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September 10, 2006, 19:36 
steady and unsteady

#1 
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As we know ,the steady flow and unsteady flow is different in the CFD,but the governing equations are the same ,jwhich are named as NS equations. BUT my question is that :what is the diffence between stead and unsteady ?Friends ,please give me some answer.


September 10, 2006, 20:18 
Re: steady and unsteady

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The equations are not the same. The equations for steady flow are a subset of the equations for unsteady flow, arrived at by dropping the time derivatives of the field quantities. In physical terms most flows are unsteady, and can be represented using a steady flow approximation to varying degrees of success  if the time variations of the field variables are small, then the approximation is good. What kind of answer are you looking for beyond that?


September 10, 2006, 20:39 
Re: steady and unsteady

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i want to know what is the unsteady quantifies ,and what is the steady quantifies, specially ,how to deal with the time term in steady treatment in CFD? because the materials in my hand just some papers and these paper presents same equations of steady and unsteady flows, i make mistakes and confused.
thanks for your answers 

September 10, 2006, 21:04 
Re: steady and unsteady

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You should note that in the steady case du/dt=0, which will arrive what ag explained. In other words, the steady state condition is reached when the solution stops to change in time, so, du/dt vanishes.
Hope it help you Renato. 

September 10, 2006, 22:05 
Re: steady and unsteady

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Thanks,i see.


September 11, 2006, 10:03 
Re: steady and unsteady

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the reason why you see the same equations in some unsteady and steady methods is that you can use (or abuse?) the unsteady equations to get to a steady state. if the flow is such that there exists a steady solution as time goes to infinity, you can run CFD on the unsteady equations until any initial unsteadiness (and error) has died out and the unsteady term, as Renato says, is reduced to an acceptable residual. since you're not interested in the time history, these computations don't have to be accurate in time, i.e. the time steps can be as large as your iteration scheme allows. that's simply one way to get to a steady state.


September 12, 2006, 07:57 
Re: steady and unsteady

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Which 'steady state' are you refering to  Eulerian, or Lagrangian? They are different.
desA 

September 12, 2006, 09:14 
Re: steady and unsteady

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I am assuming the Eulerian point of view.


September 19, 2006, 09:49 
Re: steady and unsteady

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Hi Diaw,
What is the different between Eulerian or Lagrangian steady and unsteady state? Regards 

September 20, 2006, 22:45 
Re: steady and unsteady

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In simple terms:
Lagrangian steady => substantial derivative = zero. Eulerian steady => temporal derivative portion of substantial derivative = zero. So, if you look at the substantial derivative, you can have cases where the whole term is zero, but the temporal & convective portions are nonzero. Another way of thinking about things: Lagrangian steady => observer moving on a fluid particle observes no change in time for the property he is observing. Eulerian steady => observer fixed in space observes how the flow field changes with time (say a series of time snapshots). If this 'picture' does not change in time => temporal term is zero => Eulerian steady. Hope that helps... desA 

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