Numerical Evaluation of Free-Surface Green Functions

1994 ◽  
Vol 38 (03) ◽  
pp. 193-202
Author(s):  
B. Ponizy ◽  
F. Noblesse ◽  
M. Ba ◽  
M. Guilbaud
Keyword(s):  
Coordinate Transformation ◽  
Near Field ◽  
Finite Domain ◽  
Diffraction Radiation ◽  
Green Functions ◽  
Offshore Structure ◽  
Local Flow ◽  
Ship Waves ◽  
Function Transformation ◽  
Singular Functions

A very simple and efficient method for computing the nonoscillatory near-field terms in the expressions for the Green functions, and their gradients, for wave diffraction/radiation by an offshore structure and steady ship waves in deep water is presented. The Green functions are decomposed into three terms corresponding to simple (Rankine) singularities, wave fields, and nonoscillatory near-field (local) flow components. The method which is presented for approximating the latter nonoscillatory near-field components is based on the use of a coordinate-transformation and a function-transformation. The coordinate-transformation maps the unbounded domain of definition of the Green function into a finite domain (unit square or cube) of transformed coordinates. The function-transformation expresses the near-field components, which are singular at the origin, in terms of functions that are regular everywhere. Proper coordinate and function transformations reduce the problem of approximating singular functions in unbounded domains into that of approximating smoothly varying functions within finite domains. The latter task can be accomplished in a number of ways, including the use of linear table interpolation presented in the study.

Rankine and Fourier-Kochin Representation of Near-Field Ship Waves

2002 ◽  
Vol 46 (01) ◽  
pp. 63-79
Author(s):  
Francis Noblesse
Keyword(s):  
Free Surface ◽  
Water Waves ◽  
Near Field ◽  
Boundary Surface ◽  
Diffraction Radiation ◽  
Offshore Structure ◽  
Ship Waves ◽  
Time Harmonic ◽  
The Mean ◽  
Elementary Waves

New fundamental analytical representations of the near-held potential how that corresponds to a given how at a surface bounding a potential-how region are given for three classes of free-surface hows in deep water: diffraction-radiation of regular water waves by an offshore structure, steady ship waves, and time-harmonic ship waves (diffraction-radiation with forward speed). These near-held how representations, called Rankine and Fourier-Kochin representations, define the how in terms of distributions of Rankine singularities and Fourier-Kochin distributions of elementary waves over the boundary surface and its intersection with the mean free surface. The Rankine and Fourier-Kochin near-held how representations involve only simple ordinary functions. These how representations extend the previously given Fourier-Kochin representations of waves.

Near-field mean flow dynamics of a cylindrical canopy patch suspended in deep water

2018 ◽  
Vol 858 ◽  
pp. 634-655 ◽  
Author(s):  
Jian Zhou ◽  
Subhas K. Venayagamoorthy
Keyword(s):  
Deep Water ◽  
Near Field ◽  
Flow Dynamics ◽  
Flow Diversion ◽  
Circular Cylinders ◽  
Published Data ◽  
Global Flow ◽  
Mean Flow ◽  
Near Wake ◽  
Local Flow

The time-averaged flow dynamics of a suspended cylindrical canopy patch with a bulk diameter of $D$ is investigated using large-eddy simulations (LES). The patch consists of $N_{c}$ constituent solid circular cylinders of height $h$ and diameter $d$, mimicking patchy vegetation suspended in deep water ($H/h\gg 1$, where $H$ is the total flow depth). After validation against published data, LES of a uniform incident flow impinging on the canopy patch was conducted to study the effects of canopy density ($0.16\leqslant \unicode[STIX]{x1D719}=N_{c}(d/D)^{2}\leqslant 1$, by varying $N_{c}$) and bulk aspect ratio ($0.25\leqslant AR=h/D\leqslant 1$, by varying $h$) on the near-wake structure and adjustment of flow pathways. The relationships between patch geometry, local flow bleeding (three-dimensional redistribution of flow entering the patch) and global flow diversion (streamwise redistribution of upstream undisturbed flow) are identified. An increase in either $\unicode[STIX]{x1D719}$ or $AR$ decreases/increases/increases bleeding velocities through the patch surface area along the streamwise/lateral/vertical directions, respectively. However, a volumetric flux budget shows that a larger $AR$ causes a smaller proportion of the flow rate entering the patch to bleed out vertically. The global flow diversion is found to be determined by both the patch geometrical dimensions and the local bleeding which modifies the sizes of the patch-scale near wake. While loss of flow penetrating the patch increases monotonically with increasing $\unicode[STIX]{x1D719}$, its partition into flow diversion around and beneath the patch shows a non-monotonic dependence. The spatial extents of the wake, the flow-diversion dynamics and the bulk drag coefficients of the patch jointly reveal the fundamental differences of flow responses between suspended porous patches and their solid counterparts.

