DETAILED FULL SCALE SIMULATIONS OF NEAR FIELD OVERFLOW PLUME MIXING

摘要

Detailed near field simulations of the mixing of a full scale TSHD (Trailing Suction Hopper Dredger) overflow plume are presented. Important near field processes as air entrainment, overflow discharge pulsing (a discontinuous overflow flux caused by ship motions), and interaction with TSHD hull and propellers are taken into account. The CFD model is validated with experimental data and two cases of a full scale measured overflow plume. The two cases of full scale measured overflow plumes show different sediment concentration downstream of the TSHD: 200-500 mg/l for the first case, and 10-20 mg/l for the second case. Although both cases differ substantially, the CFD model is capable to reproduce both. This gives confidence that in the model the most important near field processes are captured accurately. With the model it is shown that air entrainment and overflow discharge pulsing are important processes which can increase the amount of the overflow plume which ends up near the water surface by a factor 3-10. The near surface part of an overflow plume can be important for the environmental impact of dredging projects. Hence, it is hoped that the detailed near field overflow mixing simulations presented in this paper might improve the quality of environmental impact assessments of dredging projects. Given the importance of air entrainment and pulsing on the amount of the overflow plume which ends up near the surface, it is recommended to minimise air entrainment and pulsing at the overflow of a TSHD.