Journal of Fluids and Structures最新文献

In the manufacturing processes of thin flexible plates (sheets) such as polarizing films, the flutter can be caused to the sheets due to the interaction between the motion of the sheets and fluid flow. Then, the flutter can cause serious damage to the sheets, leading to the wrinkles and scratches. Thus, it is crucial to investigate a detailed characteristics and excitation mechanism of the flutter. In the present study, a detailed flutter characteristics and excitation mechanism of a rectangular sheet with an elastic support is investigated. The elastic support is implemented using a fine flexible wire. The influence of bending stiffness of the support section on the flutter velocity and frequency is clarified through wind-tunnel experiments and numerical analysis. Moreover, the work done by the fluid force on the sheet surface was determined.

The flutter velocity and frequency drastically decrease owing to decrease in bending stiffness of the support section regardless of the aspect and mass ratio. Then, the positive work region around the leading edge of the sheet expands owing to the large-amplitude oscillation around the leading edge. The expansion of positive work region owing to a large-amplitude oscillation around the leading edge of the sheet destabilizes the system.

In fluid dynamics, a planar starting flow through a narrow slit gives rise to a distinctive fluid mass in the form of counter-rotating vortex pairs, which do not undergo any propulsive detachment, known as ‘pinch-off’, from the tip-attached fluid layer. Our study envisions instigating the ‘pinch-off’ phenomenon in these vortex pairs using flexible plates as the slit edges to enhance momentum transport and self-propagation. In this study, considering a flow evolution model, we show that the growth rate of such ejected vortex pair scales as proportional to the square root of time. Using flexible plates to form the slit, we unearth a critical plate flexibility case with the Cauchy number, $Ca=0.01$, which induces a ‘pinch-off’ of the resultant vortex pair, a phenomenon absent in the case of rigid plates. We observe a train of vortex pairs generating one after the other, and the time period closely matches the plates’ oscillation period as the plates’ oscillation frequency locks-in with the shedding frequency of the vortex pairs. The streamwise speed of the leading vortex pair varies non-monotonically with $Ca$, showing an increase in the speed up to $Ca\approx 0.04$, and thereafter decreased speed due to upstream propagation of small-sized vortices. The new insights into inducing and controlling vortex pair behaviours pave the way for innovative applications in fluid transport and advanced flow manipulation techniques.

Owing to the availability of the jet flow control strategy in mitigating strongly alternating vortex motion and shedding in the wake flow structure, an active jet produced by an air velocity regulator installed on a box girder is proposed to alleviate the fluctuating aerodynamic force imposing on a fixed box main girder model. The pressure distributions on the top and bottom surfaces of the deck's two sections were recorded using a digital miniature pressure scanner system. The investigation manifests that the fluctuation of the outer surface pressure (OSP) distribution of the deck is alleviated, and the mean value is stable in all test cases. Wavelet coherence analysis of the OSP between two sections on the deck was performed to determine the relationship between the surface pressures varying with frequency over time. Based on the OSP distribution, the aerodynamic force was obtained to macroscopically display the availability of the active jet. Moreover, an active jet was applied to a free-vibrating box girder model to study its ability in suppressing vortex-induced vibration (VIV). The results of the oscillation response obtained by a laser displacement apparatus show that the box girder with the active jet has a lower vibration amplitude, and the VIV can be entirely suppressed when the non-dimensional jet momentum coefficient reaches a certain value. In addition, a coupled model of VIV was developed to predict the vibration response of the deck. The calculated results of the vibration response of the deck obtained by the coupled model are close to those of the experiments.

This study presents an approach for analyzing the hydroelastic response of membrane-based floating photovoltaic (PV) platforms. The structural deformation of the platform’s main components, including a floater and a membrane, is further described through a comprehensive set of in-plane and out-of-plane modes. This analysis employs potential flow theory and 3D hydroelasticity theory to evaluate the hydrodynamic loads. Additionally, the Morison equation is utilized to express the drag term associated with the floater’s in-plane motion. Addressing the connection between the floater and the membrane is achieved through the Lagrange multiplier method. Ultimately, this study establishes a frequency-domain coupled dynamic equation for the platform. The response results provide modal amplitudes and displacement data for test points, revealing that under low-frequency conditions, the flexible floater and the membrane conform to wave profiles. As the frequency increases, the impact of the floater’s stiffness becomes prominent, resulting in a substantial three-dimensional interaction effect. In addition, this study examines various structural parameters, specifically the membrane pretension, elastic modulus, and the bending stiffness of the floater, to illustrate their influence on the motion and deformation of the platform. This work contributes to a deeper comprehension of membrane-based floating PV systems and their practical applications.

The interactions between fluid flow and structural components of collapsible tubes are representative of those in several physiological systems. Although extensively studied, there exists a lack of characterization of the three-dimensionality in the structural deformations of the tube and its influence on the flow field. This experimental study investigates the spatio-temporal relationship between 3D tube geometry and the downstream flow field under conditions of fully open, closed, and slamming-type oscillating regimes. A methodology is implemented to simultaneously measure three-dimensional surface deformations in a collapsible tube and the corresponding downstream flow field. Stereophotogrammetry was used to measure tube deformations, and simultaneous flow field measurements included pressure and planar Particle Image Velocimetry (PIV) data downstream of the collapsible tube. The results indicate that the location of the largest collapse in the tube occurs close to the downstream end. In the oscillating regime, sections of the tube downstream of the largest mean collapse experience the largest oscillations in the entire tube that are completely coherent and in phase. At a certain streamwise distance upstream of the largest collapse, a switch in the direction of oscillations occurs with respect to those downstream. Physically, when the tube experiences constriction downstream of the location of the largest mean collapse, this causes the accumulation of fluid and build-up of pressure in the upstream regions and an expansion of these sections. Fluctuations in the downstream flow field are significantly influenced by tube fluctuations along the minor axes. The fluctuations in the downstream flowfield are influenced by the propagation of disturbances due to oscillations in tube geometry, through the advection of fluid through the tube. Further, the manifestation of the LU-type pressure fluctuations is found to be due to the variation in the propagation speed of the disturbances during the different stages within a period of oscillation of the tube.

