Transient Boundary Condition in Steady state solution

RAJDEEP · March 30, 2021, 00:32

Can constant magnitude of transient boundary condition provide well converged steady state solution?

TurBoris · March 30, 2021, 01:32

Hi,

Do you mean using a transient BC in a steady state simulation ?

If not, in a transient simulation, constant magnitude of transient BC will yield in a "statistically steady state" solution with a sufficient run time.

RAJDEEP · March 30, 2021, 02:24

Hi,

I mean by using Transient BC in Steady State Simulation like shaft rotation at particular constant speed which i am specifying in the CFX pre in rad/sec.

ghorrocks · March 30, 2021, 02:36

Fluid flow is by its definition transient - the stuff is moving. But in a finite volume solver like CFX, if you can assume that when you define a volume you are simulating that the boundary conditions do not change with time and the fluid condition at all points in the volume do not change with time then it is a steady state simulation.

So think about whether the boundary condition changes the simulation volume. For example, if a wall has a tangential velocity this can be modelled by a tangential velocity on the wall but the simulation volume is not changed - therefore it is steady state. If you have a wall with a normal velocity this will change the simulation volume - therefore it is transient.

March 30, 2021, 00:32	Transient Boundary Condition in Steady state solution	#1
RAJDEEP New Member RAJDEEP JAGDALE Join Date: Oct 2018 Posts: 26 Rep Power: 7	Can constant magnitude of transient boundary condition provide well converged steady state solution? Last edited by RAJDEEP; March 30, 2021 at 02:10.

March 30, 2021, 02:36		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Fluid flow is by its definition transient - the stuff is moving. But in a finite volume solver like CFX, if you can assume that when you define a volume you are simulating that the boundary conditions do not change with time and the fluid condition at all points in the volume do not change with time then it is a steady state simulation. So think about whether the boundary condition changes the simulation volume. For example, if a wall has a tangential velocity this can be modelled by a tangential velocity on the wall but the simulation volume is not changed - therefore it is steady state. If you have a wall with a normal velocity this will change the simulation volume - therefore it is transient. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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March 30, 2021, 01:32		#2
TurBoris Member Bora Join Date: Nov 2016 Posts: 32 Rep Power: 9	Hi, Do you mean using a transient BC in a steady state simulation ? If not, in a transient simulation, constant magnitude of transient BC will yield in a "statistically steady state" solution with a sufficient run time.

March 30, 2021, 02:24		#3
RAJDEEP New Member RAJDEEP JAGDALE Join Date: Oct 2018 Posts: 26 Rep Power: 7	Hi, I mean by using Transient BC in Steady State Simulation like shaft rotation at particular constant speed which i am specifying in the CFX pre in rad/sec.