Journal of Fluids and Structures最新文献

This paper investigates the effect of the aerodynamic shape on nonlinear dynamics of a cable induced by vortex-induced vibration (VIV) with theoretical and computational fluid dynamics (CFD) method. Firstly, a two-dimensional (2-D) CFD model is established and calculated by using the shear stress transport (SST) k-ω model to supply the necessary drag and lift coefficients for perturbation analysis. Secondly, the planar transverse vibration equation of the cable is established and the wind load is described by the van der Pol wake oscillator. Then, the above two equations are discretized by the Galerkin method, and the modulation equations are obtained by the multiscale method. The results are verified by Runge-Kutta method. Thereafter, three different aerodynamic shapes of the cable are considered and five wind attack angles are defined to extensively investigate the influence on nonlinear dynamic behaviors. The results show that the variation of the cable cross-section significantly changes its aerodynamic performance, and the aerodynamic coefficient of the asymmetric structure is very sensitive to the wind attack angle. Arranging aerodynamic measures (such as soft tail) in the downwind of the cable can effectively reduce the aerodynamic coefficient and dynamic response of the structure.

In this study, we explore the problem of an airfoil in a harmonic and arbitrary periodic pitching motion and seek to understand how theoretical estimation with first-order potential theory match experiments. We adopt Jones’ approximation for the Wagner function for lift computations in the time domain and Theodorsen’s model in the frequency domain. We experimentally investigate a two-dimensional symmetric NACA 0018 airfoil in pitching motion under attached flow with reduced frequencies up to 0.25 and Reynolds number varying from $1.5\times 10^5$ to $3\times 10^5$. To eliminate the influence of the laminar separation bubble, free-stream turbulence intensity was elevated above the baseline level by placing turbulence generating grid, thus, obtaining better compatibility between the flow conditions in the experiments and the theoretical assumptions. Time-resolved sectional lift is determined by simultaneous static pressure measurement with miniature pressure transducers. The transient response is captured with accuracy in the time domain by quantifying the non-circulatory contribution. Excellent agreement is achieved in the time and frequency domains due to the highly accurate measurement of the instantaneous magnitude and phase of the time-resolved surface pressures. We further explore the applicability of the theory for high-pitching amplitudes, which results in a temporary deviation of the measured lift from the theoretical predictions. As the frequency domain unsteady theory is more widely studied than its time domain counterpart, this study provides a unique opportunity to highlight the significance of time-domain analysis in estimating instantaneous lift in non-harmonic motion.

A fluttering flag in a uniform laminar flow exhibits complex fluid–structure interaction (FSI) behavior. This investigation experimentally studied aerodynamic load behavior and its connection to the oscillation behavior, flag envelope and fluid flow parameters, and application of Proper Orthogonal Decomposition (POD) to fluid flow data to elucidate changes in flow behavior associated with the changes in oscillation modes and observed drag coefficient values. For this purpose, three flag models with varying lengths were used. The aerodynamic load forces were measured using a high-precision load cell, and two-dimensional Particle Image Velocimetry (PIV) was used to quantify the flow field around the flag. The Reynolds number ($Re$), mass ratio ($R_1$), and dimensionless rigidity ($R_2$) values were varied between $4.4\times 10^4-12.3\times 10^4$, $1.48-2.77$, and $1.3\times 10^{-3}-15.1\times 10^{-3}$, respectively. The results showed the linear relationship between drag coefficients with normalized amplitude of oscillation and Strouhal number. The results also showed the connection between observed drag and change in flag oscillation modes. The POD analysis showed that the energy content of the POD modes changed with the change in flag oscillation from mode-2 to mode-3 oscillations. The phase portrait of the first four POD modes also showed a unique interplay of POD modes, resulting in a change in the velocity flow field associated with the change in oscillation modes. The low-order reconstruction using select POD modes and control volume analysis of the velocity flow field showed a connection between POD modes and observed drag.

The Bragg scattering due to an undulated elastic bottom in the presence of a uniform current is discussed in the present paper. The conditions of Bragg resonance are derived analytically and discussed numerically for various values of physical parameters. The mathematical techniques used for discussing the analysis are the perturbation and Fourier transform methods. It is seen that the amplitude of the Bragg reflection increases with the increase in the flexibility factor in the versatile floor of the sea, and there is a significant impact of the current speed on Bragg reflection and transmission coefficients. The maximum value of the amplitude of Bragg reflection is observed to decrease as the Froude number increases. Again, it is seen that the transmitted energy produced in the presence of uniform current does not fluctuate considerably when the nature of an elastic bottom gets closer to that of a stiff floor. The blocking resonance in the presence of current and elastic bottom is analyzed by neglecting the small bottom undulation. A significant effect of blocking resonance on Bragg reflection and transmission is observed.

We present a theoretical model of the hydrodynamic behaviour of a floating flexible plate of variable flexural rigidity connected to the seabed by a spring/damper system. Decomposition of the response modes into rigid and bending elastic components allows us to investigate the hydroelastic behaviour of the plate subject to monochromatic incident free-surface waves of constant amplitude. We show that spatially dependent plate stiffness affects the eigenfrequencies and modal shapes, with direct consequences on plate dynamics and wave power extraction efficiency. We also examine how plate length and Power Take-Off (PTO) distribution affect the response of the system and its consequent absorbed energy. This work highlights the need to improve existing models of flexible floating energy platforms, especially given their importance in the Offshore Renewable Energy (ORE) sector.

