[ari003]

What is the difference between steady state and transient solution?
Based on the application how can I figure out that which solution to use?
It will be really helpful if someone helps.

[FMDenaro]

Originally Posted by ari003

What is the difference between steady state and transient solution?
Based on the application how can I figure out that which solution to use?
It will be really helpful if someone helps.

Steady state solution means you solve the equations with all the time derivatives disregarded from the system.
Unsteady solution assume that you want to start from some initial condition and prescribe boundary conditions to simulate a time-dependent fluid dynamics by considering all the time derivatives in the system. Depending on the physics of the problem, the transient solution can or cannot tends towards a steady state.

Generally, steady state solution appears at very low Reynolds number as well as for statistical solutions of turbulence (RANS formulations)

[Jayant Kumar]

i have a cfd problem setup in which i have to insert a udf for soil temperature distribution with respect to depth and real time as an initial condition. I have the equation for it right now i have insert a constant soil temperature patch., please help in how may i proceed

[ari003]

Originally Posted by FMDenaro

Steady state solution means you solve the equations with all the time derivatives disregarded from the system.
Unsteady solution assume that you want to start from some initial condition and prescribe boundary conditions to simulate a time-dependent fluid dynamics by considering all the time derivatives in the system. Depending on the physics of the problem, the transient solution can or cannot tends towards a steady state.

Generally, steady state solution appears at very low Reynolds number as well as for statistical solutions of turbulence (RANS formulations)

Thanks a lot sir for your reply but what I tried to convey through my question is that how this transient and steady state solution is related to the convergence of the solution in fluent?

I had performed one transient simulation of an airfoil where after a certain time-step the solution converged and it doesn't change velocity(for example) with time, so in this case what can I predict from this solution?

And last thing, suppose I have an airfoil at whatever AOA which kind of simulation is perfect is it transient or staedy?

[FMDenaro]

Originally Posted by ari003

Thanks a lot sir for your reply but what I tried to convey through my question is that how this transient and steady state solution is related to the convergence of the solution in fluent?

I had performed one transient simulation of an airfoil where after a certain time-step the solution converged and it doesn't change velocity(for example) with time, so in this case what can I predict from this solution?

And last thing, suppose I have an airfoil at whatever AOA which kind of simulation is perfect is it transient or staedy?

This is not the Fluent section of the forum.

However, a convergence to a steady state means that your numerical solution produces all time derivatives vanishing (in practice they are below a threshold value).

[ari003]

But in transient at each time-step there should be a convergence between two successive time-step. In general convergence means when the two values of residual converge below tolerance value then why does convergence takes place in transient?

[FMDenaro]

Originally Posted by ari003

But in transient at each time-step there should be a convergence between two successive time-step. In general convergence means when the two values of residual converge below tolerance value then why does convergence takes place in transient?

Just as example, consider the solution of a linear time-dependent equation using an implicit method. At each time step you have an algebric system to solve. Therefore, you must ensure convergence of the iterative method at each time step. And that must be repeated until the time derivatives vanish.
That idea generalizes to the coupled non-linear NSE.

[ari003]

I had previously done one transient analysis of airfoil where the transient nature changed into steady after few time-steps. I ve found that the case is quasi-steady where the transient finally changed into steady. So whats the reason behind this? When does this happen generally( in terms of convergence)?

[FMDenaro]

- Convergence of iterative methods in a time step is a numerical issue.
- Convergence to a steady state is a physical issue and depends on the flow problem.



Errors in the numerical setting can alter the physics of the flow problem, for example a too high numerical viscosity can produce a non-physical steady state.
A further example can be the non-convergence of the RANS solution caused by a not suitable turbulence model.


Your case means nothing without the full details of the simulation

[LuckyTran]

Originally Posted by ari003

I ve found that the case is quasi-steady where the transient finally changed into steady. So whats the reason behind this? When does this happen generally( in terms of convergence)?

It's related to whatever was the transient and how long it takes the transient to disappear. For example, if there was a bug (like the insect) in your simulation running around causing trouble, then the times it takes is how long it takes you to find and kill the bug.

If the transient is caused by residual effect of an initialization/initial guess, then it's related to how long it takes for the initial guess to die out. This can mean how long it takes stuff to flow from inlet to outlet. But it also could be shorter. It depends what was the transient. Not all transients are the same transient.