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In-line loading and response of a compliant cylinder in oscillatory and steady flow at critical Reynolds numbers

Lakkoju, V. N. M. (1994). In-line loading and response of a compliant cylinder in oscillatory and steady flow at critical Reynolds numbers. (Unpublished Doctoral thesis, City, University of London)


A detailed investigation has been carried out to study loading and response of a compliant cylindrical member in oscillatory flow. While many previous studies have been confined to fixed cylindrical members and limited to a sub-critical Reynolds number range, in the present investigation studies are directed towards higher Reynolds numbers, up to 3 x 105 and more. One of the important factors for compliant members in oscillatory flow, the frequency ratio (i.e. the ratio of natural frequency of the test cylinder to the frequency of oscillation of the member) has been investigated especially to identify its effect on Morison drag and inertia force coefficients. This formed a basis for investigating the validity of the Mori-son equation for a compliant cylinder. At at odd integer frequency ratios, Morison force predictions based on measured cylinder displacements showed good correlation with experimental results. They also showed limitations of Morison relative velocity equation, particularly at Keulegan Carpenter numbers less than 25, where vortex excited lift forces are dominant.

To predict both in-line forces and responses, a numerical model based on the linear acceleration approach has been developed and used over all experimental conditions. Even though some limitations of this model were observed at certain fluid loading conditions, at higher KC numbers and at odd frequency ratios, the model in-line force and response predictions were relatively good.

Another set of experiments was designed to study the effect of in-line high frequency external oscillations superimposed on sinusoidal motion. A series of tests with a range of superharmonic amplitudes was conducted for every frequency ratio and at certain fixed KC numbers. Interestingly, the test results revealed that there is a dependence of drag and inertia Morison coefficients on in-line superharmonic oscillations. At lower KC numbers it was observed that the superharmonic excitation has a profound effect on the coefficients, presumably through changes in the vortex dynamics.

Finally a set of steady flow experiments was conducted in a towing tank. Similar in-line external oscillations were imposed at frequencies close to the test cylinder’s natural frequency. Hydrodynamic damping and drag coefficients were calculated and compared with previous investigations.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Lakkoju thesis 1994 PDF-A.pdf]
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