A confusing problem at making a wave tank

divoone1985 · June 14, 2015, 10:21

Hello.
I modeled a water tank in cfx.
i tried to make waves in the tank by make a moving wall.
i move the left wall of the tank around it's bottom point.
as like this image: http://i58.tinypic.com/14o7ywh.jpg

as you know the wave height must be increased by increasing the angular frequency. (or decreasing the period)
after solving the problem every thing is right.
but the wave height decreased by decreasing the period.
Expressions i defined are these:

Xdisp = (y/wavelen)*Af*sin(angularfrec*t)
angularfrec = (2*pi)/period
period = 1.2[s]
wavelen = 2.5[m]

Af is "S" in above image.

Does anyone Know where my mistake is?

ghorrocks · June 14, 2015, 20:28

Obviously decreasing the period will not increase the wave height for all cases. Do you think that a very short period would generate waves hundreds of metres high?

When you start getting to short periods then the liquid does not have enough time to refill the volume emptied by the moving wall and the wave height gets small again. I will leave as an exercise for you to work out the maximum wave height versus period

divoone1985 · June 20, 2015, 08:51

Quote:

Originally Posted by ghorrocks

Obviously decreasing the period will not increase the wave height for all cases. Do you think that a very short period would generate waves hundreds of metres high?

When you start getting to short periods then the liquid does not have enough time to refill the volume emptied by the moving wall and the wave height gets small again. I will leave as an exercise for you to work out the maximum wave height versus period

I saw some experimental data by this method from a wave pool.
those results show increasing in wave height by decreasing the period.
but my work with same conditions shows reverse results.
my results for all periods and all courses are same.
and waves are in centimeter scale.

ghorrocks · June 21, 2015, 07:30

Across the entire spectrum, increasing period will not always give larger waves. Across the range of conditions tested in that report it might show increasing period gives larger waves.

Now your question is different - why is my simulation inaccurate? FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

June 14, 2015, 10:21	A confusing problem at making a wave tank	#1
divoone1985 Member M S Join Date: Feb 2015 Posts: 30 Rep Power: 11	Hello. I modeled a water tank in cfx. i tried to make waves in the tank by make a moving wall. i move the left wall of the tank around it's bottom point. as like this image: http://i58.tinypic.com/14o7ywh.jpg as you know the wave height must be increased by increasing the angular frequency. (or decreasing the period) after solving the problem every thing is right. but the wave height decreased by decreasing the period. Expressions i defined are these: Xdisp = (y/wavelen)Afsin(angularfrect) angularfrec = (2pi)/period period = 1.2[s] wavelen = 2.5[m] Af is "S" in above image. Does anyone Know where my mistake is?

June 14, 2015, 20:28		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Obviously decreasing the period will not increase the wave height for all cases. Do you think that a very short period would generate waves hundreds of metres high? When you start getting to short periods then the liquid does not have enough time to refill the volume emptied by the moving wall and the wave height gets small again. I will leave as an exercise for you to work out the maximum wave height versus period divoone1985 likes this.

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June 21, 2015, 07:30		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Across the entire spectrum, increasing period will not always give larger waves. Across the range of conditions tested in that report it might show increasing period gives larger waves. Now your question is different - why is my simulation inaccurate? FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F