wave making resistance

leviathan · March 19, 2012, 18:42

hi all, I'm trying to use ansys for evaluating wave making resistance on catamaran for my final assignment, but as I've done half of the work, I just know that ansys cfx would only calculate total resistance, not any spesific resistance. Is there any other way so that I could get the value of wave making resistance?
thx

ghorrocks · March 19, 2012, 19:35

Splitting total drag into wave making and viscous is a little artifical anyway.

You might be able to something which looks like viscous drag by integrating the wall shear stress over the hull, and the wave resistance from the intergal of pressure.

leviathan · March 21, 2012, 20:56

Could you give me some more explanations please? I mean, I don't really get it...Am I have to calculate shear stress and pressure instead resistance with cfx-post? does the integration would give valid value comparing to towing tank test results? and if I have to generate the wave, colud you give me some advice please?

thx

ghorrocks · March 21, 2012, 22:12

My point is that you simply tow the model along and get a drag. You have to define something which splits it into wave and viscous and that split is a little artificial. And likewise, splitting the total drag in a CFD simulation into wave and viscous drag is a little artificial.

Integrating the wall shear over the body is a reasonable start, leaving the difference to total drag as the wave making resistance is one approach.

leviathan · March 25, 2012, 07:38

that someting is the part that I don't understand....Does integrating the wall shear could give me valid result? and is that acceptable?

is it nothing I could do with CEL to solve it?

ghorrocks · March 25, 2012, 07:56

I think you are missing my point. If you can mathematically define what wave drag is then you can do it in CFX and extract the number. If you have wishy-washy definitions like "it is the drag from the making the waves" you are not going to get anywhere. So yo uhave to define mathematically what it is, then it is usually fairly simple to calculate it from a CFX result.

leviathan · March 26, 2012, 14:06

hi ghorrocks ,

I think I'm starting to understand your point...you're right, my previous definitions really2 something like "it is the drag from the making the waves" (that's what I've got from class)..then would you mind to tell me how it should be?

regards,
thx

ghorrocks · March 26, 2012, 19:08

I do not know as this is not my field - and I do not have time to search for you. Google will help, also naval architecture textbooks. You need to find a solid definition of what it is then you consider implementing it.

leviathan · March 27, 2012, 03:25

Well, I'll try to figure it out somehow, but then, would you mind to help me with the software afterwards? Cause I'm really newbie in CFD

If you feel like doing it, please tell me your email address so it will be easier for me to contact you later.

regards

Thx

(ah one more thing, is it possible to see the wave spectrum? cause I guess I could figure something out if I could generate the wave...thx again)

Cluain · March 27, 2012, 04:59

Leviathan, you can get a visual output in Post, Also if you record a time series of wave heights then you can do a spectral analysis on it

ghorrocks · March 27, 2012, 06:46

If you have further questions feel free to start a new thread on the forum. I do not give out my email address for the forum.

leviathan · March 27, 2012, 13:39

Quote:

Originally Posted by Cluain

Leviathan, you can get a visual output in Post, Also if you record a time series of wave heights then you can do a spectral analysis on it

Where could I learn about it (making visual outpot and record time series) ? do you have links to any tutorial?

leviathan · March 27, 2012, 13:39

Quote:

Originally Posted by ghorrocks

If you have further questions feel free to start a new thread on the forum. I do not give out my email address for the forum.

okay...I'll try to learn about wave drag first...thx

March 19, 2012, 18:42	wave making resistance	#1
leviathan New Member ludi Join Date: Sep 2011 Posts: 21 Rep Power: 15	hi all, I'm trying to use ansys for evaluating wave making resistance on catamaran for my final assignment, but as I've done half of the work, I just know that ansys cfx would only calculate total resistance, not any spesific resistance. Is there any other way so that I could get the value of wave making resistance? thx

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March 19, 2012, 19:35		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Splitting total drag into wave making and viscous is a little artifical anyway. You might be able to something which looks like viscous drag by integrating the wall shear stress over the hull, and the wave resistance from the intergal of pressure.

March 21, 2012, 20:56		#3
leviathan New Member ludi Join Date: Sep 2011 Posts: 21 Rep Power: 15	Could you give me some more explanations please? I mean, I don't really get it...Am I have to calculate shear stress and pressure instead resistance with cfx-post? does the integration would give valid value comparing to towing tank test results? and if I have to generate the wave, colud you give me some advice please? thx

March 21, 2012, 22:12		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	My point is that you simply tow the model along and get a drag. You have to define something which splits it into wave and viscous and that split is a little artificial. And likewise, splitting the total drag in a CFD simulation into wave and viscous drag is a little artificial. Integrating the wall shear over the body is a reasonable start, leaving the difference to total drag as the wave making resistance is one approach.

March 25, 2012, 07:38		#5
leviathan New Member ludi Join Date: Sep 2011 Posts: 21 Rep Power: 15	that someting is the part that I don't understand....Does integrating the wall shear could give me valid result? and is that acceptable? is it nothing I could do with CEL to solve it?

March 25, 2012, 07:56		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I think you are missing my point. If you can mathematically define what wave drag is then you can do it in CFX and extract the number. If you have wishy-washy definitions like "it is the drag from the making the waves" you are not going to get anywhere. So yo uhave to define mathematically what it is, then it is usually fairly simple to calculate it from a CFX result.

March 26, 2012, 14:06		#7
leviathan New Member ludi Join Date: Sep 2011 Posts: 21 Rep Power: 15	hi ghorrocks , I think I'm starting to understand your point...you're right, my previous definitions really2 something like "it is the drag from the making the waves" (that's what I've got from class)..then would you mind to tell me how it should be? regards, thx

March 26, 2012, 19:08		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I do not know as this is not my field - and I do not have time to search for you. Google will help, also naval architecture textbooks. You need to find a solid definition of what it is then you consider implementing it.

March 27, 2012, 03:25		#9
leviathan New Member ludi Join Date: Sep 2011 Posts: 21 Rep Power: 15	Well, I'll try to figure it out somehow, but then, would you mind to help me with the software afterwards? Cause I'm really newbie in CFD If you feel like doing it, please tell me your email address so it will be easier for me to contact you later. regards Thx (ah one more thing, is it possible to see the wave spectrum? cause I guess I could figure something out if I could generate the wave...thx again)

March 27, 2012, 04:59		#10
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 15	Leviathan, you can get a visual output in Post, Also if you record a time series of wave heights then you can do a spectral analysis on it

March 27, 2012, 06:46		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	If you have further questions feel free to start a new thread on the forum. I do not give out my email address for the forum.