Second-Order Difference-Frequency Loads on FPSOs by Full QTF and Relevant Approximations

2020 ◽  
Author(s):  
Espen Engebretsen ◽  
Zhiyuan Pan ◽  
Nuno Fonseca
Keyword(s):  
Near Field ◽  
Time Domain Analysis ◽  
Second Order ◽  
Difference Frequency ◽  
Irregular Waves ◽  
Mooring Line ◽  
Diffraction Radiation ◽  
Surface Integral ◽  
Order Difference ◽  
Order Diffraction

Abstract This paper investigates three different approximations of the full Quadratic Transfer Function (QTF) for calculating horizontal plane second-order difference-frequency loads on FPSOs, namely Newman’s approximation, full QTF without free surface integral and the white-noise approximation. Second-order excitation loads obtained from approximated QTFs are compared in frequency-domain with those obtained by the full QTFs computed from second-order diffraction/radiation analysis in WADAM. The comparison is performed for a new-build FPSO in a range of water depths and environmental combinations. The full QTFs from second-order diffraction/radiation analysis are further compared to empirical QTFs as identified from cross bi-spectral analysis of model test results in irregular waves. A mesh convergence study is presented for calculating full QTFs by the near-field approach in a second-order diffraction/radiation analysis. The importance of including viscous damping in heave, roll and pitch is illustrated for the mean wave-drift force in surge and sway. FPSO motions and mooring line tensions from fully-coupled time-domain analysis in OrcaFlex is compared when using approximated QTFs and full QTFs from second-order diffraction/radiation analysis.

scholarly journals Far- and near-field approximation for diffraction radiation

2013 ◽  
Vol 309 ◽  
pp. 194-197
Author(s):  
V. Shpakov ◽  
S.B. Dabagov ◽  
M. Castellano ◽  
A. Cianchi ◽  
K. Honkavaara ◽  
...  
Keyword(s):  
Near Field ◽  
Field Approximation ◽  
Diffraction Radiation

scholarly journals Assessment of impinged flame structure in high-pressure direct diesel injection

2019 ◽  
Vol 21 (2) ◽  
pp. 391-405 ◽  
Author(s):  
Zhihao Zhao ◽  
Xiucheng Zhu ◽  
Jeffrey Naber ◽  
Seong-Young Lee
Keyword(s):  
Soot Formation ◽  
Near Field ◽  
Flame Structure ◽  
Combustion Process ◽  
Injection Pressure ◽  
Flux Measurement ◽  
Air Entrainment ◽  
Mie Scattering ◽  
Local Flow ◽  
Spray Impingement

Spray impingement often occurs during cold-start in direct-injection diesel engines, affecting the subsequent combustion process by altering the local flow condition. This work has investigated the impinged flame structure by examining local expansion distance and planar curvature of the boundary in details. The experiments were carried out in a constant volume combustion chamber. The injection pressure and ambient density were varied from 120 to 180 MPa and 14.8 to 30.0 kg/m3 under non-vaporizing conditions, respectively. For reacting conditions, the injection pressure and ambient density were fixed at 150 MPa and 22.8 kg/m3 but with different ambient temperatures from 800 to 1000 K. Unlike orthogonal spray impingement, the profile of expansion distance along the radial direction at the 60° impinging angle is non-uniform but the profile is comparable between the non-vaporizing and reacting conditions under the same injection pressure and ambient density. With the help of Intensity-aXial-Time method, the most intensive soot luminosity region and Mie scattering intensity region are identified and the region has been found to be along the impinged spray axial direction. Outmost boundary of an impinged flame is found to have wrinkles attributed to air entrainment. The temporal level of flame wrinkles is higher in reacting conditions than in non-vaporizing conditions. The scatter distribution of the boundary curvature and near-field soot formation illustrates an inverted “S” shape correlation with time. High flame luminosity is found to be formed in concave regions while less soot is formed in convex regions. This inverted S-shape is a new finding of the state relationship at the solid–liquid–gas impinged flame propagation. Finally, heat flux measurement through the plate is examined.

High Resolution PIV and CH-PLIF Measurements and Analysis of a Shear Layer Stabilized Flame

2015 ◽  
Author(s):  
C. W. Foley ◽  
I. Chterev ◽  
J. Seitzman ◽  
T. Lieuwen
Keyword(s):  
High Resolution ◽  
Shear Layer ◽  
Flow Velocity ◽  
Near Field ◽  
Flame Stabilization ◽  
Flow Conditions ◽  
Mean Flow ◽  
Local Flow ◽  
High Bulk ◽  
Flow Velocities

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical towards the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous PIV and CH-PLIF measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.

Scattering by a surface inhomogeneity on an elastic half-space, with application to fluid-structure interaction noise

1990 ◽  
Vol 429 (1876) ◽  
pp. 203-226 ◽  
Keyword(s):  
Turbulent Flow ◽  
Half Space ◽  
Near Field ◽  
Elastic Solid ◽  
Viscous Sublayer ◽  
Elastic Half Space ◽  
Diffraction Radiation ◽  
Frequency Spectra ◽  
Low Mach Number ◽  
Surface Inhomogeneity

A harmonic point source is situated in fluid bounded by a nominally plane interface with an elastic half-space. The source is close to a small protrusion of the elastic medium into the fluid, and it is required to determine the interaction (‘diffraction’) radiation, i. e. the acoustic, elastic-body and surface (Scholte) waves produced by the scattering of the near field of the source by the protrusion. The solution of this canonical problem is applied to the prediction of acoustic and structural noise generated by low Mach number turbulent flow over an inhomogeneity on the boundary of an elastic solid. Estimates are presented of the frequency spectra of the power delivered to the various wave modes and their dependence on the elastic properties of the solid, and a comparison is made with empirical predictions of excitation of the same modes in the absence of the inhomogeneity. The scattered radiation can be significant even when the surface inhomogeneity does not penetrate beyond the viscous sublayer into the turbulent flow.

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