
[Sponsors] 
May 24, 2013, 11:54 
Transient state or unsteady state?

#1 
New Member
Irv
Join Date: May 2013
Location: Mexico
Posts: 2
Rep Power: 0 
Hi everybody.
I have a doubt. It´s the next: On the conductive heat transfer topic what is the difference between transient state and unsteady state? could I use them indiscriminately? Thanks. Conde de la Fère. 

May 29, 2013, 09:05 

#2 
Senior Member
Rami BenZvi
Join Date: Mar 2009
Posts: 154
Rep Power: 12 
Dear Conde de la Fère,
In conductive HT (as well as in other fields) the term "transient" indicates the problem is timedependent (in your case, for example, the temperature depends on time in addition to space location), while "steady" means the problem does not depend on time. The term "unsteady" is therefore equivalent to "transient". 

May 29, 2013, 10:00 

#3 
Senior Member
Join Date: Dec 2011
Location: Madrid, Spain
Posts: 134
Rep Power: 11 
Hi, in general terms I'd consider the terms "transient" and "unsteady" to be equivalent. Both indicate that the problem is dependent on time, as Rami just said.
However, I think that there's a subtle difference between both terms: On the one hand, the term "transient" is usually employed to indicate the evolution over time of the solution from an initial state until it reaches the steady state (the solution does not change any more). On the other hand, the term "unsteady" could indicate that the solution does not reach such steadystate solution, it varies over time always. Such situation may arise for example when you have a source term within the solid, or boundary conditions that vary over time. Cheers, Michujo. 

June 11, 2013, 10:26 
Thanks

#4 
New Member
Irv
Join Date: May 2013
Location: Mexico
Posts: 2
Rep Power: 0 
Thank everybody for helping me in this question.
Best regarts, Conde de la Fère. 

June 12, 2013, 10:49 

#5 
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 126
Rep Power: 13 
Suppose one has a problem with spatial boundary conditions that are periodic in time. There may be a solution that is periodic in time, invariant from cycle to cycle, but varying within a cycle. This would seem to be a sort of steady state. If one starts with (nonperiodic) initial conditions which evolve over time to the periodic state, then one might, without question, call the evolving system transient. But what would you call the timeperiodic state? The "state" would seem to be steady as it is not evolving, though it varies within a cycle. Would you call it "periodsteady", "quasisteady", or what?
What I have in mind is peristaltic flow in a channel or a leaky piston in a cylinder, serving as a damper. 

June 12, 2013, 12:13 

#6  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,049
Rep Power: 54 
Quote:
however, such flows are unsteady, the forcing frequency is fixed in your example but the cycles are only statistically equivalent each other. Actually, this is the classical framework for UnsteadyRANS. 

June 12, 2013, 13:05 

#7 
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 126
Rep Power: 13 
F. M. Denaro posted:
however, such flows are unsteady, the forcing frequency is fixed in your example but the cycles are only statistically equivalent each other. Actually, this is the classical framework for UnsteadyRANS. Consider this problem of 2D incompressible peristaltic flow with moving upper boundary given by Yu=Y0(1+a*cos(2*pi*(xv0*t))) and bottom boundary the negative of this, or simulating half of the domain with full slip, nopenetration BC on lower boundary. We treat the time using a finite element method, so we construct a 3D mesh with 2 space dimensions and time. We truncate the domain in the time dimension to, say, one period and apply periodic boundary conditions in time. Now we have a simple (nonlinear) boundary value problem with the fluid flow being driven by the "moving" boundary. We solve this using "stationary" methods. At any fixed time the mesh boundary is moving with velocity v0 as it progresses. If a stationary solution exists (and I am confident that it does at small Re), I contend the cycles on the full domain will necessarily periodic, and obviously without statistical fluctuations. This brings me back to the question of how do we name this flow? 

June 12, 2013, 13:22 

#8  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,049
Rep Power: 54 
Quote:
Assuming a laminar condition this flow repeats the solution in a deterministic sense. But it is unrealistic that such flow is really laminar as, in generla, the Reynolds number is usually high. Therefore you can realize a URANS simulation, you still truncate the time integration to a single period but the solution has a statistically meaning. However, laminar o turbulent, both problem are timedependent in the characteristic period. This is only my opinion 

June 12, 2013, 13:42 

#9 
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 126
Rep Power: 13 
But the Reynolds number can be quite small for peristaltic flows which describe the motion of fluids in organisms, the movement of food through the intestines for example.


Tags 
heat transfer, transient state, unsteady state 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
mass flow in is not equal to mass flow out  saii  CFX  12  March 19, 2018 06:21 
is there any Differnce between results of Steady state and unsteady state  sreenivas  FLUENT  0  February 20, 2012 11:13 
question about steady state& unsteady state particle  liuyuxuan  Main CFD Forum  0  February 26, 2010 10:19 
TwoPhase Buoyant Flow Issue  Miguel Baritto  CFX  4  August 31, 2006 13:02 
About the difference between steady and unsteady problems  Lisa  Main CFD Forum  11  July 5, 2000 15:37 