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need to model an oscillating porous medium, with a layer of newtonian fluid on it...please help.

Posted Oct 28, 2011, 11:35 p.m. EDT Microfluidics, Chemical Reaction Engineering, Materials, Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions 35 Replies

Urmi Roy
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The basic model that I'm trying to create is that of a porous medium of given thickness.......

......It carries a layer of newtonian fluid on it.

.....The porous medium oscillates...and due to its motion, the overlying layer of fluid is moved along the top.

COMSOL 3.5a, that I have does not have any tutorials on porous media, so its difficult to even start off...!

Please suggest what steps I should follow for this and any reference reading material.
35 Replies Last Post Dec 24, 2011, 7:39 p.m. EST
Mayur
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Posted: 1 decade ago Oct 28, 2011, 11:45 p.m. EDT
you should first try to model flow through porous media, then add bell and whistle and make it complicate.
you should first try to model flow through porous media, then add bell and whistle and make it complicate.
Urmi Roy
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Posted: 1 decade ago Oct 29, 2011, 12:39 a.m. EDT
Ok, so I made a model according to www.comsol.com/showroom/gallery/170/
...basically a gas moving above a surface below which there is a porous medium....

Now, how do i model the movement to the porous medium?....
Ok, so I made a model according to http://www.comsol.com/showroom/gallery/170/ ...basically a gas moving above a surface below which there is a porous medium.... Now, how do i model the movement to the porous medium?....
Mayur
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Posted: 1 decade ago Oct 29, 2011, 12:52 a.m. EDT
If you can describe what kind of problem you are solving then i can be in better position to say something. I think you need moving mesh for porous media.
If you can describe what kind of problem you are solving then i can be in better position to say something. I think you need moving mesh for porous media.
Urmi Roy
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Posted: 1 decade ago Oct 29, 2011, 8:59 a.m. EDT
Hi,

its hard to say exactly what kind of problem I'm solving....I'll be more elaborate in the description:

I'm trying to model a bunch of cilia(hair like structures....(like those cilia that line the inside of you nose)...these cilia are attached to the wall and stay submersed in a gel-like fluid......this ENTIRE stucture, consisting of the cilia and the gel-like fluid that it is inside form the POROUS LAYER.....ONE MORE IMPORTANT THING: each cilia has its own independent SWAYING movement....so the oscillation of the entire porous layer depends on the SUM-TOTAL of movements of the cilia.
Note also that the cilia do not translate.....their bottom remain fixed to the wall, only top portions sway.

Now, on top of the porous layer,there is a layer of low-viscosity fluid.....due to the swaying of the cilia, this layer of fluid is pushed along...much like a carpet being moved along a floor.

So this is the entire description...please give me suggestions, especially for the moving porous medium part, since that part is going to cause me the most trouble!
Hi, its hard to say exactly what kind of problem I'm solving....I'll be more elaborate in the description: I'm trying to model a bunch of cilia(hair like structures....(like those cilia that line the inside of you nose)...these cilia are attached to the wall and stay submersed in a gel-like fluid......this ENTIRE stucture, consisting of the cilia and the gel-like fluid that it is inside form the POROUS LAYER.....ONE MORE IMPORTANT THING: each cilia has its own independent SWAYING movement....so the oscillation of the entire porous layer depends on the SUM-TOTAL of movements of the cilia. Note also that the cilia do not translate.....their bottom remain fixed to the wall, only top portions sway. Now, on top of the porous layer,there is a layer of low-viscosity fluid.....due to the swaying of the cilia, this layer of fluid is pushed along...much like a carpet being moved along a floor. So this is the entire description...please give me suggestions, especially for the moving porous medium part, since that part is going to cause me the most trouble!
Mayur
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Posted: 1 decade ago Oct 29, 2011, 9:57 a.m. EDT
As i mentioned before, you need moving mesh. So have a look at few examples given in documentation on moving mesh. Modeling work depends on objectives or goals, so if your objectives permit any kind of simplification, you have to make few assumptions and develop the model. Based on your description i don't understand your goals!
As i mentioned before, you need moving mesh. So have a look at few examples given in documentation on moving mesh. Modeling work depends on objectives or goals, so if your objectives permit any kind of simplification, you have to make few assumptions and develop the model. Based on your description i don't understand your goals!
Urmi Roy
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Posted: 1 decade ago Oct 29, 2011, 11:01 a.m. EDT
Hi,

Lets forget about all the complexity for a while....suppose I simply want to model a shearing solid .....like I show in the diagram.....actually the porous medium behaves like a solid in that it does not flow...with the movement of the cilia, it just shears, and elastically returns to place...

