Vortex-induced vibration and flow characteristics of a circular cylinder attached with inverted flexible and rigid splitter plates

Flow-induced vibration control of a circular cylinder by using flexible and rigid splitter plates

Flow-induced vibration (FIV) of an elastically mounted circular cylinder with an attached splitter plate in uniform flow is studied numerically via a stabilized space–time finite element method. The Reynolds number based on the cylinder diameter $D$ and the free-stream speed is restricted to 150. The ratio of the density of the body to that of the fluid, for the major part of the study, is 10. Two different reduced speeds are defined to quantify the compliance of the elastic support and flexibility of the splitter plate, respectively: $U_{s}^{\ast }$ based on the natural frequency of the spring–mass system and $U_{p}^{\ast }$ based on the fundamental natural frequency of the plate. Flow past a stationary cylinder ($U_{s}^{\ast }=0$) with a flexible splitter plate of length $3.5D$ is studied at different values of $U_{p}^{\ast }$. The vibration response of the plate exhibits lock-in with various eigenmodes of the plate in different ranges of $U_{p}^{\ast }$. The onsets of these lock-in regions are abrupt and hysteretic. The elastically mounted cylinder, without the splitter plate, undergoes large-amplitude vortex-induced vibration (VIV) for $4<U_{s}^{\ast }<7$. These large-amplitude oscillations are a consequence of synchronization, wherein the vortex shedding frequency locks in to the cylinder oscillation frequency. A rigid splitter plate attached to the cylinder reduces significantly the peak amplitude during VIV. Increasing the length of the plate from $1.5D$ to $3.5D$ only marginally affects the peak amplitude. It, however, leads to a wider range of lock-in. Unlike the case of an isolated cylinder, the lock-in and desynchronization regimes are not well demarcated in the presence of the splitter plate. Further, galloping is observed beyond a critical value of $U_{s}^{\ast }$; the amplitude of vibration increases with an increase in $U_{s}^{\ast }$ while the vibration frequency is relatively low and remains nearly constant. Increase in plate length delays, in terms of $U_{s}^{\ast }$, the onset of galloping. It is also found that the flexibility of the plate affects the maximum oscillation amplitude in the VIV regime. It also dictates the presence/absence of galloping. The system behaves similar to an isolated cylinder for a very flexible plate. The response is devoid of galloping, but relatively large amplitude of oscillation is observed during lock-in. The behaviour of the cylinder with a stiff plate is similar to that with the rigid one. The galloping instability sets in when the flexibility of the plate is less than a certain value ($U_{p}^{\ast }<4.7$, approximately for $U_{s}^{\ast }=22$). The VIV and galloping are separated by a range of $U_{s}^{\ast }$ in which the flow is either steady, for longer plates, or exhibits very weak vortex shedding. In the VIV regime, the plate tip and cylinder vibrate in phase for low $U_{p}^{\ast }$; their motion is out of phase for larger $U_{p}^{\ast }$. The change in phase is also associated with change in the frequency of vibration. At low $U_{p}^{\ast }$, the frequency of vibration is close to the first natural frequency of the system, while at high $U_{p}^{\ast }$ it becomes closer to the second natural frequency. The vibration amplitude of the cylinder is close to maximum in the VIV regime for $U_{s}^{\ast }=6$. Computations for various $U_{p}^{\ast }$, for $U_{s}^{\ast }=6$ and $22$, are utilized to determine optimal flexibility that leads to minimal FIV. The effect of the length of the flexible splitter plate, mass ratio and damping ratio is studied. A strategy is proposed to utilize the computations from various combinations of $U_{s}^{\ast }$ and $U_{p}^{\ast }$ to choose the appropriate flexibility of the attached splitter plate to minimize FIV.

A comparative study is performed on a circular cylinder with both flexible and rigid splitter plates (SPs). This study has the novelty of using single and dual detached SPs located downstream of the cylinder. The dimensionless gap distance between the first splitter plate and the cylinder as well as the distance between the SPs are varied. The strain of flexible SPs can be used for energy harvesting from the flow. Therefore, a parametric study is performed to find the optimal design for placing piezoelectric polymers. The two-dimensional fluid–structure-interaction analysis is performed based on the arbitrary Lagrangian–Eulerian scheme using COMSOL Multiphysics. Flow characteristics quantities, tip amplitude, and strain are evaluated at different arrangements of the SPs. The results reveal that wake control enhances effectively by doubling the number of SPs. Strain assessments indicate that the strain of dual SPs increases by more than 100% compared to the single plate case. In addition, the amplitude of the dual SPs increases by a remarkable ratio of 18.29 compared to the single plate. In the case of rigid and flexible SPs at a certain arrangement, dramatic reductions of 97.8% and 76.35% in the Strouhal number are obtained compared to a bare cylinder. In addition, 18% drag reduction compared to the bare cylinder is recorded for the rigid SPs. The presented passive method can be used as an attractive approach in flow control as well as energy harvesting from ocean waves and sea currents.

Vortex-induced vibration and structure instability for a circular cylinder with flexible splitter plates

Vortex induced vibration (VIV) of circular and square cylinders with flexible splitter plates is studied at low Reynolds numbers. Finite element based flow and structure solvers, coupled using a partitioned approach, are used for simulating the fluid-structure interaction. Effect of flexibility of an attached flexible plate on its ability to suppress the VIV of a circular cylinder is considered. Flexibility of the plate is found to adversely affect the reduction in amplitude of the vibration of the cylinder. Next, flow past two square cylinders with deformable splitter plates placed side-by-side is considered. Vibration response of the two plates is studied for different values of flexibility. Initially, the plates vibrate out-of-phase with each other, but eventually settle for an in-phase fully developed response. Large amplitude of vibrations in the fully developed response is observed when its dominant frequency is close to the first natural frequency of the plate vibrations.

