Hydrodynamic Damping on Flexible Cylinders in Sheared Flow

ABSTRACT Results are given from flow-induced vibration experiments conducted in sheared flows. The site was an historic 1848 textile mill canal with a width of 58 feet. Variable head gates allowed a controllable horizontal shear to be produced. The paper emphasizes the dramatic effect of hydrodynamic modal damping on the response. Means for predicting hydrodynamic damping are presented and results are compared to the observed response. INTRODUCTION The prediction of the flow-induced vibration response of long, cylindrical structures deployed in sheared flows is a problem of great practical importance. The applications vary from extremely long cables that exhibit the dynamic behavior of systems of infinite length [1] to relatively short and stiff risers or pilings which respond to vortex shedding in only a few of the lowest natural modes of vibration. Recently, considerable attention has been given to the use of "compliant" production risers. These hose like cylinders are usually deployed with rather low tension and are thought to be rather highly damped. However, such hopeful speculation is inadequate for serious design purposes and a closer look is necessary. The authors have recently concluded a series of field tests using a 58 foot long, 1 1/8 inch diameter rubber hose, strengthened with longitudinal kevlar strands. The cable had a specific gravity with a flooded interior of 1.34. The cable was deployed in a controllable sheared flow. By varying the tension of the cable, dynamical properties of a wide variety of cylinders could be simulated. At high tensions, only a few of the lowest modes were excited, thus simulating the behavior of a short, nearly rigid riser. At very low tensions, the behavior of a rubber hose with many responding modes was observed. Experimental results are presented in a preliminary way, as much data processing and evaluation remains to be done. More complete results will be in an MIT Department of Ocean Engineering Ph.D. thesis by T. Y. Chung due for completion in June of 1987. The prediction of the response of a flexible cylinder to vortex shedding may be thought of as consisting of four major components: an excitation model, a structural model, a damping model, and a solution technique. In this paper the emphasis is on the measurement and prediction of hydrodynamic damping. Excitation models are only addressed here to the extent that they are needed to understand damping. The conclusions regarding damping have important implications for appropriate solution techniques. These conclusions are discussed briefly. More extensive discussion of excitation models and solution techniques as well as damping will be available in the before-mentioned doctoral dissertation. Experimental data is provided to support the conclusions regarding hydrodynamic damping.