Methodology to Evaluate Slamming Loads on Ocean Platforms

2008 ◽  
Author(s):  
Marcio Domingues Maia Junior ◽  
Antonio Carlos Fernandes ◽  
Marcela Trindade ◽  
Andre Ramiro
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Pressure Sensor ◽  
Potential Flow ◽  
Experimental Methods ◽  
The Body ◽  
Platform Motion ◽  
Air Pocket ◽  
Accuracy And Precision ◽  
The Impact

The purpose of the study is suggest a methodology to be applied in ocean platforms and ships in order to appraise the maximum impact pressure due to the slamming occurrence in the hull shape near its bottom or horizontal regions. This methodology uses a theory based on potential flow. However, there are some phenomena such as creation of a compressible air pocket between the body and free surface at the impact moment that requires a more complete theory and or experimental methods. This gives rise to experimental coefficients to reduce the theoretical errors. The procedure presented here goes by the platform motion dynamics and “impact topology” to allow the potential to be used. Due to the complexity of the phenomenon studied and need for certifying accuracy and precision of the results, tank tests at the LabOceano model basin were carried out. The results showed a good fitting between numerical results and experiments. It should also be pointed out that the pressure sensor used in these experiments gives a pressure distribution over the instrumented area what brings more reliability on the results and a better visibility to the slamming phenomenon. Lastly the methodology in this work stands out as an important tool to evaluate slamming loads.

scholarly journals Analysis of students’ foot pressure distribution on the ground, as well as their body balance before and after exercise

2020 ◽  
Vol 24 (4) ◽  
pp. 194-204
Author(s):  
Jarosław Jaszczur-Nowicki ◽  
Joanna Bukowska ◽  
Dariusz Kruczkowski ◽  
Michał Spieszny ◽  
Magdalena Pieniążek ◽  
...  
Keyword(s):  
Body Composition ◽  
Pressure Distribution ◽  
Physiological Parameters ◽  
The Body ◽  
Foot Pressure ◽  
Body Balance ◽  
Balance System ◽  
Functional Efficiency ◽  
Before And After ◽  
The Impact

Background and Study Aim: The article presents the results of analyses of students’ foot pressure distribution on the ground, as well as their body balance before and after exercise (Harvard Step Test). The aim of the paper was to carry out a comparative analysis of foot pressure distribution on the ground, as well as assess the degree of body balance before and after exercise. With that purpose in view, the following research hypothesis was formulated: in the students participating in the study, the distribution of foot pressure on the ground and the degree of body balance differ significantly after physical effort compared with the at-rest conditions. Material and Methods: The study encompassed n=48 students, including 37 women and 11 men. The tests were carried out using such tools as: an EPS/R1 podobarographic mat and the impedance methods – i.e. the InBody 270 body composition analyser. An analysis was performed for the parameters concerning body composition, the distribution of foot pressure on the ground, and the level of body balance. Results: The results obtained revealed statistically significant differences in the physiological parameters of foot arching and the functional efficiency of the body balance system under different measurement conditions that reflected the impact of effort stimuli. Conclusions: Significant differences reflecting the impact of the effort stimuli were expected to be achieved during the mathematical analysis of the results of podobarographic tests that allow for the assessment of the physiological parameters of foot arching and the functional efficiency of the body balance system under different measurement conditions. The authors’ assumption was mathematically and statistically confirmed by significant differences foe most of the parameters arising out of the possibilities offered by the research method applied. Comparative assessment unquestionably revealed a negative change in foot arching, as well as lower body posture stability in the female and male subjects, resulting from the physical exercise applied.

A Second-Order Theory for the Potential Flow About Thin Hydrofoils

1984 ◽  
Vol 28 (01) ◽  
pp. 55-64
Author(s):  
Colen Kennell ◽  
Allen Plotkin
Keyword(s):  
Integral Equation ◽  
Free Surface ◽  
Potential Flow ◽  
Order Theory ◽  
Wave Drag ◽  
Second Order ◽  
The Body ◽  
Surface Shape ◽  
Surface Boundary ◽  
Free Surface Boundary Condition

This research addresses the potential flow about a thin two-dimensional hydrofoil moving with constant velocity at a fixed depth beneath a free surface. The thickness-to-chord ratio of the hydrofoil and disturbances to the free stream are assumed to be small. These small perturbation assumptions are used to produce first-and second-order subproblems structured to provide consistent approximations to boundary conditions on the body and the free surface. Nonlinear corrections to the free-surface boundary condition are included at second order. Each subproblem is solved by a distribution of sources and vortices on the chord line and doublets on the free surface. After analytic determination of source and doublet strengths, a singular integral equation for the vortex strength is derived. This integral equation is reduced to a Fredholm integral equation which is solved numerically. Lift, wave drag, and free-surface shape are calculated for a flat plate and a Joukowski hydrofoil. The importance of free-surface effects relative to body effects is examined by a parametric variation of Froude number and depth of submergence.

