simple questions on water in Fluent

me3840 · November 17, 2010, 14:42

Hey guys. I know a little bit about Fluent in air, but none at all about water. I don't know much of anything about liquid dynamics.
I'm doing a pretty basic simulation of a shell under water.
I've got a couple questions:

I've used the ke turbulence and the kklw transitional models in air - are these still valid for water? I think they are, but I want to be sure.
Are the boundary layer settings for water the same as air?
I know how to change the fluid material from air to water in Fluent; is there anything else I need to do to get a fully under water model?
Since the density of liquids don't change much with depth, does depth matter in the simulation?
For most of my in-air simulations, I use a cv with velocity-inlet and pressure-outlet. Should I change these in light of the liquid?

Any help is much appreciated!

Philipov · November 18, 2010, 07:13

Are you serious with these questions?!?!?!?!?!
Read books friend, read books....
CFD=Computational Fluid Dynamics (water, air, particular gas.... all these are fluids)
Difference is when you deal with Newtonian and Non-Newtonian fluids....

me3840 · November 18, 2010, 14:23

Yes, I'm serious; generally I try not to waste my time asking questions I don't want answers to.

The CFD books I have don't answer these questions, and it's not like high-level information is available widely on the Internet or at your local library. I don't recall it ever being a crime to ask a question.

I understand what fluid means, and both water and air in this case are Newtonian, so I'm not sure what you were getting at there.

Philipov · November 18, 2010, 16:25

I give up... if anyone wants to help - good luck

me3840 · November 18, 2010, 18:03

Well, thanks for reading, even if you gave up before trying.

andyross33 · November 18, 2010, 21:23

Wow, what welcoming and helpful comments from a senior member. I'm new to this forum and frankly I expect a more professional demeanour. Please save the bashing for youtube forums.

I'm new to Fluent and still getting up to speed so I doubt I'll be much help for you. However, I thought I'd pass on that the instructor at Ansys' course I attended a few weeks ago highly recommended an introductory CFD book which I'm about to order myself. You can get it at Amazon... http://www.amazon.com/Introduction-C...0128625&sr=8-4

According to the instructor it briefs over the basics behind CFD, when to use different turbulence models and solver methods and includes Fluent codes in its examples.

BTW, I'm in no way affiliated with this book I just thought maybe it might be of some help to you. Good luck and all the best.

KristianEtienne · November 19, 2010, 10:25

Hi,

As pointed out by Philipov and yourself, both air and water are newtonian fluids. Hence, their ("dimensionless") behaviour is essentially identical. Parameters (i.e. viscosity and density) are neccesarily different, which for instance results in larger pressures in water compared to air. As you said in your post, these material properties are changed in the material panel in Fluent.

From a CFD point of view the solution strategy is indentical, set aside compressibility effects which might occur for high Mach-number air-flows. Thus, you can safely use the same turbulence model and boundary condition which you have used for air (again, given that your air-simulations were incompressible).

The k-epsilon model I know is ok for water. I am uncertain about the kklw transitional models, but a quick check in the literature should give you an indication if it has been validated or not. Most likely it is ok also (turbulence modelling being what it is).

Concerning depth, this could be an issue. Not due to density, but due to pressure. Hydrostatically, pressure increases linearly with depth which could have importance for your simulations, depending on what you are looking for. For instance, the force on an object placed at 10m water depth is significantly different from the force on an object placed at 1m.

Finally, regarding boundary layers, these will have to changed when changing your working fluid as they typically are a function of the Reynolds-number (which depends upon your material properties). See for instance chapter 7 in the book Fluid Mechanics by Frank M. White, or Viscous Fluid Flow by the same author for an introduction.

Good luck!

