Oscillating Bluff Bodies Research Articles

Mapping vortex-induced forces of oscillating bluff bodies from subcritical to critical Reynolds numbers

Mapping vortex-induced forces of oscillating bluff bodies from subcritical to critical Reynolds numbers

Smart interference galloping energy harvester with optimally tunable gap between the bluff body and interference

This study focuses on the development of a piezoelectric wind energy harvester that utilizes the wake interference galloping phenomenon. We found that depending on the applied wind velocity, there exists an optimal gap distance between the oscillating bluff body and the interference, which produces the required wake interference galloping. Accordingly, we aimed to design a novel interference galloping–based energy harvester that can smartly adjust the bluff body location to its optimal position in response to the external wind velocity. The optimal gap distance between the two bodies for the wind velocity range considered in this study was found to be 9–15 mm, which strongly depends on the applied wind velocity. It was also observed that the phase delay of the lift force with respect to the displacement of the oscillating body plays a major role in determining the state of the system stability and that the wake interference galloping can also occur due to the memory effect. The phase angle of π/2 rad was found to be an important indicator for determining whether wake interference galloping occurs. The balance of forces on the bluff body is vital for determining the optimal location of the bluff body. The mathematical modeling was performed using the extended Hamilton principle, the Galerkin method, eigen- and multi-modal analyses, the extended Fourier theorem, and novel energy/power balance conditions that yielded the four additional equations required to compute the aerodynamic lift in an autonomous form. To identify the physical parameters for analyzing the proposed energy harvester system, a main experiment and several preliminary experiments were conducted. The analytical results were compared with the experimental data in terms of both the dynamic behavior and energy production, and they were observed to be sufficiently close (the maximum error in the averaged output power was 9.8%). The average output power from the proposed energy harvester was also compared with that from conventional interference galloping–based energy harvesters having an immobile bluff body; the results showed that the proposed energy harvester outperformed the conventional harvesters by up to 128.8%. Specifically, it generated an average output voltage of 71 V (rms) and an output power of 6.88 mW at a wind velocity of 10 m/s.

Vortex-induced forces of crossflow and inline oscillating bluff bodies at moderate Reynolds numbers

The Reynolds number (Re) is found to play an important role in the vibration response of a cross-flow (CF) only oscillating cylinder in the uniform flow that the maximum CF displacement (Ayd) increases with the increase of Re in the sub-critical regime, from 0.6 diameter (d) at Re = 1, 000 to 1.1d at Re = 20, 000. However, no existing research comprehensively studied the Re effect on the rigid cylinder vibrating in both the In-line (IL) and the CF directions. In this paper, with the help of the intelligent towing tank using an adaptive GPR sampling strategy for automatic sequential experimentation, we studied the vortex forces on a rigid cylinder forced to vibrate in the CF-only direction and the combined-IL-and-CF direction. We constructed a functional representation of the hydrodynamic coefficient database for the cylinder forced CF-only vibration (Re from 1200 to 19,000) and the cylinder combined-IL-and-CF vibration (Re from 1200 to 14,000). The results reveal that the maximum Ayd associated with the zero value of the lift coefficient in phase with velocity Clv = 0 increases significantly with Re for the CF-only vibrating cylinder, which compares well with and explains the trend observed in the free vibration experiment. In the meantime, Re is shown to have a less stronger effect on Ayd when the cylinder is allowed to move in both the IL and the CF directions in the sub-critical Re regime.

Experimental Investigation of Bladeless Power Generator from Wind-induced Vibration

The power harvester unit from flow-induced vibration (FIV) was designed to harness energy from low flow velocity based on the magnetostrictive effect on the galfenol (Fe – Ga alloy) strip induced by the oscillating bluff body. This study aimed to investigate the cross-section variation’s effect on the FIV characteristics and the magnetostrictive material’s performance for the bladeless power generator. The generator model’s vibration characteristics and performance tests were conducted in the wind tunnel test using the wind-receiving unit (WRU) variation. The results showed that the resonance reduced-velocity (Vr) were around 3.7 and 4.0 for rectangular and circular cylinders, respectively. Furthermore, the effect of rectangular depth variation on the power generation output is linear to the test models’ displacement rate and vibration frequency. The harvester’s maximum power generation was 5.25 mW, achieved using the rectangular prism with depth D = 0.4H. The power coefficient was also evaluated for different wind-receiving models. The harvester model lit up 54 LED lamps in the wind tunnel test. The voltage output is sufficient to provide electric power resources for an IoT system, sensor, and wearable or wireless devices. The harvester model successfully generated a voltage signal under the initial field test with an ambient wind velocity of 0.9 – 2.71 m/s. Therefore, this study recommends the development of bladeless power generators in the future.