scholarly journals Comparison of Linear Vortex Panel Method and Finite Volume Method for Calculation of Generated Lift in Potential Flow Over Two-Dimensional Car Bodies

2020 ◽  
Author(s):  
Saeid Moammaei ◽  
Mehran Khaki Jamei ◽  
Morteza Abbasi
Keyword(s):  
Pressure Distribution ◽  
Lift Coefficient ◽  
Control Point ◽  
Panel Method ◽  
The Body ◽  
Aerodynamic Load ◽  
Two Dimensional ◽  
Algebraic Equations ◽  
Linear Algebraic Equations ◽  
The Impact

Abstract This paper describes one of the aspects of the panel method to analyze the aerodynamic characteristics of a sedan. The linear vortex panel method has been developed to simulate the ideal flow over a two-dimensional arbitrary car and, it also calculates the aerodynamic load on the body. By satisfying the boundary conditions on each control point, our linear algebraic equations are obtained. The results are sensitive to the distribution of the panels over the body thus the body is broken up equally into very small panels. After solving the set of equations, the vortices strength is obtained and the pressure distribution for the upper and the lower surface of the body is calculated. The impact of the angle of attack on the aerodynamic behavior of the intended car is investigated and it is found that the lift coefficient increases with the free stream angle from -4 to 4. The accuracy of the results has been determined by checking them against the standard CFD data. The pressure distribution trend is found very much in confirmation with the CFD results, however, a discrepancy at the rear end is observed. Therefore, it can be concluded that this method does not seem practical for geometries with steep slopes in the rear part of the car. Finally, both methods are applied to the other modified geometries with lower slopes at the rear section and the results compare well with the fluent.

Hydroelastic Analysis of a Simple Oscillator Impacting the Free Surface

2000 ◽  
Vol 44 (04) ◽  
pp. 278-289
Author(s):  
A. lafrati ◽  
A. Carcaterra ◽  
E. Ciappi ◽  
E. F. Campana
Keyword(s):  
Free Surface ◽  
Theoretical Model ◽  
Boundary Integral ◽  
The Body ◽  
Elastic Response ◽  
Closed Form Expression ◽  
Suitable Model ◽  
Spring Stiffness ◽  
Elastic Forces ◽  
The Impact

The coupling between the hydrodynamic and elastic forces arising when a simple oscillator impacts the free surface is considered. The system is a two-mass oscillator, the lower mass being wedge-shaped, free falling on the free surface. Attention is devoted to a parametric investigation of the maximum of both hydrodynamic and elastic forces induced by the impact. The study is performed by a simplified theoretical model and by a numerical simulation of the fluid-structure interaction. The theoretical model suggested here provides an efficient tool for the computation of the hydrodynamic and elastic forces and of the corresponding maxima as a function of some parameters such as deadrise angle of the wedge, entry velocity, spring stiffness, and the masses. In particular, a closed-form expression for the critical value of the spring constant leading to the maximum elastic response is achieved as a function of the other parameters. Numerically, a panel method is adopted to solve the boundary integral formulation for the velocity potential. A suitable model is introduced to deal with the flow singularity at the intersection point between the free surface and the body contour. Time histories of the hydrodynamic and elastic forces are computed for different values of the spring stiffness and are compared with the corresponding results provided by the simplified theoretical model. The comparison shows that, despite the strong assumptions, the theoretical model allows a good estimate of the system critical condition.

Nonlinear Computation of Water Impact of Axisymmetric Bodies

2011 ◽  
Vol 55 (01) ◽  
pp. 29-44
Author(s):  
Hongmei Yan ◽  
Yuming Liu
Keyword(s):  
Free Surface ◽  
Flow Separation ◽  
Impact Force ◽  
Surface Profile ◽  
The Body ◽  
Gravity Effect ◽  
Water Impact ◽  
Free Surface Profile ◽  
Axisymmetric Bodies ◽  
The Impact

A fully nonlinear numerical simulation based on a boundary element method was used to investigate water impact of axisymmetric bodies that strike vertically the horizontal free surface from the air. The main objective was to understand the gravity effect on flow/wave kinematics and dynamics and to quantify the range of validity of existing theories and computations that are based on the infinite Froude number assumption. Two body geometries were considered: inverted cone and sphere. For the inverted cone, we obtained detailed dependencies of free-surface profile and impact pressure and load on the body on the generalized Froude number (Fr(V/gt)1/2, where V is the impact velocity, g is the gravitational acceleration, and t is time) and deadrise angle a. Based on these, we developed an approximate formula for evaluating the contribution of the gravity effect to the total impact force on the body in terms of a similarity parameter Fr/a1/2. For the sphere, we developed and applied a pressure-based criterion to follow the evolution of flow separation on the body and to obtain an appropriate description of the free-surface profile near the body and accurate evaluation of the impact pressure and load on the body during the entire impact process. The numerical result of impact force on the body agreed well with existing experimental measurements. We confirmed that the gravity effect is unimportant in initial impact of the sphere. Significantly, we found that in a later stage of impact, flow separation remains at an almost fixed position at an angle u 62.5 deg to the bottom of the sphere for a wide range of Froude numbers, Fr V/(gR)1/2 1, where R is the radius of the sphere.

