Cesar M. Freire

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

Defining a Parameter of Effectiveness for the Suppression of Vortex-Induced Vibration

The vortex-induced vibration (VIV) phenomenon is a very important issue in ocean and offshore engineering. When bluff bodies are immersed in a current, the boundary layers separate and form shear layers that can interact causing a periodic vortex shedding. In such condition, the forces acting on the body can make it vibrate. In the offshore industry, among other engineering applications, the vibration of structures can cause fatigue problems, reducing the life span of the element. To reduce the vortex-induced vibration effect suppressors can be employed in order to avoid the body to move, or at least, reduce the amplitude of vibration. There is a great number of suppressor types, like strakes, splitter plates, shrouds, etc; and each one has its own flow mechanisms to avoid VIV. Until now there is no simple way to compare the results and effectiveness of each suppressor. The purpose of this work is to define a parameter called suppressor effectiveness, based on the amplitude of vibration and the reduced velocity range, and use the suppressor effectiveness to compare different suppressors. The data used in the comparisons was obtained from experiments by the authors and collected from the literature.Copyright

EXPERIMENTAL INVESTIGATION OF THE VORTEX-INDUCED VIBRATION OF A CURVED CYLINDER

Experiments have been conducted in a water channel in order to investigate the vortex-induced vibration (VIV) response of a rigid section of a curved circular cylinder. Two curved configurations were tested regarding the direction of the approaching flow, a concave or a convex cylinder, in addition to a straight cylinder that served as reference. Amplitude and frequency response are presented versus reduced velocity for a wide Reynolds number range between 750 and 15,000. Trajectories in the cross-flow and streamwise direction are presented as well for several reduced velocities. Results show a distinct behaviour from the typical VIV of a straight cylinder highlighting the effect of curvature on vortex formation and excitation. The concave configuration presents relatively high amplitudes of vibration that are sustained beyond the typical synchronisation region. The mechanism behind the response is not yet clear, although authors suggest it might be related to some kind of buffeting excitation due to the disturbed flow from

Experimental Comparisons to Assure the Similarity Between VIM (Vortex-Induced Motion) and VIV (Vortex-Induced Vibration) Phenomena

Vortex-Induced Motion (VIM) is another way to denominate the Vortex-Induced Vibration (VIV) in floating units. The main characteristics of VIM in such structures are the low aspect ratio (L/D < 4.0) and the unity mass ratio (m* = 1.0, i.e. structural mass equal water displacement). The VIM can occur in MPSO (Monocolumn Production, Storage and Offloading System) and spar platforms. These platforms can experience motion amplitudes of around their characteristic diameters. In such cases, the fatigue life of mooring and riser systems can be greatly reduced. Typically, the VIM model testing campaigns are carried out in the Reynolds range between 200,000 and 400,000. VIV model tests with low aspect ratio cylinders (L/D = 1.0, 1.7 and 2.0) and unity mass ratio (m* = 1.0) have been carried out at the Circulating Water Channel facility available at NDF/EPUSP. The Reynolds number range covered in the experiments was between 10,000 and 50,000. The characteristic motions (in the transverse and in-line direction) were obtained using the Hilbert-Huang Transform method (HHT) and then compared with results obtained in experiments found in the literature. The aim of this investigation is to definitely establish the similarity between the VIM and VIV phenomena, making possible to increase the understanding of both and, at same time, allowing some analytical models developed for VIV to be applied to the VIM scenario on spar and monocolumn platforms, logically under some adaption.Copyright

Suppression of the Vortex-Induced Vibration of a Circular Cylinder With Permeable Meshes

