Galloping Of Square Cylinder Research Articles

Effect of pulsating flow on flow-induced vibrations of circular and square cylinders in the laminar regime

Through fluid-structure interaction simulations, this study assesses the dynamic response characteristics of elastically mounted circular and square cylinders subjected to pulsating inflow conditions, providing valuable insights into the analysis and optimization of these systems. The main focus of the present work is on analyzing the effects of two factors: (i) the ratio of the oscillatory velocity component to the steady velocity component in pulsating flow (flow ratio) and (ii) the ratio of the oscillation frequency of pulsating flow to the natural frequency of the structure (frequency ratio). The simulation results for different parameters of interest are analysed using Fourier analysis and Poincaré maps of time series data, and contour plots of vorticity. For the circular cylinder, it is found that cylinder loses synchronization in lock-in as the flow and frequency ratios are increased. Three distinct vibration patterns of vortex-induced vibration are observed for selected combinations of flow and frequency ratios at a Reynolds number of 110 for circular cylinder. For the galloping of square cylinder at a Reynolds number of 250, it is found that the instability and nonlinearity of vortex shedding become more pronounced as the flow ratio increases.

Influence of magnetically-induced nonlinear added stiffness on the lift galloping of square cylinders at low Reynolds number

A square cylinder may gallop if subjected to fluid flow, experiencing a self-excited vibration mode that can harvest energy for low-power applications. The harvested power is typically low and depends on the upstream flow velocity and system dynamic parameters. In this study, the influence of nonlinear stiffness induced by two repulsive magnetic poles on the galloping response of square rods is investigated at a mass ratio of 10 and a Reynolds number of 200. The vibration response of two identical coaxial square rods with magnetic poles attached to their opposite ends is analyzed. Two simplified configurations are selected to model the magnetic forces: a pair of identical monopoles and a pair of identical dipoles. Results show that the magnet configuration and strength significantly affect the vibration amplitude and the onset flow velocity. A weak magnetic force can enhance vibration and reduce onset flow velocity, while a stronger pair of dipoles may push the bistable system to a state of low-amplitude vibration with a mean displacement. The influence of magnets in this configuration has potential as a control technique for energy harvesting applications.

New Viewpoint on the Mechanism of Laminar Separation Flutter

At transitional Reynolds numbers, an elastically supported airfoil oscillating in pitch can undergo laminar separation flutter (LSF), which is characterized by self-sustained small-amplitude oscillations. To gain insight into the mechanism of LSF, we conduct wind tunnel tests for to investigate the LSF response of a freely rotating NACA0015 airfoil with various structural natural frequencies and positions of the elastic axis. The experimental results show that the dominant mode of flow around the NACA0015 airfoil abruptly changes at , resulting in a change in the trend of LSF response. Then, an aeroelastic model is constructed to explain how the instability of LSF arises. This model can accurately predict the LSF frequency of airfoils with various structural natural frequencies. Moreover, based on the aeroelastic model, we perform linear stability analysis on the aeroelastic system and propose the instability criterion for LSF as . This instability criterion is identical to that for rotational galloping of square cylinder, indicating that LSF and rotational galloping of square cylinder are essentially the same aeroelastic phenomenon that appears in different aerodynamic profiles. This finding identifies the underlying cause for the remarkable similarities between the two phenomena.

Power harvesting from transverse galloping of square cylinder

The concept of exploiting galloping of square cylinders to harvest energy is investigated. The energy is harvested by attaching a piezoelectric transducer to the transverse degree of freedom. A representative model that accounts for the coupled cylinder displacement and harvested voltage is used to determine the levels of the harvested power. The focus is on the effect of the Reynolds number on the aerodynamic force, the onset of galloping, and the level of the harvested power. The quasi steady approximation is used to model the aerodynamic loads. A linear analysis is performed to determine the effects of the electrical load resistance and the Reynolds number on the onset of galloping, which is due to a Hopf bifurcation. We derive the normal form of the dynamic system near the onset of galloping to characterize the type of the instability and to determine the effects of the system parameters on its outputs near the bifurcation. The results show that the electrical load resistance and the Reynolds number play an important role in determining the level of the harvested power and the onset of galloping. The results also show that the maximum levels of harvested power are accompanied with minimum transverse displacements for both low- and high-Reynolds number configurations.

Galloping of square cylinders in cross-flow at low Reynolds numbers

Galloping of square cylinders is studied at low values of the Reynolds number using a two-dimensional finite element method. A sinusoidal quasi-steady model allows determination of the occurrence of galloping and its amplitude. Parameters of this model are obtained via FEM unsteady simulations at different angles of incidence between 0° and 10°. The model efficiency is validated by comparing its predictions to those of unsteady simulations of fluid–structure interaction of a spring mounted square constrained to move in the direction transverse to the flow. Results show that the model yields good predictions of both the onset of galloping and its amplitude as a function of the Reynolds number at high values of the mass ratio. However, the quasi-steady model fails to reproduce the sudden change of amplitudes observed in finite element simulations at mass ratios below a critical value. Modifications to the model are introduced to reproduce this low mass ratio effect.

1001 角柱の流動誘起振動に及ぼすT型フィン設置による振動抑制効果(OS10-1 流体関連振動・騒音,オーガナイズドセッション)

Wind tunnel experiments are carried out to investigate the interference effect of T-shape fin on the galloping of square cylinder with a side length d. As the T-shape fin, a rectangular-plate of spanwise width w=d and cross flow height l_d is fixed to the downstream side of the square cylinder at the mid span with a thin bracket plate. The gap between the plate and the cylinder is adjusted by the bracket length s. T-shape fin of l_d/d<2.0 effectively suppress galloping when 1.4<s/d<2. While, T-shape fin of 1.0<l_d/d<1.5 induce the large amplitude vibration when s/d<1.4. Maximum amplitude of this vibration becomes about 2.2 times of galloping at around s/d=0.3. The amplitude at s/d=03 increases divergently with flow velocity. This oscillation is called "Wake Body Interference Fluid Elastic Vibration (WBIFEV)" to be reasonable, because this vibration is caused by the fluid-elastic instability but not by a periodic vortex shedding.

221 下流平板による角柱のギャロッピングの抑制効果(GS1-6 三次元流れ,流力振動,一般セッション:GS1 流体工学)

221 下流平板による角柱のギャロッピングの抑制効果(GS1-6 三次元流れ,流力振動,一般セッション:GS1 流体工学)