scholarly journals Asymmetric impact between liquid and solid wedges

2013 ◽  
Vol 469 (2150) ◽  
pp. 20120203 ◽  
Author(s):  
Y. A. Semenov ◽  
G. X. Wu
Keyword(s):  
Free Surface ◽  
Velocity Potential ◽  
Mapping Function ◽  
Solution Procedure ◽  
Contact Angles ◽  
The Body ◽  
Surface Shape ◽  
Parameter Plane ◽  
Complex Velocity Potential ◽  
The Impact

The hydrodynamic problem of impact between a solid wedge and a liquid wedge is analysed. The liquid is assumed to be ideal and incompressible; gravity and surface tension effects are ignored. The flow generated by the impact is assumed to be irrotational and therefore can be described by the velocity potential theory. The solution procedure is based on the analytical derivation of the complex-velocity potential in a parameter plane and the function mapping conformally the parameter plane onto the similarity plane. The mapping function is found as a combination of the derivatives of the complex potential in the similarity and parameter planes, through the integral equations for mixed and homogeneous boundary-value problems in terms of the velocity modulus and the velocity angle with the fluid boundary, together with the dynamic and kinematic boundary conditions. These equations are solved through a numerical method. The procedure is first verified through comparisons with some known results. Simulations are then made for a variety of cases, and detailed results are presented in terms of the free surface shape, streamlines, pressure distribution on the wetted solid surface, and contact angles between the free surface and the body surface.

scholarly journals Water entry of a wedge with rolled-up vortex sheet

2017 ◽  
Vol 835 ◽  
pp. 512-539 ◽  
Author(s):  
Yuriy A. Semenov ◽  
G. X. Wu
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Body Surface ◽  
Vortex Shedding ◽  
Vortex Sheet ◽  
Water Entry ◽  
The Body ◽  
Surface Shape ◽  
Force Coefficients ◽  
Local Singularity

The problem of asymmetric water entry of a wedge with the vortex sheet shed from its apex is considered within the framework of the ideal and incompressible fluid. The effects due to gravity and surface tension are ignored and the flow therefore can be treated as self-similar, as there is no length scale. The solution for the problem is sought through two mutually dependent parts using two different analytic approaches. The first one is due to water entry, which is obtained through the integral hodograph method for the complex velocity potential, in which the streamline on the body surface remains on the body surface after passing the apex, leading to a non-physical local singularity. The second one is due to a vortex sheet shed from the apex, and the shape of the sheet and the strength distribution of the vortex are obtained through the solution of the Birkhoff–Rott equation. The total circulation of the vortex sheet is obtained by imposing the Kutta condition at the apex, which removes the local singularity. These two solutions are nonlinearly coupled on the unknown free surface and the unknown vortex sheet. This poses a major challenge, which distinguishes the present formulation of the problem from the previous ones on water entry without a vortex sheet and ones on vortex shedding from a wedge apex without a moving free surface. Detailed results in terms of pressure distribution, vortex sheet, velocity and force coefficients are presented for wedges of different inner angles and heel angles, as well as the water-entry direction. It is shown that the vortex shedding from the tip of the wedge has a profound local effect, but only weakly affects the free-surface shape, overall pressure distribution and force coefficients.

CFD Verification and Validation Study for a Captive Bullet Entry in Calm Water

2017 ◽  
Author(s):  
António Maximiano ◽  
Guilherme Vaz ◽  
Jule Scharnke
Keyword(s):  
Free Surface ◽  
The Body ◽  
Load Test ◽  
Verification And Validation ◽  
Vertical Force ◽  
Time Step ◽  
Pitch Moment ◽  
The Impact ◽  
Modelling Error ◽  
Quantities Of Interest

