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-   -   Cany anyone tell me how to define oscillations using field functions?? (http://www.cfd-online.com/Forums/star-ccm/114251-cany-anyone-tell-me-how-define-oscillations-using-field-functions.html)

arun7328 March 7, 2013 13:27

Cany anyone tell me how to define oscillations using field functions??
 
Hi all,

I have tried every possible way other than morphing, to define an oscillatory motion to a cylinder in free surface. The field function shows no syntax error but when i try to define it in translation motion it gives an error.

Is it only possible to define translational velocities as numbers [x,y,z]? Is it not possible to assign a field function?

Can any one please tell me how to define oscillatory motion? I am looking at SHM of amplitude 0.1 mts and frequency 1 rad/s.I will then have to change the frequency and get results for various frequencies?

Please help me if you have any idea

Regards
Arun

AndrewLBloxom March 7, 2013 14:44

Could you post a screenshot of your definition and the property window?

arun7328 March 7, 2013 15:01

Hi andrew.

I have been getting an error the whole day while trying to define it as a vector field function. That is the translation velocity was defined as vector field function which is a SHM. But now i have imported a table of 2 columns time and velocity and defined this in a field function and later assigned this function to translational velocity. Its running now but i am not sure of this definition and what i am going to get.

If you send me your mail id i can share it from my drop box..I have put the model in the dropbox. Mine is arun7328@gmail.com.

Send a mail.
thanks
Arun

arun7328 March 8, 2013 08:09

Hi all,

Thanks a lot for every one who viewed my post.

I have successfully defined an oscillatory motion to a cylinder and thought of answering my own thread as it could be useful for others who may be trying to simulate simple problems like these while learning CFD.

So my problem was to oscillate a cylinder on the free surface. The oscillations can be either in horizontal or vertical directions. I have used overset mesh for my problem and refined the overlap and the free surface region. Now is the refinement good enough, I will have to check from my results.

1. Create a new motion 'Translation' and assign the same to the motion specification of the overset. I am starting with an SHM of 1 mt amplitude and 1 rad/s frequency. My task is evaluate the hydrodynamic forces on the body and further the added mass and damping coefficients for various frequencies ranging from 1 to 12.

So a simple SHM of x=Asinwt and velocity=Aw coswt was assumed.

2. Create an excel sheet with time and velocity for a comparatively long time frame. I have started with 10 secs with a time interval of 0.1 secs. The file has 4 colums in total time, vel_x,vel_y,vel_z and 0.0 entered for y and z velocities. Save this as a .crv file and import the table into star ccm+.

3.Create a new field function and give a function name and type. In this case i gave WallVel as the name and type->velocity. In definition specify such

interpolateTable(@Table("velocity"),"TIME","LINEAR ","VEL_X","VEL_Y","VEL_Z",$Time)

This command interpolates the components of velocity over the time interval. Make sure the terms under quotes are defined exactly the same way as you have defined your column name. They are case sensitive(LINEAR should be in caps, refer manual for other options)


4. In the motion node->Translation define the velocity as the function name of the user defined field. Since i have defined a vector i referred it as $$WallVel

5. decide on your time steps, scenes,plots and start solution. :)

I could not do it with field function alone but i will try sometime later and reply if I accomplish.

I am new to CFD and that is the reason i had posted this simple doubt. I had been trying for 2-3 days, before i thought of asking people in the forum.

Would require expert advice again in future. :)

Thanks a lot
Regards
Arun

Ollie West June 8, 2015 09:51

Hi arun7328,

Thank you very much for the guide you provided. I have followed it as closely as possible but seem to get the following error when trying to run the simulation.

"Cannot evaluate expression $$velocity on
Simulation>MotionManager>TranslatingMotion("veloci ty")>VectorPhyscialQuantity>InlineFieldFunction("I nlineFieldFunction").

Do you know why this might be happening and what I can do to fix it?

Many thanks,

Oliver.

Ollie West June 9, 2015 11:45

Managed to fix it.

There's an error in your field function arun7328. LINEAR should not be in quotes, so the correct definition is as follows.

interpolateTable(@Table("velocity"), "TIME", LINEAR, "VEL_X", "VEL_Y", "VEL_Z", $Time)

Thanks.


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