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 September 10, 2006, 19:36 steady and unsteady #1 superfool Guest   Posts: n/a 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 N-S 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 #2 ag Guest   Posts: n/a 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 #3 superfool Guest   Posts: n/a 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 #4 Renato. Guest   Posts: n/a 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 #5 superfool Guest   Posts: n/a Thanks,i see.

 September 12, 2006, 07:57 Re: steady and unsteady #7 diaw Guest   Posts: n/a Which 'steady state' are you refering to - Eulerian, or Lagrangian? They are different. desA

 September 12, 2006, 09:14 Re: steady and unsteady #8 Mani Guest   Posts: n/a I am assuming the Eulerian point of view.

 September 19, 2006, 09:49 Re: steady and unsteady #9 Brostan Guest   Posts: n/a Hi Diaw, What is the different between Eulerian or Lagrangian steady and unsteady state? Regards

 September 20, 2006, 22:45 Re: steady and unsteady #10 diaw Guest   Posts: n/a 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 non-zero. 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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