Frequently Asked Question

• Check mesh quality first — high skewness or poor orthogonality is a frequent root cause
• Lower under-relaxation factors
• Verify boundary conditions are physically consistent (e.g., inlet and outlet conditions that don't create an impossible mass imbalance)
• Consider whether the flow is genuinely unsteady and needs a transient approach instead

• This is often fine if monitored engineering quantities have stabilized — see Article 8 on interpreting convergence properly
• If quantities are also still drifting, investigate mesh independence (Article 4) and boundary condition placement (Article 5)

• Double-check units throughout the setup — a common source of order-of-magnitude errors
• Verify boundary condition values are correctly specified (gauge vs. absolute pressure is a frequent mix-up)
• Check for mesh errors near boundaries — degenerate cells at walls or inlets can cause local nonphysical results even if overall convergence looks fine

• Mesh may be unnecessarily fine in regions that don't need it — review refinement zones
• Consider whether a coupled solver would converge faster than a segregated one for this case
• Check hardware/resource allocation for the run if using parallel processing

• Usually indicates the outlet boundary is too close to a recirculation zone or wake — move the outlet further downstream
• Can also indicate the outlet boundary condition type is inappropriate for the physical situation

• This means you haven't yet reached mesh independence (Article 4) — results from the coarser mesh shouldn't be trusted for final conclusions until finer meshes show the output value stabilizing

Engineering interpretation

Use the simplest model that captures the dominant mechanism, then check whether omitted effects could change the decision. Dimensional consistency, limiting cases, sensitivity to the dominant input, and comparison with an independent estimate are practical safeguards. If the result is used for a release decision, the measurement method, acceptance criterion, configuration, and evidence owner should be recorded with the result.

The expression is a reporting framework. It does not replace the governing relation for the specific problem. Inputs should have units and a declared source; assumptions should state what is neglected and why that omission is acceptable for the intended use.

Worked example

Suppose the requirement is a characteristic of 10.00 ± 0.10 mm. A production study records a mean of 10.02 mm and a within-process standard deviation of 0.02 mm. The nearest specification limit is 0.08 mm from the mean, or four standard deviations. The nominal result appears capable, but the engineer must still confirm measurement-system variation, process stability, material condition, and whether the sample represents the intended production window.

Engineering check: record the input data, revision, calculation, uncertainty, and reaction plan. A result is not engineering-grade merely because a formula produces a number.