Linear Dynamic Stability Analysis of a Surface Piercing Plate Advancing at High Forward Speed

2013 ◽  
Author(s):  
Babak Ommani ◽  
Odd M. Faltinsen
Keyword(s):  
Stability Analysis ◽  
Dynamic Stability ◽  
Domain Boundary ◽  
Vortex Sheet ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Hydrodynamic Coefficients ◽  
Forward Speed ◽  
Linear Distribution ◽  
Linear Dynamic

The dynamic stability of a surface-piercing plate advancing with high forward speed in horizontal plane is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface and a vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results are presented. Using linear dynamic stability analysis the influence of hydrodynamic coefficients on the plate’s stability is investigated as a simplified alternative to a semi-displacement vessel.

Linear Dynamic Stability Analysis of a Surface-Piercing Plate Advancing at High Forward Speed

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Babak Ommani ◽  
Odd M. Faltinsen
Keyword(s):  
Stability Analysis ◽  
Dynamic Stability ◽  
Domain Boundary ◽  
Vortex Sheet ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Hydrodynamic Coefficients ◽  
Forward Speed ◽  
Linear Distribution ◽  
Linear Dynamic

The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horizontal plane, is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed-dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time-domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface, and vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results is presented. Using linear dynamic stability analysis, the influence of hydrodynamic coefficients on the plate's stability is investigated as a simplified alternative to a semidisplacement vessel.

Dynamic Stability Response of Piggyback Pipelines

2010 ◽  
Author(s):  
Hammam Zeitoun ◽  
Masˇa Brankovic´ ◽  
Knut To̸rnes ◽  
Simon Wong ◽  
Eve Hollingsworth ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Dynamic Response ◽  
Dynamic Stability ◽  
Large Diameter ◽  
Hydrodynamic Forces ◽  
Model Testing ◽  
Equivalent Diameter ◽  
Pipeline System ◽  
Hydrodynamic Coefficients ◽  
Hydrodynamic Loads

One of the main aspects of subsea pipeline design is ensuring pipeline stability on the seabed under the action of hydrodynamic loads. Hydrodynamic loads acting on Piggyback Pipeline Systems have traditionally been determined by pipeline engineers using an ‘equivalent pipeline diameter’ approach. The approach is simple and assumes that hydrodynamic loads on the Piggyback Pipeline System are equal to the loads on a single pipeline with diameter equal to the projected height of the piggyback bundle (the sum of the large diameter pipeline, small diameter pipeline and gap between the pipelines) [1]. Hydrodynamic coefficients for single pipelines are used in combination with the ‘equivalent diameter pipe’ to determine the hydrodynamic loads on the Piggyback Pipeline System. In order to assess more accurately the dynamic response of a Piggyback Pipeline System, an extensive set of physical model tests has been performed to measure hydrodynamic forces on a Piggyback Pipeline System in combined waves and currents conditions, and to determine in-line and lift force coefficients which can be used in a dynamic stability analysis to generate the hydrodynamic forces on the pipeline [2]. This paper describes the implementation of the model testing results in finite elements dynamic stability analysis and presents a case study where the dynamic response of a Piggyback Pipeline System was assessed using both the conventional ‘equivalent diameter approach’ and the hydrodynamic coefficients determined using model testing. The responses predicted using both approaches were compared and key findings presented in the paper, in terms of adequacy of the equivalent diameter approach, and effect of piggyback gap (separation between the main line and the secondary line) on the response.

Particle encapsulation due to thread breakup in Stokes flow

2008 ◽  
Vol 617 ◽  
pp. 141-166 ◽  
Author(s):  
M. G. BLYTH ◽  
C. POZRIKIDIS
Keyword(s):  
Growth Rate ◽  
Stability Analysis ◽  
Stokes Flow ◽  
Normal Modes ◽  
Stability Diagram ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Spherical Particles ◽  
Particle Spacing ◽  
Particle Separations

