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Fluid flow

Posted Aug 12, 2014, 1:02 p.m. EDT 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 Sep 26, 2014, 7:24 p.m. EDT
Pieter Westerik
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Posted: 10 years ago Aug 14, 2014, 3:18 a.m. EDT
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: 10 years ago Aug 14, 2014, 1:13 p.m. EDT
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: 10 years ago Aug 15, 2014, 4:01 a.m. EDT
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: 10 years ago Aug 17, 2014, 3:05 p.m. EDT
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: 10 years ago Aug 19, 2014, 3:25 a.m. EDT
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: 10 years ago Aug 20, 2014, 11:14 a.m. EDT
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: 10 years ago Aug 21, 2014, 3:06 a.m. EDT
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: 10 years ago Aug 24, 2014, 4:44 p.m. EDT
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: 10 years ago Aug 25, 2014, 3:38 a.m. EDT
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: 10 years ago Aug 26, 2014, 1:29 p.m. EDT
thanks...now I got it!
thanks...now I got it!
Md Fayaz Ahamed
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Posted: 10 years ago Sep 26, 2014, 7:24 p.m. EDT
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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