Experiments have been carried out on models of rigid circular cylinders fitted with three different types of permeable meshes to investigate their effectiveness in the suppression of vortex-induced vibrations (VIV). Measurements of the dynamic response are presented for models with low mass and damping which are free to respond in the cross-flow direction. Reynolds number ranged from 1,000 to 10,000 and reduced velocity was varied between 2 and 13. Also presented are measurements of the wake of static models with Particle Image Velocimetry (PIV) at Reynolds number equal to 4000. Results for two meshes made of ropes and cylindrical tubes are compared with the VIV response of a bare cylinder and that of a known suppressor called the “ventilated trousers” (VT). All three meshes achieved an average 50% reduction of the response when compared with that of the bare cylinder. The sparse mesh configuration presented a similar behaviour to the VT, while the dense mesh produced considerable VIV response for an indefinitely long range of reduced velocity. Visualisation of the flow by PIV around static cylinders revealed that all suppressors disrupt the vortex shedding and increase the formation length when compared to the bare cylinder. The VT mesh, which presented the best suppression, also presented the largest vortex formation length.© 2014 ASME

The Effect of Plate Length on the Behaviour of Free-to-Rotate VIV Suppressors With Parallel Plates

Experiments have been carried out on free-to-rotate parallel plates fitted to a rigid section of circular cylinder to investigate the effect of plate length on the stability of this type of VIV (vortex-induced vibration) suppressor. Measurements of the dynamic response and trajectories are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown that, depending on a combination of geometric and strucutral parameters, parallel plates might not be able to completely suppress VIV for the whole range of reduced velocities investigated. Plates with length between 1.0 and 2.0 diameters showed instabilities and induced high-amplitude vibrations for some specific reduced velocities. Rotational friction was increased for a second run and all plates stabilised and suppressed VIV for the whole range of reduced velocities tested. An undesirable steady lateral force was also observed to occur for all configurations. Experiments with a plain cylinder in the Reynolds number range from 1,000 to 20,000 have been performed to serve as reference.Copyright

Interference Effect in the Flow Around Two Aligned Cylinders and the Reduction of a Helical Strakes Suppressor Effectiveness

It is well known that the interference effect reduces the effectiveness of vortex induced vibration (VIV) suppressors such as helical strakes. Usually the major concern about structures fitted with VIV suppressors is its own vibration. In this paper it is shown how an upstream structure fitted with helical strakes can induce higher vibrations than bare cylinders in a structure mounted downstream in a tandem configuration. Experiments were conducted in a water channel facility and demonstrate that the vortex wake generated by an cylinder fitted with strakes can induce higher amplitudes of oscillation than a bare cylinder. Particle image velocimetry was employed to measure the velocity field in the wake behind a smooth cylinder and behind a cylinder fitted with strakes. The Reynolds number for the experiments goes from 1000 up to 10.000. The flow field visualization was conducted for fixed cylinder at Re = 10.000.Copyright

Comparison Between the Koopman Modes for the Flow Around Circular Cylinder and Circular Cylinder Fitted with Helical Strakes

The helical strake is a device commonly employed for vortex-induced vibration attenuation or suppression. It works by changing and affecting the flow patterns around a circular cylinder and, thereby, reducing the interaction of the shear layers formed in this massive separated type of flow. In this paper the wake of vortices generated by a bare circular cylinder is compared to the wake generated by a circular cylinder fitted with helical strake with pitch equal to ten cylinder diameters and height equal to 0.2 of the cylinder diameter. Besides the analysis of the measured velocity field, a Koopman decomposition is made and the most energetic modes obtained for each condition are compared. The Reynolds number for the visualizations is \(\text {Re} = 10{,}000.\)

Vortex-Induced Vibrations and Wake Structures of Isolated Plain and Straked Cylinders Mounted on a 2DOF Elastic Base

This paper presents experimental results concerning the response of plain and straked circular cylinders. The isolated cylinders are mounted in a two degrees of freedom elastic base. Two straked cylinders are tested and they have the same pitch p = 10d and two different heights h = 0.1d and h = 0.2d. The longitudinal and transverse amplitude responses and wake structures of plain and helically straked cylinders are compared. The wake visualization uses the stereoscopic digital particle image velocimetry (SDPIV) technique. Comparing to the plain cylinder response, the p = 10d and h = 0.1d strakes moderately reduce the maximum amplitude response, while the p = 10d and h = 0.2d strakes suppress the vortex-induced vibrations. The strake effectiveness is directly related to the strake height. The Reynolds number varies from 1000 up to 7500 in the experiments.© 2010 ASME