The Proceedings Fifth International Conference on Numerical Ship Hydrodynamics (1990) / Chapter Skim
Currently Skimming:
Recent Developments in a Ship Stern Flow Prediction Code
Recent Developments in a Ship Stern Flow Prediction Code
Pages 87-102
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 87... ...
The free surface disturbances created by the ship are assumed to be of little significance so that it is allowed to replace the undisturbed free surface by a symmetry plane. Thus, taking a reference frame moving with the ship, we consider actually the double model of the submerged part of a ship, held fixed in a uniform flow directed from bow to stern.
|
From page 88... ...
The contravariant formulation of the momentum equations is adopted, which gives three relations expressing momentum conservation along each of the three coordinate lines. The velocity components and the pressure are the dependent variables but a particular choice for the velocity variables is made as described in Section 3.
|
From page 89... ...
along the waterline when the frame line of the hull does not meet the symmetry plane (undisturbed free surface) at a right angle - but the mapping technique used excludes their occurrence in the interior of the domain.
|
From page 90... ...
84. However, if diffusion along the coordinate associated with the predominant flow direction is neglected, the equations exhibit Euler character in that direction with a consequent change in the boundary condition requirements.
|
From page 91... ...
Let us next turn to the boundary conditions on the external boundary. The old practice was to specify the pressure and the two tangential velocity components.
|
From page 92... ...
As a measure for the rate of convergence we use the rate of decay of the maximum pressure change between successive iterations anywhere in the computation domain. Besides two lines representing the results for the old and the new boundary conditions, respectively, a dashed line is shown indicating the maximum pressure change on the external boundary.
|
From page 93... ...
Notably in the initial phase of the calculations the source term may assume appreciable values, generating a significant overshoot in the velocity profiles or provoking flow separation. In the first case the turbulence model may show pathological behaviour in the determination of the outer length scale, which deteriorates the global convergence.
|
From page 94... ...
of downstream pressure changes resulting from the basic method. Thus we have incorporated the desirable feature of an infinite propagation speed of pressure influences in upstream direction (a property of the continuum equations)
|
From page 95... ...
Potential flow and boundary-layer calculations provided us with the non-trivial part of the boundary conditions. On the external boundary, conventional conditions (prescription of tangential velocity components and pressure from the potential flow)
|
From page 96... ...
8. Contour plots of axial velocity distributions 96
|
From page 97... ...
A preliminary exercise with vorticity boundary conditions on the external boundary indicated that the inclusion of viscous-inviscid interaction does not bring about either a change in the near-hull flow to the extent required for a reconciliation of measurements and predictions. Still to be investigated are the effects of grid refinement and of a variation in the turbulence model.
|
From page 98... ...
f ~~ ill_ ~ :-V~\ \ ~ \ ~ lilititt\Ntt ~ \ \ \ (3alc~llate:d at '2~/L -- 0.920 Measured at 2x/L = 0.926 rig. · ~ ~ ~ Calculated at 2x/L = 0.980 Measured at 2x/L = 0.977 Calculated at 2x/L = 1.020 Measured at 2~/L = 1.015 Fig.
|
From page 99... ...
Concluding remarks . In this paper we have given a detailed description of new developments in certain aspects of a computer code for the prediction of the stern flow field of a ship: · We have explained how coordinate-oriented velocity variables may be chosen so as to maintain a high numerical accuracy near the grid singularity in the wake; ~ We have presented some results of our first application of a new set of boundary conditions on the external boundary of the computation domain, which allow this boundary to be chosen close to the edge of the boundary layer and wake i We have shown how the convergence rate of the global relaxation of the solution may be improved by an easy algebraic update of the pressure field after every iteration cycle.
|
From page 100... ...
6. Hoekstra, M., "Coordinate generation in symmetrical interior, exterior and annular 2-D domains using a generalised Schwarz-Christoffel transformation", in Numerical Grid Generation in Computational Fluid Dynamics, (ea.
|
From page 101... ...
They do however inf luence the boundary condition requirements on the out l e t boundary.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.