| Thu October 12th 2017 14:00 – 15:00 ZH286 | Seminar | Wave forces in sheared flows Simen Ådnøy Ellingsen |
Details:Wave forces are often a primary concern in engineering projects in coastal waters. The designer typically face as scenario where three factors interact: water waves, solid bodies and currents varying with depth. At present, virtually no tools exist to to analyse such systems, and their physical behaviour is largely unknown. We demonstrate the importance of accounting for shear in calculation of wave-loads and motions on floating, moored and fixed objects considering forces on a ship in steady motion on a shear current measured in the Columbia River delta.As much as 50% of the fuel consumption of medium to small size vessels is due to making waves which radiate energy. The corresponding force on the vessel is called wave resistance. Within the limitations of linear wave theory we develop a theoretical framework for predicting wave patterns and wave resistance for ships in arbitrary motion on top of horizontal currents whose magnitude and direction can both vary arbitrarily with depth. The theory is applied to calculating wave resistance on realistic vessels operating in the Columbia River mouth. Results show that wave resistance can differ by a factor of 3 between upriver and downriver motion at the same velocity relative to the water surface, for Froude numbers typical of smaller vessels. For ships travelling across the current, a lateral radiation force also acts, amounting in our case to about 20% of the sternward resistance. No equivalent force exists in currents of uniform depth dependence. We also study transient forces for a ship starting suddenly from rest, and a ship in circular motion. Transient of the lateral force are particularly strong, and could well affect seakeeping and operational safety. Even for large tanker ships, on which wave-making forces are generally negligible, the transient lateral force could be significant, acting as it does on the ship's sides rather than the much smaller bow and stern. | ||
