Hydrodynamic Forces on Circular Cylinders in Sinusoidal and Random Oscillating Flow
Raul G. Longoria, University of Texas at Austin
An experimental apparatus has been developed which can be used to generate a general time-dependent planar flow across a cylinder. A mass of water enclosed with no free surface within a square cross-section tank and two spring pre-loaded pistons is oscillated using a hydraulic actuator. A circular cylinder is suspended horizontally in the tank by two X-Y force transducers used to simultaneously measure the total inline and transverse forces. Fluid motion is measured using a differential pressure transducer for instantaneous acceleration and an LVDT for displacement. This investigation provides a measurement of forces on cylinders subjected to planar fluid flow velocity with a time (and frequency) dependence which accurately represents the random conditions encountered in a natural ocean environment. The use of the same apparatus for both sinusoidal and random experiments provides a quantified assessment of the applicability of sinusoidal planar oscillatory flow data in offshore structure design methods.
The drag and inertia coefficients for a Morison equation representation of the inline force are presented for both sinusoidal and random flow. Comparison of the sinusoidal results is favorable with those of previous investigations. The results from random experiments illustrates the difference in the force mechanism by contrasting the force transfer coefficients for the inline and transverse forces. It is found that application of sinusoidal results to random hydrodynamic inline force prediction using the Morison equation incorrectly predicts the drag and inertia components, and the transverse force is over-predicted. The use of random planar oscillatory flow in the laboratory, contrasted with sinusoidal planar oscillatory flow, quantifies the accepted belief that the force transfer coefficients from sinusoidal flow experiments are conservative for prediction of forces on cylindrical structures subjected to random sea waves and the ensuing forces. It is concluded that the large-scale vortex activity prominent in sinusoidal oscillatory flow is subdued in random flow conditions. Further analysis of the random data is conducted in the frequency domain to illustrate models used for predicting the power spectral density of the inline force, including a nonlinear describing function method.