The capillary instability of a liquid thread containing a regular array of spherical particles along the centreline is considered with reference to microencapsulation. The thread interface may be clean or occupied by an insoluble surfactant. The main goal of the analysis is to illustrate the effect of the particle spacing on the growth rate of axisymmetric perturbations and identify the structure of the most unstable modes. A normal-mode linear stability analysis based on Fourier expansions for Stokes flow reveals that, at small particle separations, the interfacial profiles are nearly pure sinusoidal waves whose growth rate is nearly equal to that of a pure thread devoid of particles. Higher harmonics suddenly enter the normal modes for moderate and large particle separations, elevating the growth rates and yielding a stability diagram that consists of a sequence of superposed pure-thread lobes. A complementary numerical stability analysis based on the boundary integral formulation for Stokes flow reveals the strong stabilizing effect of particles whose radius is comparable to the thread radius. Numerical simulations of the finite-amplitude motion based on the boundary integral method demonstrate that thread breakup leads to particles coated with annular layers of different thicknesses.

An Inverse Method for the Design of Bodies of Revolution by Boundary Integral Modelling

1991 ◽  
Vol 205 (2) ◽  
pp. 91-97 ◽  
Author(s):  
R I Lewis
Keyword(s):  
Inverse Method ◽  
Body Shape ◽  
Axisymmetric Flow ◽  
Vortex Sheet ◽  
Boundary Integral ◽  
The Body ◽  
Boundary Integral Method ◽  
Bodies Of Revolution ◽  
Final Design ◽  
Induced Flow

A surface vorticity boundary integral method is presented for the design of bodies of revolution in axisymmetric flow. The analysis finds the desired body shape to deliver a prescribed surface potential flow velocity or pressure distribution. To achieve this the body surface is simulated by a flexible vorticity sheet of prescribed strength. Starting from an arbitrary first guess for the body shape, normally an ellipsoid, the flexible vortex sheet is successively realigned with its own self-induced flow field during an iterative process which converges accurately onto the desired shape. A well-proven analysis method is also presented for back-checking the final design.

Numerical studies of two-dimensional Faraday oscillations of inviscid fluids

2000 ◽  
Vol 402 ◽  
pp. 1-32 ◽  
Author(s):  
JEFF WRIGHT ◽  
STEVE YON ◽  
C. POZRIKIDIS
Keyword(s):  
Integral Method ◽  
Density Ratio ◽  
Extended Period ◽  
Vortex Sheet ◽  
Boundary Integral ◽  
The Other ◽  
Boundary Integral Method ◽  
Two Dimensional ◽  
Inviscid Fluids ◽  
Mathieu’S Equation

The dynamics of two-dimensional standing periodic waves at the interface between two inviscid fluids with different densities, subject to monochromatic oscillations normal to the unperturbed interface, is studied under normal- and low-gravity conditions. The motion is simulated over an extended period of time, or up to the point where the interface intersects itself or the curvature becomes very large, using two numerical methods: a boundary-integral method that is applicable when the density of one fluid is negligible compared to that of the other, and a vortex-sheet method that is applicable to the more general case of arbitrary densities. The numerical procedure for the boundary-integral formulation uses a global isoparametric parametrization based on cubic splines, whereas the numerical method for the vortex-sheet formulation uses a local boundary-element parametrization based on circular arcs. Viscous dissipation is simulated by means of a phenomenological damping coefficient added to the Bernoulli equation or to the evolution equation for the strength of the vortex sheet. A comparative study reveals that the boundary-integral method is generally more accurate for simulating the motion over an extended period of time, but the vortex-sheet formulation is significantly more efficient. In the limit of small deformations, the numerical results are in excellent agreement with those predicted by the linear model expressed by Mathieu's equation, and are consistent with the predictions of the Floquet stability analysis. Nonlinear effects for non-infinitesimal amplitudes are manifested in several ways: deviation from the predictions of Mathieu's equation, especially at the extremes of the interfacial oscillation; growth of harmonic waves with wavenumbers in the unstable regimes of the Mathieu stability diagram; formation of complex interfacial structures including paired travelling waves; entrainment and mixing by ejection of droplets from one fluid into the other; and the temporal period tripling observed recently by Jiang et al. (1998). Case studies show that the surface tension, density ratio, and magnitude of forcing play a significant role in determining the dynamics of the developing interfacial patterns.

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