So now I need to use mesh movement... So I can use structural analysis module? then I define the solid with various parameters like rigidity modulus....then how do I apply force and model the shear deformation...and more importantly, how do i show the deformation visually (not only by means of a graph) with time...more like a movie?
Hi, Lets forget about all the complexity for a while....suppose I simply want to model a shearing solid .....like I show in the diagram.....actually the porous medium behaves like a solid in that it does not flow...with the movement of the cilia, it just shears, and elastically returns to place... So now I need to use mesh movement... So I can use structural analysis module? then I define the solid with various parameters like rigidity modulus....then how do I apply force and model the shear deformation...and more importantly, how do i show the deformation visually (not only by means of a graph) with time...more like a movie?
Mayur
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Posted: 1 decade ago Oct 29, 2011, 3:03 p.m. EDT
I have attached one model documentation, which describes how to do simulation for fluid structure interaction with deformation mesh. This is for comsol 4.2, but if you are working with comsol 3.5a, you can just go through this example and redo it in comsol 3.5a.
cheers
I have attached one model documentation, which describes how to do simulation for fluid structure interaction with deformation mesh. This is for comsol 4.2, but if you are working with comsol 3.5a, you can just go through this example and redo it in comsol 3.5a. cheers
Urmi Roy
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Posted: 1 decade ago Oct 30, 2011, 9:30 a.m. EDT
Thanks, I'm sure this will be helpful!

I'll let you know if I have any more problems.
Thanks, I'm sure this will be helpful! I'll let you know if I have any more problems.
Urmi Roy
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Posted: 1 decade ago Oct 30, 2011, 12:32 p.m. EDT
Hi,

I did the example and I also looked through the part on ALE on the manual for 3.5a (pg 455):
math.nju.edu.cn/help/mathhpc/doc/comsol/modeling.pdf

Here, it says we can define the mesh movt. using subdomain settings.....now I selected the porous medium subdomain and went to subdomain settings...there is no mesh option!!

Please suggest what might be wrong.
Hi, I did the example and I also looked through the part on ALE on the manual for 3.5a (pg 455): http://math.nju.edu.cn/help/mathhpc/doc/comsol/modeling.pdf Here, it says we can define the mesh movt. using subdomain settings.....now I selected the porous medium subdomain and went to subdomain settings...there is no mesh option!! Please suggest what might be wrong.
Mayur
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Posted: 1 decade ago Oct 30, 2011, 1:43 p.m. EDT
have you added Moving mesh (ALE) module from model navigator? I have created domain with one channel and one more subdomain inside it, and I am getting options for mesh displacement under subdomain menu. if you can attach your model or pics of it i may suggest something..
have you added Moving mesh (ALE) module from model navigator? I have created domain with one channel and one more subdomain inside it, and I am getting options for mesh displacement under subdomain menu. if you can attach your model or pics of it i may suggest something..
Urmi Roy
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Posted: 1 decade ago Oct 30, 2011, 4:19 p.m. EDT
Hi,
Thanks!
I also read through the documentation, and I found how to bring the moving mesh frame into the porous medium module...I'm still working on how to implement it...

Thanks for your help...I really couldn't have gone anywhere without it!

I''l let you know how its going!
Hi, Thanks! I also read through the documentation, and I found how to bring the moving mesh frame into the porous medium module...I'm still working on how to implement it... Thanks for your help...I really couldn't have gone anywhere without it! I''l let you know how its going!
Urmi Roy
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Posted: 1 decade ago Nov 3, 2011, 6:46 p.m. EDT
Hi...we've decided to add a volumetric force to a bulk of newtonian fluid....so do u know how I could do this? Do you have any examples of this nature?...it basically means that at each point inside the bulk of the fluid, there is an applied force....
Hi...we've decided to add a volumetric force to a bulk of newtonian fluid....so do u know how I could do this? Do you have any examples of this nature?...it basically means that at each point inside the bulk of the fluid, there is an applied force....
Mayur
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Posted: 1 decade ago Nov 3, 2011, 8:15 p.m. EDT
Under subdomain section of navier stokes module, you will find the source term which is nothing but the volumetric force term.
Under subdomain section of navier stokes module, you will find the source term which is nothing but the volumetric force term.
Urmi Roy
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Posted: 1 decade ago Nov 4, 2011, 1:32 a.m. EDT
Thanks...I located it....so this is the volumetric force as in the specified Fx value acts on the entire fluid...say if I draw it in a rectangular domain?

