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

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

Hi Lailai,

I suggest initially starting with a single run, using steady Stokes.
(Note, steady Stokes works only for Pn-Pn-2, so set lx2,ly2 = lx1-2, etc.)

Paul


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

> 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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