Computational analysis of naturally oscillating tandem square and circular bluff bodies: a GPU based immersed boundary – lattice Boltzmann approach

ABSTRACT In this study, a fluid-structure interaction solver is employed to explore the flow physics of tandem oscillating square bluff bodies by varying the corner radius. Based on previous research, dynamic behavior, and fluid forces only square and circular bluff bodies are chosen for a supplementary investigation. This study investigates the effects of the streamwise displacement, , of two elastically mounted square and circular bluff bodies in a tandem arrangement. Fluid flows at a Reynolds number of 100 and a fixed mass ratio are used for the upstream and downstream bluff bodies. As the most complex oscillations and wake flows occur adjacent to the approximate natural frequency ratio, thus a constant natural frequency of is defined. To achieve excellent parallel performance, an immersed boundary lattice Boltzmann method code coupled with a structural equation is developed and accelerated using a graphical processing unit. The results for flow-induced vibrations suggest that the flow response characteristics of the tandem oscillating bluff bodies are strong functions of the streamwise displacement. Four different flow regimes of the bluff body arrangements are identified on the basis of response amplitude. Both the square and circular downstream bluff bodies exhibit two peak transverse amplitudes in the existing scrutinized spacing ratios. In addition, the critical spacing ratio values are dissimilar for the two bluff bodies. Moreover, at the end of the spacing ratio (), the wake effects for the downstream square body are amplified, and it oscillates near a single oscillating circular bluff body. Finally, the flow physics, as well as the hydrodynamic force coefficients of the tandem oscillating bluff bodies are also reviewed.

Development of a novel vibro-wind galloping energy harvester with high power density incorporated with a nested bluff-body structure

A two degrees of freedom (DOF) vibro-wind galloping energy harvester with a nested structure is proposed to improve wind energy harvesting efficiency. Unlike traditional wind energy harvesters configured in a single bluff body-beam structure, the proposed system comprises outer and inner square cross-sectional bluff bodies in tandem, connected with elastic cantilevered beams to allow separate oscillations. The generated power performance of a conventional single-DOF wind galloping energy harvester and the proposed system is compared through wind tunnel tests. Aerodynamic interactions between the two oscillating bluff bodies are analyzed by varying the gap distances and the coupling mechanism is determined by analyzing their power spectral density (PSD). Experimental results show that the outer bluff body has a dominant influence on the inner bluff body at low wind speed, which is a secondary cause of inner body vibration after the plain galloping directly induced by the incoming wind. The proposed system exhibits excellent performance with a significant power density increase of 27.8%. Moreover, the interference effects between the two bluff bodies can be reinforced by reducing their gap distance. We conclude that the inner bluff body plays a key role in enhancing output power, specifically at relatively high wind speed.

The use of artificial compressibility to improve partitioned semi-implicit FSI coupling within the classical Chorin–Témam projection framework

Over the last decade the classical Chorin–Témam projection method has been utilized to address fluid-structure interaction in a semi-implicit manner. In previous studies the fluid projection step is fully coupled with the structural motion due to the divergence-free constraint. A set of simultaneous equations thus have to be iteratively solved. To overcome this difficulty, a simple and accurate partitioned semi-implicit coupling method is proposed based on the artificial compressibility (AC) in this paper. The iterated AC parameter decouples the pressure, end-of-step velocity and structural motion within the characteristic-based split scheme. The present approach is completely matrix-free and has unlimited access to the finite elements. Its performance is demonstrated for an oscillating bluff body subjected to uniform flows.

