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CFD to structural analysis: mapping temperature and pressure

The input to a thermal stress calculation is the temperature field. What mapping usually loses is the gradient — and the gradient sets the stress.

In a boiler, an exhaust manifold or a turbine blade the problem is not the temperature itself but the temperature difference: the material expands differently in different regions, that expansion is restrained, and stress arises. The quality of the field mapped from CFD to structural analysis therefore sets the stress result directly.

What is lost in the mapping

The gradient. The two meshes differ; the CFD mesh is built for the flow, the structural mesh for the stress. During interpolation, sharp temperature gradients are smoothed and the stress is underpredicted. In the most critical regions — weld seams, section changes, cooling hole edges — the structural mesh must be fine enough as well.

Surface mismatch. If the surfaces of the two meshes do not coincide, the mapping leaves gaps. On curved surfaces this produces a systematic shift.

Time. A steady-state CFD result gives one instant. The moment of highest thermal stress is usually not steady state but a transient — start-up or sudden cooling. See the steady versus transient note.

The right sequence

  1. Solve the flow and the solid together to obtain the temperature field. A field obtained by applying h alone gets the gradient wrong. See the conjugate heat transfer note.
  2. Select the most critical time steps — not the peak temperature but the peak gradient.
  3. Map the temperature and pressure fields onto the structural mesh.
  4. Check after mapping: do the maximum and minimum temperatures on the structural mesh match the CFD values? If not, the interpolation is faulty.

Do not forget pressure

In thin-walled structures in particular, thermal stress and pressure-induced stress can be of the same order and can reinforce each other. Assessing them separately means missing the critical combination.

References

  1. Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605
  2. Celik, I. B., Ghia, U., Roache, P. J., Freitas, C. J., Coleman, H. & Raad, P. E. (2008). Procedure for estimation and reporting of uncertainty due to discretization in CFD applications. Journal of Fluids Engineering, 130(7), 078001. DOI: 10.1115/1.2960953

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