Creating Custom Shape Shells in FWsim with the Shape Shell Generator

Shape Shell Generator for FWsim — Templates, Tips, and TricksThe Shape Shell Generator for FWsim is a versatile tool designed to speed up the creation of geometric shells used in FWsim simulations. Whether you’re modeling thin-walled structures, aerodynamic surfaces, or custom membranes, this tool helps convert parametric shapes into discretized shells ready for analysis. This article covers available templates, step-by-step usage, advanced tips, and common pitfalls to help you get consistent, high-quality shells for your FWsim workflows.

What the Shape Shell Generator does

The Shape Shell Generator converts high-level geometric descriptions (parametric curves, primitives, or imported geometry) into finite-element-like shell representations compatible with FWsim. It typically generates:

• A discretized surface mesh of triangular or quadrilateral elements.
• Thickness and material property assignments for shells.
• Edge and vertex tagging for boundary conditions and constraints.
• Optional mid-surface extraction or offset surfaces for multi-layer shells.

Key benefit: it automates repetitive meshing and tagging tasks, letting you focus on physics setup rather than low-level geometry prep.

Templates — starting points to save time

Templates are pre-configured generator settings that match common use cases. Using templates speeds up model setup and promotes consistency across simulations.

Common template types:

• Basic Plate — uniform rectangular shell with user-set dimensions and mesh density. Good for bending/tension benchmarks.
• Circular Membrane — radial mesh with center constraint options. Useful for drum-like membranes and axisymmetric tests.
• Aerofoil Surface — parametric airfoil profile with spanwise division and controlled chordwise mesh grading. Use for aerodynamic shell approximations.
• Tubular/Annulus — concentric ring templates for pipes or pressure vessels; supports inner/outer boundary conditions and seam definitions.
• Custom Imported Shell — settings optimized for converting CAD surfaces (NURBS) into FWsim shells (defines tolerances, smoothing, and remeshing parameters).

Each template typically presets:

• Element type (tri/quad)
• Target element size and grading
• Smoothing and curvature-based refinement thresholds
• Default thickness/material assignment
• Edge labeling scheme (e.g., boundary, free, fixed)

Use templates as a launch point, then tweak parameters for your particular physics or mesh quality targets.

Workflow: step-by-step guide

1. Choose a template or start from scratch.
2. Import or define the geometry:
   • Parametric curves, primitives, or load CAD (STP/IGES) surfaces.
3. Set global mesh parameters:
   • Target element size, anisotropy controls, and element type.
4. Configure refinement rules:
   • Curvature-adaptive refinement, boundary layers, and user-defined regions.
5. Assign thickness and material properties:
   • Uniform thickness or spatially varying fields; associate material IDs for FWsim.
6. Tag edges and vertices:
   • Name boundaries for later BC/specification in FWsim (e.g., clamp_edge, symmetry_plane).
7. Preview and quality-check:
   • Skew, aspect ratio, min/max angle, and Jacobian metrics.
8. Export to FWsim format:
   • Ensure tags map to FWsim boundary conditions and material definitions.
9. Run a quick verification simulation:
   • Light static or modal check to confirm expected behavior before full runs.

Mesh quality: practical tips

• Target element size relative to geometry curvature: use element edge length ≈ radius_of_curvature / 4 for smooth curvature capture.
• Avoid abrupt grading transitions — limit size ratio between neighboring elements to ≤ 1.5–2.0.
• Prefer quad-dominant meshes on shell-like surfaces when bending accuracy is important; triangles are fine for complex topology but often need more refinement.
• Use curvature-based refinement near high curvature or load-concentration regions (holes, fillets, sharp edges).
• Check element aspect ratio and minimum angle; aim for angles between 30°–120° where possible.
• For thin shells, ensure at least 3–4 elements across thickness or use appropriate shell formulation in FWsim if single-layer shell is assumed.

Thickness and material assignment strategies

• Uniform thickness: simplest and works for many problems.
• Spatially varying thickness: define via analytic function or scalar field when shells vary across the surface (useful for tapered panels).
• Multi-layer shells: model composite layups by stacking multiple shell layers or using an equivalent single-layer with homogenized properties.
• Map material IDs by regions or via vertex weight fields to simplify batch assignment in FWsim.

Boundary conditions and tagging best practices

• Use semantic tags: clamp_edge, roller_edge, symmetry_plane, load_region — human-readable names reduce setup errors.
• Tag both edges and adjacent faces where needed; FWsim mappings depend on expected input.
• Define small transitional regions for applied loads rather than point loads on single nodes to avoid stress singularities.
• For periodic or cyclic models (e.g., blades), tag seam edges consistently and ensure node ordering matches FWsim’s periodic constraints.

Automation and scripting

• Use the generator’s scripting API (if available) to:
  • Batch-generate shells across parameter sweeps (vary thickness, mesh density, geometry scale).
  • Enforce company-wide templates and naming conventions.
  • Integrate shell generation into CI pipelines for regression testing of simulation setups.
• Example automation tasks:
  • Auto-generate aerofoil shells for multiple Reynolds number cases.
  • Create parametric test samples for validation (rectangular plates with varying aspect ratio/thickness).

Common pitfalls and how to avoid them

• Poor element quality near CAD defects — heal geometry first (small gaps, flipped normals).
• Over-refinement — leads to long solve times with marginal accuracy gain. Balance between physics needs and computational cost.
• Incorrect tag mapping — verify that exported tag names exactly match FWsim’s BC/material references.
• Thin-shell assumptions violated — if through-thickness effects matter, consider 3D solid modeling or layered shells with appropriate constitutive models.
• Ignoring symmetry — modeling only a sector can massively reduce run times if loads and geometry allow.

Debugging checklist

• Visual check: normals orientation, duplicated faces, tiny edges.
• Quality metrics: min angle, skew, Jacobian; fix or remesh problem areas.
• Tag verification: export a tag map and compare against FWsim input file references.
• Small sanity-run: run a low-resolution static or modal test to uncover unexpected constraints or flipped normals.
• Compare to analytical benchmarks (e.g., cantilever plate deflection) when possible.

Example use cases

• Aerospace: create wing shell approximations for aeroelastic coupling studies.
• Automotive: thin panels and crash-related shell preprocessing.
• Civil: membrane and thin-panel roof modeling.
• Research: parametric studies of shell stability and buckling using automated template sweeps.

Final notes

Templates and sensible defaults dramatically reduce setup time and errors. Combine visual inspection, automated quality metrics, and lightweight verification runs to ensure shells behave as expected in FWsim. Over time, refine templates to capture domain-specific needs (e.g., composite layups, periodic seams) and automate repetitive tasks with scripts.