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What does 'Compute Implicit Height Change From Base Velocity' mean in the Thin Film Flow module?

Posted Jan 2, 2024, 11:40 a.m. EST Fluid & Heat, Computational Fluid Dynamics (CFD), User Interface Version 6.1 2 Replies

Jake Liu
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Dear all,

I am trying to predict the experimental results of plain bearings with the COMSOL thin film flow module. However, I am confused by the option 'Compute Implicit Height Change From Base Velocity' in Fluid-Film Properties, which is not explained in the documentation and can have a significant impact on the results. (The attachment shows the location of this option)

What does 'Compute Implicit Height Change From Base Velocity' mean in the Thin Film Flow module?

Thanks in advance.

The same question has been asked before but not answered:

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2 Replies Last Post Jan 5, 2024, 1:04 p.m. EST
Mats Nigam COMSOL Employee

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Posted: 12 months ago Jan 5, 2024, 12:23 p.m. EST
Updated: 12 months ago Jan 5, 2024, 12:26 p.m. EST

Dear Jake,

I regret that this had not been documented. I have added the documentation which will be included in future versions of COMSOL. When a user-defined wall or base velocity is specified, it induces a "squeezing" effect on the flow which results in an effective mass source. If, for example, the base moves with velocity , the implicit height change becomes,

where the first part comes from the motion normal to the reference plane and the second part is due to topology changes of the base. This results in an effective mass source on the right hand side of the continuity equation,

and correspondingly for the wall velocity.

Best regards, Mats

Dear Jake, I regret that this had not been documented. I have added the documentation which will be included in future versions of COMSOL. When a user-defined wall or base velocity is specified, it induces a "squeezing" effect on the flow which results in an effective mass source. If, for example, the base moves with velocity {\bf v}_{b} , the implicit height change becomes, \frac{\partial h_{b}}{\partial t}={\bf v}_{b}\cdot{\bf n}_{ref}-{\bf v}_{b}\cdot\nabla_{t}h_{b} where the first part comes from the motion normal to the reference plane and the second part is due to topology changes of the base. This results in an effective mass source on the right hand side of the continuity equation, Q_{b}^{eff}=-\rho({\bf v}_{b}\cdot{\bf n}_{ref}-{\bf v}_{b}\cdot\nabla_{t}h_{b}) and correspondingly for the wall velocity. Best regards, Mats
Jake Liu
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Posted: 12 months ago Jan 5, 2024, 1:04 p.m. EST
Updated: 12 months ago Jan 5, 2024, 2:48 p.m. EST

Dear Mats,

Thank you for your reply. I didn't fully understand the second part, but it sounds like it's an option that should be enabled all the time (similar to the concept of inertial forces), could you tell me when it should be turned off? After testing, I've found that enabling this option always tends to overestimate the pressure (several times greater than not enabling it), even if the film is thick.

Also, I don't find this option in the hydrodynamic bearing module (in theory it is the same as the Thin Film Flow module), is it enabled by default?

I’m sorry for asking so many questions. I appreciate your time in responding to whatever you can, when you are able to find the time.

Best regards,
Jake

Dear Mats, Thank you for your reply. I didn't fully understand the second part, but it sounds like it's an option that should be enabled all the time (similar to the concept of inertial forces), could you tell me when it should be turned off? After testing, I've found that enabling this option always tends to overestimate the pressure (several times greater than not enabling it), even if the film is thick. Also, I don't find this option in the hydrodynamic bearing module (in theory it is the same as the Thin Film Flow module), is it enabled by default? I’m sorry for asking so many questions. I appreciate your time in responding to whatever you can, when you are able to find the time. Best regards, Jake

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