Journal of Fluids and Structures最新文献

This study numerically investigates the aerodynamic and aeroelastic characteristics of a square prism (aeroelastic model) and wind field around it in sinusoidal oscillatory flows (SOFs). The reliability of the fluid-solid interaction (FSI) simulation is validated by a free vibration test and wind tunnel tests in smooth flow and SOF. The effects of the amplitude and frequency of SOFs are studied at the mean wind speed of vortex-induced resonance. The results show that increasing the amplitude and frequency of SOFs will amplify the root mean square (RMS) along-wind and across-wind base shear forces of the aeroelastic model but decrease the RMS across-wind displacement at the top of the aeroelastic model. The spectral analysis of the base shear forces indicates that the influence of vortex shedding on the across-wind base shear force is reduced by either increasing the amplitude or increasing the frequency of SOFs. The mean and instantaneous wind fields around the aeroelastic model in SOFs and smooth flow are compared, and the wake characteristics of the aeroelastic model in SOFs are analysed by dynamic mode decomposition. It is observed that when the frequency of SOFs is 1.5 times as large as the fundamental natural frequency of the aeroelastic model, the regular vortex shedding process is substantially affected.

Fluidic circuits are a promising recent development in embodied control of soft robots. These circuits typically make use of highly non-linear soft components to enable complex behaviors given simple inputs, such as constant flow or pressure. This approach greatly simplifies control, as it removes the need for external hardware or software. However, detailed fundamental understanding of the non-linear, coupled fluidic and mechanical behavior of these components is lacking. Such understanding is needed to guide new designs and increase the reliability of increasingly autonomous soft robots. Here, we develop an analytical model that captures the coexistence of a pressure regulation mode and an oscillatory mode in a specific soft hysteretic valve design, that we previously used to achieve reprogrammable activation patterns in soft robots. We develop a model that describes the mechanics, fluidics and dynamics of the system by two coupled non-linear ordinary differential equations. The model shows good agreement with the experimental evidence, as well as correctly predicts the effect of design changes. Specifically, we experimentally show that we can remove the regulation mode at low input flow rates by changing the fluidic response of the valve. Taken together, the present study contributes to better understanding of system-level behavior of fluidic circuits for controlling soft robots. This may contribute to the reliability of soft robots with embodied control in future applications such as autonomous exploration and medical prosthetic devices.

Mesh-based Eulerian and particle-based Lagrangian models are common computational fluid dynamics (CFD) tools for simulating wave-structure interactions. While Eulerian models are efficient in terms of computational time, they are limited in their ability to handle large interface discontinuities between two flows and complex structure motion responses. Conversely, Lagrangian models are suitable for such discontinuities and motion responses but can be computationally expensive. However, there is a lack of comprehensive discussion on the (dis)advantages of hybrid Eulerian-Lagrangian models, which have the potential to achieve both numerical efficiency and flexibility through a combined use of mesh and particles. This paper presents a comparative study of a hybrid Eulerian-Lagrangian Particle-In-Cell (PIC) model and the widely-used OpenFOAM model, applied to a variety of complex wave interactions with floating offshore renewable energy structures in both 2D and fully 3D domains. We found that both models demonstrate good performance in simulating complex floating structures. Additionally, it is the first time that the two models have been compared in parallel on the same computing facility, allowing us to directly show their computational efficiency. The PIC model has the advantage of using staggered grids, which enables it to achieve computational efficiency comparable to the pure mesh-based OpenFOAM. The findings of this study provide researchers and practitioners in the field of computational fluid dynamics with a clear understanding of the performance of the hybrid Eulerian-Lagrangian PIC model and OpenFOAM for simulating complex fluid-structure interaction problems.

Given the current context of changes in aeronautics to reduce emissions, it is also necessary to modernise the computation methods to anticipate future cases where disciplines which are now calculated separately (i.e. manoeuvers and gusts) should be computed at the same time including flexible effects and using a time-domain approach. In this work, a static aeroelasticity formulation is adapted to compute wind gust loads. This static method uses aerodynamic matrices to calculate an effective angle of attack (used to recover the local pressure coefficients) from a structural deformation. The approach has been to define this deformation including unsteady effects influence in order to use the same formulation as the static case. Three gust cases (two unsteady and one quasi-steady) have been tested in a rectangular wing, and the proposed method has been compared to the aeroelastic high-fidelity solution and to an uncorrected version of the Doublet Lattice Method (Nastran Solution 146). The proposed solution benefits from the use of the lookup tables to accurately estimate the peak lift coefficient value (maximum error of 6.7%) at least 2.5 times faster than the Doublet Lattice Method. Nevertheless, using a limited model with only two degrees of freedom prevents the proposed method from capturing complex dynamics coming from highly unsteady gust excitation or from aerodynamic instabilities.

The study of the vibration characteristics of the liquid-filled pipeline has important academic significance and practical value for analyzing the dynamic behavior of the pipeline system, ensuring its stability and improving its reliability. The fluid-structure interaction transfer matrix method (FSITMM) is regarded as an effective method for the study of these vibration characteristics. Nonetheless, there are relatively few studies concerning the theoretical basis, especially stability and orthogonality, of the FSITMM for liquid-filled piping systems. The existing studies cannot adequately address computational failure issues in models based on the FSITMM, cannot determine whether the results are credible, and even more, cannot predict whether the new models will be computationally successful. The weighted orthogonality of the eigenvectors is a necessary condition for the modal synthesis method to determine the transient (or time-domain) response of the pipeline, and the stability is crucial as it guarantees the accuracy of the solution results. In this paper, the weighted orthogonality of the modes of the FSITMM for liquid-filled piping systems is validated, the stability of this transfer matrix is examined, and enhanced by the reduced transfer matrix method. Numerical simulation results demonstrate the ability of stability validation to predict the success of computational results, while weighted orthogonality validation can determine the accuracy of computational results. The results obtained from the fluid-structure interaction model using the approach of this paper are more accurate.

Vortex induced vibrations is a withstanding ubiquitous problem in the marine industry. Although seemingly simple, cylindrical structures in cross-flows originate extremely complex and, at times, chaotic hydrodynamics which are not fully understood nowadays. One of the biggest industries driving economic development that has had to deal which this problem is Offshore Oil & Gas. Key to a safe oil extraction, marine risers have to operate and withstand the erratic process that arises from the fluid–structure interaction of marine risers with vortex induced hydrodynamic forces.

In the following paper we put forward a methodology to assimilate large amounts of data into empirical models. In doing so, we hope to bring attention to the potential that sensors and data collected by them can have in improving predictions of VIV phenomena. Although we leverage a semi-empirical VIV prediction tool (VIVA), the optimization methods used to extract robust hydrodynamic databases for a Steel Catenary Riser (SCR) are not limited to this method. The performance of the extracted databases are systematically cross-validated. To the authors’ best knowledge, an extensive cross-validation of this methodology has not been performed in previous studies.

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.