On splitter plate coverage for suppression of vortex-induced vibrations of flexible cylinders

doi:10.1016/j.apor.2014.09.002

Applied Ocean Research

Volume 48, October 2014, Pages 244-249

https://doi.org/10.1016/j.apor.2014.09.002 Get rights and content

Highlights

•
Experimental results obtained with a flexible cylinder with low mass ratio show the effectiveness of splitter plates for reducing VIV.
•
Splitter plates covering less than half of the flexible cylinder model length yield VIV reductions of up to 90%.
•
The VIV reduction is accompanied by a 50% drag reduction.

Abstract

Experimental results show how vortex-induced vibration (VIV) amplitudes of flexible cylinders can be reduced up to a 90% by covering less than half of the length of the cylinder with splitter plates elastically mounted to the surface of the cylinder.

The VIV amplitude reduction takes place along with drag coefficient reductions of up to a 50% for the reduced velocities investigated.

Introduction

There is a wide variety of situations in offshore and marine engineering applications in which vortex-induced vibrations (VIV) are prone to occur. In general, structures designed for ocean engineering systems have inherently low structural damping and low mass if compared to the fluid surrounding them. In such systems, the excitation caused by vortex shedding can result in very large amplitudes. These large amplitudes combined with high structural responding modes end up in fatigue and reduced life span for the structure. Large amplitudes also result in increased hydrodynamic forces that yield large mean deflections of the structures, and therefore clashing and contact risks are increased.

A considerably large effort has been lead by the industry in order to overcome the limitations imposed by VIV in the design of marine systems, with the objective of finding ways to passively suppress structural response without increasing hydrodynamic forces. A large quantity of works have been published in the open literature with novel methods to passively suppress VIV. Helical strakes are the device that has probably received more attention, see for example recent works by Huang [7]; Quen et al. [15]; Trim et al. [18]; Zhou et al. [20]; Huang [7]; Huang and Sworn [8]; Korkischko and Meneghini [13]. Other passive suppression methods have been investigated in order to overcome the problem of drag increase produced by the helical strakes [14]. Splitter plates and fairings have been analysed recently for VIV suppression with drag reduction by Assi et al. [1], [2].

In Assi et al. [1] the authors show how splitter plates allowed to rotate around the axis of an elastically mounted rigid cylinder, can be used for suppressing VIV if an adequate rotary stiffness is used. Other devices for suppressing VIV were investigated in that work, such as double oblique and parallel splitter plates and splitter plates with gaps. In Gu et al. [6] the authors studied the stable positions, forces, correlation and flow structures in the wake of a rigid cylinder fitted with a splitter plate of different lengths. Previous to those works Cimbala and Garg [5] had already found the stable configurations of splitter plates in the wake of cylinder as a function of the length of the plate. The present work is concerned with the spanwise splitter plate coverage needed in a flexible cylinder in order to produce an acceptable level of VIV suppression. If rigid free-to-rotate (about the axis of the cylinder) splitter plates are used, the rotary stiffness has to be inside a certain range in order to allow the splitter plate to get oriented with the flow and not being unstable. In fact there is a stable position at a certain angle, for which VIV suppression will happen reducing drag. If the rigid splitter plates are fixed the system will experience combinations of galloping and VIV, with amplitudes larger than those seen on plain cylinders [1].

The results presented here, based on a configuration similar to the one used by Shukla et al. [17], show how motions can be suppressed with splitter plates fixed at the base of the cylinder, but able to deform as in a hinge. The effectiveness in the minimisation of VIV with drag reduction of this type of splitter plate is analysed. The work also shows the splitter plate coverage needed in a flexible cylinder, in order to obtain an adequate level of response reduction. More research is planned in order to investigate new concepts based on the proposed here, but able to passively orientate to the flow. Also the effect of the incoming flow angle of incidence needs to be investigated.

Section snippets

Experimental set-up

Experiments were conducted in the Free Surface Water Channel (FSWC) at the Department of Mechanical Engineering at Universitat Rovira i Virgili in Tarragona. The water channel is a closed loop recirculating facility made of stainless steel with a transparent working section made of glass with dimensions 1 m × 1.1 m × 2.5 m. With the water at its maximum height of 1 m, flow speeds of more than 0.6 m/s can be achieved with low levels of turbulence intensity.

The cylinder model used in the experiments was

Results and discussion

In this section, the results obtained in the experiments are summarised. An example of the data obtained for one of the experiments appears in Fig. 3. The plot in the second row shows the cross-flow amplitudes along the axis of the cylinder. Each line is for an instant in time separated one tenth of a second apart. In the upper row, the time series of the amplitude of the point at the middle of the model during 25 s is shown together with its spectrum after FFT computations. Finally in the

Conclusions

The experimental results presented in this work demonstrate how the VIV dynamic response of flexible cylinders can be reduced by approximately a 90% in maximum amplitude, if splitter plates covering less than half of the length of the model, are used. This drastic amplitude reduction introduced by the splitter plates is accompanied by a reduction in drag coefficients, up to a 50%.

The mechanism for VIV suppression has been confirmed to be the modification of shear layer interaction in the near

Acknowledgement

Funding from Spanish Ministry of Science through grant DPI2012-37904 is gratefully acknowledged.

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Suppression of wake-induced vibration of tandem cylinders with free-to-rotate control plates
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View full text

On splitter plate coverage for suppression of vortex-induced vibrations of flexible cylinders

Highlights

Abstract

Introduction

Section snippets

Experimental set-up

Results and discussion

Conclusions

Acknowledgement

Low drag solutions for suppressing vortex-induced vibration of circular cylinders

J Fluids Struct

Suppression of wake-induced vibration of tandem cylinders with free-to-rotate control plates

J Fluids Struct

Investigation of the flow behind a two dimensional model with a blunt trailing edge and fitted with splitter plates

J Fluid Mech

Laboratory measurements of vortex-induced vibrations of a vertical tension riser in a stepped current

J Fluids Struct

Flow in the wake of a freely rotatable cylinder with splitter plate

AIAA J

Pressure distribution, fluctuating forces and vortex shedding behavior of circular cylinder with rotatable splitter plates

J Fluids Struct

VIV suppression of a two-degree-of-freedom circular cylinder and drag reduction of a fixed circular cylinder by the use of helical grooves

J Fluids Struct

Hydrodynamic coefficients of two fixed circular cylinders fitted with helical strakes at various staggered and tandem arrangements

Appl Ocean Res

Towing tank experiments on the vortex-induced vibrations of low mass ratio long flexible cylinders

J Fluids Struct

An optical instrument based on defocusing for dynamic response model testing in water or wind tunnels

Ocean Eng