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CFD - Surface roughness

Posted Dec 11, 2012, 7:15 a.m. EST Fluid & Heat, Computational Fluid Dynamics (CFD), Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3a 15 Replies

Mikael Noerregaard Nielsen
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Hey guys

I really need your help on this one!

I was wondering if any of you could tell me about the default settings of surface roughness's in COMSOL?
All I've found about it in the documentation is Materials 3D Model Default Appearance Settings but I can't find anything about the use of surface roughness in the calculation of the pressure loss.
But the surface roughness does have an impact on the pressure loss however, I don't know where to find it nor do I know how to change it? Does COMSOL even use the material surface roughness or is it like Blasius --> surface roughness = 0?

Best Regards

Mikael

15 Replies Last Post Jan 28, 2013, 11:43 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 11, 2012, 7:31 a.m. EST
Hi

but I believe that is only the "appearance of the graphics" not any surface physical material property value, you must check the Equations for your physics and check if there is any surface roughness parameter. I'm not sure this is defined as a PDE anyhow, more as engineering correction factors from the literature.
Have you tried a search on the doc about surface roughness under the CFD doc ?

--
Lykke til
Ivar
Hi but I believe that is only the "appearance of the graphics" not any surface physical material property value, you must check the Equations for your physics and check if there is any surface roughness parameter. I'm not sure this is defined as a PDE anyhow, more as engineering correction factors from the literature. Have you tried a search on the doc about surface roughness under the CFD doc ? -- Lykke til Ivar
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Dec 11, 2012, 7:44 a.m. EST
Mange tak Ivar (thanks a lot)

Yep that is the very problem, that it is only graphics and has nothing to do with physical surface of the walls in the model. I've searched the CFD-doc as well - still trying though as I really have to figure it out.
I don't think there's the surface roughness is in the equations but Im still checking those as well. Currently going through those in the turbulent model window - I believe that's where I should find the surface roughness, under wall functions.

Best regards

Mikael
Mange tak Ivar (thanks a lot) Yep that is the very problem, that it is only graphics and has nothing to do with physical surface of the walls in the model. I've searched the CFD-doc as well - still trying though as I really have to figure it out. I don't think there's the surface roughness is in the equations but Im still checking those as well. Currently going through those in the turbulent model window - I believe that's where I should find the surface roughness, under wall functions. Best regards Mikael
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 11, 2012, 8:50 a.m. EST
Hi

The first two books on Fluid flow I took out from my bookshelf (Fluid Mechanics F.M.White Mc Graham Hill, 4th Ed, and Basic fluid and mass transfer, A.F. Mills 2nd Ed, Prentice Hall) both present roughness as "engineering" empirical corrections to the skin friction layer

So you must add these in via some modifications to the equations if you want them.

But in the Pipe module there seem to be Surface roughness factors included with different friction models
In the Geothermal models you have a Darcy friction factor
So I assume COMSOl has added some of these equations, but I have none of these modules ;(

--
Good luck
Ivar
Hi The first two books on Fluid flow I took out from my bookshelf (Fluid Mechanics F.M.White Mc Graham Hill, 4th Ed, and Basic fluid and mass transfer, A.F. Mills 2nd Ed, Prentice Hall) both present roughness as "engineering" empirical corrections to the skin friction layer So you must add these in via some modifications to the equations if you want them. But in the Pipe module there seem to be Surface roughness factors included with different friction models In the Geothermal models you have a Darcy friction factor So I assume COMSOl has added some of these equations, but I have none of these modules ;( -- Good luck Ivar
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Dec 11, 2012, 9:09 a.m. EST
Ahh, I see that the used terms are different from my textbooks compared to COMSOL - no wonder I could not find any of variables. I'll go through the equations yet again. The value epsilon (surface roughness [mm]) is used, although I'm uncertain whether or not I'm able to define it in the parametres without COMSOL crashing.

Either way. Thanks a lot for your help Ivar

I'll post the answer to the problem (if I find the solution) so others can use it.

Best regards

Mikael

EDIT. epsilon is the relative permittivity (of course), so if Darcy's Law of Friction is indeed applied, it uses different terms than the textbook you mention Ivar.
Ahh, I see that the used terms are different from my textbooks compared to COMSOL - no wonder I could not find any of variables. I'll go through the equations yet again. The value epsilon (surface roughness [mm]) is used, although I'm uncertain whether or not I'm able to define it in the parametres without COMSOL crashing. Either way. Thanks a lot for your help Ivar I'll post the answer to the problem (if I find the solution) so others can use it. Best regards Mikael EDIT. epsilon is the relative permittivity (of course), so if Darcy's Law of Friction is indeed applied, it uses different terms than the textbook you mention Ivar.
Niklas Rom COMSOL Employee

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Posted: 1 decade ago Dec 12, 2012, 10:07 a.m. EST
Hello Mikael,
as Ivar points out, in the 1D Pipe Flow module, empirical correlations are used for pressure drop calculations, which in turn makes use of the surface roughness as an input.

