex street, vortex rings materialize only in the downstroke stage, while the attachment effect of leading-edge vortices is noticeable in several models. In the same phase flapping, the two wings combine to form a giant wing, which generates significant forward flight momentum. In out-of-phase flapping mode, the series wings generate two lifts and two or three thrust peaks to attain the required forward flight speed while sustaining a high lift.

Forced transverse oscillations are used to investigate the quasi-steady behavior of a rectangular cylinder with a side ratio of 2 at Reynolds numbers less than 10,000 (based on cylinder thickness). To this end, phase-averaged measurements of the transverse force acting on the oscillating cylinder at different phases of motion are compared to the mean force acting on the static cylinder at different angles of attack. The force data are complemented with boundary-layer-resolved measurements of the streamwise velocity using single-component molecular tagging velocimetry. The experiments are conducted for two Reynolds numbers of 2,500 and 7,500, two oscillation amplitudes of 20 % and 50 % of the cylinder thickness, and reduced velocities in the approximate range of 2–30 times the reduced velocity corresponding to the vortex shedding frequency. The results show that the reduced velocity threshold required to attain quasi-steady behavior is strongly dependent on Reynolds number in the investigated range. The effect is such that the lower the Reynolds number, the higher the threshold. This behavior is linked to viscous effects which render the shear layer too slow to adapt to the cylinder motion with decreasing Reynolds number. A most pronounced effect is observed at the lowest reduced velocity and Reynolds number, where the shear layer reattaches on the side of the cylinder during the cycle at an angle of attack less than half that exhibited for the static cylinder. Overall, the study shows that in addition to the time scale associated with vortex shedding, when the Reynolds number is sufficiently low, a viscous time scale characteristic of the shear layer dynamic response should be included in determining the validity of quasi-steadiness.

Physics informed neural networks (PINNs) have been explored extensively in the recent past for solving various forward and inverse problems for facilitating querying applications in fluid mechanics. However, investigations on PINNs for unsteady flows past moving bodies, such as flapping wings are scarce. Earlier studies mostly relied on transferring the problems to a body-attached frame of reference, which could be restrictive towards handling multiple moving bodies/deforming structures. The present study attempts to couple the benefits of PINNs with a fixed Eulerian frame of reference, and proposes an immersed boundary aware framework for developing surrogate models for unsteady flows past moving bodies. Specifically, high-resolution velocity reconstruction and pressure recovery as a hidden variable are the main goals. The framework has been developed by using downsampled velocity data obtained from prior simulations to train the PINNs model. The efficacy of the velocity reconstruction has been tested against high resolution IBM simulation data, whereas the efficacy of the pressure recovery has been tested against high resolution simulation data from an arbitrary Lagrange Eulerian (ALE) solver. Under the present framework, two PINN variants, (i) a moving-boundary-enabled standard Navier–Stokes based PINN (MB-PINN), and, (ii) a moving-boundary-enabled IBM based PINN (MB-IBM-PINN) have been formulated.

Relaxation of physics constraints in PINNs models has been identified to be a useful strategy in improving the predictions. A fluid-solid partitioning of the physics losses in MB-IBM-PINN has been allowed, in order to investigate the effects of solid body points while training. This strategy enables MB-IBM-PINN to match with the performance of MB-PINN under certain loss-weighting conditions. Interestingly, MB-PINN is found to be superior to MB-IBM-PINN when a priori knowledge of the solid body position and velocity is available. To improve the data efficiency of MB-PINN, a physics based data sampling technique has also been investigated. It is observed that a suitable combination of physics constraint relaxation and physics based sampling can achieve a model performance comparable to the case of using all the data points, under a fixed training budget.

This study aims to analyze the dynamics of a thin Newtonian liquid film on a uniformly heated compliant substrate. We consider the violation of time-reversal symmetry in the liquid, resulting in an additional non-zero term in the liquid stress tensor. Using the long-wave expansion technique, we derive a set of coupled equations governing the film thickness and substrate deformation, accounting for inertia, surface tension, thermocapillarity, and odd viscosity. Through linear stability analysis and spatiotemporal simulations, we observe that the compliant substrate enhances instability, while wall heating exacerbates it. However, the introduction of odd viscosity effectively suppresses these instabilities, as confirmed by the agreement between simulation and theoretical predictions.