Frequency Domain Analysis of In-Line Forces on Circular Cylinders in Random Oscillatory Flow

Experiments were conducted to study random hydrodynamic loading of smooth, rigidly mounted circular cylinders exposed to random oscillatory flow conditions. The experiments were conducted in a water tunnel for statistical Keulegan-Carpenter numbers ranging from KC_r = 6.8 to 11.6 and statistical Reynold’s numbers from Re_r = 8211 to 20169 under broad and narrow-band flow conditions. A two-input/single-output frequency domain model for the in-line force is used to directly identify frequency-dependent inertia and drag coefficients. This model was also used to show the relative strength of the inertia and drag components in the frequency domain for different random flow conditions parameterized by KC_r, Re_r and the velocity spectral width, q. This analysis illustrates the effect of statistical flow parameters (KC_r, Re_r and q) on the nonlinear behavior of the in-line forces. Estimates of the in-line force power spectral density under random flow conditions are determined using the two-input/single-output model and compared to the method for estimating wave forces on a cylindrical structure by Borgman (1967, 1972).