Simulation of water entry of a wedge through free fall in three degrees of freedom

2010 ◽  
Vol 466 (2120) ◽  
pp. 2219-2239 ◽  
Author(s):  
G. D. Xu ◽  
W. Y. Duan ◽  
G. X. Wu
Keyword(s):  
Degrees Of Freedom ◽  
Velocity Potential ◽  
Free Fall ◽  
Water Entry ◽  
Auxiliary Function ◽  
The Body ◽  
Body Motion ◽  
Time Stepping ◽  
The Impact ◽  
Three Degrees Of Freedom

The water entry problem of a wedge through free fall in three degrees of freedom is studied through the velocity potential theory for the incompressible liquid. In particular, the effect of the body rotation is taken into account, which seems to have been neglected so far. The problem is solved in a stretched coordinate system through a boundary element method for the complex potential. The impact process is simulated based on the time stepping method. Auxiliary function method has been used to decouple the mutual dependence between the body motion and the fluid flow. The developed method is verified through results from other simulation and experimental data for some simplified cases. The method is then used to undertake extensive investigation for the free fall problems in three degrees of freedom.

Theoretical Study of the Water Entry of a Body in Waves: Application to Safety of Occupants in Free-Fall Lifeboats

2009 ◽  
Author(s):  
Thomas Sauder ◽  
Se´bastien Fouques
Keyword(s):  
Water Waves ◽  
Degrees Of Freedom ◽  
Mass Matrix ◽  
Free Fall ◽  
Theoretical Method ◽  
Emergency Evacuation ◽  
Momentum Conservation ◽  
The Body ◽  
Water Impact ◽  
The Impact

The safety of occupants in free-fall lifeboats (FFL) during water impact is addressed. The first part of the paper describes a theoretical method developed to predict the trajectory in six degrees of freedom of a body entering water waves. Slamming forces and moments are computed, based on momentum conservation, long wave approximation and a von Karman type of approach. The added mass matrix of the body is evaluated for impact conditions by a boundary element method. The second part of the paper focuses on the application of the method to free-fall lifeboats, which are used for emergency evacuation of oil platforms or ships. Acceleration loads on FFL occupants during water impact are dependent on numerous parameters, especially the hull shape, the mass distribution, the wave heading relative to the lifeboat, and the impact point on the wave surface. Assessing operational limits of FFL by means of model tests only has therefore been costly and time consuming. This issue is addressed here by applying the theoretical method described in the first part. The model has been validated for FFL through extensive model testing in calm water and regular waves, and statistical estimates of acceleration levels for lifeboat occupants, as well as acceleration time series were obtained that can be used as inputs to numerical human response models.

Free fall water entry of a cone in three degrees of freedom

2020 ◽  
Vol 101 ◽  
pp. 102273 ◽  
Author(s):  
Shi-Li Sun ◽  
Yong Cheng ◽  
Jie Cui ◽  
Shi-Yan Sun
Keyword(s):  
Degrees Of Freedom ◽  
Free Fall ◽  
Water Entry ◽  
Three Degrees Of Freedom

Some insights into slamming forces: Compressible and incompressible phases

2000 ◽  
Vol 214 (6) ◽  
pp. 881-888 ◽  
Author(s):  
E F Campana ◽  
A Carcaterra ◽  
E Ciappi ◽  
A Iafrati
Keyword(s):  
Impact Force ◽  
Mass Parameter ◽  
Free Fall ◽  
The Body ◽  
Body Motion ◽  
Closed Form Expression ◽  
Surface Evolution ◽  
Entry Velocity ◽  
Wetted Area ◽  
The Impact

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.

scholarly journals On the Effect of Cavity Formation during the Water Entry of Flexible Bodies

2018 ◽  
Author(s):  
Riccardo Panciroli ◽  
Tiziano Pagliaroli ◽  
Giangiacomo Minak
Keyword(s):  
Fluid Motion ◽  
Free Fall ◽  
Time Frame ◽  
Water Entry ◽  
The Body ◽  
Structural Deformation ◽  
Cavity Formation ◽  
Experimental Conditions ◽  
Flexible Bodies ◽  
The Impact

Elastic bodies entering the water might experience Fluid-Structure Interaction phenomena introduced by the mutual interaction between the structural deformation and the 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 forms beneath the wedge. Their generation mechanism is found to be ruled by a differential between structural and fluid velocities, which is introduced by the structural vibrations. Results show that the impact force during water entry of stiff bodies is always opposing gravity, while in case of flexible bodies might temporarily reverse its direction, with the body that is being sucked into the water within the time frame between the cavity formation and its collapse. Severe impacts might also generate a series of cavity generation and collapses.

scholarly journals Two-Dimensional Impact Modelling with Three Degrees of Freedom and Its Application in the Dynamics of Planing Hulls

2020 ◽  
Vol 10 (3) ◽  
pp. 1072
Author(s):  
Roberto Algarín ◽  
Antonio Bula
Keyword(s):  
Degrees Of Freedom ◽  
Numerical Solutions ◽  
Cross Flow ◽  
Free Fall ◽  
Three Dimensions ◽  
Two Dimensional ◽  
Slender Body Theory ◽  
Impact Modelling ◽  
The Impact ◽  
Three Degrees Of Freedom

