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.

NUMERICAL SOLUTIONS OF 2D AND 3D SLAMMING PROBLEMS

2021 ◽  
Vol 153 (A2) ◽  
Author(s):  
Q Yang ◽  
W Qiu
Keyword(s):  
Free Surface ◽  
Numerical Solutions ◽  
Stokes Equations ◽  
Type Equation ◽  
The Body ◽  
Navier Stokes ◽  
Time Step ◽  
Navier Stokes Equations ◽  
Highly Nonlinear ◽  
2D And 3D

Slamming forces on 2D and 3D bodies have been computed based on a CIP method. The highly nonlinear water entry problem governed by the Navier-Stokes equations was solved by a CIP based finite difference method on a fixed Cartesian grid. In the computation, a compact upwind scheme was employed for the advection calculations and a pressure-based algorithm was applied to treat the multiple phases. The free surface and the body boundaries were captured using density functions. For the pressure calculation, a Poisson-type equation was solved at each time step by the conjugate gradient iterative method. Validation studies were carried out for 2D wedges with various deadrise angles ranging from 0 to 60 degrees at constant vertical velocity. In the cases of wedges with small deadrise angles, the compressibility of air between the bottom of the wedge and the free surface was modelled. Studies were also extended to 3D bodies, such as a sphere, a cylinder and a catamaran, entering calm water. Computed pressures, free surface elevations and hydrodynamic forces were compared with experimental data and the numerical solutions by other methods.

The Vertical Mean Force and Moment of Submerged Bodies Under Waves

1971 ◽  
Vol 15 (03) ◽  
pp. 231-245 ◽  
Author(s):  
C. M. Lee ◽  
J. N. Newman
Keyword(s):  
Free Surface ◽  
Added Mass ◽  
Cross Sectional Area ◽  
The Body ◽  
Slender Body ◽  
Longitudinal Distribution ◽  
Vertical Force ◽  
Sectional Area ◽  
Cross Sectional ◽  
The Mean

A neutrally buoyant slender body of arbitrary sectional form, submerged beneath a free surface, is free to respond to an incident plane progressive wave system. The fluid is assumed inviscid, incompressible, homogeneous and infinitely deep. The first-order oscillatory motion of the body and the second-order time-average vertical force and pitching moment acting on the body are obtained in terms of Kochin's function. By use of slender-body theory for a deeply submerged body, the final expressions for the mean force and the moment are shown to depend on the longitudinal distribution of sectional area and added mass and on the amplitude and the frequency of the ambient surface waves. The magnitude of the mean force for various simple geometric cylinders is compared with that of a circular cylinder of equal cross-sectional area. The mean force on a nonaxisymmetric body is often approximated by replacing the section with circular profiles of equivalent cross-sectional area. A better scheme of approximation is presented, based on a simple way of estimating the two-dimensional added mass. It is expected that the effect of the cross-sectional geometry on mean vertical force and moment will be more significant when the body is very close to the free surface.

Development of a Time Domain Boundary Element Method for the Seakeeping Behavior of a Cruise Ship Under Combined Strong Wind and Extreme Irregular Beam Seas

2014 ◽  
Author(s):  
G. D. Gkikas ◽  
F. van Walree
Keyword(s):  
Free Surface ◽  
Time Domain ◽  
Model Tests ◽  
The Body ◽  
Computational Method ◽  
Green Functions ◽  
Strong Wind ◽  
Cruise Ship ◽  
Time Step ◽  
Beam Seas

A computational method for the seakeeping behavior of a cruise ship at zero speed and under severe wind and oblique wave loads is presented. The proposed methodology is a time-domain panel method where the transient Green functions used for the estimation and implementation of the free surface effects on the vessel’s motions are estimated assuming constant low lateral speed, instead of the common practice zero speed influence functions. For the evaluation of the overall hydrodynamic forces, the so called “blended approach” is followed in the sense that the induced hydrodynamic pressures due to the scattering and radiation phenomena are calculated over the linearized position of the body, ignoring any displacements with respect to its mean position, while the hydrostatic and non-linear Froude-Krylov forces are considered at the actual body location and taking into account the free surface elevation at each time step. For the validation of the proposed methodology, heave and roll motions, the drift velocity as well as lateral accelerations of the vessel were investigated for two cases of severe beam seas combined with a constant strong wind load and the results were compared against experimental model tests. The model tests were performed to investigate the vessel’s behavior under extreme weather conditions. The low lateral speed Green functions were estimated for a speed similar to the one that the vessel was expected to drift, an estimation based on the model tests, as well as for the case where the input speed corresponded to the half of the expected speed. Good agreement was presented for both cases, showing that accurate and computationally efficient numerical simulations of the vessel’s motions under severe wind and wave excitations can be obtained by using low lateral speed transient Green functions.

Computational fluid dynamics study of human-induced wake and particle dispersion in indoor environment

2016 ◽  
Vol 26 (2) ◽  
pp. 185-198 ◽  
Author(s):  
Yao Tao ◽  
Kiao Inthavong ◽  
Jiyuan Tu
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Indoor Environment ◽  
Particle Dispersion ◽  
The Body ◽  
Wake Flow ◽  
Time Step ◽  
Directed Flow ◽  
Stagnant Region ◽  
The Impact

