wave maker

wave maker · February 16, 2015, 10:15

hello...i want to use flap wavemaker method but i don't know how i can define the volume fraction between the two fluids: air and water.
i used like boundary condition for the wavemaker, for the bottom and for the endwall part.

if I was not clear ask me more explanations.
thanks a lot for your help and excuse me for my not perfect english

ghorrocks · February 16, 2015, 17:30

Have you looked at the tutorials? It explains how to do basic things like this. The "Free surface flow over a bump" tutorial would be relevant here.

wave maker · February 18, 2015, 13:39

i try to do solve my problem following the tutorial of "modellig a buoy using cfx rigid body solver" but i have some problem with the volume fraction at the initial time. this is the situation at the beginning of the simulation:

https://www.dropbox.com/s/8ugncusvh5...ction.png?dl=0

why at the inclined surface do i have this situation and no a straight line like before?????

Please help me

ghorrocks · February 18, 2015, 18:39

The jagged line is because you have defined the initial volume fraction as 1 when it is below some y value and 0 when it is above. This means any variation in your mesh will create jagged lines.

The best solution for this is to use a blurring function to blur the interface over a few elements. CFX blurs the free surface over about 4 elements, so that is a good blur to target. Then small variations int he underlying mesh will not create jagged lines. And you simulation will run better as well, as CFX will not need to resolve the ripples which these jagged initial conditions create.

wave maker · February 19, 2015, 04:11

sorry but how can i compute a blurring function????

ghorrocks · February 19, 2015, 06:58

tanh is a good base function for this, with the input variable being a function of the y height.

wave maker · February 25, 2015, 07:03

i had this situation with the initial volume fraction:

https://www.dropbox.com/s/ufxd2h2ks8...ction.png?dl=0

you told me to use a blurring function like tanh.
i'm sorry but i'm not able to solve my problem.
could you give me more information and advices to solve my problem and how to express blurring function please???
thanks a lot

P.S. sorry for my not percfect english

ghorrocks · February 25, 2015, 07:13

If you want to put the free surface at z=10[m], then a function like volume fraction = tanh((z-10[m])/1[m]) will blur the free surface over about 2 metres. Note the divide by 1[m] is just to make the argument dimensionless for the tanh function.

wave maker · February 25, 2015, 11:27

sorry but if i set in the water volume fraction tanh((y-1,5[m])/1[m]), where 1,5m is my free surface, the program doesn't give solution because there is a division by zero.

ghorrocks · February 25, 2015, 18:13

tanh(x) is defined for all x so it will not be creating the division by zero error. The error is coming from something else.

wave maker · March 18, 2015, 11:49

hello, i have a problem with the extrapolation of the data from CFD post. I am simulating a wave tank and i want to plot the free surface at some fixed times but i am not able to do that. can you help me?
thanks a lot

Jonas_cfd · July 10, 2020, 04:12

Hello Foamers!
I'm trying to understand the "WavePistonMaker" tutorial (OF_19.12). where can I find documentation about it?

ghorrocks · July 10, 2020, 05:35

We are not Foamers. This is the CFX forum, so try the Open Foam forum.

February 16, 2015, 10:15	wave maker	#1
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	hello...i want to use flap wavemaker method but i don't know how i can define the volume fraction between the two fluids: air and water. i used like boundary condition for the wavemaker, for the bottom and for the endwall part. if I was not clear ask me more explanations. thanks a lot for your help and excuse me for my not perfect english

July 10, 2020, 04:12	WaveMaker_Piston	#12
Jonas_cfd New Member Join Date: Jun 2020 Posts: 3 Rep Power: 6	Hello Foamers! I'm trying to understand the "WavePistonMaker" tutorial (OF_19.12). where can I find documentation about it?

July 10, 2020, 05:35		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	We are not Foamers. This is the CFX forum, so try the Open Foam forum. Stel likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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February 16, 2015, 17:30		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you looked at the tutorials? It explains how to do basic things like this. The "Free surface flow over a bump" tutorial would be relevant here.

February 18, 2015, 13:39		#3
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	i try to do solve my problem following the tutorial of "modellig a buoy using cfx rigid body solver" but i have some problem with the volume fraction at the initial time. this is the situation at the beginning of the simulation: https://www.dropbox.com/s/8ugncusvh5...ction.png?dl=0 why at the inclined surface do i have this situation and no a straight line like before????? Please help me

February 18, 2015, 18:39		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The jagged line is because you have defined the initial volume fraction as 1 when it is below some y value and 0 when it is above. This means any variation in your mesh will create jagged lines. The best solution for this is to use a blurring function to blur the interface over a few elements. CFX blurs the free surface over about 4 elements, so that is a good blur to target. Then small variations int he underlying mesh will not create jagged lines. And you simulation will run better as well, as CFX will not need to resolve the ripples which these jagged initial conditions create.

February 19, 2015, 04:11		#5
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	sorry but how can i compute a blurring function????

February 19, 2015, 06:58		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	tanh is a good base function for this, with the input variable being a function of the y height.

February 25, 2015, 07:03		#7
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	i had this situation with the initial volume fraction: https://www.dropbox.com/s/ufxd2h2ks8...ction.png?dl=0 you told me to use a blurring function like tanh. i'm sorry but i'm not able to solve my problem. could you give me more information and advices to solve my problem and how to express blurring function please??? thanks a lot P.S. sorry for my not percfect english

February 25, 2015, 07:13		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	If you want to put the free surface at z=10[m], then a function like volume fraction = tanh((z-10[m])/1[m]) will blur the free surface over about 2 metres. Note the divide by 1[m] is just to make the argument dimensionless for the tanh function.

February 25, 2015, 11:27		#9
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	sorry but if i set in the water volume fraction tanh((y-1,5[m])/1[m]), where 1,5m is my free surface, the program doesn't give solution because there is a division by zero.

February 25, 2015, 18:13		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	tanh(x) is defined for all x so it will not be creating the division by zero error. The error is coming from something else.

March 18, 2015, 11:49		#11
wave maker New Member wave maker Join Date: Feb 2015 Posts: 14 Rep Power: 11	hello, i have a problem with the extrapolation of the data from CFD post. I am simulating a wave tank and i want to plot the free surface at some fixed times but i am not able to do that. can you help me? thanks a lot