Boundary layer influence on ship model tests in extremely shallow and confined water
Lataire, E.; Raza, A.; Vantorre, M.; Delefortrie, G. (2023). Boundary layer influence on ship model tests in extremely shallow and confined water. J. Hydrodynam. (Print) online: [1-14]. https://dx.doi.org/10.1007/s42241-023-0024-0
In: Journal of Hydrodynamics. Elsevier: Amsterdam. ISSN 1001-6058; e-ISSN 1878-0342, meer
Ships sailing in shallow and/or confined water (when calling a harbour or other berthing areas), will experience a different behaviour due to the interaction with vertical and/or horizontal boundaries. Among other hydrodynamic changes induced in confined water, the lateral ship-bank interaction force changes its sign at a critical distance between ship and bank or bottom. However, this distance and its effects on model test results have not been quantified in the past. To investigate the shallow water hydrodynamics coupled with bank effects, systematic model tests were carried out at Flanders Hydraulics Research (FHR) with different ship models. The following parameters were systematically varied: water depth, lateral position, speed, and propeller rate. The change of the ship-bank induced lateral force from an attraction force in medium-deep and shallow water to a repulsion force in extremely shallow water conditions, can be ascribed to the interaction of the boundary layers of the ship model and the environment (tank and installed banks). In this article, a mathematical model is proposed for the critical distance in terms of boundary layer influence thickness. This indicates the range where the model tests are influenced by the horizontal or vertical restrictions combined with the propeller’s dynamic effects. Moreover, the expression has also been extended to describe the relationship between full-scale ship length and water depth with the boundary layer influence thickness. Due to lower Reynolds numbers and relatively thicker boundary layers at model scale, upscaling of the model test results, according to Froude’s law, may provide erroneous results. The influence of the boundary layer initiates at a relatively higher under keel clearance (UKC) for a smaller ship model compared with a larger ship model. Therefore, the boundary layer’s influence with respect to ship model length should be considered during model testing.
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