Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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A Method for the Optimization of Ship Hulls from a Resistance Point of View
A Method for the Optimization of Ship Hulls from a Resistance Point of View
Pages 680-696
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From page 680... ...
Hi 680 (k) Total resistance Friction resistance Viscous pressure resistance Wave resistance Design variable, master variable Upper bound for the design variable Lower bound for the design variable Objective function Constraint function Approximating function Bounds for the constraint Number of constraints Number of design variables Upper moving asymptote Lower moving asymptote Lagrange function Dual objective function Lagrange multiplier Iteration number
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From page 681... ...
The second zone is a thin layer at the hull surface and a boundary layer method of the momentum integral type is used. The momentum integral equations are solved along streamlines traced from the potential flow solution.
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From page 682... ...
Since the velocity in the normal direction must be much smaller than the tangential velocities the equation for the normal velocity degenerates to the statement that the pressure is constant across the boundary layer. If the boundary layer is thin, it disturbs the potential flow very little, so the potential flow pressure distribution over the hull surface may be used for He pressure across the layer.
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From page 683... ...
and (23~. 2.4 Computation of resistance There are three types of resistance components that can be computed in the present method, wave resistance, friction resistance and viscous pressure resistance.
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From page 684... ...
A dual objective function can be defined by minimizing the Lagrange function with respect to the design variables.
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From page 685... ...
are the coordinates of the original position and A, pyj, ,Bzj are linking factors between the offset-point and each of the master variables. Once the hull surface is defined, grids suitable for the potential flow calculation, the boundary layer calculation and the Navier-Stokes calculation may be generated using grid generators developed for each computational method, figure 4, 5 and 6.
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From page 686... ...
Sinkage and trim effects were not included. The hull was optimized with respect to the total resistance including wave resistance from a linear potential-flow solution and viscous resistance from boundary layer and Navier-Stokes solutions.
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From page 687... ...
The computed total resistance for the optimized hull was 1 loo less than for the original hull when the larger master variable range was used and To less for the smaller. Hull modifications outside the hull main dimensions were allowed for the larger range while the modifications were kept inside the main dimensions for the small range.
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From page 688... ...
00000 `, n 0 2 ~ 6 8 10 ITERATION HISTORY Fig. 10 Iteration history for the optimization A - Total resistance B- Friction resistance C - Viscous pressure resistance D - Wave resistance The second case where the hull modifications were kept inside the main dimensions was selected for the discussion in the following sections.
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From page 689... ...
The resistance measurements for the model free condition, figure 12, did not verify the resistance reduction obtained in the computations. Based on the computations the increased resistance at low speeds was expected due to the higher viscous pressure resistance and the larger form factor for the optimized hull, but the residuary resistance was expected to be much lower at speeds close to the Froude number 0.316.
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From page 690... ...
To make it possible to add the displacement thickness also at parts covered by the wave in the nonlinear computations, a boundary layer computation was performed for a draught larger than the design draught. The potential flow computations were then repeated using the modified hull shapes and the finest grid.
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From page 691... ...
Non-linear free-surface boundary conditions in the potential-flow solution may also improve the results. The routine for hull modifications must be improved to ensure hull forms of practical use.
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From page 692... ...
and Cong L.Z., "Optimal Ship Forms for Minimum Total Resistance With the Consideration of Boundary Layer and Wake," 5th International SYmcosium on the Practical Desian of Ships and Mobile Units9 Newcastle upon Tyne9 UK, Vol.
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From page 693... ...
and Uberoi, S.B.S., "A Study of Wave Resistance Characteristics Through the Analysis of Wave Height and Slope Along a Longitudinal Track," Report No. Hy-15, 1971, fIydro og Aerodynamisk Laboratorium, Lyngby, Denmark.
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From page 694... ...
This, however, requires that viscous effects and nonlinear wave effects are of minor importance and that the derivatives of the wave resistance with respect to the shape modifications are close to the derivatives computed by more accurate methods. In the paper, both fore body and aft body modifications were included and the total resistance was considered for the optimization.
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From page 695... ...
But before going to such extreme calculation effort, one should not neglect to examine approximate ways for indirectly, perhaps semiempirically, allowing for viscous effects including trim and linkage. AUTHORS' REPLY Thank you for pointing out the importance of sinkage and trim and the difference in sinkage and trim between potential flow and real fluid flow.
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From page 696... ...
The authors show great courage to present the rather disappointing results of Fig 12, where the measured resistance of the original hull is below the corresponding one of the optimized hull. The above figure would greatly gain in clarity if the authors would add the computational results for the total resistance and its components for the original and the optimized hull.
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