The dynamics of curved tubes with fluid flow fixed on both sides was studied experimentally. Two cases were studied in which the curved tubes differed in curvature. Tests were performed with the flow without pulsation to determine the natural frequencies and with pulsation to determine the interesting dynamic phenomena occurring in the system. Two alternative measurement systems were used to study the motion of the tubes, a contact one based on accelerometers and a non-contact one based on optical technology. Acceleration and displacement time-series were analysed to determine the nature of the motion of the tubes. It was confirmed that the eigenfrequencies of the curved systems are insensitive to flow velocity which is consistent with the extensible centreline hypothesis. The interesting dynamics of tubes with pulsatile flow, the possibility of excitation of simple, combination parametric resonances, including several different combination resonances simultaneously, were identified. The vibrations in the simple resonances were polyharmonic and in the combination resonances quasi-periodic. The motion of the tubes was three-dimensional in nature and its complexity increased with increasing excitation amplitude. It was found that increasing the curvature of the tube reduced the possibility of excitation of parametric vibration.

Power extraction performance of a fully-passive oscillating-plate hydrokinetic turbine prototype was investigated experimentally using measurements of the kinematics of the flat plate and the estimated power extraction. Two configurations were considered: a flat plate with a 6${}^{\circ }$ sweep angle and an unswept plate (control configuration), which were undergoing fully-passive pitching and heaving motions in uniform inflow at Reynolds numbers ranging from 15,000 to 30,000. The resulting kinematic parameters and the power extraction performance were evaluated for both plates. The influence of the bottom endplate was also studied to investigate the extent of the spanwise flow and its effects at the tip of the plate. The swept plate experienced significant flow-induced forces acting in the direction of the heaving motion over a larger portion of the oscillation cycle, compared to the unswept plate. Consequently, the swept plate reached comparatively larger heaving amplitudes at high values of the inflow velocity, which resulted in higher power coefficient values but comparable efficiency values, relative to the unswept plate. Moreover, the energy-harvesting performance of the swept plate was consistent over a wider range of inflow velocities, compared to the unswept plate. The presence of the bottom endplate significantly impacted the kinematics of the swept plate, suggesting a dominant role of the spanwise convection of vorticity, which induced substantial tip losses.

Two dimensional pressure field was acquired at 10 kHz to study the post flutter oscillations of a thin elastic panel placed beneath a Mach 2.5 turbulent boundary layer. The panel was made of aluminum and is secured to the mounting fixture using a collection of rivets, which resulted in a boundary condition that was between both ideally clamped and pinned boundaries. Direct comparison of the mean and unsteady pressure fields were made for the panel executing post flutter oscillations and oscillations away from the flutter boundary. Whereas the mean pressure fields were largely similar during and away from post-flutter oscillations, the unsteady pressure fields showed a significant increase in the $p_{r.m.s.}$ during post flutter oscillations. The spectral content of the pressure oscillations and panel oscillations revealed that the tonal aeroelastic frequency dominate the post flutter oscillations. This tonal frequency was determined to lie at the close vicinity of the (2,1) panel elastic mode. The r.m.s. panel deflection field during post flutter oscillations were also reconstructed from the unsteady pressure fields and the reconstructed panel deflection also corresponded to the (2,1) elastic mode.Further coherence and cross-correlation analyses provided insights into possible mechanisms that control the transition of the panel oscillations away from the flutter boundary. The analyses suggest that the transition away from the flutter boundary is possibly initiated by the decoherence of the organized pressure field during the post flutter region by the turbulent boundary layer.

Efficiency of a torsional-flutter energy harvester in thunderstorm-like wind flows is studied. The apparatus has been proposed in recent years as an alternative to other flapping windmills of medium size due to its simple installation and operational mechanism. The paper investigates a rapid change of wind load intensity, induced by a thunderstorm, which may either reduce the output energy conversion or damage the apparatus. Aeroelastic load is replicated through unsteady Wagner’s indicial theory. The study explores the influence of both slowly-varying wind velocity variations, non-stationary turbulence of variable intensity and load perturbations. A stochastic model is derived to investigate both pre-critical and post-critical regimes. Various configurations are considered.

The paper presents an experimental investigation of the flow around two bluff bodies in a tandem arrangement. Two different configurations were investigated. The first one concerns two square cylinders (SS case) whereas in the second setup, a triangular cylinder was used as an upstream object (TS case). Particular attention was devoted to examining the effect of the gap space $G$ between cylinders (from 1 to 6) on the flow structure and dynamics of vortices. The study also includes measurements conducted for single square and single triangular cylinders to show how the second object modifies the flow field. It was found that the wake behind each object is always wider for the SS case regardless of $G$. When $G$ is less than or equal to 3 for the TS case, the slender body regime is seen in the flow field which for the SS case is normally present for $G<1.5$. Consequently, distinctive vortex shedding frequencies cannot be distinguished in the gap for the TS case when $G\le 3$. When $G\ge 4$, twice as high values of the Strouhal number $St$ are reported in the gap for the TS case compared to the SS case. Downstream the second object, $St$ gradually decreases from $\sim$0.245 to $\sim$1.14 with increasing $G$ from 1 to 4 for the TS case, whereas for $5\le G\le 6$ distinctive vortex shedding frequencies are no longer observed.