November 17, 2010, 14:42	simple questions on water in Fluent	#1
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	Hey guys. I know a little bit about Fluent in air, but none at all about water. I don't know much of anything about liquid dynamics. I'm doing a pretty basic simulation of a shell under water. I've got a couple questions: I've used the ke turbulence and the kklw transitional models in air - are these still valid for water? I think they are, but I want to be sure. Are the boundary layer settings for water the same as air? I know how to change the fluid material from air to water in Fluent; is there anything else I need to do to get a fully under water model? Since the density of liquids don't change much with depth, does depth matter in the simulation? For most of my in-air simulations, I use a cv with velocity-inlet and pressure-outlet. Should I change these in light of the liquid? Any help is much appreciated!

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November 18, 2010, 07:13		#2
Philipov Senior Member Svetlin Filipov Join Date: Mar 2009 Location: United Kingdom Posts: 176 Rep Power: 17	Are you serious with these questions?!?!?!?!?! Read books friend, read books.... CFD=Computational Fluid Dynamics (water, air, particular gas.... all these are fluids) Difference is when you deal with Newtonian and Non-Newtonian fluids....

November 18, 2010, 14:23		#3
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	Yes, I'm serious; generally I try not to waste my time asking questions I don't want answers to. The CFD books I have don't answer these questions, and it's not like high-level information is available widely on the Internet or at your local library. I don't recall it ever being a crime to ask a question. I understand what fluid means, and both water and air in this case are Newtonian, so I'm not sure what you were getting at there.

November 18, 2010, 16:25		#4
Philipov Senior Member Svetlin Filipov Join Date: Mar 2009 Location: United Kingdom Posts: 176 Rep Power: 17	I give up... if anyone wants to help - good luck

November 18, 2010, 18:03		#5
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	Well, thanks for reading, even if you gave up before trying.

November 18, 2010, 21:23		#6
andyross33 New Member Andrew Join Date: Nov 2010 Posts: 12 Rep Power: 16	Wow, what welcoming and helpful comments from a senior member. I'm new to this forum and frankly I expect a more professional demeanour. Please save the bashing for youtube forums. I'm new to Fluent and still getting up to speed so I doubt I'll be much help for you. However, I thought I'd pass on that the instructor at Ansys' course I attended a few weeks ago highly recommended an introductory CFD book which I'm about to order myself. You can get it at Amazon... http://www.amazon.com/Introduction-C...0128625&sr=8-4 According to the instructor it briefs over the basics behind CFD, when to use different turbulence models and solver methods and includes Fluent codes in its examples. BTW, I'm in no way affiliated with this book I just thought maybe it might be of some help to you. Good luck and all the best.

November 19, 2010, 10:25		#7
KristianEtienne New Member Kristian Etienne Einarsrud Join Date: Oct 2010 Location: Trondheim Posts: 29 Rep Power: 16	Hi, As pointed out by Philipov and yourself, both air and water are newtonian fluids. Hence, their ("dimensionless") behaviour is essentially identical. Parameters (i.e. viscosity and density) are neccesarily different, which for instance results in larger pressures in water compared to air. As you said in your post, these material properties are changed in the material panel in Fluent. From a CFD point of view the solution strategy is indentical, set aside compressibility effects which might occur for high Mach-number air-flows. Thus, you can safely use the same turbulence model and boundary condition which you have used for air (again, given that your air-simulations were incompressible). The k-epsilon model I know is ok for water. I am uncertain about the kklw transitional models, but a quick check in the literature should give you an indication if it has been validated or not. Most likely it is ok also (turbulence modelling being what it is). Concerning depth, this could be an issue. Not due to density, but due to pressure. Hydrostatically, pressure increases linearly with depth which could have importance for your simulations, depending on what you are looking for. For instance, the force on an object placed at 10m water depth is significantly different from the force on an object placed at 1m. Finally, regarding boundary layers, these will have to changed when changing your working fluid as they typically are a function of the Reynolds-number (which depends upon your material properties). See for instance chapter 7 in the book Fluid Mechanics by Frank M. White, or Viscous Fluid Flow by the same author for an introduction. Good luck!