[Nek5000-users] angular velocity to the mesh velocity

Fri Feb 9 09:02:34 CST 2018

Sorry for the slow response but was on travel yesterday.

For moving meshes with rigid-body motion of pitch+translation, say,

you essentially need a smooth function that is 1 at the moving boundary and 0 at the far field boundaries.  Call this function h(X,t)=h(x,y,z,t).

Then your mesh velocity (W=[wx,wy,wz]), which is required by Nek's ALE formulation, is given by

     Translational Velocity(t)*h + Rotational Velocity*h

Where h==1 (i.e., on the boundary) you will have the correct velocity.

Where h==0 (on the far field), you will have no mesh velocity.

h is in effect a blending function between these regions.

My favorite way of generating h is to solve a Laplace equation with a variable diffusivity.   A high diffusivity near the object ensures h=1 near

the object and thus elements near the object move _with_ the object, which means that they do not get excessively deformed.

I attach a .usr file from a case I ran several years ago -- I've not tested it recently so please take it as a guide and not a solution to your problem.

Look for "Greens Function" to see how to compute h.

Here is a paper describing the approach

http://www.mcs.anl.gov/~fischer/pubhtml/ccfd.pdf

Paul

PS - Note - Nek requires _only_ the mesh velocity -- mesh position

is updated by Nek.

Note that you may, or may not, need to recompute the Green's function.... It is often a smooth function even after the mesh is deformed, so you can keep using it.... I suggest doing some experiments for your particular case.

________________________________
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: Friday, February 9, 2018 8:36:39 AM
To: nek5000-users
Subject: Re: [Nek5000-users] angular velocity to the mesh velocity

Thank you for your replay Marco.

I will explain from the begening clearly. My current task is to validate Fluid structre interaction on aero foil(NACA profile). The equations which i have to solve I have attached for aero foil. Before  this case, I had succesfully done a case of vortex induced vibration of cylinder in NEK5000. And the code below shows a part of the forced oscillating cylinder. From this code I learned that the body motion prescribed through the mesh velocities but I also think that I can only give translational velocity components in NEK5000 to presribe a body motion (mesh as well).

Since my aero foil has pitching (it rotates up and down about an axis at angle theta). Since it has to be a FSI problem my plan is by using  " call torque_calc(scale,x0,.true.,.true.)" I will get the M (moment about the elastic axis (x0)) and it gives as an input to the Equation of angular motion (attached with this email). Then using Range Kutta method I get the angular velocity of the profile (r, theta,z co ordinates). Then I am planning to conver this angular velocity into linear velocity components (x, y, z direction) then impose into the wx,wy,wz components and I hope it leads to the movement of mesh thus the geometry.

My question is could you think that its possible? Or is there any other simple way to give this angular velocity to my NACA profile about the elastic axis (x0)? And  I feel difficulty to convert the angular velocity into linear velocty and gives its to each nodes which is on "mv" B.C .
Could you please help me with this situation by an example if you have.

Thanks in advance

 c     Cylinder velocity components

      ucx = 0.0
      ucy = amplitude * omega * cos(omega*time)
      ucz = 0.0

      n = nx1*ny1*nz1*nelv
      do i=1,n                          ! Translational velocity
         wx(i,1,1,1) = basev(i)*ucx     ! component.
         wy(i,1,1,1) = basev(i)*ucy
         wz(i,1,1,1) = basev(i)*ucz

         umeshx(i,1,1,1) = wx(i,1,1,1)
         umeshy(i,1,1,1) = wy(i,1,1,1)
         umeshz(i,1,1,1) = wz(i,1,1,1)

      enddo

________________________________
De: "nek5000-users" <nek5000-users at lists.mcs.anl.gov>
À: "nek5000-users" <Nek5000-users at lists.mcs.anl.gov>
Envoyé: Vendredi 9 Février 2018 15:03:26
Objet: Re: [Nek5000-users] angular velocity to the mesh velocity

Sijo,

The mv_cyl example is a 2-D moving body example. As for the mesh velocity, can you please be more specific?

Marco

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>
Reply-To: "nek5000-users at lists.mcs.anl.gov" <Nek5000-users at lists.mcs.anl.gov>
Date: Friday, February 9, 2018 at 8:56 AM
To: nek5000-users <Nek5000-users at lists.mcs.anl.gov>
Subject: [Nek5000-users] angular velocity to the mesh velocity

Hi Neks,

Its a long time that i came up with something. So here we go,

SInce I was doinf the FSI of aero foil, I have to deal with 2 degrees of freedom (its a 2D case) one in vertical and another in angular direction about the elastic axis of the profile. I could code the vertical motion of the foil but before starting the angular motion I have some doubts. Is there anyway to give the angular velocities directly to the mesh velocities? Apart from the translational velocity components Or Do you want me to covert the angluar velocities into translational components and give to mesh velocities? Do you have any example with a body which is rotating?

Thanks in advance

Sijo GEORGE

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