On point vortex models of exotic bluff body wakes

Exotic vortex wakes, in which three or more vortices are generated during each shedding cycle, are frequently found in the wake of an oscillating bluff body. Two common examples are P+S wakes (with 3 vortices) and 2P wakes (with 4 vortices). We consider mathematical models of these wakes consisting of N = 3 or 4 point vortices with constant strengths in an inviscid fluid that is otherwise at rest in a singly-periodic domain. By enforcing constraints on the vortex strengths and, in the case of N = 4, on the symmetry of the vortex locations, the mathematical models reduce to integrable Hamiltonian systems. We compare the point vortex trajectories with two exotic wake patterns reported in the literature. Results support the use of point vortex modeling to investigate vortex dynamics in exotic wakes and suggest the need for additional classification of experimental wake patterns.

The New Micro Water Power Generation by Use of the Oscillating Bluff Body

The New Micro Water Power Generation by Use of the Oscillating Bluff Body

Noisy inflows cause a shedding-mode switching in flow past an oscillating cylinder.

Vortex streets formed behind oscillating bluff bodies consist of arrays of groups of two, three, or four vortices classified as 2S, P+S, and 2P shedding modes, respectively. The prevailing dominant mode depends primarily on the amplitude and the frequency of the oscillation and on the Reynolds number. We investigate the effect of noise at the inflow on the stability of these vortex modes in laminar flow past a circular cylinder. We employ stochastic simulations based on a new polynomial chaos method to study the shedding-mode switching from a P+S pattern to a 2S mode in the presence of noise.

Vortical Flow and Fluiddynamic Characteristics of an Oscillating Bluff Body

Vortical Flow and Fluiddynamic Characteristics of an Oscillating Bluff Body

Unsteady pressure evaluation on oscillating bluff body by vortex method

Abstract This paper describes the unsteady pressure evaluation on an oscillating bluff body using the unsteady Bernoulli's formula. Flow around a body is simulated by the vortex method. A 2-D rectangular cross section is forced to keep heaving oscillation at reduced wind velocity U/fD = 5. According to wind tunnel experiments, the movement-induced-type vortex-induced oscillation is observed in this cross section at the focused reduced wind velocity region. And the unsteady pressure on the body side surface is known to show convection nature due to the movement-induced vortices which generate at the leading edge and travel towards the trailing edge in exactly one natural period. In this study, this pressure convection on the side surface is focused and its reproduction using vortex method is investigated. The contribution of each term which is part of the time varying rate of the velocity potential is discussed. It is concluded that the term relating to the shedding vortex movement plays the most important role in giving reliable pressure convection nature.

Simulation of unsteady flow around oscillating bluff bodies using a hybrid Eulerian Lagrangian scheme

Simulation of unsteady flow around oscillating bluff bodies using a hybrid Eulerian Lagrangian scheme

Unsteady aerodynamic force characteristics on 2-D oscillating bluff body

Unsteady aerodynamic force characteristics on 2-D oscillating bluff body

Unsteady aerodynamic force characteristics on 2-D oscillating bluff body

Unsteady aerodynamic lift force and pitching moment for a 2-D rectangular cross section are evaluated by the discrete vortex method. The section is in coupled oscillation for heaving/torsional 2 d.o.f. The results are discussed by comparison with the data from wind tunnel experiments and with the analytical values determined by the non-stationary thin airfoil theory (Theodorsen function).

Critical geometry of oscillating bluff bodies

Measurements are presented of the mean pressures around rectangular and D-section cylinders, with a flat front face normal to the flow, forced to oscillate transversely at an amplitude of 10% of the length of the front face. The ratio of depth (streamwise dimension) to height (cross-stream dimension) of the cross-section ranges from 0.2 to 1.0 for rectangular cylinders and from 0.5 to 1.5 for D-section cylinders. The range of reduced velocities investigated, 3 to 11, includes the vortex-resonance region. When increasing the depth, an oscillating bluff cylinder shows a critical depth where base suction attains a peak. The value of a critical depth is lowered with decreasing reduced velocity. In particular, an extraordinarily low critical depth with a very high base suction is obtained on cylinders oscillating at vortex resonance. For cylinders with depths beyond the critical, a reattachment-type pressure distribution is established on the afterbody due to the shear-layer/edge direct interaction. The shear-layer/edge direct interaction can also occur on oscillating cylinders with a fixed splitter plate. At low reduced velocities, the reattachment-type pressure distributions on cylinders with and without a splitter plate are similar except for the mean level. A remark is made on the critical geometry of bluff bodies under various flow conditions.

In-line forces on oscillating bodies in a fluid flow.

