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December 3, 2009, 12:06 
URANS for unsteady flow?

#1 
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karine
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Hi all
i have some little questions: 1can URNANS models predict unsteadiness in flows? like vortex schedding??? 2The average solution for URANS flows is it the same than the solution of steady RANS ??? thanks and regards 

December 9, 2009, 19:28 

#2  
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Quote:
1. Yes they can, but not with complete confidence. You need to do some validation against experimental or perhaps higher accuracy, trusted simulations to make sure your results are acceptable. 2. Not necessarily. It depends on the problem. There might be nonlinear interactions between 'organised' unsteadiness and the random fluctuations that alter the timemean solution. In such cases, you need to be wary of URANS results. 

December 10, 2009, 14:42 

#3 
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karine
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Thank u very much, but i didt understand too much the second point
In fact, what u are saying is true: URANS will not give a trusted result, but this will be the same for steady RANS. My question is if we do the same simulation with steady RANS, than with Unsteady Rans and we take the average of the URANS solution, will the 2 solutions be the same???? I know that the mean solution of nonlinear equations is not necessarely a solution of the equation , but i am not 100% sure.... Thank u 

December 11, 2009, 22:12 

#4  
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Quote:
But even if you do get a converged solution using a steady RANS solver, it may not be the same as the timemean of the solution obtained with an URANS solver. If you have a quasisteady rate of variation, where the largescale variations of flow conditions over time are slow enough such that at each instant in time the turbulence is able to readjust to the new conditions as if it were at a new equilibrium state, then you might expect the to get agreement between the timemean and the steady solution. But if not, i.e. if the rate of variation is too fast for the turbulence to adjust to new conditions, then you probably will not have such an agreement. There can be other complicating factors, such as periodic laminarisation and retransition in pulsatile pipe flow (happens under certain conditions), for example, which might throwoff the turbulence models in a cyclical way (models are less than perfect in predicting transition). In which case, the timemean solution you get will be different from the corresponding steady solution without forced pulsations. This is to name one example. Last edited by sfalsharif; December 11, 2009 at 22:39. 

December 12, 2009, 17:12 

#5 
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karine
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OK thank u very much for ure answer


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