[Nek5000-users] stokes flow with time-dependent boundary

[nek5000-users at lists.mcs.anl.gov]

Hi Lailai,

To switch to unsteady Stokes, you simply set the flag
IFNAV to "F" in the .rea file, which turns off the convective
term and simultaneously eliminates the CFL timestep constraint.

[  Set:

   T F F F F F F F F F F IFNAV & IFADVC (convection in P.S. fields)

to

   F F F F F F F F F F F IFNAV & IFADVC (convection in P.S. fields)

.]

It's still not clear to me if you are solving an unsteady
Stokes problem, or a series of steady Stokes problems.
(There is a difference...)

Nek can handle either case with equal ease.

I hope this helps.

Paul


On Wed, 21 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:

> On 12/19/2011 06:17 PM, nek5000-users at lists.mcs.anl.gov wrote:
>> 
>> Hi Lailai,
>> 
>> I have used the approach you proposed for solving multiple
>> steady stokes problems... you use an artificially large
>> timestep.  That works.
>> 
>
> thank for your reply, if understand correctly, here you are talking about the 
> second approach i proposed. For each time step, we solve a transient problem 
> with very large internal timestep to quickly get to the steady-state 
> solution.
> since i am very new to nek5000, thus  i am not sure how to implement this 
> method which seems not trivial.
>
> On the other hand, i started from the first example of the Kovasznay problem. 
> I remove the time-derivative and convection term by setting the density in 
> .rea file to zero, the numerical results agree very well with the analytical 
> solution with zero Re number. I guess here the solver is just inverting a 
> matrix which seems feasible for a 2D problem but might be too expensive or 
> inefficient for a 3D problem.
>
>
>> If you really have a time-dependent boundary condition, there
>> is no reason you can't just use the unsteady Stokes solver
>> with whatever timestep is required to accurately resolve your
>> temporal bcs.   Note that, in this case, you would indeed have
>> the inertial term rho du/dt present in the physics.
>> 
>
>
>> Paul
>> 
>> 
>> On Mon, 19 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:
>> 
>>> Hello, guys,
>>> 
>>> i am a new guy to Nek5000, i saw the manual of nek5000 that it can solve 
>>> the steady stokes flow.
>>> i guess when i solve such a flow, do i need to set it as a transient 
>>> simulation with time-derivative term included to get  a steady-state 
>>> solution? or, i can solve it by a direct solver method to get the solution 
>>> immediately?
>>> 
>>> since i want to add some time-dependent boundary condition for the steady 
>>> stokes flow, so it will be pretty nice if i can solve it using the direct 
>>> solver, for each time step, i solve one stokes flow; if nek5000 cannot 
>>> solve it in such a way, i guess i have to use the first way; then for each 
>>> time step i have to solve a transient problem to approach the steady state 
>>> with some artificial time step used.
>>> 
>>> i am not sure if i have stated my problem clearly. hopefully you guys have 
>>> some experience on the feasibility of the two ways mentioned above. Thank 
>>> you in advance.
>>> 
>>> lailai
>>> 
>>> 
>>> 
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