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

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

thank you for your tip, Paul.

i think we are solving a series of Stokes problems linked by a 
time-dependent boundary condition.
The time-derivative term is zero and we are not solving an unsteady 
Stokes problem.  I think we are not clear how to solve a series of 
Stokes problems.

lailai





On 12/21/2011 06:47 PM, nek5000-users at lists.mcs.anl.gov wrote:
> 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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