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Electrokinetic flow in a tube with a fixed Surface Charge Density
Posted Apr 26, 2012, 10:50 a.m. EDT 3 Replies
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I am trying to simulate blood flow in a vessel and ultimately the streaming potential generated when flow passes by the negatively charged vessel wall.
I have had a number of issues getting this to work. I am attaching a simple version of the model. In this model I have left the parameters as I would like them to be even though the attached model will not solve(failed to find consistent initial values). This is a highly simplified version of a vessel but would be a good starting point if I can get it to solve.
Some key features of the model are:
- The Boundaries surrounding Domain 2 are defined with a fixed Surface Charge Density. I have tried this on a different Geometry with no width in the fixed Surface Charge Density region - i.e. I defined only the outter-most Boundary with a fixed charge density. This didn't help though and the model would be more representitive with a width in the fixed charge density region.
- I define the Space Charge Density as F*(c1-c2)
- I make the material in Domain 2 similar to water but with 1000x more Density and Dynamic Viscosity.
- I make the Mobility and Diffusion in Domain 2 small compared to Domain 1(neither are set to real-world values).
The issue that is causing the biggest problem for the solver is the fact that I do not define a voltage reference under the Electrostatic interface. When I do add a Ground to the model it will solve - this was done by adding an additional Domain very close to the axisymmetric line and making it Ground. For some reason COMSOL will not allow me to define the axisymmetic boundary itself as Ground - do you know why this is? Regardless, I do not want to define a Ground in my model as this is not accurate. I could define a region near the axisymmetric line with zero Electrical Field though. Is there a way I could do this under the Electrostatic interface?
One other issue I have run into(when having the Ground in the model) is that COMSOL will change my initial conditions such that they are very close to steady state. This makes it virtually impossible to determine whether I am observing a streaming potential.
I welcome any suggestions regarding how to take this basic model and set up it such that I can observe a streaming potential as the fluid passes by the fixed charge density region.
I have had a number of issues getting this to work. I am attaching a simple version of the model. In this model I have left the parameters as I would like them to be even though the attached model will not solve(failed to find consistent initial values). This is a highly simplified version of a vessel but would be a good starting point if I can get it to solve.
Some key features of the model are:
- The Boundaries surrounding Domain 2 are defined with a fixed Surface Charge Density. I have tried this on a different Geometry with no width in the fixed Surface Charge Density region - i.e. I defined only the outter-most Boundary with a fixed charge density. This didn't help though and the model would be more representitive with a width in the fixed charge density region.
- I define the Space Charge Density as F*(c1-c2)
- I make the material in Domain 2 similar to water but with 1000x more Density and Dynamic Viscosity.
- I make the Mobility and Diffusion in Domain 2 small compared to Domain 1(neither are set to real-world values).
The issue that is causing the biggest problem for the solver is the fact that I do not define a voltage reference under the Electrostatic interface. When I do add a Ground to the model it will solve - this was done by adding an additional Domain very close to the axisymmetric line and making it Ground. For some reason COMSOL will not allow me to define the axisymmetic boundary itself as Ground - do you know why this is? Regardless, I do not want to define a Ground in my model as this is not accurate. I could define a region near the axisymmetric line with zero Electrical Field though. Is there a way I could do this under the Electrostatic interface?
One other issue I have run into(when having the Ground in the model) is that COMSOL will change my initial conditions such that they are very close to steady state. This makes it virtually impossible to determine whether I am observing a streaming potential.
I welcome any suggestions regarding how to take this basic model and set up it such that I can observe a streaming potential as the fluid passes by the fixed charge density region.
Attachments:
3 Replies Last Post Jun 13, 2012, 3:18 a.m. EDT
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Posted:
1 decade ago
I've made some progress with this model but I'm still having trouble with something that I think should be easy to do - I want to define the change in potential with respect to r (in domain 1, and at boundaries 2,3,4) to be equal to zero in the attached model.
I've tried a number of things under Definitions/Global Definitions but can't seem to define a change in potential. I'd assume that I could do something like this:
myV = mod1.es.Vr (under variables or parameters)
and then I could create a formula(under Analytic Function maybe?) which gave:
d(myV,r) = 0
However I can't get COMSOL to recognize myV, it gives an "Undefined variable" regardless of what I do. I've searched the Documentation and Forum and it seems like I should be able to access mod1.es.Vr but for some reason I can't get around this.
Could someone explain how I can define d(es.Vr,r) = 0 ?
Assuming that is working I need to figure out how to assign it to domain 1, so that the change in potential with respect to r in domain 1 is zero. Any help there would also be appreciated.
I've tried a number of things under Definitions/Global Definitions but can't seem to define a change in potential. I'd assume that I could do something like this:
myV = mod1.es.Vr (under variables or parameters)
and then I could create a formula(under Analytic Function maybe?) which gave:
d(myV,r) = 0
However I can't get COMSOL to recognize myV, it gives an "Undefined variable" regardless of what I do. I've searched the Documentation and Forum and it seems like I should be able to access mod1.es.Vr but for some reason I can't get around this.
Could someone explain how I can define d(es.Vr,r) = 0 ?
Assuming that is working I need to figure out how to assign it to domain 1, so that the change in potential with respect to r in domain 1 is zero. Any help there would also be appreciated.
Attachments:
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Posted:
1 decade ago
The voltage, is a scalar i.e. there is no r-component attached to it. myV=V, and than it would be something like d(myV,r)=0 in the analytic section. Where do you use this function in your model?....what do you want to achieve with it?
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Posted:
1 decade ago
Hi
Vz is no global variable, it's a local field Vz(x,y,z,t) so you need to define a Model1 Definitions Variable on all your domains myV = Vz
if you want the second derivative it's Vzz but with 2nd order elements it will be rather "ragged" perhaps you should use hiogher order discretisazion
Finally I do not believe your geoemtry should be in meters, perhaps um is a better scale
Also for constant values, use Parameters (with units) instead of global variables
--
Good luck
Ivar
Vz is no global variable, it's a local field Vz(x,y,z,t) so you need to define a Model1 Definitions Variable on all your domains myV = Vz
if you want the second derivative it's Vzz but with 2nd order elements it will be rather "ragged" perhaps you should use hiogher order discretisazion
Finally I do not believe your geoemtry should be in meters, perhaps um is a better scale
Also for constant values, use Parameters (with units) instead of global variables
--
Good luck
Ivar