Flow-induced vibration of a circular cylinder with rigid splitter plate

Combined effect of bending stiffness and streamwise length of the attached flexible splitter plate on the vortex-induced vibration of a circular cylinder

A numerical investigation on effects of structural flexibility on aerodynamic far field sound

A detailed flow field behind a stationary square cylinder with attached rigid and flexible splitter plates has been studied using particle image velocimetry, constant temperature anemometry, and flow visualization techniques. A wide range of lengths of the splitter plate (L/B = 0–8) are considered, and their respective wake interference is reported. The investigation is carried out at an intermediate flow regime at three Reynolds numbers 600, 1000, and 2000 (based on blocking width “B” of the cylinder). The literature seriously lacks the information on a passive flow control of bluff body wakes in this flow regime. This study shows that the wake frequency and mean drag coefficient vary nonmonotonically to splitter plate lengths. The length of the splitter plate is a critical parameter, which, apart from flow control, can also bring a significant wake transition. At L/B > 3 to L/B = 4, strong secondary vortices are shed from the trailing edge. The shedding of the secondary vortex leads to a sudden shrinkage in the recirculation bubble and an increase in the periodicity of the unsteady flow. The onset of high amplitude flapping occurs in a flexible splitter plate (L/B = 3) at Re = 2000. The vortex shedding frequency becomes higher than the first mode natural frequency of the flexible splitter plate for this length and remains in the same regime for L/B > 3. The amplitude of flapping increases up to L/B = 5 and then again recedes. The high amplitude flapping of the flexible splitter plate adversely affects the mean drag coefficient of the bluff body.

While it is known that rigid splitter plates play significant roles in flow control, the exact roles of them in flow-induced vibration (FIV) have not been systematically investigated. This has motivated the present work to experimentally investigate the FIV of a cylinder equipped with an upstream rigid splitter plate (USP), a downstream plate (DSP), and symmetrically arranged splitter plates in a water tunnel with Reynolds number of 1100–7700. The length of the plate is in a range of L* = 0–3.6 (L*=L/D, L is the plate length, D is the cylinder diameter). The response characteristics, vortex evolution, fluid force, and pressure fields are thoroughly analyzed. Both USP and DSP can succeed in oscillation mitigation and drag reduction. However, dramatic galloping is observed for DSP with L* = 0.4–3.2. The low-pressure region forms near the downstream plate is beneficial to trigger galloping. For USP, only vortex-induced vibration is found, and the transition of response branches corresponds to the variation in oscillation frequency and phase jumps in total transverse force and vortex force. However, the vortex mode transition from 2S to 2P disappears with long plate length. Flow visualization reveals that the upstream vortex induced by USP alters the downstream vortex shedding. Furthermore, a high-pressure region forms near the tip of USP, yielding an obstructive force that suppresses the growth of oscillation. With the combination of USP and DSP, weak galloping is excited in a narrow range of L* = 1.0–1.8, and the linear increase is also broken due to the existence of USP.

Effect of splitter plate length on FIV of circular cylinder

We present experiment and analysis of the effectiveness of a flexible splitter plate in the suppression of vortex-induced vibration (VIV) of a circular cylinder. The important finding is that it is not necessary to suppress VIV by a full-span flexible splitter plate, while efficient control can be achieved by a finite-span one. The fluid-structure interaction and control mechanism are revealed by analysis of wake evolution and force characteristics.

With the micro-miniaturization of offshore wireless sensors, signal lights, and other devices and the emergence of the problem of self-powering in the distant sea, how to harvest energy from low-speed currents has become a hot spot of research nowadays. To improve the energy output power and conversion efficiency of low-speed water flow, we propose a vertical cantilever beam circular cylinders fitted with a rigid splitter plate piezoelectric energy harvester (CSPPEH). In this paper, the influence of the length and the attack angle of the splitter plate on CSPPEH has been experimentally investigated. The vibration response mechanism involving the mutual transition between vortex-induced vibration and galloping was analyzed through particle image velocimetry flow field visualization. The experimental results indicate that the vibration and piezoelectric characteristics of the CSPPEH increase initially and then decrease with the length of the splitter plates (L/D = 0–2.4) at the attack angle of 0°, which can be explained by the theoretical model of the energy harvester. It is found that the optimal vibration and piezoelectric characteristics occur at a rigid splitter plate length of 1.40D with an attack angle of 90°. The maximum values for amplitude, vibration swing angle, voltage, power, and power density are 4.96D, 21.7°, 42.68 V, 910.81 μW, and 1.94 mW/cm3, respectively. Efficiency was up to 2.2% at 0.4D length and 90° attack angle of the splitter plate. Compared to the bare circular cylinder energy harvester, the output power and efficiency are significantly improved. The demonstration of continuous charging and discharging of capacitors and light emitting diode lights is performed to show the practicability of the designed CSPPEH. Overall, the present study enables the applications of CSPPEH for realizing self-powered wireless sensing and signal lights under low-water-speed environments.

Study on the parameters of detached splitter plate for VIV suppression

The effect of spacing on the flow-induced vibration of one-fixed-one-free tandem cylinders with a rigid splitter plate