The impact of human-induced wake flow and particle re-dispersion from floors in an indoor environment was investigated by performing computational fluid dynamics simulations with dynamic mesh of a moving manikin model in a confined room. The manikin motion was achieved by a dynamic layering mesh method to update new grids with each time step. Particle transport from the floors and its re-dispersion was tracked by a Lagrangian approach. A series of numerical simulations of three walking speeds were performed to compare the flow disturbance induced by the walking motion. The significant airflow patterns included: an upward-directed flow in front of the body combined with a high velocity downward-directed flow at the rear of the body; a stagnant region behind the gap between the legs and counter-rotating vortices in the wake region. The airflow momentum induced by the moving body disturbed PM2.5 particles that were initially at rest on the floor to lift and become re-suspended due to its interaction with the trailing wake. The residual flow disturbances after the manikin stopped moving continued to induce the particle to spread and deposit over time. The spatial and temporal characteristics of the particle dispersion and concentration showed that higher walking speed was conducive to reducing human's exposure to contaminants in breathing region.

scholarly journals Energetically optimal running requires torques about the centre of mass

2012 ◽  
Vol 9 (73) ◽  
pp. 2011-2015 ◽  
Author(s):  
James R. Usherwood ◽  
Tatjana Y. Hubel
Keyword(s):  
Large Body ◽  
The Body ◽  
Moment Of Inertia ◽  
Mechanical Work ◽  
Vertical Force ◽  
Centre Of Mass ◽  
Pitch Moment ◽  
Tail Structure ◽  
Reaction Forces ◽  
Long Head

Bipedal animals experience ground reaction forces (GRFs) that pass close to the centre of mass (CoM) throughout stance, first decelerating the body, then re-accelerating it during the second half of stance. This results in fluctuations in kinetic energy, requiring mechanical work from the muscles. However, here we show analytically that, in extreme cases (with a very large body pitch moment of inertia), continuous alignment of the GRF through the CoM requires greater mechanical work than a maintained vertical force; we show numerically that GRFs passing between CoM and vertical throughout stance are energetically favourable under realistic conditions; and demonstrate that the magnitude, if not the precise form, of actual CoM-torque profiles in running is broadly consistent with simple mechanical work minimization for humans with appropriate pitch moment of inertia. While the potential energetic savings of CoM-torque support strategies are small (a few per cent) over the range of human running, their importance increases dramatically at high speeds and stance angles. Fast, compliant runners or hoppers would benefit considerably from GRFs more vertical than the zero-CoM-torque strategy, especially with bodies of high pitch moment of inertia—suggesting a novel advantage to kangaroos of their peculiar long-head/long-tail structure.

scholarly journals An assessment of the impact of special training of inspiratory muscles in a fitness classes programme on physical capacity of 20-25-year-old women

2015 ◽  
Vol 7 (4) ◽  
pp. 37-47 ◽  
Author(s):  
AGNIESZKA CYBULSKA ◽  
PAWEL DROBNIK
Keyword(s):  
Aerobic Capacity ◽  
Direct Method ◽  
Training Session ◽  
Physical Capacity ◽  
The Body ◽  
Load Test ◽  
Control Group ◽  
Special Training ◽  
Inspiratory Muscles ◽  
The Impact

Background: In the process of modernising and improving physical fitness classes on offer, there is a growing trend to complement them with extra means characterized by different directions of interactions. Training of respiratory muscles (TRM) is one of them used in sports training but also increasingly more often in health training. The aim of the study was to as- sess changes in physical capacity under the influence of 6 weeks' training of a different type of inspiratory muscles incorporated into the programme of fitness classes for women aged 20-25 years. Material/Methods: The study involved 33 not training professionally young women aged 20-25 years. To assess the capacity of the respiratory sytem, dynamic spirometry was performed with a use of the K4b2 Spirometry apparatus by Cosmed company, and inspiratory muscle strength was measured with a use of a respiratory pressure meter by Micro Medical. Aerobic capacity was evaluated based on a direct method, using an incremental load test until exhaustion. Measurements were taken before and after a training session throughout six weeks with a frequency of three sessions per week. Results: As a result of a six-week special training in the POWERbreathe group there was a significant increase in spirometric indices compared to the initial measurement: FVC l (BTPS), FEV1 l (BTPS), PEF (l·secˉ1), MVV (l·minˉ1), MIP (-mH2O). However, apart from the above-mentioned effects in the group of those training with bands also a significant increase in MEP (cmH2O) was noted compared to the control group. Conclusions: The special inspiratory muscles training programme improved the functionality of the respiratory system, which is revealed in the increase in indicators characterizing aerobic capacity (with no significant influence on changing the body composition). The results obtained in our study indicate the possibility of practical application of respiratory training simulators in health and recreational physical activities enabling the growth of aerobic capacity of those exercising.

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.

Time-Domain Simulations of Radiation and Diffraction Forces

2010 ◽  
Vol 54 (02) ◽  
pp. 79-94 ◽  
Author(s):  
Xinshu Zhang ◽  
Piotr Bandyk ◽  
Robert F. Beck
Keyword(s):  
Free Surface ◽  
Time Domain ◽  
Three Dimensional ◽  
The Body ◽  
Surface Boundary ◽  
Two Dimensional ◽  
Forward Speed ◽  
Time Step ◽  
Surface Boundary Conditions ◽  
Strip Theory

Large-amplitude, time-domain, wave-body interactions are studied in this paper for problems with forward speed. Both two-dimensional strip theory and three-dimensional computation methods are shown and compared by a number of numerical simulations. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength flat panels on the exact body surface, the boundary integral equations are solved numerically at each time step. The strip theory method implements Radial Basis Functions to approximate the longitudinal derivatives of the velocity potential on the body. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous changing wetted body geometry. Extensive results are presented to validate the efficiency of the present methods. These results include the added mass and damping computations for a Wigley III hull and an S-175 hull with forward speed using both two-dimensional and three-dimensional approaches. Exciting forces acting on a Wigley III hull due to regular head seas are obtained and compared using both the fully three-dimensional method and the two-dimensional strip theory. All the computational results are compared with experiments or other numerical solutions.

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.

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