In the 2D and 3D CFD physics interfaces, like Turbulent Flow k-epsilon, smooth walls are assumed by default. However, the B turbulence model parameter in the k-epsilon model is related to surface roughness. The default value 5.2 corresponds to smooth walls. Surface roughness can theoretically be included by exchanging the B-parameter for a function C=C(roughness). For a smooth wall, C=B, but the variation is in general solution dependent. See for example "Boundary Layer Theory" by Schlichting for details.

kind regards
Niklas

Hello Mikael, as Ivar points out, in the 1D Pipe Flow module, empirical correlations are used for pressure drop calculations, which in turn makes use of the surface roughness as an input. In the 2D and 3D CFD physics interfaces, like Turbulent Flow k-epsilon, smooth walls are assumed by default. However, the B turbulence model parameter in the k-epsilon model is related to surface roughness. The default value 5.2 corresponds to smooth walls. Surface roughness can theoretically be included by exchanging the B-parameter for a function C=C(roughness). For a smooth wall, C=B, but the variation is in general solution dependent. See for example "Boundary Layer Theory" by Schlichting for details. kind regards Niklas
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Dec 15, 2012, 5:45 a.m. EST
Dear Niklas

Thanks a lot for your answer, it was most useful!

Best regards

Mikael
Dear Niklas Thanks a lot for your answer, it was most useful! Best regards Mikael
Ahmad Syahrir Ayub
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Posted: 1 decade ago Jan 12, 2013, 9:15 a.m. EST

Hello Mikael,
as Ivar points out, in the 1D Pipe Flow module, empirical correlations are used for pressure drop calculations, which in turn makes use of the surface roughness as an input.

In the 2D and 3D CFD physics interfaces, like Turbulent Flow k-epsilon, smooth walls are assumed by default. However, the B turbulence model parameter in the k-epsilon model is related to surface roughness. The default value 5.2 corresponds to smooth walls. Surface roughness can theoretically be included by exchanging the B-parameter for a function C=C(roughness). For a smooth wall, C=B, but the variation is in general solution dependent. See for example "Boundary Layer Theory" by Schlichting for details.

kind regards
Niklas


Hi Niklas and everyone.

Thank you for the explanation.

A little bit of introduction, I am trying to simulate an airflow around an airfoil to which I will be studying its aerodynamics properties (i.e. lift and drag coefficients) for various wind speeds and angle of attack.

First, I will be looking into the aerodynamic properties with clean/smooth surface of an airfoil. Later, I will be looking on the effect of surface roughness (i.e.height) on an airfoil. The simulations will be done in 2D to start with, I may be doing it in 3D too later as for comparison.

So, looking at your comment that we can change/modify parameter B in k-epsilon to simulate surface roughness, hence, it would be great if you could advise me on the following:
1) Is there any limitation in terms of its (i.e. parameter 'B') usage? I mean, does it only valid to be used in pipe flow module? Can it be also used for airfoil studies?
2) Apart from modifying its roughness height, Could I also change the location of the roughness itself over airfoil surfaces (i.e. upper and lower camber)? or does it just represent an uniform roughness throughout an object (e.g. pipeflow, flat plate, airfoil) under study?

I thank you very much in advance for any comments given regarding my aforementioned questions.

Regards,
Ahmad
[QUOTE] Hello Mikael, as Ivar points out, in the 1D Pipe Flow module, empirical correlations are used for pressure drop calculations, which in turn makes use of the surface roughness as an input. In the 2D and 3D CFD physics interfaces, like Turbulent Flow k-epsilon, smooth walls are assumed by default. However, the B turbulence model parameter in the k-epsilon model is related to surface roughness. The default value 5.2 corresponds to smooth walls. Surface roughness can theoretically be included by exchanging the B-parameter for a function C=C(roughness). For a smooth wall, C=B, but the variation is in general solution dependent. See for example "Boundary Layer Theory" by Schlichting for details. kind regards Niklas [/QUOTE] Hi Niklas and everyone. Thank you for the explanation. A little bit of introduction, I am trying to simulate an airflow around an airfoil to which I will be studying its aerodynamics properties (i.e. lift and drag coefficients) for various wind speeds and angle of attack. First, I will be looking into the aerodynamic properties with clean/smooth surface of an airfoil. Later, I will be looking on the effect of surface roughness (i.e.height) on an airfoil. The simulations will be done in 2D to start with, I may be doing it in 3D too later as for comparison. So, looking at your comment that we can change/modify parameter B in k-epsilon to simulate surface roughness, hence, it would be great if you could advise me on the following: 1) Is there any limitation in terms of its (i.e. parameter 'B') usage? I mean, does it only valid to be used in pipe flow module? Can it be also used for airfoil studies? 2) Apart from modifying its roughness height, Could I also change the location of the roughness itself over airfoil surfaces (i.e. upper and lower camber)? or does it just represent an uniform roughness throughout an object (e.g. pipeflow, flat plate, airfoil) under study? I thank you very much in advance for any comments given regarding my aforementioned questions. Regards, Ahmad
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Jan 20, 2013, 3:59 p.m. EST
Hey Ahmad

Sorry for the late reply. I've been really busy with my latest project and the following presentation.

