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-   -   Optimize for laminar flow, assume it valid for turbulent flow? (http://www.cfd-online.com/Forums/main/93650-optimize-laminar-flow-assume-valid-turbulent-flow.html)

Chander October 21, 2011 20:15

Optimize for laminar flow, assume it valid for turbulent flow?
 
Hi,

I want to optimize a design for convective heat transfer and pressure drop.
I have four designs..design 1 ,2,3 and 4.
I want to judge how these designs compare at different inlet Re numbers.
The Re numbers are in laminar region at lowest level and sweep through transition range on the higher side.
I am finding it difficult to get convergence at higher Re both with or without using turbulence models.
However i am able to get converged results at low Re i.e. in the laminar flow regime.

I analyse through CFD and rank the available designs in terms of their performance at low Re i.e. in the laminar flow regime and get a ranking say 2>3>1>4 in terms of their performance for pressure drop and conjugate heat transfer. Now how safe will it be to assume that the same ranking of designs will also hold at higher Re i.e. in the transition and turbulent flow regimes ( and thus avoid CFD analysis at higher Re)?

cfdnewbie October 22, 2011 05:43

Hello,
I would consider that very dangerous. Turbulent flows can behave significantly different from laminar ones, and extrapolating things like heat or momentum transfer usually goes awfully wrong: that's why Airbus and Boeing are not relying on laminar simulations to build their big birds.

My advice: do the turbulent simulations, maybe you will need to increase your grid resolution, but do not extrapolate laminar results!

Chander October 24, 2011 07:43

Thanks cfdnewbie for your advice!


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