New Hull Form Design and Fairing Overview

Transcription

1 1 (6), Napa Ltd 1. Background In the old surface definition method in NAPA all curves in the grid have the same importance, in other words, they are used as a patch boundaries. When new curves are added, they affect the surface topology. The curve definition controls the patches in the surface and the user should understand both the lines design and the behaviour of surface patches. Especially in Napa Ltd s customer countries in Asia the hull surface is typically designed through the lines plan made of waterlines, buttocks and stations. On the contrary, this results in very many surface patches and unsuitable patch topology. Surface patches do not behave well if there are only three corners or more than four corners in a surface patch. This can cause problems in most cases where final production fairing is concerned. For almost the whole surface the curvature should be continuous, but this is practically impossible between the surface patches. Every patch edge essentially acts as a knuckle line as it is not possible to maintain tangency continuity along the entire length of the patch edge. 2. Requirements for the new method There have been two main reasons why Napa Ltd wants to introduce a totally new method to define the hull surface: 1. The general way of working in several of our customer countries is very different from our current hull surface design method 2. Importing of NAPA surface to general CAD systems should be well-supported with the new method Both of the reasons presented above lead to the following requirements: 1. The surface should be made of not more than a few tens of surface patches 2. One patch should be a continuous 3rd degree B-spline surface having continuous curvature within the patch 3. Neighbouring patches should have the same parameterisation for an easy check of surface continuity, for allowing manual editing of the surface and for avoiding numerical inaccuracies during the file transfer between systems 4. The number of control points should be minimized as much as possible 5. Flat areas, such as bottom, side and transom, should be described with surfaces that automatically guarantee the flat surface. Simple B-spline surface could also be used where the surface is 1 st order to one direction. 6. The surface is defined with the aid of a set of curves as before but there will be three curve types with different roles in the hierarchy 2009 Napa Ltd NAPA User Meeting 2009

2 2 3. Curve hierarchy Also in the new method the hull surface is defined with a grid of curves. The biggest difference is that the curves are divided in the hierarchy with three different types; primary, secondary and boundary curves. Of course, the Hull Surface Editor can be used also with the new method. The curve definition syntax is exactly the same as in the old definition method but the curve types are separated in the surface definition. For example: SUR, HULLF, P THR FRF, FBF, CLF, FSF, DECKF, STEM, KNF, FR12, FR13, FR14, FR15, FR16, FR17, FR18, FR19, FRB2, FRB3, FR12.5, FR19.75, FRB4, FRB5, NZ1, NZ2, NZ3, NZ3.5, NZ4, NZ5, NZ6, NZ7, NZ8, NZ9, NZ10, NZ11, NZ12, NZ16, NZ18, NZ.2, NY.1, NY.2, NY.5, NY1, NY4, NY3, NY6, NY8, NY10, NY12, NY14, NY15, NY15.5, NY15.8, NZ13.5, DF1, FRF1, DF2, NZ.05, NY15.95, FRF2, FR14.5, FRF1.1, FR13A, WLF1, FRF55, FRF56, FRF41, FRF57, FRF48, FRF49, FRF50, FRF52, FRF51, FRF53, FRF54, FRF46, WLF4, WLF5 BND KNF, DECKF, FRF, CLF, STEM, FBF, FSF, FR19, NZ9, FR13, FRF1, FRB3 SND FRF55, FRF56, FRF41, FRF57, FRF48, FRF49, FRF50, FRF52, FRF51, FRF53, FRF54, FRF46, FRF1.1, WLF4, WLF5, DF2, DF1, NY3, NY4, NY8, NY10, NY12, NY14, NY15, NY15.5, NY15.8, NY6, NY1, FR14.5 PO, BTOL=0.005, KTOL=2, K2TOL=2, ETOL=0.01, ERED=30, TTOL=0.002, NTOL=-0.5, TSC=0, WSC=0, G=2, IMAX=4 Command BND defines the boundary curves and command SND the secondary curves. The rest of the curves are primary ones. Command PO is related to the preparation options. 3.1 Primary curves Primary curves are those the user normally works with. Those are used to define the surface and edit its shape. The primary curves are topologically connected to each other and the final surface follows the primary curves exactly at the nodes.

3 3 Figure 1 Primary curves 3.2 Boundary curves A subset of primary curves is used as edges of the surface patches. Edges define the boundaries of the final B-spline surfaces. Figure 2 Boundary curves

4 4 3.3 Secondary curves Secondary curves are used to support the surface definition process and to interpolate the surface for B- spline surface fitting process. Secondary curves always refer to all primary and boundary curves but they do not refer to each other in general, but case by case the user may build this kind of reference when needed. Figure 3 Secondary curves Figure 4 Prepared surface shown with the equiparameter curves

5 5 4. Preparing the surface When the grid of curves is ready, the actual surface is created by preparing it the same way as with the old method. The biggest difference is in the preparation options with which the user can control the surface generation to result in the best possible B-spline surface. All the preparation options are explained in detail in the paper related to Workshop N1 in this User Meeting. 5. Fairing tools In case the surface preparation result is not perfectly smooth, NAPA has a new automatic surface fairing tool to give the surface that last finishing touch. As the new preparation produces a NURBS surface, the fairness of the surface can be improved by adjusting the control points. The fairing can be applied either to the whole surface or only to a part of it. The fairing results can be checked right away and the process can be repeated several times. The result of the fairing operation is a new NURBS surface which can be exported from NAPA e.g. to IGES or DXF file. The automatic fairing tool is included in the Hull Surface Editor but it can be used also in the Geometry Window. 6. Benefits of the new method The surface defined with the new method is a continuous and smooth B-spline surface with lots of benefits when compared to the old patch surface: The surface definition is more flexible because references between all the curves are not necessary anymore. The number of patches is much smaller which gives the user lots of freedom with the definition curves. Fairing the surface is easier and faster because of the flexible grid of curves, using of cross fairing and new automatic fairing tools and functions. Much better results in exporting surfaces from NAPA to other design softwares. 7. Future development The new hull definition method published in NAPA Release is the first step in the development. The method will be developed further and new functions and options will be included in the coming releases. At least the following issues are on the development list: End roundings for curves can be defined in the Hull Surface Editor without using command syntax. Tangent function can be defined along a curve by selecting the nodes of the curve to use and the angles at the nodes. Grid curve references are explicitly visible in the Hull Surface Editor. This makes it easy to understand the reference hierarchy and avoid cross-referencing. Improvements in the Hull Surface Editor

6 6 8. Compatibility with old NAPA Releases When the surface is defined with the new method, NAPA actually uses the same geometry twice and generates two surfaces, B-spline and patch surface. Patch surface can be used as such in the older NAPA Releases. Surface defined with older NAPA Releases can be used without any modifications in NAPA Release or it can be converted to the new method by defining the boundary curves. 9. More information More information about the new hull form definition method is available in Workshop N1 New NAPA Hull Form in this User Meeting NAPA Online Manuals Example project D-CONT091 (version A), installed automatically with NAPA Release