An experimental investigation was performed to study the in-line forces acting on oscillating bluff bodies -a sphere, a cylinder and a disc- in a fluid flow. The energy dissipations calculated with fluid resistance were well correlated by the following correlation regardless of body shape over the whole range investigated. E2/3fv=E2/3f+E2/3vwhere Efv, Ef and Ev are energy dissipations for the oscillating body in a fluid flow, for the stationary body in a fluid flow and for the oscillating body in a liquid at rest.

The flow around an oscillating flat plate with splitter in stationary fluid. Numerical analysis by discrete vortex model.

It is very interesting to examine fluid forces on an oscillating bluff body with splitter plates in stationary fluid in connection with wave-induced forces on ocean structures and others. The discrete vortex model together with the conformal mapping method has been a;;lied to analyze the flow around an oscillating flat plate with splitter plates in a stationary fluid. Numerical calculations are performed for different splitter lengths and inclined angles of plates in various oscillating conditions, and vortex patterns and unsteady forces are obtained. From the calculations, it has been found that the splitter prevents the development of the shedding vortices and that the forces on the plate are influenced by the splitter length and inclined angle remarkably.

Vortex Shedding from Oscillating Bluff Bodies

When placed ih a fluid stream, some bodies generate separated flow over a substantial proportion of their surface and hence can be classified as bluff. On sharp-edged bluff bodies, separation is fixed at the salient edges, whereas on bluff bodies with continuous surface curvature the location of separation depends both on the shape of the body and the state of the boundary layer. At low Reynolds numbers, when separation first occurs, the flow around a bluff body remains stable, but as the Reynolds number is increased a critical value is reached beyond which instabilities develop. These instabilities can lead to organized unsteady wake motion, dis­ organized motion, or a combination of both. Regular vortex shedding, the subject of this article, is a dominant feature of two-dimensional bluff-body wakes and is present irrespective of whether the separating boundary layers are laminar or turbulent. It has been the subject of research for more than a century, and many hundreds of papers have been written. In recent years vortex shedding has been the topic of Euromech meetings reported on by Mair & Maull (1971) and Bearman & Graham (1980), and a comprehensive review has been undertaken by Berger & Wille (1972). Vortex shedding and general wake turbulence induce fluctuating pres­ sures on the surface of the generating bluff body, and if the body is flexible this can cause oscillations. Oscillations excited by vortex shedding are usually in a direction normal to that of the free stream, and amplitudes as large as 1.5 to 2 body diameters may be recorded. In addition to the generating body, any other bodies in its wake may be forced into oscillation. Broad-band force fluctuations, induced by turbulence produced in the flow around a bluff body, rarely lead to oscillations as severe as those caused by vortex shedding. Some form of aerodynamic instability, such that move-

Vortex Shedding from Oscillating Bluff Bodies

When placed ih a fluid stream, some bodies generate separated flow over a substantial proportion of their surface and hence can be classified as bluff. On sharp-edged bluff bodies, separation is fixed at the salient edges, whereas on bluff bodies with continuous surface curvature the location of separation depends both on the shape of the body and the state of the boundary layer. At low Reynolds numbers, when separation first occurs, the flow around a bluff body remains stable, but as the Reynolds number is increased a critical value is reached beyond which instabilities develop. These instabilities can lead to organized unsteady wake motion, dis­ organized motion, or a combination of both. Regular vortex shedding, the subject of this article, is a dominant feature of two-dimensional bluff-body wakes and is present irrespective of whether the separating boundary layers are laminar or turbulent. It has been the subject of research for more than a century, and many hundreds of papers have been written. In recent years vortex shedding has been the topic of Euromech meetings reported on by Mair & Maull (1971) and Bearman & Graham (1980), and a comprehensive review has been undertaken by Berger & Wille (1972). Vortex shedding and general wake turbulence induce fluctuating pres­ sures on the surface of the generating bluff body, and if the body is flexible this can cause oscillations. Oscillations excited by vortex shedding are usually in a direction normal to that of the free stream, and amplitudes as large as 1.5 to 2 body diameters may be recorded. In addition to the generating body, any other bodies in its wake may be forced into oscillation. Broad-band force fluctuations, induced by turbulence produced in the flow around a bluff body, rarely lead to oscillations as severe as those caused by vortex shedding. Some form of aerodynamic instability, such that move-