Geometric Model Preparation

Learn the best practices for developing a geometric model surface mesh that is compatible with ultraFluidX.

These practices help make a model that is easy to analyze while maintaining the required model fidelity for accuracy. They are common to all types of analyses (external aerodynamics, aeroacoustics, and thermal management). The chapter topics are as follows:
  • Grouping of Parts
  • Watertight Cabin
  • Powertrain Modeling
  • Front End Airflow /Chassis / Other External Modeling
  • Simulation Model Assembly and Check
  • Component Naming Convention
  • Cut and Stitch

Key Points

For accurate simulations correctly representing the geometry is the primary criteria for geometry creation and further preparation. The most important points to keep in mind are as follows:
  1. Capture details of sensitive regions:
    1. There are three types of geometry which have their own requirements for preparation:
      1. Flow-critical surfaces, such as the vehicle exterior in an external aerodynamic simulation, require the highest quality surfacing and meshing
      2. Non-flow-critical surfaces, such the vehicle underbody, suspension and powertrain components, do not require the highest quality surfacing and meshing
      3. Refinement zone surfaces, these surfaces are used to define areas of resolution and tolerate the lowest quality
    2. The geometry must exactly represent the intent of the simulation
    3. Seemingly small details can have significant effects on the results
    4. Oversimplification is not necessary, even if detailed effects are not desired
  2. Keep modeled geometry watertight:
    1. Cabin and powertrain and other part should not have leaks or at least not have gaps bigger than intended refinement size
    2. Make sure desired parts are not missing
    3. Properly close out gaps and patch bigger holes
  3. Mesh quality requirements/checks:
    1. All surface mesh normal must face outward
    2. Baffle components should have thickness of 0.1 mm or should be organized properly
    3. Chordal deviation is a good measure of mesh quality:
      1. Fine mesh is required in areas of high curvature
      2. Coarser mesh can be used in large, flat areas
    4. No extremely high aspect ratio elements (> 100), folded elements

Meshing Considerations

Before starting to create and prepare geometry for a simulation, consider the following:
  • All surface normal must face outward
  • Baffle components should have thickness of 0.1mm. Do not need to be watertight
  • Common Coordinate System for components is required
  • CAD units need to be in meters
  • For closed bodies and shells, remove the internal components
  • If any components have been used in previous simulations, reuse them
  • If the input geometry is incomplete (for example, early stage design), use or adapt production geometry from previous or similar platforms
  • You can create new parts by free-form surface design using Inspire Studio or equivalent software
  • It is acceptable to convert scanned data to meshes
  • In all cases, if any needed components have similar parts that have been used before, morph the existing meshes with HyperMesh rather than redoing the entire process

Element Quality

Visually inspect the mesh to detect regions where the mesh can be improved. Look for shape quality and smooth-shaded model appearance. Review each component separately, and hide element edges to look for discontinuities, rotating the model to catch the light from different angles.

Unacceptable areas include:
  • Elements that stick out visually
  • A mesh that looks irregular in certain spots
  • Extremely dense regions
You might need to increase the element refinement for critical areas in components such as:
  • Leading edge of the hood
  • Side mirrors
  • Areas of pressure-induced separation
  • Regions that are likely to be morphed
Within each component, check for the following:
  • Overlaps — Elements of the same shell must not overlap.
  • Wrinkles or near overlaps — To detect these problems, give elements an artificial thickness of about 0.01 mm and find the elements that intersect this volume.
  • Aspect ratio or small angles (< 1°) — Although this is not mandatory for aerodynamics, it might help point out trouble spots.
  • Unexpected free or multi-bounded edges

Note that two closed shells can intersect because they each represent a solid. This is useful, for example, if the vehicle body is a closed shell and a mirror (also a closed shell) is introduced later. Here, the mirror can (and should) intersect the vehicle body. If they did not completely intersect, fluid could travel between the mirror and vehicle body.

If there are relatively few elements that do not meet the quality criteria, you could make minor manual adjustments to the mesh, if they can be done in a reasonable amount of time. If there are more significant problems, or if there are too many areas to manually correct in a reasonable time amount of time, it might be necessary to re-mesh.

Common Mistakes

Common mistakes in automotive applications include the following:
  • Size and position of the wheels and tires
  • Vehicle posture, position, and ride height
  • Incorrect representation of gaps and holes
  • Oversimplification of geometry, such as not including many of the under-hood components
  • Missing or misplaced parts
  • Incorrect fillets/radii and or bridging from wrapping in flow critical areas
  • When comparing to physical wind tunnel tests:
    • The virtual simulation must match the physical simulation regarding model geometry and wind tunnel boundary conditions
    • Dynamic pressure must be calculated equivalent to the wind tunnel
  • When comparing between two models:
    • The simulation models must be identical except for the areas of comparison