Mastering Simulation Troubleshooting
A comprehensive, interactive guide based on best practices for identifying, diagnosing, and resolving the most common pitfalls in finite element and computational fluid dynamics simulations.
The Methodical Approach
Simulating physical reality on a computer is an inherently complex task involving approximations, numerical methods, and idealized boundary conditions. When a simulation fails—whether it crashes, takes an eternity to solve, or produces physics-defying results—panic is the enemy. The key to resolution is a structured, methodical diagnostic process.
This application is designed to walk you through that process. We categorized the most common issues into distinct areas: the foundation (Meshing), the mathematics (Convergence), the physics (Results), and the software's communication (Diagnostics).
Meshing Errors: The Foundation
Addressing reports of bad elements, gaps, and geometry issues.
The mesh is the discretization of your physical domain. It translates continuous geometry into a finite number of equations the solver can handle. If PoligonSoft (or any solver) reports "bad elements," "negative Jacobians," or "unclosed volumes," your simulation is dead on arrival.
1. The Symptoms of a Bad Mesh
When you encounter a meshing failure, the error logs will typically point to specific metric violations. Understanding these metrics is crucial:
- Aspect Ratio: The ratio of the longest dimension of an element to its shortest. Highly stretched elements (high aspect ratio) lead to ill-conditioned stiffness matrices.
- Skewness: How far the element deviates from an ideal shape (e.g., an equilateral triangle or a perfect cube). High skewness degrades interpolation accuracy.
- Jacobian Ratio: A measure of element distortion. A negative Jacobian indicates the element has folded in on itself, making the volume calculations mathematically impossible.
2. The Root Cause: It's Usually the CAD
While meshing algorithms have improved, they cannot magically fix terrible geometry. 90% of meshing errors originate from the imported CAD model. Before tweaking mesh controls, you must interrogate the geometry.
Mandatory Geometry Checks
Check for Slivers: Extremely narrow faces or tiny edges created during complex Boolean operations in CAD. The mesher tries to place elements on these features, resulting in highly skewed, microscopic elements.
Check for Interferences/Overlaps: In assemblies, parts that mathematically intersect (even by 0.001mm) will cause the mesher to fail when trying to create a conformal mesh across the boundary.
Check for Gaps: Surfaces that look connected visually but have a microscopic gap. The mesher may try to treat them as separate boundaries, leading to disconnected fluid domains or structural bodies.
3. Resolution Strategy
When Poligon reports meshing errors, follow this workflow:
- Locate the Error: Most software allows you to highlight the specific elements that failed criteria. Look at where they are located.
- Return to CAD / Repair Tools: If the bad elements are clustered around a tiny, irrelevant chamfer or a sliver face, remove that feature. Use "defeaturing" tools or Virtual Topology to merge small faces.
- Local Refinement: If the geometry is valid but highly curved, apply local mesh sizing (refinement) to that specific area to allow the mesher to capture the curvature without distorting the elements.
- Switch Element Types: If tet-meshing fails on a thin-walled structure, consider extracting the mid-surface and using shell elements instead.
Convergence & Runtime Problems
When simulations crash, diverge, or take infinite time to solve.
You have a good mesh, but the solver starts spitting out numbers, and suddenly it crashes, or the residuals flatline and never reach the target. This is a convergence failure. It means the mathematical solver (typically Newton-Raphson for non-linear problems) cannot find a balanced solution.
Diagnostic Tool: Solver Residuals Tracker
Interact with the options below to compare the behavior of a healthy solving process versus one experiencing convergence failure due to instability.
The Geometry/Mesh Factor
Simplify Geometry: Every degree of freedom (node) adds to the computational matrix. If a simulation takes too long, are you modeling threads on a bolt when a simple cylinder with a bonded contact would suffice? Remove non-structural or non-flow-critical details (fillets, small holes, logos).
Refine Mesh (Strategically): Paradoxically, a mesh that is too coarse in areas of high gradient (stress concentrations, boundary layers) can cause convergence failure because the element cannot capture the steep change in values. Refine the mesh locally where gradients are highest, not globally.
Boundary Condition Impossibilities
The most common cause of sudden divergence is asking the physics engine to do something impossible.
- Point Loads: Applying a massive force to a single node creates a mathematical singularity (infinite stress). Distribute loads over faces.
- Over-constraining: Fixing too many faces can prevent a structure from naturally deforming (e.g., thermal expansion), leading to artificially massive internal stresses that crash the solver.
- Rigid Body Motion: In statics, if a part is not fully constrained (it can fly off into space), the stiffness matrix becomes singular. Ensure all 6 degrees of freedom are accounted for.
Unexpected Results
When the simulation finishes, but the output contradicts reality.
This is the most dangerous failure mode. The software gives you a pretty color plot, and if you aren't skeptical, you might base engineering decisions on garbage data. Always apply the "Sanity Check."
Verify Units (The classic error)
Did you import CAD in millimeters but set the solver to meters? A pressure applied in Pascals on a geometry scaled up by 1000x will yield virtually zero deformation. Always check reaction forces and total volume to verify scale.
Check Material Data
Are you using a linear elastic material model for a rubber component undergoing 200% strain? The solver will output numbers, but they will be completely divorced from reality. Verify Yield strength, Young's Modulus, and non-linear curves.
Case Study: The "Silly Simulation" Indicator
Injection Molding CFDSometimes, to find an error, you must simulate something obviously wrong to see if the software catches it. If it doesn't, your setup is fundamentally flawed.
Scenario:
We are simulating plastic injection into a mold cavity. We have defined the inlet (injection gate) and the fluid properties.
The Deliberate Error:
We have intentionally sealed the entire cavity. We did not define any air vents (outlets) for the air inside the cavity to escape as the fluid rushes in.
Poligon Diagnostics & Logs
Reading the Matrix: How to extract meaning from output files.
When a simulation fails, the GUI might just say "Error." The real story is always in the output log files (.out, .log, .err). Learning to read these is the most critical troubleshooting skill.
Detail
Wrap-Up & Best Practices
Save time and preserve sanity.
The Golden Rule: Methodical Checks
Never trust a simulation blindly. Build complexity incrementally. Do not start by throwing a fully detailed CAD assembly with non-linear materials and complex contact definitions into the solver.
- Start with a highly simplified "blocky" version of your geometry.
- Apply basic linear materials.
- Verify boundary conditions work and the model runs quickly.
- Then, step-by-step, add the real geometry, complex materials, and finer mesh.
Pro-Hacks to Save Time
Symmetry
If your geometry, material, and loads are symmetrical, cut the model in half (or quarters). Apply symmetry boundary conditions on the cut faces. You instantly reduce solve time by 50-75% without losing accuracy.
Sub-modeling
Run a coarse mesh on the entire assembly to get global displacements. Then, cut out the small area of interest, apply a highly refined mesh, and use the global displacements as boundary conditions on the cut edges. Massive time saver.