For ship analysis, Free Slip or No Slip?

alfianalfarisi · September 9, 2020, 00:14

I have trouble, I want to analyze ship resistance using Ansys CFX, I have defined the wall, inlet, outlet, then what should I choose for the ship part? is it a wall with slip or free slip?

Thank you

karachun · September 9, 2020, 05:29

If you choose Free Slip you will neglect friction resistance, I don't think you want to do this.

urosgrivc · September 9, 2020, 05:29

If you want to include the drag forces which occur because of the thin boundary layer near the wall than you need to include slip.

Free slip means that there is no near-wall transition from wall speed to the flow near the wall.

If you include slip this means that the 'fluid particles' which are touching the wall have the same velocity as the wall, which is in most cases.

Your drag value will probably be higher with slip than without it. And the difference would be dependant on the viscosity of the fluid.
For example, honey would have a much higher drag with slip than without.
For a nonvisus fluid there would be almost no diference

Opaque · September 9, 2020, 08:55

Disclosure: I am not an expert on ship modeling by any means.

However, if I recall correctly from the fluid mechanics course, there are two fundamentals components of the total ship resistance: wave and viscous resistances. Froude's work on similarity explained that.

Now, if you use a Free Slip wall option at the boundary, you will definitely ignore the viscous forces at that specific wall, but they will still be present in the fluid. I am not sure how you will compute the "wave resistance" component though.

karachun · September 9, 2020, 10:28

Quote:

Originally Posted by Opaque

wave and viscous resistances

Small clarification

Total Resistance = Friction Resistance + Shape Resistance + Wave Resistance.

Friction resistance can be estimated using friction formula for flat plate.

Here is a useful link to "Practical Guidelines for Ship CFD Applications".

https://ittc.info/media/4196/75-03-02-03.pdf

For TS question - No Slip corresponds to real life behavior of fluids and gases in most cases, except low pressure gases.

Free Slip condition is physical simplification, not presented in real life, except superfluids but I'm not sure. Free slip is good to model top, bottom and side walls of your domain if you model ships in an infinite volume of fluid. At last for the first try.

You should have some good reasons (want calculate components of resistance) to set wall to Free Slip.

alfianalfarisi · September 10, 2020, 21:50

Thanks for all your response, I'll continue to the next analysis.

September 9, 2020, 00:14	For ship analysis, Free Slip or No Slip?	#1
alfianalfarisi New Member M. Alfian Alfarisi Habsya Join Date: Sep 2020 Posts: 12 Rep Power: 6	I have trouble, I want to analyze ship resistance using Ansys CFX, I have defined the wall, inlet, outlet, then what should I choose for the ship part? is it a wall with slip or free slip? Thank you

September 9, 2020, 08:55		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Disclosure: I am not an expert on ship modeling by any means. However, if I recall correctly from the fluid mechanics course, there are two fundamentals components of the total ship resistance: wave and viscous resistances. Froude's work on similarity explained that. Now, if you use a Free Slip wall option at the boundary, you will definitely ignore the viscous forces at that specific wall, but they will still be present in the fluid. I am not sure how you will compute the "wave resistance" component though. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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September 9, 2020, 05:29		#2
karachun Senior Member Join Date: Nov 2015 Posts: 246 Rep Power: 12	If you choose Free Slip you will neglect friction resistance, I don't think you want to do this.

September 9, 2020, 05:29		#3
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	If you want to include the drag forces which occur because of the thin boundary layer near the wall than you need to include slip. Free slip means that there is no near-wall transition from wall speed to the flow near the wall. If you include slip this means that the 'fluid particles' which are touching the wall have the same velocity as the wall, which is in most cases. Your drag value will probably be higher with slip than without it. And the difference would be dependant on the viscosity of the fluid. For example, honey would have a much higher drag with slip than without. For a nonvisus fluid there would be almost no diference

September 10, 2020, 21:50		#6
alfianalfarisi New Member M. Alfian Alfarisi Habsya Join Date: Sep 2020 Posts: 12 Rep Power: 6	Thanks for all your response, I'll continue to the next analysis.