As a step towards complex impact loads cases, e.g. lifeboat drop tests or ship/platform slamming in waves, a verification and validation (V&V) study is carried out with an open-usage community based CFD code ReFRESCO for a simple impact load test case: a captive axisymmetric generic lifeboat shape (bullet) that penetrates the water surface at a constant velocity and angle of attack. The quantities of interest are the body fixed longitudinal force FX, vertical force FZ, and pitch moment MYY. The influence of the iterative convergence level, domain size and free surface modelling are investigated. Seven different grids and four time steps were used to assess the grid and time step sensitivity, in a total of 28 calculations. For the tested grids and time steps it was found that the results are more sensitive to the grid resolution than to the time step. The pressure distribution on the hull is correlated with the trends observed in the loads, and the relation between between relative and static pressure is found to be important for the calculated loads. An experimental test campaign was previously carried out by MARIN, and its results are used to validate the simulations performed. A very good match between experiments and simulations is found. A V&V study is performed for the quantities of interest at nine different time instants covering the impact phase. The numerical uncertainties are obtained from a solution verification procedure [1]. The experimental uncertainties are estimated, and a validation exercise carried out according to the ASME standards [2]. The outcome of the validation exercise is an estimated 95 % confidence interval for the modelling error, δM. For FX the modelling error is below 15 N, for 8 out of 9 time instants. For FZ the modelling error is below 14 N, except at the time instants where, due to vibrations in the experimental setup, a larger value (up to 23 N) is found. For MYY the modelling error is under 5N m. These results provide confidence in ReFRESCO for the simulation of free surface impact flows.

Water entry of rounded cylindrical bodies with different aspect ratios and surface conditions

2019 ◽  
Vol 863 ◽  
pp. 757-788 ◽  
Author(s):  
Nayoung Kim ◽  
Hyungmin Park
Keyword(s):  
Free Surface ◽  
Body Surface ◽  
Surface Deformation ◽  
Side Wall ◽  
Water Film ◽  
The Body ◽  
Cavity Formation ◽  
Surface Conditions ◽  
Aspect Ratios ◽  
The Impact

In the present study, we experimentally investigate water surface deformation due to the impact of rounded cylindrical projectiles with different aspect ratios (1.0–8.0). The subsequent jet and splash formation is closely related to the dynamics of an underwater cavity. To control the cavity formation, two kinds of surface conditions (smooth and rough) are applied to the front parts of the projectiles, and two impact speeds are considered. The Froude, Reynolds and Weber numbers are in the ranges of 32–90, $5\times 10^{4}{-}8.4\times 10^{4}$ and 1700–5000, respectively. When the front is smooth, the water film rises up along the body surface immediately after impact, and the temporal variation of its height is analytically estimated. The film converges at the rear pole to create an apex jet at lower aspect ratios and simply rises up and falls with the body at higher aspect ratios. The jets could be further distinguished as thin and thick jets, whose breakdown is found to be a function of the viscous force and surface tension, i.e. the Ohnesorge number. On the other hand, when the front is rough, the water film cannot rise up along the body surface, and instead early separation occurs to make the splash above a free surface. The splash size is quantified to assess the effects of the aspect ratio and impact speed. Upon splash formation, a cavity is created under the free surface, which emanates from the nose of the projectile. As the body sinks, the cavity pinch-off occurs due to the imbalance between the hydrostatic pressure and air pressure inside the cavity. At higher aspect ratios, cavity pinch-off occurs on the side wall of the projectile and leaves a portion of the cavity bubble on it. When the surface is smooth, no underwater cavity forms. Finally, we compare the hydrodynamic force acting on the sinking bodies with and without cavity formation, based on the underwater trajectory of each projectile. It is found that the underwater cavity reduces the drag force on the sinking body when it fully encapsulates the body; however, if the air bubbles are partially attached to the body after pinch-off, they tend to detach irregularly or impose additional drag on the body.

A wake source model for bluff body potential flow

1970 ◽  
Vol 40 (3) ◽  
pp. 577-594 ◽  
Author(s):  
G. V. Parkinson ◽  
T. Jandali
Keyword(s):  
Experimental Data ◽  
Pressure Distribution ◽  
Critical Points ◽  
Potential Flow ◽  
Bluff Body ◽  
Pressure Coefficient ◽  
Present Theory ◽  
Source Model ◽  
The Body ◽  
Body Potential

A theory is presented for two-dimensional incompressible potential flow external to a symmetrical bluff body and its wake. The desired flow-separation points are made the critical points of a conformal transformation to a complex plane in which surface sources in the wake create stagnation conditions at the critical points. The stagnation streamlines then transform to tangential separation streamlines in the physical plane, with separation at the desired pressure. The position and strength of the sources are determined by the requirements of separation position and pressure coefficient. The flow inside the separation streamlines is ignored and base pressure is assumed constant at the separation value. Features of the theoretical model include a finite wake width, a pressure distribution on the separation streamlines decreasing asymptotically towards the free stream value at infinity and a simple analytic expression for the pressure distribution on the body. Comparisons of the theory with experimental data and with other theories are presented for the normal plate, the circular cylinder, the 90° wedge, and the elliptical cylinder. Although simpler to apply than the other theories, the present theory produces at least as good agreement with the experimental data.

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