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Fluid flow

Posted 12 août 2014, 13:02 UTC−4 11 Replies

Md Fayaz Ahamed
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Hi,
I am trying to do fluid flow simulation of a peristaltic pump by following COMSOL model example(www.comsol.com/model/download/156649/models.sme.peristaltic_pump.pdf).But when I set fluid flow global equation 1:netflowt-(outflow+inflow)/2 and global equation 2:Vpumpt-outflow in the table of equation section,they become orange and inconsistent unit.
why?can someone help me to fix it?

Regards
Ahamed
11 Replies Last Post 26 sept. 2014, 19:24 UTC−4
Pieter Westerik
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Posted: 1 decade ago 14 août 2014, 03:18 UTC−4
Hi Ahamed,

Can you maybe upload your model so that we can see how you have defined the different variables?
Hi Ahamed, Can you maybe upload your model so that we can see how you have defined the different variables?
Md Fayaz Ahamed
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Posted: 1 decade ago 14 août 2014, 13:13 UTC−4
Hello Peter,
yes,here I have attached the mph file.Actually,it seems to be a simple problem but I am quite new of using this software.anyway,thanks in advance.

Regards
Ahamed
Hello Peter, yes,here I have attached the mph file.Actually,it seems to be a simple problem but I am quite new of using this software.anyway,thanks in advance. Regards Ahamed

Attachments:

Pieter Westerik
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Posted: 1 decade ago 15 août 2014, 04:01 UTC−4
outflow en inflow have unit m^3/s. outflowt and inflowt therfore have unit m^3/s^2. netflow and Vpump (I guess) should also have units m^3/s, however, the expression you give has units m^3/s^2. So you need to multiply both global equations with something that has units s (or divide by something with units 1/s) for the units to be consistent.
outflow en inflow have unit m^3/s. outflowt and inflowt therfore have unit m^3/s^2. netflow and Vpump (I guess) should also have units m^3/s, however, the expression you give has units m^3/s^2. So you need to multiply both global equations with something that has units s (or divide by something with units 1/s) for the units to be consistent.
Md Fayaz Ahamed
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Posted: 1 decade ago 17 août 2014, 15:05 UTC−4
Hi,
I did so,but still it remains inconsistent unit.For instance,If I multiply netflowt by Ttot(Total time for a pump cycle=1.5[s]) it should become consistent(unit is then m^3/s).why?any idea??

Regards
Ahamed
Hi, I did so,but still it remains inconsistent unit.For instance,If I multiply netflowt by Ttot(Total time for a pump cycle=1.5[s]) it should become consistent(unit is then m^3/s).why?any idea?? Regards Ahamed
Pieter Westerik
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Posted: 1 decade ago 19 août 2014, 03:25 UTC−4
That is not enough, you should multiply the whole expression for netflow [=netflowt-(outflow+inflow)/(2*Ttot)] by Ttot, because outflow and inflow have units m^3/s but in this expression you also divide them by Ttot, so the units for that part of the expression are also m^3/s^2, like netflowt. I guess in the expression for Vpump [=Vpumpt-outflow] it is enough to multiply only Vpumpt by Ttot.

Also, in the settings for the global equation nodes you must select in the 'units' part 'Volume per time' for both the dependent variable and the source term quantity.
That is not enough, you should multiply the whole expression for netflow [=netflowt-(outflow+inflow)/(2*Ttot)] by Ttot, because outflow and inflow have units m^3/s but in this expression you also divide them by Ttot, so the units for that part of the expression are also m^3/s^2, like netflowt. I guess in the expression for Vpump [=Vpumpt-outflow] it is enough to multiply only Vpumpt by Ttot. Also, in the settings for the global equation nodes you must select in the 'units' part 'Volume per time' for both the dependent variable and the source term quantity.
Md Fayaz Ahamed
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Posted: 1 decade ago 20 août 2014, 11:14 UTC−4
Hi Pieter,
I did so,and now they become consistent.But still I ain't getting proper result(e.g.velocity magnitude,pressure etc.)when I am computing the study for fluid flow.But when I am computing the study for solid mechanics it's giving me proper result.Why?any idea?

Regards
Ahamed
Hi Pieter, I did so,and now they become consistent.But still I ain't getting proper result(e.g.velocity magnitude,pressure etc.)when I am computing the study for fluid flow.But when I am computing the study for solid mechanics it's giving me proper result.Why?any idea? Regards Ahamed
Pieter Westerik
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Posted: 1 decade ago 21 août 2014, 03:06 UTC−4
Multiplying by Ttot is actually quite arbitrary, any constant with unit time would work to fix the units, but if it is a much larger constant you will find that netflow and Vpump become much more sensitive to changes in inflow and outflow. What do you mean by a 'proper result'?
Multiplying by Ttot is actually quite arbitrary, any constant with unit time would work to fix the units, but if it is a much larger constant you will find that netflow and Vpump become much more sensitive to changes in inflow and outflow. What do you mean by a 'proper result'?
Md Fayaz Ahamed
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Posted: 1 decade ago 24 août 2014, 16:44 UTC−4
Hello,
Sorry I am a bit late to reply you.I was not getting proper simulation result that I meant by 'proper result'.Now it is OK.I am getting proper result now.But still I was not able to understand some parts of my model.I am following peristaltic pump model id:985 from comsol model gallery(www.comsol.com/model/peristaltic-pump-985).Here in variable section the expression of inflow is defined by intop1(2*pi*r*w_fluid) where w_fluid is the velocity of the fluid but w_fluid is not defined in parameters section.then how the model is working?how does it get the fluid velocity?any idea?

Regards
Ahamed
Hello, Sorry I am a bit late to reply you.I was not getting proper simulation result that I meant by 'proper result'.Now it is OK.I am getting proper result now.But still I was not able to understand some parts of my model.I am following peristaltic pump model id:985 from comsol model gallery(http://www.comsol.com/model/peristaltic-pump-985).Here in variable section the expression of inflow is defined by intop1(2*pi*r*w_fluid) where w_fluid is the velocity of the fluid but w_fluid is not defined in parameters section.then how the model is working?how does it get the fluid velocity?any idea? Regards Ahamed
Pieter Westerik
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Posted: 1 decade ago 25 août 2014, 03:38 UTC−4
w_fluid is a dependent variable of the 'fluid-structure interaction' physics, so during and after simulation this variable will be available for visualisation and calculations. It is a location dependent variable, hence the integration.
w_fluid is a dependent variable of the 'fluid-structure interaction' physics, so during and after simulation this variable will be available for visualisation and calculations. It is a location dependent variable, hence the integration.
Md Fayaz Ahamed
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Posted: 1 decade ago 26 août 2014, 13:29 UTC−4
thanks...now I got it!
thanks...now I got it!
Md Fayaz Ahamed
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Posted: 1 decade ago 26 sept. 2014, 19:24 UTC−4
Hi Pieter,
I need to increase (maximum) load of my above mentioned model.But when I am increasing the load ,non linear solver is not converging.Also,sometimes I have same problem when I am changing the material linear elastic to hyper elastic.If are you still there,do you have any idea how I can fix those problems.Thanks in advance.

Kind Regards
Ahamed Md.fayaz
Hi Pieter, I need to increase (maximum) load of my above mentioned model.But when I am increasing the load ,non linear solver is not converging.Also,sometimes I have same problem when I am changing the material linear elastic to hyper elastic.If are you still there,do you have any idea how I can fix those problems.Thanks in advance. Kind Regards Ahamed Md.fayaz

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