How to estimate total time and timestep for unsteady simulations?

Queiroz · February 22, 2018, 09:42

Hi fellows,

I'm having the hardest time

to simulate a turbulent separation bubble induced by a combination of adverse to favourable pressure gradients.

Some of the experimental data known that could help to carry out the unsteady simulation are: two Strouhal number based on bubble length (separation to reattachment point) and reference velocity, Uref = 25 m/s. The first Strouhal number describes the breathing motion of the bubble, St = 0.01 and the second one describes the emission of coherent structures in (by?) the shear layer, St = 0.35.

Could anyone please give me a hint on how to get to see, for example, u' x t???

Of course I need to first find out the appropriate timestep and total time for the unsteady simulation.

Thanks in advance!

Queiroz · February 22, 2018, 10:05

I have been using BSL EARSM model on CFX to steady simulation. This was the only model that best showed the flow pattern, compared to experiments (oil-film technique), in the upper wall of the wind tunnel test section in the region of the separated flow.

ghorrocks · February 22, 2018, 18:52

You should do some careful research on turbulence models to help with this work. Most (if not all) 2 eqn turbuelnce models will be unsuitable for this as they have known deficiencies in getting separation bubbles the correct size. RSM models may help, but you may well have to use an LES style model.

You need to spend a merry day at the library doing a literature review on all this before starting, I would suggest.

February 22, 2018, 09:42	How to estimate total time and timestep for unsteady simulations?	#1
Queiroz New Member Join Date: May 2013 Posts: 10 Rep Power: 13	Hi fellows, I'm having the hardest time to simulate a turbulent separation bubble induced by a combination of adverse to favourable pressure gradients. Some of the experimental data known that could help to carry out the unsteady simulation are: two Strouhal number based on bubble length (separation to reattachment point) and reference velocity, Uref = 25 m/s. The first Strouhal number describes the breathing motion of the bubble, St = 0.01 and the second one describes the emission of coherent structures in (by?) the shear layer, St = 0.35. Could anyone please give me a hint on how to get to see, for example, u' x t??? Of course I need to first find out the appropriate timestep and total time for the unsteady simulation. Thanks in advance!

February 22, 2018, 18:52		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You should do some careful research on turbulence models to help with this work. Most (if not all) 2 eqn turbuelnce models will be unsuitable for this as they have known deficiencies in getting separation bubbles the correct size. RSM models may help, but you may well have to use an LES style model. You need to spend a merry day at the library doing a literature review on all this before starting, I would suggest. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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February 22, 2018, 10:05		#2
Queiroz New Member Join Date: May 2013 Posts: 10 Rep Power: 13	I have been using BSL EARSM model on CFX to steady simulation. This was the only model that best showed the flow pattern, compared to experiments (oil-film technique), in the upper wall of the wind tunnel test section in the region of the separated flow.