By the way, as I said earlier, I want to basically apply a volumetric force on a rectangular domain of fluid...then there is a layer of fluid of different viscosity on top of it...so I want to plot how much the layer of above-lying fluid displaces/translates when the volumetric force is applied on the layer below.....is this possible? Any idea how I can specify the boundary conditions?

Thanks,
Urmi.
Thanks...I located it....so this is the volumetric force as in the specified Fx value acts on the entire fluid...say if I draw it in a rectangular domain? By the way, as I said earlier, I want to basically apply a volumetric force on a rectangular domain of fluid...then there is a layer of fluid of different viscosity on top of it...so I want to plot how much the layer of above-lying fluid displaces/translates when the volumetric force is applied on the layer below.....is this possible? Any idea how I can specify the boundary conditions? Thanks, Urmi.
Mayur
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Posted: 1 decade ago Nov 4, 2011, 10:57 a.m. EDT
You can look at the problems on multiphase flow. Displacement of one fluid due to another require careful analysis of force term, displacement of the interface, interface tension etc...you will get some idea after looking at multiphase flow theory.
You can look at the problems on multiphase flow. Displacement of one fluid due to another require careful analysis of force term, displacement of the interface, interface tension etc...you will get some idea after looking at multiphase flow theory.
Urmi Roy
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Posted: 1 decade ago Nov 11, 2011, 10:11 p.m. EST
I tried out the model and it works just fine! Thanks for the guidance!!

Only one thing is that I'm trying to vary the value of the Volume force based on the point in the bulk of the fluid i.e. to define the force in terms of x and y..

So I went to options, expressions and put F....Comsol couldn't identify F, and there was no option to define F as a force, since the description place could not be changed.
Then I tried defining the for as say F=5(x+y) Newton....and it kept giving an error!
I tried the same in Options--> Functions, and again with no luck.

Please tell me what could be wrong!
I tried out the model and it works just fine! Thanks for the guidance!! Only one thing is that I'm trying to vary the value of the Volume force based on the point in the bulk of the fluid i.e. to define the force in terms of x and y.. So I went to options, expressions and put F....Comsol couldn't identify F, and there was no option to define F as a force, since the description place could not be changed. Then I tried defining the for as say F=5(x+y) Newton....and it kept giving an error! I tried the same in Options--> Functions, and again with no luck. Please tell me what could be wrong!
Mayur
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Posted: 1 decade ago Nov 12, 2011, 9:04 a.m. EST
I can see no error in the expression..have a look at the attached pic file...
cheers
I can see no error in the expression..have a look at the attached pic file... cheers

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Urmi Roy
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Posted: 1 decade ago Nov 12, 2011, 11:58 p.m. EST
Thanks! It worked...just 2 quick questions:

1. in the in-compressible navier stokes module, in the subdomain settings, there is also a section for porous flow, where u can introduce brinkmann number etc....what does that do? Does it make my given fluid into a porous medium with those assigned properties?

2. If I want to introduce a time-varying volume force, and I don't use moving mesh, is there any way to see the output at a particular time, say at t=5sec?

Thanks a lot for your help, and sorry for the bother :-)
Thanks! It worked...just 2 quick questions: 1. in the in-compressible navier stokes module, in the subdomain settings, there is also a section for porous flow, where u can introduce brinkmann number etc....what does that do? Does it make my given fluid into a porous medium with those assigned properties? 2. If I want to introduce a time-varying volume force, and I don't use moving mesh, is there any way to see the output at a particular time, say at t=5sec? Thanks a lot for your help, and sorry for the bother :-)
Mayur
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Posted: 1 decade ago Nov 13, 2011, 12:13 a.m. EST
Hi,
Regarding 1, yes...by mentioning porosity and permeability your domain becomes porous.....regarding 2, you can look at the postprocessing section, where you can select specific time to see the result.
Hi, Regarding 1, yes...by mentioning porosity and permeability your domain becomes porous.....regarding 2, you can look at the postprocessing section, where you can select specific time to see the result.
Urmi Roy
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Posted: 1 decade ago Nov 14, 2011, 12:29 p.m. EST
V=(sin(5[1/s]t))[m/s]

