E. Ciappi

Prediction of the compressible stage slamming force on rigid and elastic systems impacting on the water surface

In this paper some models focusing on hydrodynamic and elasticforces arising during the impact of rigid and elastic systems on thewater surface are investigated. In particular, the supersoniccompressible stage of the impact is considered by modelling the slammingphenomenon through the Skalk–Feit acoustic approximation. The dynamicequations of the dropping system are coupled to those of the fluid and anonlinear fluid-solid interaction problem is stated. Generalrelationships between the bodys shape, slamming force and body motionare determined. These equations are applied to the wedge water entrycases, and a closed-form expression for the maximum hydrodynamic forceis found. Moreover the theoretical correlation between the hydrodynamicforce and the body geometry allows us to control the inverse problem andthe shape associated to a constant slamming force is determined.Due to some simplifications allowed in the supersonic compressibleimpact, the results of the hydrodynamic analysis hold in closed form.This permits us to focus on the basic result of the paper addressed to asystematic correlation between hydrodynamic and elastic maximum forcesin terms of some characteristic dimensionless quantities involved influid-solid interaction.In particular, ‘critical conditions’corresponding to those hydorelastic parameters combinations areinvestigated, leading to severe elastic response of the impactingsystem.

Some insights into slamming forces: Compressible and incompressible phases:

Abstract In the present paper the slamming force occurring in the free-fall impact of cylindrical bodies over the water surface is analysed in both compressible and incompressible stages. In the compressible phase the hydrodynamic analysis is carried out by the acoustic approximation and a closed-form expression for the impact force is recovered. The incompressible stage is approached through an unsteady boundary element method to compute the free surface evolution and the slamming force on the body. In both cases the hydrodynamic force is coupled to the rigid body motion to update the entry velocity of the body. The combined effect of the increasing wetted area and the reducing entry velocity leads to a maximum in the impact force that depends on the body mass. A parametric investigation shows that in the impact of a wedge section, if the maximum is reached either in the compressible or in the incompressible stages, a similar square root trend characterizes the dependence of this maximum on a non-dimensional mass parameter.

Experimental Evidence Of Critical Phenomena In Structure-water Impact

The design of modem high-speed vessels makes clear that extreme care must be devoted to fluid-structure interaction problems. It is a matter of fact that dangerous impact phenomena occur in fast marine vehicles, leading to severe pressure load conditions of the hull structure. In this paper a new theoretical and experimental approach to the water impact problem is proposed. The basic idea relies on the water shock spectral response able to predict the occurrence of critical impact phenomena in terms of the characteristic parameters as the impact velocity, the body impacting mass and the natural frequencies of the structure. The experimental set up consists of a wedge shaped body dropped on the water surface by a vertical slide system and of a test elastic structure installed on board the wedge. Systematic experimental measurements show that when varying the characteristic first natural frequency of the on board structure, critical impact conditions appear characterized by very large amplitudes of the structural response confirming the predictions of the theoretical model.