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February 21, 2014, 02:48 
difference between steady and transient solution?

#1 
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Huang Xianbei
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Hi,all
I'm a little confused about the difference between steady and transient solution.As we all know, the transient NS and Reynolds timeaveraged equations both contain the term corresponding to the time(t). Usually, we depart the value such as Velocity U into Uave(time average) and u(the fluctuation). Q1:Is that means the steady state solution represents the Uave? While only the transient solution can get u. Does there any difference in the equations been solved? Q2:If it's required to specify the Pressure gradient force(ave) in a transient simulation, how can we implement this? Through momentum source or some way else? 

February 21, 2014, 07:49 

#2 
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Glenn Horrocks
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Good question
For a steady state run the Reynolds averaging assumes the flow has an averaged component and a time varying component, where the time varying component averages over time to zero. Then it is obvious to simulate the averaged flow component and use a turbulence model to handle the time varying component. But in a transient flow with RANS things are a bit more subtle, as the simulated flow also has a time varying component as it is transient. The fundamental assumption being made is that there is a distinction in the spectrum of the turbulent variations and the average flow variations. As long as that distinction can be made and the turbulence model is correctly scaled to that distinction you are OK. But in many flows there is not a clear distinction  for instance IC engine flows, flows off bluff bodies. That is one key reason why RANS models do not do very well in these applications and LES based approaches are often more accurate. You have implement a momentum source to do whatever you want, but the pressure gradient is already part of the momentum equation so why do you want to include it again? 

February 21, 2014, 08:12 

#3  
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According to your description, the steady state gets the average solution, ,and the equations been solved have no difference from the transient case, is my understanding right? As for the second question, well ,I'd like to describe it in details: First, in the rotating tunnel flow(rectangular intersection), many researches were done with DNS. In the DNS calculation, the NS equations under rotating frame were used which including the centrifugal force and Coriolics force terms. Usually, the centrifugal force term is included into the pressure gradient term and the pressure gradient term Grad(P)becomes effective pressure gradient term, Grad(Peff). In order to drive the flow, the mean effective pressure gradient was used as a constant value, that is Grad(Peff)ave, so I don't know how to implement this. 

February 22, 2014, 05:27 

#4 
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Glenn Horrocks
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A better way to think of it is that both approaches use the RANS approach. In the steady state situation what is the avergae flow and what is the turbulent fluctuations is obvious as anything transient is assumed turbulent, but in transient simulation the RANS approach assumes a distinction between turbulent and average flow time scales. In this case some transient flow is assumed avergae flow and some turbulent.
Second question> CFX already has a rotating frame of reference model built in which already has the centripetal and coriolis forces applied. Why not use that? 

February 22, 2014, 08:23 

#5  
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according to the classical research, the Grad(Peff)ave is specified as a constant as referred before. Just as you say ,in the rotating frame in CFX, the centrifugal force is applied already, so in the momentum source we are going to specify is needed to exclude these effects, but in the NS equations, the Grad(P) and F(centrifugal) both exist, so I don't know what do the formal researchers mean in the specification of Grad(Peff)ave. 

February 23, 2014, 07:38 

#6 
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Glenn Horrocks
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I have no idea what you are talking about. If you are modelling a rotating region then why is the default rotating frame of reference model not suitable? It includes all necessary terms, you should not need to add anything.


February 23, 2014, 07:53 

#7  
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Grad(P)+centrifugal force=const. But in fact, this is impossible to guarantee this condition(I think) So I think a momentum source is needed instead,which can be written like this: S=const+Grad(P)centrifugal force This means to use a momentum source to make sure that the SGrad(P)+centrifugal force=const 

February 23, 2014, 17:51 

#8 
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Glenn Horrocks
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Are you saying the implementation of rotating frames of reference in CFX is wrong?


February 23, 2014, 22:34 

#9  
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Grad(P)ave + centrifugal force(ave)=const, as in the steady state, the Grad(P)ave=Grad(P), centrifugal force(ave)=centrifugal force so the problem turns to how to set Grad(P) + centrifugal force=const 

February 26, 2014, 01:59 

#10  
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I reviewed many papers in this field of rotating channel and find that their main method is to set the pressure gradient adjusted to keep the flow rate constant. I referred to the CFXhelp and got to know how to get the pressure gradient through Fortran. But I still don't know how to make the pressure gradient to adjust the flow rate. Can you give me some advice? 

February 26, 2014, 17:43 

#11 
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Glenn Horrocks
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I still have little idea of what you are talking about, and even less idea why you are doing this when CFX already has a rotating frame of reference model.
If you are talking about the pressure gradient at each control volume  you cannot set that. The solver does not work that way. If you are talking about the pressure gradient from the inlet to outlet  that is easy, you simply set the boundary conditions to the pressure drop you wish. 

February 26, 2014, 20:48 

#12  
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In order to do this in CFX, I chose the RANS, which in the equations to be solved is timeaveraged, so if I want to get the same result, I should also set the grad(Peff) to be a constant. The problem I'm facing is how to ensure this? In the RANS equation, how can I guarantee the sum of the pressure gradient term and the centrifugal force term being a constant? 

February 26, 2014, 21:00 

#13 
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Glenn Horrocks
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The rotating domain model in CFX already includes coriolis and centripetal components. The effect you are talking about should be already in the model. It is not imposed as you describe (a constraint on grad(Peff)) but as source terms on the momentum equations to account for the rotation. This is all described in the documentation for further details.
You cannot impose a constraint in CFX such that grad(Peff) is a constant. You would need to write a custom solver to handle this constraint. My suggestion is you use the built in rotating domain model and check that grad(Peff) is constant (or nearly so) in post processing. This will be a check on the accuracy of your simulation  and of the assumption that grad(Peff) is in fact constant. 

February 27, 2014, 04:20 

#14  
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