Deciding the flow of the models

Faris Qusyairi · May 16, 2022, 08:42

Currently I am working on simulation for lubrication film thickness simulation analysis from different surface roughness on different sliding sizes. So basically I decided to run the simulation of lubricant film between a piston ring and cylinder liner. But now I am having a problem on how to calculate the Reynolds number so I can decide if the flow is laminar or turbulence. I will attach the picture of the model below.

https://drive.google.com/file/d/1DHB...ew?usp=sharing
Density of oil: 856 kg/m3
Viscosity of oil: 0.106715 kg/ms

LuckyTran · May 16, 2022, 09:40

In general it should be laminar or else the film loses its load carrying capacity. If you are simulating a working piston-cylinder engine, it will be laminar.

Faris Qusyairi · May 16, 2022, 11:29

May i know what will be the suitable 2D model if i would like to run a turbulence flow?

LuckyTran · May 16, 2022, 12:05

If you're wondering if a lubrication film should be laminar or turbulent, then it should be laminar or it wouldn't be a lubricating film (or at least it would be a bad one).

If you're asking how to determine if it is laminar or turbulent based on Reynolds number, then you're looking for the Couette flow Reynolds number which is the shear (sliding) velocity and gap height, and that has a critical Reynolds number around 2000.

If you're asking where in the universe you can find a lubricating film that is turbulent, they exist in many places at non-optimal conditions for typical oil bearings that people get paid to design out of their systems. They also exist in non-lubricating film s(i.e. water films). So I must ask why? Is this some sort of academic exercise?

Faris Qusyairi · May 16, 2022, 13:38

Thank you for the reply and info, now I have better understanding of what I am doing, Yes this is some sort of academic exercise. I am having trouble because i dont know how am I supposed to find the effects of lubrication film thickness on different surface roughness if the flow is laminar. Because in ansys fluent surface roughness option is off for laminar flow. Or maybe you have any ideas on how am I able to solve the problem? I am new to this software.

LuckyTran · May 17, 2022, 03:45

Have you done any literature survey?

Surface roughnesses are usually much smaller than the diffusive length scale in a laminar flow and laminar flows (from a wall shear stress perspective) are basically unaffected by typical surface roughnesses–unless your roughness is an elephant. If the flow is affected by the roughness then you should grid it and just brute force it with DNS. If you try to look up the friction factor in a laminar pipe flow for example, it's independent of surface roughness.

On the roughness options available in Fluent, they are for modeling turbulent flows and no options exist for laminar flows. Even for turbulent flows, they barely have any effect.

May 16, 2022, 08:42	Deciding the flow of the models	#1
Faris Qusyairi New Member Faris Join Date: May 2022 Posts: 5 Rep Power: 5	Currently I am working on simulation for lubrication film thickness simulation analysis from different surface roughness on different sliding sizes. So basically I decided to run the simulation of lubricant film between a piston ring and cylinder liner. But now I am having a problem on how to calculate the Reynolds number so I can decide if the flow is laminar or turbulence. I will attach the picture of the model below. https://drive.google.com/file/d/1DHB...ew?usp=sharing Density of oil: 856 kg/m3 Viscosity of oil: 0.106715 kg/ms

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May 16, 2022, 09:40		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,846 Rep Power: 68	In general it should be laminar or else the film loses its load carrying capacity. If you are simulating a working piston-cylinder engine, it will be laminar.

May 16, 2022, 11:29		#3
Faris Qusyairi New Member Faris Join Date: May 2022 Posts: 5 Rep Power: 5	May i know what will be the suitable 2D model if i would like to run a turbulence flow?

May 16, 2022, 12:05		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,846 Rep Power: 68	If you're wondering if a lubrication film should be laminar or turbulent, then it should be laminar or it wouldn't be a lubricating film (or at least it would be a bad one). If you're asking how to determine if it is laminar or turbulent based on Reynolds number, then you're looking for the Couette flow Reynolds number which is the shear (sliding) velocity and gap height, and that has a critical Reynolds number around 2000. If you're asking where in the universe you can find a lubricating film that is turbulent, they exist in many places at non-optimal conditions for typical oil bearings that people get paid to design out of their systems. They also exist in non-lubricating film s(i.e. water films). So I must ask why? Is this some sort of academic exercise?

May 16, 2022, 13:38		#5
Faris Qusyairi New Member Faris Join Date: May 2022 Posts: 5 Rep Power: 5	Thank you for the reply and info, now I have better understanding of what I am doing, Yes this is some sort of academic exercise. I am having trouble because i dont know how am I supposed to find the effects of lubrication film thickness on different surface roughness if the flow is laminar. Because in ansys fluent surface roughness option is off for laminar flow. Or maybe you have any ideas on how am I able to solve the problem? I am new to this software.

May 17, 2022, 03:45		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,846 Rep Power: 68	Have you done any literature survey? Surface roughnesses are usually much smaller than the diffusive length scale in a laminar flow and laminar flows (from a wall shear stress perspective) are basically unaffected by typical surface roughnesses–unless your roughness is an elephant. If the flow is affected by the roughness then you should grid it and just brute force it with DNS. If you try to look up the friction factor in a laminar pipe flow for example, it's independent of surface roughness. On the roughness options available in Fluent, they are for modeling turbulent flows and no options exist for laminar flows. Even for turbulent flows, they barely have any effect.