Alexander Korobkin (UEA) - Water entry and exit

In collaboration with: T. Khabakhpasheva (UEA), K. Maki (UoM), J. Rodriguez (UC3M), P. Vega- Martinez (UC3M), S. Seng (BV)

The problem of a body lifted from water surface is studied by theoretical, numerical and experimental means. The linearised model of water exit was been developed further to account for gravity, non-linear effects and elasticity of the body.

The two-dimensional problem of a symmetric wedge entering water and exiting from it thereafter is considered. The pressure distribution along the wetted part of the wedge and the total hydrodynamic force acting on the wedge are calculated by three simplified models: the Original Wagner Model of water entry and the Linearized Model of water exit, the Modified Logvinovich Model (MLM) of entry and exit, and Generalized Wagner Model (GWM) of entry and exit. Computational Fluid Dynamics simulations are used to generate reference results for the development and elaboration of the water exit models with large displacements, and to match these models with the GWM of water entry. It is shown that the simplified models can be significantly improved by including gravity effects without increasing complexity of the models.

Unsteady axisymmetric problem of a circular disc, which initially touches the flat water surface and is lifted then suddenly with high acceleration up to 20g, is studied experimentally and theoretically. The disc drags the liquid behind it. No cavitation occurs in the conditions of the performed experiments. This is also confirmed by theoretical estimates. Both the external force applied at the centre of the disc and the disc acceleration at the same place are measured during an initial transient stage of the process. It is shown that the external force and the disc acceleration are well related initially to each other by the added mass approach. Later on, the measured acceleration decays even the applied force continues to increase. The disc displacement and the radius of the wetted area of the disc are recorded by high-speed cameras. The records reveal that the wetted area of the disc does not change before the disc acceleration reaches its maximum value. It is argued that these phenomena are caused by the elastic deflection of the disc during the initial transient stage. The linearised model of water exit is generalised to account for elastic behaviour of the lifted body. The results obtained with the generalised exit model fairly well agree with the experimental results. These findings are confirmed finally by experiments with circular discs of different rigidity.