Ivan Korkischko

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

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

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

On the asymptotic solution of the flow around a circular cylinder

This study presents new advances concerning the asymptotic solution derived for the viscous flow around a circular cylinder. It provides arguments, based on numerical and experimental (DPIV) results, to suport the validity of the solution for Reynolds numbers far beyond the critical value of the first Hopf bifurcation (Re cr ≈ 46). Results are then related to recent studies regarding the construction of reduced models employing POD techniques.

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

Experimental Investigation of Vortex-Induced Vibration on an Inclined Circular Cylinder

This paper presents experimental results of vortex-induced oscillations of an inclined circular cylinder mounted on an elastic base. Models are mounted on an air-bearing elastic base, instrumented with strain gages, accelerometers and a load cell. The experiments were carried out on a water channel facility at NDF-EPUSP. The elastic base has low structural damping and is free to oscillate only in the cross-flow direction. The cylinder axis is inclined in relation to the current. New measurements on the dynamic response oscillations of this inclined cylinder, due to vortex-induced vibrations (VIV), are compared with previous experiments on a vertical cylinder. VIV is investigated by conducting experiments in two ways: first, the cylinder is maintained vertical on the elastic base, with a uniform current normal to its axis, and the response curve is obtained; subsequently, the investigation is carried out changing the angle of inclination from 0 to 45 degrees in relation to vertical. The results for a vertical cylinder are in accordance with other literature measurements for mass ratio m*=2. For the inclined model, using the decomposition of the flow on the direction normal to cylinder axis, the results for amplitude, drag and lift coefficients are consistent with the vertical cylinder.Copyright