Model reduction is the quickest time saving in CFD. Put in a symmetry plane and the cost halves; take a quarter sector and it drops to a quarter. It looks free. It is not [1].
The condition for symmetry is the flow, not the geometry
The geometry may be symmetric while the flow is not. A symmetry assumption imposes that no flux crosses that plane. If the flow does cross it in reality, the model forbids it and the solution stops being physical.
Typical cases where symmetry breaks:
- Vortex shedding. The wake behind a symmetric cylinder is not symmetric; it sheds alternately. A half model cannot show this at all and returns zero oscillating side force.
- Natural convection. A symmetry plane aligned with gravity is usually valid; a horizontal one almost never is.
- Rotation. If a rotating component is present, geometric symmetry does not produce symmetry in the flow.
- Unstable separation. In a sudden expansion the jet can attach to one wall even in symmetric geometry. A half model forbids this from the start.
The condition for periodicity
Modelling a single passage in rotating machinery is common and usually correct — but only if the upstream and downstream flow is periodic too. In a volute pump the cutwater breaks circumferential symmetry; a single-passage model may give a reasonable result at the operating point, yet it cannot show the load fluctuation on the blade.
If the blade counts share no common divisor, the blade count has to be "rounded" for a sector model. That is an assumption, and its effect belongs in the report.
The right use
Symmetry is used not to make the computation cheaper but to tell the model a physical fact you already know. If you are not sure, the remedy is clear: run the full model once, compare it with the reduced model, measure the difference. That one-off cost lets you run every subsequent variation on the small model with confidence.
References
- Ferziger, J. H., Perić, M. & Street, R. L. (2020). Computational Methods for Fluid Dynamics, 4th ed. Springer. DOI: 10.1007/978-3-319-99693-6
- International Towing Tank Conference. ITTC Recommended Procedures and Guidelines 7.5-03-02-03: Practical Guidelines for Ship CFD Applications. ITTC.