It has been recognized for some time that sinusoidal oscillatory flow experiments generate hydrodynamic force coefficients larger than those measured in a random sea environment. The present study investigates forces induced on cylinders by random oscillatory flows. The results show that Cd and Cm differ from the values obtained under sinusoidal conditions by more than 30% to 40% in the inertia/drag regime of the Keulegan-Carpenter number.
It has been known for some time that force coefficients obtained in periodic or sinusoidal oscillatory flow experiments are larger than those measured for cylindrical structures exposed to random wave-induced flows. However, physical understanding and modeling of hydrodynamic forces induced on circular cylinders has been accomplished to a large extent by sinusoidal oscillatory flow studies in the laboratory. These studies, such as Sarpkaya (1976, 1986), Williamson (1985) and Obasaju, et al, (1988), have identified the basic influence of changes in the fluid flow parameters on the hydrodynamic forces induced on the cylinder. Ikeda, et aI, (1988), oscillated cylinders in a tank and showed the strong influence of previously shed vortices on the inline force through measurement of the drag and inertia coefficients for Morison's equation. The present investigation examines the inline and transverse forces exerted on smooth circular cylinders under random oscillatory flow conditions. The forces in random oscillatory flow are directly compared to those induced under sinusoidal oscillatory flow conditions by focusing on experimentally measured force coefficients for the inline and transverse forces. The experiments were conducted in an experimental apparatus capable of generating either sinusoidal or random planar oscillatory flow. This latter study illustrates that, although results from sinusoidal experiments may not be directly applicable to random flow conditions, the fundamental understanding gained in these experiments can lend significant insight into interpretation and modeling of forces induced in random flows.