First of all, good thing starting off in 2D. Second of all, the choice of turbulence model is also something you might wanna look into. I believe, but not entirely sure, that the Spalart Allmaras (1 equation model) is calibrated for airfoil/blade simulations. It's fast and really robust - so you can start off with a coarse mesh. You can then use the results from that simulation as an initial value in a k-epsilon model for a more complex model. Now as for your questions.


1) Is there any limitation in terms of its (i.e. parameter 'B') usage? I mean, does it only valid to be used in pipe flow module? Can it be also used for airfoil studies?

I'm sure it can be applied to any given surface however, with increasing Reynolds number the impact of surface roughness decreases.


2) Apart from modifying its roughness height, Could I also change the location of the roughness itself over airfoil surfaces (i.e. upper and lower camber)? or does it just represent an uniform roughness throughout an object (e.g. pipeflow, flat plate, airfoil) under study?


I'm no expert but I believe it makes uniform for your entire model since the surface roughness is applied in the turbulence model and not on each surface. Again I'm just guessing here, so you might wanna write to COMSOL support:-)

I hope this was helpful

Good luck

Best regards
Mikael
Hey Ahmad Sorry for the late reply. I've been really busy with my latest project and the following presentation. First of all, good thing starting off in 2D. Second of all, the choice of turbulence model is also something you might wanna look into. I believe, but not entirely sure, that the Spalart Allmaras (1 equation model) is calibrated for airfoil/blade simulations. It's fast and really robust - so you can start off with a coarse mesh. You can then use the results from that simulation as an initial value in a k-epsilon model for a more complex model. Now as for your questions. [QUOTE] 1) Is there any limitation in terms of its (i.e. parameter 'B') usage? I mean, does it only valid to be used in pipe flow module? Can it be also used for airfoil studies? [/QUOTE] I'm sure it can be applied to any given surface however, with increasing Reynolds number the impact of surface roughness decreases. [QUOTE] 2) Apart from modifying its roughness height, Could I also change the location of the roughness itself over airfoil surfaces (i.e. upper and lower camber)? or does it just represent an uniform roughness throughout an object (e.g. pipeflow, flat plate, airfoil) under study? [/QUOTE] I'm no expert but I believe it makes uniform for your entire model since the surface roughness is applied in the turbulence model and not on each surface. Again I'm just guessing here, so you might wanna write to COMSOL support:-) I hope this was helpful Good luck Best regards Mikael
Ahmad Syahrir Ayub
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Posted: 1 decade ago Jan 24, 2013, 8:44 a.m. EST
Hi,

In COMSOL's CFD, Parameter 'B' is set to 5.2 as to represent smooth surfaces, however, from the two books I have referred, which are;
1) Turbulence Modelling for CFD by D. Wilcox, 2002
2) Boundary Layer Theory by H. Schlichting, 1968

It is 5 instead of 5.2.

So, may I know why it is set up at 5.2 instead of 5 as per books?

By the way, my background is on electrical engineering. Hence, I could be wrong in tems of inferring that value from the books. So, please make any comments should I have understood it wrongly.

Thank you very much.

Regards,
Ahmad
Hi, In COMSOL's CFD, Parameter 'B' is set to 5.2 as to represent smooth surfaces, however, from the two books I have referred, which are; 1) Turbulence Modelling for CFD by D. Wilcox, 2002 2) Boundary Layer Theory by H. Schlichting, 1968 It is 5 instead of 5.2. So, may I know why it is set up at 5.2 instead of 5 as per books? By the way, my background is on electrical engineering. Hence, I could be wrong in tems of inferring that value from the books. So, please make any comments should I have understood it wrongly. Thank you very much. Regards, Ahmad
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 24, 2013, 10:22 a.m. EST
Hi

such values are often "Engineering" "rule of thumb" given values, with references to many tests and measurements, buit not to be confused with the calue of c_const or g_const ..., there might not be such a difference in results by using 5 or 5.2.