Planing boat dynamics are a complex phenomenon and the maneuver forces acting on these kind of hulls are difficult to predict. In the current work, a mathematical model of a two-dimensional impact with three degrees of freedom (3DOF) is developed. The model was used to study wedge sections with knuckles, the vertical, horizontal, and rotational motion are considered. Pressure distribution, forces, and motion during the impact, considering both free fall and forced motion, are evaluated. The commercial CFD (Computational flow dynamics) software Star-CCM+ V9.06 was used to validate the formulation. Simulations with one, two, and three degrees of freedom were carried out, and the results were compared with CFD simulations, experimental data, and numerical solutions by others authors. The results show a good agreement with the authors. The model is extended to three dimensions applying slender body theory, and the forces in the hull are computed. The formulation allows evaluating the seakeeping with cross flow, dynamic stability, and manoeuvrability of planing boats with variable sections over the length.

scholarly journals On Air-Cavity Formation during Water Entry of Flexible Wedges

2018 ◽  
Vol 6 (4) ◽  
pp. 155 ◽  
Author(s):  
Riccardo Panciroli ◽  
Tiziano Pagliaroli ◽  
Giangiacomo Minak
Keyword(s):  
Fluid Motion ◽  
Free Fall ◽  
Time Frame ◽  
Water Entry ◽  
The Body ◽  
Structural Deformation ◽  
Cavity Formation ◽  
Experimental Conditions ◽  
Fluid Velocities ◽  
The Impact

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.

scholarly journals Water entry of a body which moves in more than six degrees of freedom

2015 ◽  
Vol 471 (2177) ◽  
pp. 20150058 ◽  
Author(s):  
Y.-M. Scolan ◽  
A. A. Korobkin
Keyword(s):  
Degrees Of Freedom ◽  
Early Stage ◽  
Three Dimensional ◽  
Vertical Displacement ◽  
Oblique Impact ◽  
Water Entry ◽  
The Body ◽  
Body Motion ◽  
Elliptic Paraboloid ◽  
Over Time

The water entry of a three-dimensional smooth body into initially calm water is examined. The body can move freely in its 6 d.f. and may also change its shape over time. During the early stage of penetration, the shape of the body is approximated by a surface of double curvature and the radii of curvature may vary over time. Hydrodynamic loads are calculated by the Wagner theory. It is shown that the water entry problem with arbitrary kinematics of the body motion, can be reduced to the vertical entry problem with a modified vertical displacement of the body and an elliptic region of contact between the liquid and the body surface. Low pressure occurrence is determined; this occurrence can precede the appearance of cavitation effects. Hydrodynamic forces are analysed for a rigid ellipsoid entering the water with 3 d.f. Experimental results with an oblique impact of elliptic paraboloid confirm the theoretical findings. The theoretical developments are detailed in this paper, while an application of the model is described in electronic supplementary materials.

scholarly journals Water Entry of A Wedge Into Waves in Three Degrees Offreedom

2019 ◽  
Vol 26 (1) ◽  
pp. 117-124
Author(s):  
Shi Yan Sun ◽  
Hai Long Chen ◽  
Gang Xu
Keyword(s):  
Degrees Of Freedom ◽  
Mutual Effect ◽  
Vertical Direction ◽  
Physical Space ◽  
Auxiliary Function ◽  
Body Motion ◽  
Wave Loading ◽  
Initial Stage ◽  
The Time Domain ◽  
Three Degrees Of Freedom

Abstract The hydrodynamic problem of a two-dimensional wedge entering into a nonlinear wave in three degrees of freedom is investigated based on the incompressible velocity potential theory. The problem is solved through the boundary element method in the time domain. To avoid numerical difficulties due to an extremely small contact area at the initial stage, a stretched coordinate system is used based on the ratio of the Cartesian system in the physical space to the distance travelled by the wedge in the vertical direction. The mutual dependence of body motion and wave loading is decoupled by using the auxiliary function method. Detailed results about body accelerations, velocities and displacements at different Froude numbers or different waves are provided, and the mutual effect between body motion and wave loading is analysed in depth.

Rolling Condition and Gyroscopic Moments in Curve Negotiations

2012 ◽  
Author(s):  
Ahmed A. Shabana ◽  
Martin B. Hamper ◽  
James J. O’Shea
Keyword(s):  
Coordinate System ◽  
Degrees Of Freedom ◽  
The Body ◽  
Contact Forces ◽  
Body Motion ◽  
Global Coordinate System ◽  
Gyroscopic Moment ◽  
Absolute Angular Velocity ◽  
Pure Rolling ◽  
The Stability

In vehicle system dynamics, the effect of the gyroscopic moments can be significant during curve negotiations. The absolute angular velocity of the body can be expressed as the sum of two vectors; one vector is due to the curvature of the curve, while the second vector is due to the rate of changes of the angles that define the orientation of the body with respect to a coordinate system that follows the body motion. In this paper, the configuration of the body in the global coordinate system is defined using the trajectory coordinates in order to examine the effect of the gyroscopic moments in the case of curve negotiations. These coordinates consist of arc length, two relative translations and three relative angles. The relative translations and relative angles are defined with respect to a trajectory coordinate system that follows the motion of the body on the curve. It is shown that when the yaw and roll angles relative to the trajectory coordinate system are constrained and the motion is predominantly rolling, the effect of the gyroscopic moment on the motion becomes negligible, and in the case of pure rolling and zero yaw and roll angles, the generalized gyroscopic moment associated with the system degrees of freedom becomes identically zero. The analysis presented in this investigation sheds light on the danger of using derailment criteria that are not obtained using laws of motion, and therefore, such criteria should not be used in judging the stability of railroad vehicle systems. Furthermore, The analysis presented in this paper shows that the roll moment which can have a significant effect on the wheel/rail contact forces depends on the forward velocity in the case of curve negotiations. For this reason, roller rigs that do not allow for the wheelset forward velocity cannot capture these moment components, and therefore, cannot be used in the analysis of curve negotiations. A model of a suspended railroad wheelset is used in this investigation to study the gyroscopic effect during curve negotiations.

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