RIASSUNTO
ABSTRACT
A variety of cables were studied under controlled conditions in the ocean. Sections of cable, 76.5 ft in length, were exposed to a spatially uniform, time-varying current. Vibration response, Current velocity, and tension were simultaneously recorded. Care was taken to minimize complicating influences, such as lumped masses and non uniform tensions. The data revealed strong interaction between the vortex shedding process and the natural frequencies of each cable. Under lock-in conditions the span wise vibration mode shape corresponded correctly with the ""locked-in"" natural frequency. A cable with an antistrumming fairing was tested simultaneously with an unfaired length of the -same cable the vibration frequencies and amplitudes are compared.
INTRODUCTION
The transverse oscillation of cylinders in a current is a phenomenon that has received much treatment, both theoretical and practical. The fluctuating forces can lead to accelerated fatigue failure of cables or structural members, or. to erroneous data collected by moored or towed sensor arrays. Data obtained on full- scale systems have been difficult to interpret due to complications, such as lumped masses or no uniformity of flow velocity and tension over the length of the test section. Studies on rigid and flexible cylinders (with and without fairings, splitter plates, or the like) have been conducted in wind and water tunnels. The data are valid, but the validity of extrapolating from these very short lengths to the behavior of long cables in the field is not certain. Theoretical treatments are not adequate for the prediction of on-site systems. Work is also being done to model cable behavior numerically.
In an attempt to bridge the gap between experimental and theoretical data, a field study was conducted on 76.5-ft lengths of various cables with ends fixed and held under constant tension in a uniform flow. The frequency and amplitude of the cable vibrations were monitored as the current velocity varied over half of a tidal cycle at the test site. The lock-in phenomenon was studied by varying the tension during a period of constant current velocity. This had the effect of changing the: cable's natural frequencies, while maintaining a constant preferred Strouhal frequency. It is hoped that data obtained from this intermediate size, but manageable, experiment will be of value in relating the results of tunnel studies and complex full scale experiments. A summary of cables tested and test conditions may be found in Table 1.
THEORETICAL BACKGROUND
A bluff body positioned in a fluid flow will shed vortices. For rigidly mounted cylinders, a pair of vortices will be shed simultaneously for Reynolds numbers up to 40. For higher Reynolds numbers, up to about 105, vortices will be shed alternately from each side of the cylinder. The vortex formation is associated with, increased local now speed, and therefore reduced pressure, so that the cylinder experiences a force transverse to the flow. When a vortex is then shed from the opposite side, a force in the other direction is felt.