Cavity Formation during Asymmetric Water Entry of Rigid Bodies

This work numerically evaluates the role of advancing velocity on the water entry of rigid wedges, highlighting its influence on the development of underpressure at the fluid-structure interface, which can eventually lead to fluid detachment or cavity formation, depending on the geometry. A coupled FEM-SPH numerical model is implemented within LS-DYNA, and three types of asymmetric impacts are treated: (I) symmetric wedges with horizontal velocity component, (II) asymmetric wedges with a pure vertical velocity component, and (III) asymmetric wedges with a horizontal velocity component. Particular attention is given to the evolution of the pressure at the fluid-structure interface and the onset of fluid detachment at the wedge tip and their effect on the rigid body dynamics. Results concerning the tilting moment generated during the water entry are presented, varying entry depth, asymmetry, and entry velocity. The presented results are important for the evaluation of the stability of the body during asymmetric slamming events

Experimental evaluation of the air trapped during the water entry of flexible structures

Deformable structures entering the water might experience several fluid-structure interaction (FSI) phenomena; air trapping is one of these. According to its definition, it consists of air bubbles trapped between the structure and the fluid during the initial stage of the impact. These bubbles might reduce the peak impact force. This phenomenon is characteristic for the water entry of flat-bottom structures. Above a deadrise angle of 10°, air trapping is negligible. In this work, we propose a methodology to evaluate the amount of air trapped in the fluid during the water entry. Experiments are performed on wedges with varying stiffness, entry velocity, and deadrise angle. A digital image post- processing technique is developed and utilized to track the air trapping mechanism and its evolution in time. Interesting results are found on the effect of the impact velocity and the structural deformation on the amount of air trapped during the slamming event

Bistable morphing panels through SMA actuation

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On the use of ductile tabs as a viable strategy to test SMA and other high-strength fine wires

Experimental characterization of high-strength fine wires often arises challenges greater than expected due to the difficulties encountered gripping the wire to the fixtures. To avoid premature breakage within the fixtures, capstan grips are widely utilized, but slippage might occur unless the wire is folded many times around the capstan. Further, estimating the actual elongation of the gauge section might be an issue since wires do not deform in the gauge section only but also within the folded length. This work proposes a simple clamping approach relying on sacrificial tabs made of soft and ductile material allowing to utilize classic wedge-shaped steel jaws to characterize high-strength fine wires without encountering any slippage or break within the gripping region. The suggested tabbing strategy further allows referring to the crosshead motion to evaluate the overall strain in the wire as its deformation concentrates only over the gauge length, hence excluding the use of an extensometer. This is particularly needed in the case of testing in a climatic chamber. On top of the experimental evidence, the proposed tabbing methodology is studied through analytical solutions and finite element analyses, which result in a generalized design tool that can be utilized to choose the best material and dimensions of the sacrificial tabs to correctly test wires other than the ones considered in this work

Numerical modelling of impact on the water

Studying the impact between the moving body and a free surface of water is a well-known problem both in the marine and aeronautical fields, and concerns the phenomena of interaction between hulls, offshore structures and aeroplanes with the water\u2019s surface, sometimes considered together with waves. The main aim of this kind of research is to identify the load transferred to the structure during impact to verify its resistance or redesign it

On Air-Cavity Formation during Water Entry of Flexible Wedges

Elastic bodies entering water might experience fluid⁻structure interaction phenomena introduced by the mutual interaction between structural deformation and fluid motion. Cavity formation, often misleadingly named cavitation, is one of these. This work presents the results of an experimental investigation on the water entry of deformable wedges impacting a quiescent water surface with pure vertical velocity in free fall. The experimental campaign is conducted on flexible wedges parametrically varying the flexural stiffness, deadrise angle, and drop height. It is found that, under given experimental conditions, cavity pockets form beneath the wedge. Their generation mechanism might be ascribed to a differential between structural and fluid velocities, which is introduced by structural vibrations. Results show that the impact force during water entry of stiff wedges are always opposing gravity, while, in case flexible wedges temporarily reverse their direction, with the body that is being sucked into the water within the time frame between the cavity formation and its collapse. Severe impact might also generate a series of cavity generation and collapses

Dynamic monitoring of compliant bodies impacting the water surface through local strain measurements

The understanding and the experimental characterization of the evolution of impulsive loading is crucial in several fields in structural, mechanical and ocean engineering, naval architecture and aerospace. In this regards, we developed an experimental methodology to reconstruct the deformed shape of compliant bodies subjected to impulsive loadings, as those encountered in water entry events, starting from a finite number of local strain measurements performed through Fiber Bragg Gratings. The paper discusses the potential applications of the proposed methodology for: i) real-Time damage detection and structural health monitoring, ii) fatigue assessment and iii) impulsive load estimation

METHOD TO CONTROL THE COMBUSTION OF AN INTERNAL COMBUSTION ENGINE

A method to control the combustion of an internal combustion engine comprising a number of cylinders and a water collection and injection system to introduce water into the internal combustion engine