# VOF wave getting damped.

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September 11, 2013, 07:07
VOF wave getting damped.
#1
Member

Arun Krishnan.L.H
Join Date: Jan 2013
Posts: 75
Rep Power: 6
Dear All,

I am going to work on wave interaction on ships and the dynamic forces for various cases. Just to get the modelling and meshing right for VOF wave and to what comes out of the software for a user defined wave, I am simulating first order waves in a rectangular domain.

The domain is 75m(x), 10m(y which is the direction of wave height), 1m(z direction). The free surface is initialized so that the domain is equally divided. The coordinate of y axis of free surface is 0.

I have defined a wave of length 5 mts, current/wind velocity 1.5 m/s, amp 0.1 mts. The mesh sizes on the free surface is refined using a block of height 0.3 m. The x and z refinement are wavelength/80 and y axis is amp/20. Time step is wave period (2.4*80). I have run the simulation using 50 and 70 inner iterations but the nature of the pressure plot seems to be that of a damped oscillation. Though the waves are not becoming completely flat , which was an issue i had before, it gets damped a little. The point on water level of the wave is 0,0,0. The distance from the wave to outlet is 50mts.

I have also changed the VOF relaxation from 0.9 to 0.5 but there is no change. Can anyone please guide me regarding what is going wrong or what other parameter i could change. I am running unsteady, inviscid model and 1st order solver.

Thanks
Regards
Arun
Attached Images
 vof initial.jpg (9.5 KB, 99 views) vof after 4 sec.jpg (10.3 KB, 93 views)

December 18, 2013, 09:34
#2
New Member

saeed barzegar
Join Date: Feb 2012
Posts: 2
Rep Power: 0
Dear Arun

I had a same problem. I suppose, you are working on old version of fluent? if it is true, you can follow another simple way (by new version of fluent (in ANSYS 13 or 13))
in ANSYS13-fluent, there are options for modeling wave (Airy, stocks,...) and free surface, but it takes times to realize. however, in this way you can simulate completely your problem.
my firs your problem was like "picture 1111" (likes your problem) and my result after modeling in new version of fluent is like "picture 2222" (that wave does not damp)

I hope my suggestion will be helpful.
Attached Images
 1111.jpg (38.6 KB, 87 views) 2222.png (3.8 KB, 76 views)

 January 18, 2016, 05:47 #3 New Member   merseyside Join Date: Jan 2016 Posts: 1 Rep Power: 0 Dear saeed, Would you please also share your experience with inlet and outlet settings? Thanks a lot. Jenn

 February 13, 2017, 07:56 #4 New Member   Yi Theng Sea Join Date: Jan 2017 Posts: 3 Rep Power: 2 Dear Saeed Hi I am trying to simulate waves too but it dampens out a lot over distance. I am using ANSYS version 16.2. Are you using UDF to generate waves? Can you share with me your UDF code? yithengs@gmail.com Your help is very very much appreciated Regards, Yitheng

 February 13, 2017, 14:05 #5 New Member   Dong Seo Join Date: Aug 2010 Posts: 3 Rep Power: 9 Dear Arun, I think your result doesn't looks like a wave damping issue. It looks like that there was not enough simulation time for getting the fully developed wave - the initial wave just starts propagating into the calm water area. It might be better to apply the initialization option to fill the whole domain with the wave. If you don't want a current-wave interaction, start with the zero current/wind velocity. Also, please check the wave period and length again. It doesn't match. Regards, Don

February 13, 2017, 20:43
#6
New Member

Yi Theng Sea
Join Date: Jan 2017
Posts: 3
Rep Power: 2
Dear Wester

Attached is the screenshot of the wave I generated, T=1 sec, A = 0.05m using UDF code. As you can see the damping issue is very serious where the wave just disappeared over the distance. Could you please share with me how could I solve the damping issues in my case?

Regards,
Yitheng
Attached Images
 Untitled.png (20.5 KB, 25 views)

 February 17, 2017, 09:48 #7 New Member   Dong Seo Join Date: Aug 2010 Posts: 3 Rep Power: 9 Hello SeaTiTheng, Sorry for the late reply. I can't make sure what was the main damping source just based on the attached image. Frequently, the reflected wave on the outlet boundary might be one of the reasons. Usually, if possible, adding an extra domain (space) is recommended to resolve this wave reflection issue. Also, gradual changes in meshing size and additional numerical damping (or smoothing) is useful to suppress the unwanted wave reflections. Cheers, Dong

 February 17, 2017, 10:54 #8 New Member   Yi Theng Sea Join Date: Jan 2017 Posts: 3 Rep Power: 2 Hi Wester Thanks for the reply! But i guess reflection might not be the issue here as what I am showing in the picture is the the first three waves that are generated from the moving wall on the right. The first three waves have not reached the left wall yet for reflection to occur i think. As you can see, the first wave is moving from the right to the left and the wave height reduces a lot compared to the waves on the right. Do you have any idea why this happens? Do let me know if you need any more information on my setup. Appreciate your help Regards, Yitheng

 February 17, 2017, 11:38 #9 New Member   Dong Seo Join Date: Aug 2010 Posts: 3 Rep Power: 9 Hi SeaYiTheng, What's the physical time of the attached image? You have to wait a sufficient time for the fully developed wave field. Assuming a deep water wave, (actually your case looks like an intermediate depth), phase velocity is 1.56 m/s, and group velocity is 0.78 m/s. So you can figure it out how much time needs for the fully developed wave field. Also did you check if your wave input is valid? It seems like that the your wave input is not appropriate to obtain a very sinusoidal linear wave - not enough water depth and too steep wave profile. Thanks, Dong

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