.....quick question...in the subdomain expressions, I tried making velocity as a function of time...why is this not working? It says inconsistent units.
V=(sin(5[1/s]t))[m/s] .....quick question...in the subdomain expressions, I tried making velocity as a function of time...why is this not working? It says inconsistent units.
Mayur
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Posted: 1 decade ago Nov 14, 2011, 12:39 p.m. EST
type the expressions as you type in matlab

V=(sin(5[1/s]t))[m/s] wrong

V=(sin(5[1/s]*t))[m/s] correct
type the expressions as you type in matlab V=(sin(5[1/s]t))[m/s] wrong V=(sin(5[1/s]*t))[m/s] correct
Urmi Roy
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Posted: 1 decade ago Nov 18, 2011, 1:08 a.m. EST
I'm basically modeling a small part the lining of the lungs...and on both ends of my domain, there would be fluid flow and forces exerted by the adjacent portions....so is the symmetry boundary option suitable?

does the open boundary condition mean that it is just a open, exposed surface?

Finally, could you describe briefly what the interior conditions 'velocity', 'slip' and 'continuity' imply?
I'm basically modeling a small part the lining of the lungs...and on both ends of my domain, there would be fluid flow and forces exerted by the adjacent portions....so is the symmetry boundary option suitable? does the open boundary condition mean that it is just a open, exposed surface? Finally, could you describe briefly what the interior conditions 'velocity', 'slip' and 'continuity' imply?
Urmi Roy
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Posted: 1 decade ago Nov 18, 2011, 1:38 a.m. EST
...sorry for the bother..and just one more thing....if i wanted to add a bulk velocity...and make it vary w.r.t space, what would I do?

At present, I'm using inlet boundary conditions, but thats only at the inlet.....

thanks....sorry for the numerous questions :-(
...sorry for the bother..and just one more thing....if i wanted to add a bulk velocity...and make it vary w.r.t space, what would I do? At present, I'm using inlet boundary conditions, but thats only at the inlet..... thanks....sorry for the numerous questions :-(
Mayur
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Posted: 1 decade ago Nov 19, 2011, 4:24 a.m. EST
If you think same forces are acting on both sides then i think symmetry boundary condition is suitable.

To understand the boundary conditions, you may like to refer comsol documentations, or any fundamental book on numerical methods is useful.

Regarding your last question, you can define inlet velocity boundary condition according to what kind of bulk velocity profile you want. Again i recommend to refer any numerical methods book for more detailed explanations.
If you think same forces are acting on both sides then i think symmetry boundary condition is suitable. To understand the boundary conditions, you may like to refer comsol documentations, or any fundamental book on numerical methods is useful. Regarding your last question, you can define inlet velocity boundary condition according to what kind of bulk velocity profile you want. Again i recommend to refer any numerical methods book for more detailed explanations.
Urmi Roy
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Posted: 1 decade ago Dec 2, 2011, 12:41 a.m. EST
Hi,

Due to certain problems, I've had to shift to COMSOL 4.0 version...in this, a unique problem that I'm facing is that when I select boundary conditions, whatever I apply to any one boundary gets applied to all of the boundaries...so basically I'm not being able to specify separate boundary conditions for each boundary...please help!
Hi, Due to certain problems, I've had to shift to COMSOL 4.0 version...in this, a unique problem that I'm facing is that when I select boundary conditions, whatever I apply to any one boundary gets applied to all of the boundaries...so basically I'm not being able to specify separate boundary conditions for each boundary...please help!
Mayur
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Posted: 1 decade ago Dec 2, 2011, 2:03 a.m. EST
attach your model file..it should not be possible. Are you using Comsol 4.0 or 4.2a? the latest one is 4.2a. I recommend not to use 4.0, it had lot of problems, however i didn't face such boundary condition problem in 4.0.
attach your model file..it should not be possible. Are you using Comsol 4.0 or 4.2a? the latest one is 4.2a. I recommend not to use 4.0, it had lot of problems, however i didn't face such boundary condition problem in 4.0.
Urmi Roy
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Posted: 1 decade ago Dec 2, 2011, 8:04 a.m. EST
There's a place in the settings window, where it says 'All domains'...so whatever settings I apply to any one domain applies to all of them...all the boundaries etc etc...somehow I just can't change that....there's no option to make it from 'All Domains' to specified domains...

