Julio R. Meneghini

On the wake-induced vibration of tandem circular cylinders: the vortex interaction excitation mechanism

The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterized by a build-up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibration (VIV) response, which occurs in a limited resonance range. We suggest that WIV of the downstream cylinder is excited by the unsteady vortex–structure interactions between the body and the upstream wake. Coherent vortices interfering with the downstream cylinder induce fluctuations in the fluid force that are not synchronized with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited. An experiment performed in a steady shear flow turned out to be central to the understanding of the origin of the fluid forces acting on the downstream cylinder.

Secondary instabilities in the flow around two circular cylinders in tandem

Direct stability analysis and numerical simulations have been employed to identify and characterize secondary instabilities in the wake of the flow around two identical circular cylinders in tandem arrangements. The centre-to-centre separation was varied from 1.2 to 10 cylinder diameters. Four distinct regimes were identified and salient cases chosen to represent the different scenarios observed, and for each configuration detailed results are presented and compared to those obtained for a flow around an isolated cylinder. It was observed that the early stages of the wake transition changes significantly if the separation is smaller than the drag inversion spacing. The onset of the three-dimensional instabilities were calculated and the unstable modes are fully described. In addition, we assessed the nonlinear character of the bifurcations and physical mechanisms are proposed to explain the instabilities. The dependence of the critical Reynolds number on the centre-to-centre separation is also discussed.

Possible states in the flow around two circular cylinders in tandem with separations in the vicinity of the drag inversion spacing

The possible states in the flow around two identical circular cylinders in tandem arrangements are investigated for configurations in the vicinity of the drag inversion separation. By means of numerical simulations, the hysteresis in the transition between the shedding regimes is studied and the relationship between (three-dimensional) secondary instabilities and shedding regime determination is addressed. The differences observed in the behavior of two- and three-dimensional flows are analyzed, and the regions of bistable flow are delimited. Very good agreement is found between the proposed scenario and results available in the literature.

Two and Three-Dimensional Simulation of Sound Generated by Flow Around a Circular Cylinder

The objective of this paper is to calculate the far-field sound generated from low mach number flow around a two-dimensional and three-dimensional circular cylinder in the subcritical regime using the Lighthill acoustic analogy. For the two-dimensional case, the timedependent incompressible flow is predicted using unsteady Reynolds-averaged NavierStokes models. For the three-dimensional case, the flow was obtained by solving the filtered Navier-Stokes equations of the Large Eddy Simulation model. As a benchmark, a flow-field with a Reynolds number of 90,000 is employed. The obtained numerical results such as Strouhal number, fluctuating lift and mean drag are compared with experiments. The computed unsteady pressure fluctuations on the cylinder wall are used as a sound source for the acoustic solver. Comparison between the two-dimensional numerical results and the experiment shows that computed acoustic field overpredict the noise amplitude; however, good agreement is obtained if an appropriate correlation length is taken into account. The sound measurements obtained by Revell et al. were carried out with a much longer span cylinder length when compared to the span used for the three-dimensional LES simulations. Therefore, the far-field sound is estimated by two correction methods proposed, respectively, by Kato et al. and Seo & Moon. The aerodynamic and acoustic results obtained by the threedimensional approach agree favorably well with the corresponding experimental data.

An Experimental Investigation of the Flow Around Straked Cylinders

This paper presents experimental results concerning the response of circular cylinders with and without strakes. The longitudinal and transverse fluid forces (drag and lift), amplitude response and wake structures of plain and helically straked cylinders are compared. Six different configurations of straked cylinders with pitches (p) equal to 5D, 10D and 15D and heights (h) equal to 0.1D and 0.2D are investigated. Measurements on the dynamic response oscillations of an isolated plain and straked cylinders and flow visualization employing a PIV system are shown. Fixed cylinder drag measurements are also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges or in an air bearing base. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping and the flexor blades base an the air bearing base are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments ranges from 2000 to 10000, and reduced velocities, based on natural frequency in still water, vary up to 13. The drag coefficient is increased by 20% for the h = 0.1 D cylinder, and 60% for the h = 0.2 D cylinder, comparing both with the plain cylinder. The smaller height strokes (h = 0.1 D) do not prevent vortex formation in the region very close to the body, resulting in a decrease of about 50% of the amplitude response compared with the plain cylinder. Lowest amplitude response was found to the p = 10 D and h = 0.2 D case. The analysis of the vorticity contours shows that the shear layer does not roll close to the body (same result for the other cases with h = 0.2 D).Copyright

