[Nek5000-users] neuman bc for the normal velocity component

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

Hi Paul,

thanks a lot for your help. Now I understand why 'ON ' did
not work. The inviscid flow we're trying to compute
the boundary layer to is a pulse which first has
v > 0 (outflow) and then after the crest has passed v < 0
(inflow). Therefore 'ON ' did not work.

I helped myself by imposing U_base and V_bl
on the top boundary, where U_base is the
inviscid base flow and V_bl the normal
velocity computed by a boundary layer solver.

However, just for the fun, do you think imposing

u = U_base and dv/dy = - dU_base/dx

on the top boundary would work in this case?

Thanks a lot!

Joris


2012/9/25 <nek5000-users at lists.mcs.anl.gov>

>
> Hi Joris,
>
> The usage you describe should work.
>
> I've put an example "blasius" in the examples directory.
>
> Note that a common failure mode is where fluid is drawn _in_
> to the domain through the ON boundary.  This can happen if
> the pressure is such that flow wants to come in rather than
> leave.
>
> However, from what you describe, it sounds as if something
> else is going wrong in your case.  Perhaps looking at the
> example will help.
>
> Paul
>
>
>
> On Thu, 20 Sep 2012, nek5000-users at lists.mcs.anl.**gov<nek5000-users at lists.mcs.anl.gov>wrote:
>
>  Dear Nekians,
>>
>> I'm relatively new to nek.
>>
>> I would like to
>> compute the flow of a boundary layer and
>> I would like to impose the following
>> boundary condition on the top (2D) boundary,
>> which is parallel to the wall:
>>
>> u = U_base and dv/dy = - dU_base/dx,
>>
>> where U_base is the known inviscid base flow.
>> The velocity component u is parallel to
>> the boundary, whereas v is normal.
>>
>> I tried to use 'ON ' but this did not give
>> the result I was expecting (Am I doing smth. wrong?).
>> At the boundary nodes u was equal to U_base, but
>> at the very next node u was completly different
>> (and v was of course completly wrong).
>>
>> Thanks for any help
>>
>> Joris
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