This paper presents a method for predicting the vortex-excited crossflow vibrations of cylindrical structures which oscillate as a result of their interaction with a steady current. The predicted maximum vibration responses are compared with results obtained from a large number of experiments which were conducted both in air and in water. Good agreement is obtained between the theory and the measured responses of elastically mounted rigid circular cylinders, pivoted rods, cantilevers and taut cables over the range of structural and fluid mass and damping parameters in which vortex-induced oscillations occur. The increased drag which accompanies these vibrations was also measured and the experimental data agree with available predictions.
The prediction and measurement of vortex-excited structural vibrations and the amplified fluid forces which accompany them are important in many ocean engineering applications. The flow-induced lift and drag forces on and the response of sea floor pipelines and structures, tow and mooring cable systems, floating platforms, and suspended pipelines and risers must be known in order to implement appropriate design procedures and to prevent costly construction delays and structural failures. Examples of offshore facilities which often encounter vortex excited vibrations include oil jetties and terminals, offshore platforms, and supply ship and tanker moorings. The implementation of design procedures that consider vortex-induced motions is becoming more critical as off- shore exploration is extended into deeper waters where steady currents are more often encountered, e.g., the North Sea oil fields.
If one of the natural frequencies 'of a bluff body immersed in a moving stream of fluid is near the frequency at which vortices are naturally shed from the body, then self-excited, resonant vibrations can occur if the damping of the system is sufficiently low. There is also a range of frequencies near this so-called Strouhal frequency of vortex shedding where forced vibrations of the body cause the vortex frequency to be captured by, or to synchronize with, the body frequency. This means that the body and wake have the same characteristic frequency and that the Strouhal frequency, relating to the vortex shedding from a stationary body, is suppressed. This locking-in or wake capture phenomenon causes vortex excited oscillations to occur in directions both parallel and normal to the incident current direction over a range of flow speeds. The forces which act on a structure are amplified as a result of such vibrations, and these forces are closely related to the changes which occur in the wake flow downstream of the body.
This paper presents a method for predicting the vortex-excited, cross flow motions of cylindrical structures and cables which vibrate as a result of their interaction with a steady current. This method, the so-called "wakeoscillator" model, was introduced in two earlier OTC papers [1,2] for the case of an elastically-mounted, rigid cylinder. The purpose of the present paper is to further develop the basic wake-oscillator model and to extend it to flexible structures.