I always make simple cases and run a few example to see what are the effect of a 5 versus 5.2 ... To get a better feeling of the sensitivities, often you notice little differences, but sometimes yes ;)

--
Good luck
Ivar
Hi such values are often "Engineering" "rule of thumb" given values, with references to many tests and measurements, buit not to be confused with the calue of c_const or g_const ..., there might not be such a difference in results by using 5 or 5.2. I always make simple cases and run a few example to see what are the effect of a 5 versus 5.2 ... To get a better feeling of the sensitivities, often you notice little differences, but sometimes yes ;) -- Good luck Ivar
Ahmad Syahrir Ayub
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Posted: 1 decade ago Jan 24, 2013, 10:33 a.m. EST
Hi Ivar,

Thank you for your quick reply.

Since 5 (or 5.2) is equal to smooth surface (i.e. 0cm/mm presumably), please advise how do I vary that number (5 or 5.2) so it equals to 1cm, 2cm, 3cm, 4cm or 5cm.

By the way, what does c_const or g_const refer to?

Regards,
Ahmad

Hi Ivar, Thank you for your quick reply. Since 5 (or 5.2) is equal to smooth surface (i.e. 0cm/mm presumably), please advise how do I vary that number (5 or 5.2) so it equals to 1cm, 2cm, 3cm, 4cm or 5cm. By the way, what does c_const or g_const refer to? Regards, Ahmad
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Jan 24, 2013, 12:41 p.m. EST

1) Since 5 (or 5.2) is equal to smooth surface (i.e. 0cm/mm presumably), please advise how do I vary that number (5 or 5.2) so it equals to 1cm, 2cm, 3cm, 4cm or 5cm.

2) By the way, what does c_const or g_const refer to?


Hi

1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it.

2) I believe those are stored constants within COMSOL with c_constant = speed of light and g_const = gravitational constant.

Best of luck and regards

Mikael
[QUOTE] 1) Since 5 (or 5.2) is equal to smooth surface (i.e. 0cm/mm presumably), please advise how do I vary that number (5 or 5.2) so it equals to 1cm, 2cm, 3cm, 4cm or 5cm. 2) By the way, what does c_const or g_const refer to? [/QUOTE] Hi 1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it. 2) I believe those are stored constants within COMSOL with c_constant = speed of light and g_const = gravitational constant. Best of luck and regards Mikael
Ahmad Syahrir Ayub
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Posted: 1 decade ago Jan 24, 2013, 1:02 p.m. EST


1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it.



Hi,

Thank you for the quick reply.

By the way, Mikael, it would be really great if you could explain how did you obtain B = 3.7 = surface roughness of 4cm.

I really appreaciate it alot.

Regards,
Ahmad
[/QUOTE] 1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it. [/QUOTE] Hi, Thank you for the quick reply. By the way, Mikael, it would be really great if you could explain how did you obtain B = 3.7 = surface roughness of 4cm. I really appreaciate it alot. Regards, Ahmad
Mikael Noerregaard Nielsen
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Posted: 1 decade ago Jan 28, 2013, 11:33 a.m. EST



1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it.



Hi,

Thank you for the quick reply.

By the way, Mikael, it would be really great if you could explain how did you obtain B= 3.7 = surface roughness of 4cm.

I really appreaciate it alot.

Regards,
Ahmad


Hi Ahmad

As I said in my previous post, I don't believe you can say that B=3,7= surface roughness of 4cm. I don't think that the 5.2 has anything to do with a surface roughness in mm/cm. But again I'm not sure at all - I havent studied the theory regarding modelling of surface roughness. Again I suggest that you write to COMSOL support:-)

Best regards

Mikael
[QUOTE] [/QUOTE] 1) I don't believe it is that simple ex. I don't think that a B = 3.7 = surface roughness of 4cm. You might wanna talk to COMSOL support about it. [/QUOTE] Hi, Thank you for the quick reply. By the way, Mikael, it would be really great if you could explain how did you obtain B= 3.7 = surface roughness of 4cm. I really appreaciate it alot. Regards, Ahmad [/QUOTE] Hi Ahmad As I said in my previous post, I don't believe you can say that B=3,7= surface roughness of 4cm. I don't think that the 5.2 has anything to do with a surface roughness in mm/cm. But again I'm not sure at all - I havent studied the theory regarding modelling of surface roughness. Again I suggest that you write to COMSOL support:-) Best regards Mikael
Ahmad Syahrir Ayub
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Posted: 1 decade ago Jan 28, 2013, 11:43 a.m. EST
Hi Mikael,

Thank you for your replies and comments.

My apologies, it seems that I have misunderstood your comment in your previous post.

Anyway, I shall write it to COMSOL Support Team.

Regards,
Ahmad
Hi Mikael, Thank you for your replies and comments. My apologies, it seems that I have misunderstood your comment in your previous post. Anyway, I shall write it to COMSOL Support Team. Regards, Ahmad

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