This study uses a hybrid finite element method to predict dynamic behavior of truncated conical shells with ring stiffeners under fluid loading. The proposed approach combines classical shell theory and the finite element method, making use of displacement functions derived from exact solutions of Sanders’ shell equilibrium equations for conical shells. The analysis of the shell-fluid interface involves leveraging the velocity potential, Bernoulli’s equation, and impermeability conditions to determine an explicit expression for fluid pressure. To the best of our knowledge, this paper is the first to compare the methods applied to ring-stiffened shells against other numerical and experimental findings. Our results on conical shells in various conditions, with and without ring stiffeners, are largely consistent with previous findings. This study also explores the influence of geometric parameters, stiffener quantity, cone angle, and applied boundary conditions on the natural frequency of fluid-loaded ring-stiffened conical shells. The paper concludes with a discussion of several useful implications for further research.

The cross-flow swinging dynamics of a cube pendulum is studied experimentally in a flow at high Reynolds numbers ($Re\sim 10^5$) with a low free-stream turbulence intensity. A galloping instability is observed and results in the exponential growth of the swinging motion. The onset of galloping is found to be very sensitive to the static yaw angle of the cube. Despite the 3D geometry of the cube, flow mechanisms similar to the case of a square cylinder appear to govern the onset of the instability. A quasi-steady linear model of the motion is assessed to predict the stability of the pendulum.

For the lowest reduced velocity investigated ($U^{\ast }=18.5$), unsteady phenomena arise during the saturation phase of the pendulum oscillations. From the analysis of the unsteady loads and the pressure distribution on the faces of the cube, an unsteady phase delay between the wake and the pendulum dynamics is identified. It produces an energy loss in the pendulum motion which favors its saturation.

This study concerns a numerical investigation of new morphing concepts by means of high-fidelity simulation around a high-lift wing-flap system and a real scale Airbus A320 airplane. A new designed hybrid morphing flap is proposed based on cambering at high deformation amplitudes associated with trailing-edge flapping at an actuation frequency in the order of 300 Hz. The numerical results are obtained using the code NSMB (Navier–Stokes Multi-Block) with adapted turbulence modeling over a generated Chimera grid and dynamic grid deformations. Optimal shapes of the cambering are studied in respect of the aerodynamic performance increase based on a quasi-static approach with a parametric study of different angles of attack, Reynolds numbers and cambering positions in addition to the dynamic cambering effects. Hybrid morphing is then examined using an AIRBUS A320 airplane with morphing high-lift flaps. An increase of the aerodynamic performance is obtained using these novel designs compared to the baseline configuration.

For the first time, a fully-coupled Harmonic Balance method is developed for the forced response of turbomachinery blades. The method is applied to a state-of-the-art model of a turbine bladed disk with interlocked shrouds subjected to wake-induced loading. The recurrent partial opening and closing of the pre-loaded shroud contact causes a softening effect, leading to turning points in the amplitude–frequency curve near resonance. Therefore, the coupled solver is embedded into a numerical path continuation framework. Two variants are developed: the coupled continuation of the solution path, and the coupled re-iteration of selected solution points. While the re-iteration variant is slightly more costly per solution point, it has the important advantage that it can be run completely in parallel, which substantially reduces the wall clock time. It is shown that wake- and vibration-induced flow fields do not linearly superimpose, leading to a severe under-/overestimation of the resonant vibration level by the influence-coefficient-based state-of-the-art methods (which rely on this linearity assumption).

The dynamics of an S-shaped elastic sheet, in which the inclination angles of two clamped ends are normal to the direction of uniform flow and opposite to each other, are experimentally investigated. Flow-induced vibrations are ensured in this novel configuration by the substantial area perpendicular to the flow and the bluff shape. The motions of the sheet can be divided into three modes depending on the trends of the oscillation amplitude and frequency with respect to the flow velocity. At low flow velocities, the sheet undergoes small-amplitude oscillations with a nearly constant frequency. Beyond a certain threshold of flow velocity, the amplitude increases rapidly while the frequency declines. The dimensionless critical flow velocity is almost independent of the ratio between the clamp distance and sheet length, as predicted by simple scaling analysis. As the flow velocity increases further, the amplitude becomes saturated, while the frequency becomes almost proportional to the flow velocity. The most notable features of the sheet are the temporal and spatial distributions of its bending energy. The bending energy exhibits negligible fluctuations over time, and despite changes in the flow velocity, the time-averaged bending energy remains almost constant. However, by dividing the sheet into front, center, and rear parts, significant variations in the bending energy are found, and these are intensified at higher flow velocities. The S-shaped sheet exhibits more pronounced variations in local bending energy at lower flow velocities compared with a snap-through sheet model clamped at both ends.