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UDF DEFINE_ADJUST for pseudo-sinusoidal motion

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Old   July 2, 2010, 05:26
Default UDF DEFINE_ADJUST for pseudo-sinusoidal motion
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Hi guys,

I would like to know (1) how I could model into an UDF an oscillatory motion which is not quite sinusoidal. Imagine a sinusoidal signal with the peaks and valleys flatten out, and between them a steep ramp. So, something between sinusoidal and a series of step functions.

I have been using a UDF until now for a sinusoidal function that feeds the rotational speed into fluent. (2) Do you know how I could translate translation data into an analytical function of speed for this case?

Many thanks.

C.
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Old   July 2, 2010, 17:26
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Quote:
Originally Posted by aerospain View Post
Hi guys,

I would like to know (1) how I could model into an UDF an oscillatory motion which is not quite sinusoidal. Imagine a sinusoidal signal with the peaks and valleys flatten out, and between them a steep ramp. So, something between sinusoidal and a series of step functions.

I have been using a UDF until now for a sinusoidal function that feeds the rotational speed into fluent. (2) Do you know how I could translate translation data into an analytical function of speed for this case?

Many thanks.

C.
Have you tried using the DEFINE_GRID_MOTION macro? You set it up by defining a dynamic mesh zone and setting the zone motion with a compiled UDF. There are some good examples in the documentation, and I can provide an example if you need.
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Old   July 2, 2010, 17:56
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Thanks TDI,

I haven't tried that function you menntioned, I'll take a look at it during the weekend. In the mean time,
I would really appreciate it if you let me take a look at that example of yours.

Regards
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Old   July 5, 2010, 08:05
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Hello again TDI,

I have read a bit about that macro you mentioned. I must apologise if I didn't express myself clearly, I am not looking into grid deformation, actually, I am avoiding it by all means.

I have three circular domains, the largest and medium are concentric, and the smallest one, inside the medium size, can be concentric as well or some distance away from the center. And, I would like to be able to set different motions to the medium and smalles domains by one or two UDF, and be able to define time frames for different motions.

My input data could be in terms of velocity or position at specific times.

Regards
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Old   July 5, 2010, 11:43
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OK, I see.
This would require a DEFINE_PROFILE macro. You write the macro and compile it. Then you link that function to the boundary in question using the BC setup dialog. You can use this to set a parabolic velocity profile, for instance, or to add swirl, etc.
I think there are some examples in the documentation of a DEFINE_PROFILE that do exactly the kind of thing you are trying to do.
The programming generally goes like this. Fluent passes your function a thread variable. You use that thread variable to start a loop over the nodes of face in question. For each node, you can look up the coordinates and assign the velocity based on the coordinates.
Hope this helps.
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Old   July 5, 2010, 12:15
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Thans for following my threads TDI,

I think I have misslead you one more time. The flow has a constant horizontal velocity, defined at the most outer boundary as a velocity_inlet. I want the inner domains, which are circles, to rotate by a motion passed from the UDF. I have been using DEFINE_ADJUST for the rotational speed (omega) of the cell zone containing the aerofoil. This was done for a purely sinusoidal motion. Now, I want to define different motions at different lengths of time during the same calculation. For example, it will start at a "resting position" to ramp up at a given pitching speed towards an angle that will keep constant for some time, during which the medium domain that I mentioned before, will rotate at an approximated rotational speed proportional to the force generated by the aerofoil until it reachs a maximum, at which the medium domain will stop, the aerofoil will pitch down at a given ramp speed, and the whole motion will repeat itself in this way for a few cycles.

regards
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Old   July 5, 2010, 12:38
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Would access to the flow-time variable be helpful?
Code:
real flow_time = RP_Get_Real("flow-time");
Then your current approach could make use of the current time in the simulation.
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Old   July 5, 2010, 12:52
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Thanks TDI,

Yes, it is most helpful. I can use the "flow-time" as the independent variable in my analytical expressions.

I was wondering how I could define the pitching motion of the aerofoil if its center of pitching rotation is away from the "main" rotation axis of the system. Imaging the "main" rotation point is your elbow and the pitching rotation is your wrist. The elbow remains fixed throughout the whole motion and the wrist moves up and down along an arc. The aerofoil will pitch down or up when the arm is at the top or bottom of that arc. How could I define implicitly those two positions for the aerofoil ramping pitch?

Regards
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Old   July 5, 2010, 13:06
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You just left my comfort zone. Perhaps some others know of numerical trick to do what you are talking about.
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Old   July 5, 2010, 13:16
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Thanks for all your help TDI,

Hopefully somebody will read this tread and fell compassionate

Kind regards
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Old   July 11, 2010, 12:00
Default Dynamic meshing
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Hi

I want to learn about dynamic mechsing and need the file cyl3d.msh, does anyone have this file and if possible could you please email it to me at
davesmith_01@hotmail.co.uk.

I wanted to try and example problem before I try and do my own dynamic meshing

Thanks

Dave
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Old   May 6, 2011, 23:22
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zhang guoqing
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Hello,aerospain:
Do you have solved your problem now?
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Old   June 19, 2011, 09:30
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Alberto Pellegrino
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Hi Aerospain,

I'm trying to do your previous step, I mean a purely sinusoidal motion of a circular grid that contains the airfoil (pitching). And the circular grid (quad) is inside a tri-grid supposed to remain static.

Reading on cfd-online I have not figured out yet if I need to use DEFINE_CG_MOTION or DEFINE_GRID_MOTION...or even DEFINE_ADJUST as you mentioned above.

- Please can you explain what you did?

- Must the node at the interface (between rotating quad-mesh and static tri-mesh) be merged or not?

- What setting did you use on Fluent about dynamic mesh? I mean, smoothing, layering, remeshing, or nothing?

- Could you kindly show me your UDF? I would really appreciate your help.


Many thanks!
Kind regards,

Alberto
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Old   November 29, 2011, 06:55
Default 3D dynamic mesh tutorial
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Darren Cam
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Hi people,

I was attempting the 3D dynamic mesh tutorial however I haven't got the cyl3d_new.msh file. If anyone can send me the file please it will be greatly appreciated. Thanks.

My email: darren.camilleri90@gmail.com
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