Journal of Fluids and Structures最新文献

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.

We discuss the behavior of a flexible sheet placed in the wake of a cylinder that is forced to rotate periodically. This is done through a series of experiments conducted in a water tunnel and by simultaneously visualizing the flow behavior and tracking the motion of the flexible sheet in the wake of the cylinder. We show how the response of a flexible sheet in the wake of a fixed cylinder, which is the result of the sheet’s interaction with the vortices that are shed at a frequency predicted by the Strouhal law can be changed to a “desired” response by forcing the upstream cylinder to rotate periodically. Large-amplitude oscillations at a frequency different from the Strouhal frequency can be imposed on the flexible sheet, and the sheet’s oscillations can be suppressed if the cylinder is forced to rotate at a higher frequency. The flexible sheet finds its way in between the vortices that are shed in the wake of the cylinder, and by controlling the frequency and location of the shed vortices in the wake of the cylinder, one can impose a desired motion on the sheet. Besides imposing a symmetric oscillatory response on the sheet, we show that asymmetric responses can be imposed on the sheet when an asymmetric waveform is used to force the upstream cylinder.

Studies on bluff-body aerodynamics have emphasized that galloping instability is strongly associated with the Kármán vortex. This study discusses the aerodynamic interactions between the galloping instability and Kármán and motion-induced vortices, analyzes the effects of these vortices on vortex-induced vibration and galloping, and investigates the stabilizing effects of various corner cuts on a rectangular cylinder. Wind tunnel tests were performed on a rectangular cylinder with a side ratio of 1.5 under a smooth flow for seven different corner shapes. The rectangular cylinder with cut corners significantly reduced both the aerodynamic force coefficients and the Kármán vortex shedding intensity. Furthermore, spring-supported free vibration tests indicated that the onset reduced wind velocities were high in the response amplitudes of all corner-cut sections that were analyzed despite the significantly low onset reduced wind velocities of the Kármán vortex-induced vibration, which were denoted as the reciprocal of the Strouhal number and closely associated with the onset of galloping. This was attributed to the motion-induced vortex dominating the vibration when the Kármán vortex shedding intensity was reduced. Therefore, this study clarified one of the factors that affected the onset-reduced wind velocity of galloping.

The performance of Savonius turbines driven by flow in a pipe is experimentally investigated. The turbine is manufactured to have a spherical outline based on the pipe cross section at a small clearance. The torque and power of the turbine are obtained from the time derivative of the rotational speed measured using a high-speed camera and an equation of rotational motion. We find that the idling tip-speed ratio of the turbine exceeds 5, which is much greater than that of a turbine operating in an open free stream. This proves the dominance of pulsatile flow through the gap between two hemispherical blades in torque generation. Widely varying the gap (i.e., the overlap ratio O_R of the Savonius turbine) reveals that a turbine with O_R = 30 % has the highest power coefficient. The output efficiency exceeds 50 % for a tip-speed ratio of approximately 3. These experimental results are supported by fluid dynamics theory and computational fluid dynamics simulation, which clarify the driving mechanism of the turbine in a pipeline.

We consider the response of a flexibly-mounted cylinder placed in flow and free to oscillate on a curved path about a pivot point. The curved path could be concave (i.e., bent toward the incoming flow) or convex (i.e., bent away from the incoming flow). We consider different radii of curvature for both the concave and the convex paths and show that oscillations are observed for radii of curvature larger than one cylinder diameter. In general, we show that the oscillations are of larger amplitudes in the convex orientation, reaching angles of oscillations of up to 38° and normalized (with respect to the cylinder's diameter) amplitudes of oscillations in the transverse and inline directions of up to 1.1 and 0.35, respectively. The oscillations on the concave path are of smaller amplitudes, but last up to higher values of reduced velocities than those in the convex orientation. The shedding and oscillation frequencies increase with increasing reduced velocity for the concave orientation reaching values of around 1.8 times the system's natural frequency in water at the end of the lock-in range, while for the convex orientation, the frequencies stay close to the natural frequency and only jump from values slightly lower than the natural frequency to values slightly higher than the natural frequency when the oscillation amplitude drops from an upper branch to a lower branch in the VIV amplitude response. Two single vortices are observed in the wake when oscillation amplitudes are relatively low, and two pairs of vortices of different sizes are observed in the wake when amplitudes are relatively larger. When two pairs of vortices are observed, the cylinder carries two bound vortices with it during its transverse oscillations and sheds them in the form of a pair of vortices of different sizes at the end of each half cycle. In the cases with the largest amplitudes of oscillations, besides the vortices that are shed synchronized with the oscillation frequency, several smaller-size vortices are shed in the wake as the cylinder traverses its crossflow path. We relate the reason for observing larger amplitudes of oscillations on the convex path to the relative orientation of the fluctuating forces that are exerted on the cylinder due to the shedding of vortices in its wake with respect to its oscillation path.

In this paper, we present a computational framework based on fully Eulerian models for fluid–structure interaction for the numerical simulation of biological capsules. The flexibility of such models, given by the Eulerian treatment of the interface and deformations, allows us to easily deal with the large deformations experienced by the capsule. The modeling of the membrane is based on a full membrane elasticity Eulerian model that is capable of capturing both area and shear variations thanks to the so-called backward characteristics. In the validation section several test cases are presented with the goal of comparing our results to others present in the literature. In this part, the comparisons are done with different well-known configurations (capsule in shear flow and square-section channel), and by deepening the effect of the elastic constitutive law and capillary number on the membrane dynamics. Finally, to show the potential of this framework we introduce a new test case that describes the relaxation of a capsule in an opening channel. In order to increase the challenges of this test we study the influence of an internal nucleus, modeled as a hyperelastic solid, on the membrane evolution. Several numerical simulations of a 3D relaxation phenomenon are presented to provide characteristic shapes and curves related to the capsule deformations, while also modifying size and stiffness parameter of the nucleus.