Design and Experimental Verification of an Exit Manifold for Improved Current Generation in an Offshore Engineering Basin

Abstract

Current is generated at the National Research Council's Institute for Marine Dynamics, Offshore Engineering Basin (OEB) in St. John's, Newfoundland, using a pump thruster system in which flow is conveyed under a false floor and returns through the test section of basin. The objectives of this project were to quantify and assess the performance of the existing current generation system and to experimentally verify a design exit manifold for improved current generation. At present, current operation results in large-scale vertical vortices being produced at the current exit end of the basin where. the wave boards are installed. The exit flow is not confined and therefore flow divergence occurs at the vertical back wall of the basin. These conditions contribute to substantial energy losses at the exit region, which in turn result in low current velocities at the test region of the basin. Through the use of Computational Fluid Dynamics, preliminary designs for an exit manifold to reverse flow back into the basin were modeled to ensure an efficient system is implemented to dissipate the flow into the basin. The CFD models provided an insight into the shear flow turbulent mixing at the exit region. For operational reasons the design was constrained to a height of 0.35m and a maximum length of approximately 4. 0m. The final detailed design called for a contraction/expansion combination, J 80° turn with exit manifold and flow straighteners. A prototype of the design was fabricated of aluminum and experimental tests were conducted to assess its performance. The results of the experiments showed a significant improvement in current generation capabilities over the present current generation setup. For deep-water tests, the use of the prototype exit manifold resulted in a velocity magnitude increase of two times over the present setup. Shallow water tests resulted in a velocity magnitude increase of greater than three times over the present setup. It is envisaged that the installation of the exit manifold design in the OEB will significantly improve the current generation.