[Nek5000-users] Rotating computational domain

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

Hi Paul and Phillip,

I have validated my model at constant AOA and am now returning to my pitch
and hold scenario to do some work.  I definitely think the ALE formulation
applied to the entire domain is the way to go, but I'm feeling a little
confused about the "mv" boundary condition.  In the mv_wall example, the
"mv" boundary condition is applied to the section of the wall that is
moving, so in my mind it follows logically that to have the entire domain
pitch about the z axis, the entire domain would need the "mv" condition.
This doesn't seem possible while preserving my inlet and outlet BCs.  I
understand that for ALE to work appropriately, we need to match the normal
component of fluid velocity to that of the mesh velocity at any boundary.
Does this apply also to the inlet and outlet, or only internal fluid
boundaries such as my airfoil?

I am also looking at the mv_cyl and ocyl/ocyl2 examples, but these are in
rea/re2 format and I cannot look at the element boundary conditions
(forgive me, I'm still learning terminology so some of what I say is not
100% accurate).  While looking through the example files, I see that the
logical switch "ifusermv" is assigned in "usrchk" in some of the examples,
and in one of the "usrdat" subroutines in others.  I don't really
appreciate the difference, is there one?

I am building my mesh in ANSYS Fluent, and then converting the mesh to
ASCII .rea format with mshconvert2, and then extruding to 3D and converting
to rea/re2 format with n2to3.  Before, I would scroll to the bottom of the
.msh file and add Nek5000 BC flags to each boundary (i.e. adding an _W to
wall, _O to outlet, and _v to inlet) before converting to .rea format, and
this would successfully apply boundary conditions.  However, when
attempting to insert the _mv boundary condition on my wall, the boundary
conditions in the .rea file are unchanged.  Is there a better/more
appropriate way to assign the "mv" BC when converting a mesh?

On Tue, Aug 21, 2018 at 11:30 AM <nek5000-users at lists.mcs.anl.gov> wrote:

>
> Hi Harry,
>
>
> One simple approach is to use ALE but to have the entire domain pitch,
>
> thus preserving your mesh structure.
>
>
> That would properly account for all the accelerations in your non inertial
>
> domain.
>
>
> The slightly tricky part would be to determine what BCs you need.  You
>
> need to be careful about changing the character of the BCs (e.g., from
> inflow
>
> to outflow)  in the middle of a computation because that changes which
> quantities
>
> are prescribed and which are unknown.   Perhaps one approach would be
>
> to have a circular domain with 3/4 of it Dirichlet and 1/4 being outflow
> (i.e.,
>
> Neumann for velocity).
>
>
> hth,
>
>
> Paul
>
>
> ------------------------------
> *From:* Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> on behalf
> of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
> *Sent:* Tuesday, August 21, 2018 9:43:56 AM
> *To:* nek5000-users at lists.mcs.anl.gov
> *Subject:* [Nek5000-users] Rotating computational domain
>
> Hello Neks,
>
> I am an undergraduate student new to Nek and I am working on a simulation
> studying dynamic stall in a dynamically pitching airfoil.  I want to do a
> pitch and hold simulation at low Reynold's number (Re = 12000) where I
> rotate the entire computational domain up to about 50 degrees AOA and then
> hold at 50 degrees, with steady inflow ux = 1.  I would like to rotate the
> domain about the z axis, which passes through the quarter chord location of
> the airfoil profile (modified NACA 0012).
>
> Because of the steep AOA, I would like to stay away from pitching the
> airfoil within the stationary domain with ALE to avoid too much mesh
> deformation.  To this end, I have been scouring documentation to try and
> find a method of applying a rotational mesh velocity to the entire domain
> about the z axis while keeping the flow within the domain at ux = 1.  I am
> feeling very stuck.  Others have suggested applying a Coriolis force to the
> flow, but I don't really see how this would simulate rotation of the entire
> domain.  It seems to me that simulation of a Coriolis force would be
> acceptable to correct the flow field to always be ux = 1 inside a rotating
> domain, but I don't see how that would apply a mesh velocity anywhere.
>
> Are there any examples where an entire computational domain is rotated
> that I can look at, or has anybody done similar work who can point me in
> the right direction?
>
> Regards,
> -Harry
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>


-- 
Harry E. Werner IV
Secretary
Clarkson University SCUBA Club
*Cell:* (716) 570-4023
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