I really haven't done very much..I'm just trying to assign different boundary conditions to each of these boundaries..

Btw, what kind of problems did 4.0 have...I'm using 4.0,not 4.2....will they seriously disturb my work?
There's a place in the settings window, where it says 'All domains'...so whatever settings I apply to any one domain applies to all of them...all the boundaries etc etc...somehow I just can't change that....there's no option to make it from 'All Domains' to specified domains... I really haven't done very much..I'm just trying to assign different boundary conditions to each of these boundaries.. Btw, what kind of problems did 4.0 have...I'm using 4.0,not 4.2....will they seriously disturb my work?

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Mayur
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Posted: 1 decade ago Dec 2, 2011, 10:31 a.m. EST
you need to add extra boundary conditions; by default Comsol starts with one option for each "fluid properties 1", " Wall 1", "Initial values 1", if axial symmetry then "Axial symmetry 1". Now to add inlet, outlet or if you want to define second sub-domain with different fluid properties then right click on "Lamina Flow (spf)" and add whatever you want. e.g. after adding extra "Fluid properties 2" you can go the geometry window, select and right click whichever domain you want to add under Fluid properties 2; similar procedure apply for boundary selection.
I don't have 4.0 on my workstation and you can not open any file saved using 4.2a in 4.0, so I am not attaching the modified file..though I have attached couple of snaps for your reference.
you need to add extra boundary conditions; by default Comsol starts with one option for each "fluid properties 1", " Wall 1", "Initial values 1", if axial symmetry then "Axial symmetry 1". Now to add inlet, outlet or if you want to define second sub-domain with different fluid properties then right click on "Lamina Flow (spf)" and add whatever you want. e.g. after adding extra "Fluid properties 2" you can go the geometry window, select and right click whichever domain you want to add under Fluid properties 2; similar procedure apply for boundary selection. I don't have 4.0 on my workstation and you can not open any file saved using 4.2a in 4.0, so I am not attaching the modified file..though I have attached couple of snaps for your reference.

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Mayur
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Posted: 1 decade ago Dec 2, 2011, 10:40 a.m. EST
4.0 has lot of problems, even 4.2a has few but much better than 4.0. So I recommend not to use 4.0. If you don't have technical and product update supports, continue with 3.5a. I loved 3.5a and I am still using it.
4.0 has lot of problems, even 4.2a has few but much better than 4.0. So I recommend not to use 4.0. If you don't have technical and product update supports, continue with 3.5a. I loved 3.5a and I am still using it.
Urmi Roy
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Posted: 1 decade ago Dec 3, 2011, 1:05 a.m. EST
Hi!

Thanks...you're awesome :-)

I've now learned how to add stuff....now when I specify each boundary-type, it gets added as wall2 or open boundary 1 etc ...but the default boundary condition as you mentioned in your last post still remains....will that affect my solution, or should I try deleting it, since I really don't want it...?

I'm not sure what's happening, but I'm getting an error:

Failed to evaluate variable Jacobian.
- Variable: mat.rho
- Geometry: 1
- Domain: 1

I've attached the file..its a simple case with only one region...I'm just doing a simple case in which I apply a bulk force and all my boundaries except for the bottom one(wall) are open boundaries...
I just want some valid output...

Thanks for your help!
Hi! Thanks...you're awesome :-) I've now learned how to add stuff....now when I specify each boundary-type, it gets added as wall2 or open boundary 1 etc ...but the default boundary condition as you mentioned in your last post still remains....will that affect my solution, or should I try deleting it, since I really don't want it...? I'm not sure what's happening, but I'm getting an error: Failed to evaluate variable Jacobian. - Variable: mat.rho - Geometry: 1 - Domain: 1 I've attached the file..its a simple case with only one region...I'm just doing a simple case in which I apply a bulk force and all my boundaries except for the bottom one(wall) are open boundaries... I just want some valid output... Thanks for your help!

