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Moving wall with velocity profile

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Old   May 16, 2011, 13:44
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Hi Amir,

Thank you for this advice, I've checked: my dynamic zones are OK.

To be more precise about my problem, please find attached a picture of the velocity distribution I got.
The flow is laminar, so the velocity distribution vs the radius has to be linear, as it is when I use moving wall option. I cannot understand why there is a difference since I haven't changed any parameter when applying Dynamic mesh.
Another strange thing is the fact that the "colored layer" becomes thinner if I reduce the time step.
Have you any idea?

Thanks!

Tobard
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Old   May 16, 2011, 16:29
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Hi,
the velocity profile would be linear if you impose a constant angular velocity. (is it your case?)
it seems that the wall momentum has diffused in the domain but you have to have much more time steps to reach the linear profile; let it advances in time.
Also check that whether your wall velocity is correct or not.
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Old   May 16, 2011, 17:29
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Hi,

The angular velocity is constant after 2 seconds.
The problem is that even if I wait for more timesteps this profile doesn't change any more, which is physically impossible. I could have sent the same picture at 4 or 5 seconds, it would have been the same! And if I reduce the time step, this profile becomes thinner...
Besides, the results that I am able to have using another method (moving wall) are quite different for the same flow time. I really don't understand why moving mesh give me such inaccurate results.

Thank you for your reply.

Tobard
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Old   May 17, 2011, 09:12
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Hi,
Post your UDF and I'll check that.
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Old   May 17, 2011, 16:50
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Hi!

It is a very basic UDF:
------------------------------
#include"udf.h"
DEFINE_CG_MOTION(rot_cg, dt, velocity, omega, time, dtime)
{
if(time < 2){
omega[2]=75*time;
}
else{
omega[2]=150;
}
}
------------------------------
Thank you again!

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Old   May 18, 2011, 15:19
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Hi,
I have suggestion; consider a point on the boundary and export results during different time steps and check whether it obeys UDF or not.
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Old   May 18, 2011, 15:46
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Hi,

I tried this and this is all right. Points on the boundary are the only ones that have a correct velocity at each time step... That's why it seems really weird to have an inaccurate velocity inside the zone...

Thanks!
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Old   May 19, 2011, 02:01
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Hey,
double check your boundary conditions, i.e. in fluid zone, you have to set stationary in motion type and in wall zones, set stationary wall (relative to adjacent cell zones)
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Old   May 19, 2011, 15:31
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Dear Amir,

Everything is already stationary... Does it work for you? I don't see what is wrong...
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Old   May 19, 2011, 15:56
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Have you ever checked that with DEFINE_GRID_MOTION macro?
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Old   May 19, 2011, 17:11
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Yes, that was my initial try. And that's because it didn't work that I decided to use DEFINE_CG_MOTION... (Yeah, not really logical process order, I know...). So I have encountered the same problem with both methods!
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Old   December 6, 2011, 16:10
Default what is meant by movingWallVelocity?
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Can anyone please tell me what the keyword movingWallVelocity does in OpenFoam? My question pertains to its use in the U file in sloshingTank2D in the OpenFOAM tutorial. The walls of the tank are defined as patch type walls and the front and back walls are defined as empty. IN the velocity (U) file, all the walls are defined as movingWallVelocity. Does this mean that the walls are moving with the velocity or the mesh?
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Old   December 6, 2011, 16:17
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Quote:
Originally Posted by musahossein View Post
Can anyone please tell me what the keyword movingWallVelocity does in OpenFoam? My question pertains to its use in the U file in sloshingTank2D in the OpenFOAM tutorial. The walls of the tank are defined as patch type walls and the front and back walls are defined as empty. IN the velocity (U) file, all the walls are defined as movingWallVelocity. Does this mean that the walls are moving with the velocity or the mesh?
Dear friend,

As you know, this not OpenFOAM forum!!! but I've had experiences with this. You can move your boundaries via dynamic mesh technique with this BC.

Bests,
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Old   December 6, 2011, 17:11
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Quote:
Originally Posted by Amir View Post
Dear friend,

As you know, this not OpenFOAM forum!!! but I've had experiences with this. You can move your boundaries via dynamic mesh technique with this BC.

Bests,
Thanks and sorry for not paying attention to where it was being posted.
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Old   June 3, 2013, 08:37
Talking udf question...
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Dear Friends;
I want to ask you about my problematic UDF
I want to write a UDF in order to define the temperature distribution on a side wall.
I am very ne about UDF writing and can not create what I really want.
My aim is to define just;
T=938+40*y
it is the temperature change on the wall. there is no other BC for my case.
could you please help me??*Thank you very much.
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Old   June 3, 2013, 09:45
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Quote:
Originally Posted by angelicapeygo View Post
Dear Friends;
I want to ask you about my problematic UDF
I want to write a UDF in order to define the temperature distribution on a side wall.
I am very ne about UDF writing and can not create what I really want.
My aim is to define just;
T=938+40*y
it is the temperature change on the wall. there is no other BC for my case.
could you please help me??*Thank you very much.
Hi,

Refer to the "DEFINE_PROFILE" macro in the UDF manual.

Bests,
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Old   January 30, 2015, 13:22
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Hi guys

I want to simulate an oscillating wall in a cylinder in a tangential direction of the wall just could any one help me to figure out how can I do this?? Do I need to use dynamic mesh for this problem??

Best Regards
Mahdi
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Old   April 9, 2015, 09:54
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I am trying to simulate compression stroke in 2D axisymetric mode. using rigid motion.
I tried using the profile- in cylinder, i took data point for time vs velocity at y and notepad write in profile defone. the piston boundary moves but is not 254mm distance i needed (even though i got 254 in matlab with that points)

I used the udf option but i am geting warning error.

Can you please tell what is wrong in either profile or udf i attached.
#include "udf.h"
DEFINE_CG_MOTION(oscillate, dt, vel, omega, time, dtime)
{
Thread *t;
face_t f; /* define the variables */

t = DT_THREAD(dt); /* get the thread pointer for which the motion is defined */

/* if (!Data_Valid_P())
/* return; /* check if the values of the variables are accessible before you compute the function */

begin_f_loop(f, t) /* loop over each face in the zone to create an array of data */
{
if (time <= 0.012)
vel[0] = (637* time); /* define the velocity of the moving zone---*/
else if (0.012 < time < 0.019)
vel[0] = vel[0] + (2.4* time);
else if (0.019 < time < 0.027)
vel[0] = 14;
else if (0.027 < time < 0.03)
vel[0] = vel[0] - (6.5*time);
else if (0.03 < time)
vel[0] = 0;
}
end_f_loop(f, t)
}


thanks
sanjeet
Attached Images
File Type: jpg velocity and distance vs time(milisec).jpg (19.6 KB, 0 views)
Attached Files
File Type: txt FFF.2-Setup-Output.set - Copy.txt (55.0 KB, 2 views)
File Type: c VELOCITY.c (882 Bytes, 3 views)
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