Numerical Simulation Of Vortex Shedding From An Oscillating Circular Cylinder

The interaction between cylinder oscillation and the shedding of vortices is investigated numerically in this paper. The near wake structure is presented for different values of reduced velocity of a cylinder free to oscillate transversely. One of the objectives of this paper is to compare the numerical results with experimental data obtained by Parra [11] in the water tank facility of IPT/University of Sao Paulo. The attraction of applying numerical methods to this problem is that the way the flow is modified can be studied in closer detail. In the computer it is possible to investigate many different flow conditions more easily. The method used for the simulation is based on the Vortex-in-Cell formulation incorporating viscous diffusion. The Navier-Stokes equations are solved using the operator-splitting technique, where convection and diffusion of vorticity are treated separately. The convection part is modelled assuming that the vorticity field is carried on a large number of discrete vortices. Force coefficients are calculated by considering the normal gradient of vorticity at the wall to evaluate the pressure contribution and the vorticity at the wall to obtain the skin friction.

Two- and Three-Dimensional Simulations of the Flow Around a Cylinder Fitted With Strakes

Two- and three-dimensional simulations of the flow around straked cylinders are presented. For the two-dimensional simulations we used the Spectral/hp Element Method, and carried out simulations for five different angles of rotation of the cylinder with respect to the free stream. Fixed and elastically-mounted cylinders were tested, and the Reynolds number was kept constant and equal to 150. The results were compared to those obtained from the simulation of the flow around a bare cylinder under the same conditions. We observed that the two-dimensional strakes are not effective in suppressing the vibration of the cylinders, but also noticed that the responses were completely different even with a slight change in the angle of rotation of the body. The three-dimensional results showed that there are two mechanisms of suppression: the main one is the decrease in the vortex shedding correlation along the span, whilst a secondary one is the vortex wake formation farther downstream.Copyright

Unsteady Force Measurements on a Responding Circular Cylinder in the Wake of an Upstream Cylinder

This paper presents force measurements during flow-induced vibration of a pair of circular cylinders with low mass ratio (m ⁄ = 2:0) and low damping (z = 0:7%) aligned in a tandem arrangement. A particular case with a gap of 3 diameters centre to centre is used to examine flow-interference mechanisms occurring on a downstream cylinder, free to oscillate only in the transverse direction. The Reynolds number varies within the range 1500 < Re < 20000. A cylinder immersed in the wake of another can develop flow-induced oscillations persisting for a large range of reduced velocities. Oscillations are observed for reduced velocities, based on cylinder natural frequency measured in air, as high as 35. Apparently, the amplitude of oscillation is reaching a level of saturation of about 1:5 diameters, while the frequency of vibration is increasing at an approximate constant rate. As reduced velocity is increased two regimes of flow-induced vibration are observed: first vortex-induced vibration and then a wake-induced vibration regime. In addition, the presence of the second cylinder affects the dynamics of the upstream wake, but it is found not to synchronize the vortex shedding frequency of the upstream cylinder for the second regime of oscillations.

Numerical Simulation of Control of Bluff Body Flow Using a Discrete Vortex Method Incorporating Viscous Diffusion

Control of bluff body flow is investigated numerically. Control is achieved by applying forced oscillation to a cylinder and cases covering a range of high amplitudes and with various frequencies have been simulated. Special attention is given to the change of phase between body displacement and lift force and the phase jump that occurs near the resonance point. The near wake structure is presented for different values of reduced velocity. One of the objectives of applying open loop control is to define possible strategies for future investigations with closed loop or feed-back control.

Velocity-correction schemes for the incompressible Navier-Stokes equations in general coordinate systems

This paper presents methods of including coordinate transformations into the solution of the incompressible Navier-Stokes equations using the velocity-correction scheme, which is commonly used in the numerical solution of unsteady incompressible flows. This is important when the transformation leads to symmetries that allow the use of more efficient numerical techniques, like employing a Fourier expansion to discretize a homogeneous direction. Two different approaches are presented: in the first approach all the influence of the mapping is treated explicitly, while in the second the mapping terms related to convection are treated explicitly, with the pressure and viscous terms treated implicitly. Through numerical results, we demonstrate how these methods maintain the accuracy of the underlying high-order method, and further apply the discretisation strategy to problems where mixed Fourier-spectral/hp element discretisations can be applied, thereby extending the usefulness of this discretisation technique.