Frequently Asked Question

Engineering extraction

Post-processing converts fields into forces, moments, heat rates, profiles, averages, and balances. Define the surface or volume, direction, reference area, density, speed, averaging interval, and sign convention before calculating the result. A contour helps diagnose a mechanism, but an integrated balance is normally more defensible for a design decision. Sampling must also be fine enough to capture the variation relevant to the output.

F is force, p is pressure, n is surface normal, τ is viscous stress, and A_ref is the declared reference area.

Worked example

For F = 120 N, ρ = 1.2 kg/m³, U = 30 m/s, and A_ref = 0.5 m², C_F = 120/[0.5×1.2×30²×0.5] = 0.444. Recalculate if the reference area, force direction, or averaging definition changes.

Check: confirm surface closure, units, mesh sensitivity, and whether the average is area-, mass-, time-, or volume-weighted.

Engineering check

For Vorticity, Q-Criterion, and Coherent Structures, maintain traceability from requirement to risk, design output, evidence, and approval. Record the configuration, acceptance criterion, test or analysis conditions, open actions, and residual risk. A method is not complete when the document is filled in; it is complete when the evidence supports the decision and affected controls are updated.

Engineering note

For Vorticity, Q-Criterion, and Coherent Structures, state the intended use, input range, dominant mechanism, units, boundary conditions, acceptance criterion, and evidence owner. Use an independent balance, limiting case, repeat measurement, or sensitivity check to challenge the result. Document the configuration and uncertainty so another engineer can reproduce the reasoning and determine whether the result remains valid after a design, material, boundary, or process change.