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Mayur
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Posted: 1 decade ago Dec 3, 2011, 1:32 a.m. EST
I have looked at your model file..it seems these are not big problems and you need more practice to understand how new version of Comsol works..so i recommend to do practice by solving few already solved problems using Comsol 4.0 or 4.2 documentation.
I have looked at your model file..it seems these are not big problems and you need more practice to understand how new version of Comsol works..so i recommend to do practice by solving few already solved problems using Comsol 4.0 or 4.2 documentation.
Urmi Roy
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Posted: 1 decade ago Dec 3, 2011, 7:11 p.m. EST
Hi,

I almost figured it out...it turns out that the problem was that I didn't select any material...only thing now is...how do I see the solution at each time step?..in 3.5a, I had got it from the plot menu, but in the equivalent 'study' menu on version4.0, there doesn't seem to be any such option.
Hi, I almost figured it out...it turns out that the problem was that I didn't select any material...only thing now is...how do I see the solution at each time step?..in 3.5a, I had got it from the plot menu, but in the equivalent 'study' menu on version4.0, there doesn't seem to be any such option.
Mayur
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Posted: 1 decade ago Dec 3, 2011, 8:18 p.m. EST
right click on Results, plot whichever plot you want to plot....easy. In new versions of Comsol, right click is very important..you can go any place and right click and make your path. You can read chapter 6 of attached reference guide.
cheers
Mayur
right click on Results, plot whichever plot you want to plot....easy. In new versions of Comsol, right click is very important..you can go any place and right click and make your path. You can read chapter 6 of attached reference guide. cheers Mayur
Urmi Roy
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Posted: 1 decade ago Dec 4, 2011, 12:24 a.m. EST
after right clicking results? After i right click, there's no option to show results at different times..
after right clicking results? After i right click, there's no option to show results at different times..
Urmi Roy
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Posted: 1 decade ago Dec 24, 2011, 7:39 p.m. EST
ON VERSION 3.4
Can anyone tell me what'swrong with this?

V=(y*cos(5[1/s]*t+6[1/m]*x))[1/s]
U=0.5+u
f=(U-V)[N*s/m^4]
F= 0.396*cos(5[1/s]*t+6[1/m]*x)*(y/0.4[m])[N/m^3]
It does not show any dimensional error and the same worked on version 3.5...why doesn't it work on 3.4?

Error given:
Error: 6170
Failed to evaluate variable
- Variable: t
- Geometry: 1
- Dimension: 2
- Domains: {1}
Failed to evaluate variable
- Variable: V
- Defined as: ((cos((((x*6)/unit_m_cf)+((5*t)/unit_s_cf)))*y)/unit_s_cf)
Failed to evaluate variable
- Variable: f
- Defined as: ((unit_s_cf*(unit_N_cf*(U-V)))/(unit_m_cf^4))
Failed to evaluate variable
- Variable: F_x_chns
- Defined as: (-f)
Failed to evaluate expression
- Expression: diff(((+(-2*eta_chns*ux+p)*test(ux)+(-eta_chns*(uy+vx))*test(uy)+(F_x_chns-rho_chns*(u*ux+v*uy))*test(u)+(-test(nojac(rho_chns)*(nojac(u)*ux+nojac(v)*uy)+px-nojac(eta_chns)*(2*uxx+uyy+vxy))*taum_chns*res_u_chns))-(0))*(dvol),{test@4})

(NOTE: I can't upload the file since I'm using a crack version of the software)
ON VERSION 3.4 Can anyone tell me what'swrong with this? V=(y*cos(5[1/s]*t+6[1/m]*x))[1/s] U=0.5+u f=(U-V)[N*s/m^4] F= 0.396*cos(5[1/s]*t+6[1/m]*x)*(y/0.4[m])[N/m^3] It does not show any dimensional error and the same worked on version 3.5...why doesn't it work on 3.4? Error given: Error: 6170 Failed to evaluate variable - Variable: t - Geometry: 1 - Dimension: 2 - Domains: {1} Failed to evaluate variable - Variable: V - Defined as: ((cos((((x*6)/unit_m_cf)+((5*t)/unit_s_cf)))*y)/unit_s_cf) Failed to evaluate variable - Variable: f - Defined as: ((unit_s_cf*(unit_N_cf*(U-V)))/(unit_m_cf^4)) Failed to evaluate variable - Variable: F_x_chns - Defined as: (-f) Failed to evaluate expression - Expression: diff(((+(-2*eta_chns*ux+p)*test(ux)+(-eta_chns*(uy+vx))*test(uy)+(F_x_chns-rho_chns*(u*ux+v*uy))*test(u)+(-test(nojac(rho_chns)*(nojac(u)*ux+nojac(v)*uy)+px-nojac(eta_chns)*(2*uxx+uyy+vxy))*taum_chns*res_u_chns))-(0))*(dvol),{test@4}) (NOTE: I can't upload the file since I'm using a crack version of the software)

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