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fully developed velocity profile for an FSI problem
Posted Oct 28, 2011, 12:44 p.m. EDT Fluid & Heat, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2 3 Replies
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In COMSOL, for a laminar pipe flow problem, a fully developed velocity profile can be specified by using
umax*(1-(r/H)^2) where umax is a known constant, H is the radius of the pipe, r is the spatial coordinate in the radial direction.
But how do you specify a fully developed velocity BC when you have a fluid-structure interaction problem? It looks like I cannot use umax*(1-(r/H)^2) anymore because of the following reasons:
- when the fluid flows, it exerts a force on the solid and the solid expands. So H which was a known constant in the laminar flow problem is no longer a constant. H becomes (H+some displacement based on the solid domain). I am wondering if anyone knows how to define this displacement as an input? If I ignore the displacement then when r>H, my velocity BC is no longer fully developed. It changes and is different from what I want it to be.
- Since I don't know an answer to the above question yet, lets assume that I am considering the inlet velocity profile for a FSI problem as umax*(1-(r/H)^2). Should this be considered as umax*(1-(r/H)^2) or umax*(1-(R/H)^2). In other words, what coordiates should I use? I would think it should be spatial coordinates (r) but can someone confirm this?
Thanks,
Sirisha
umax*(1-(r/H)^2) where umax is a known constant, H is the radius of the pipe, r is the spatial coordinate in the radial direction.
But how do you specify a fully developed velocity BC when you have a fluid-structure interaction problem? It looks like I cannot use umax*(1-(r/H)^2) anymore because of the following reasons:
- when the fluid flows, it exerts a force on the solid and the solid expands. So H which was a known constant in the laminar flow problem is no longer a constant. H becomes (H+some displacement based on the solid domain). I am wondering if anyone knows how to define this displacement as an input? If I ignore the displacement then when r>H, my velocity BC is no longer fully developed. It changes and is different from what I want it to be.
- Since I don't know an answer to the above question yet, lets assume that I am considering the inlet velocity profile for a FSI problem as umax*(1-(r/H)^2). Should this be considered as umax*(1-(r/H)^2) or umax*(1-(R/H)^2). In other words, what coordiates should I use? I would think it should be spatial coordinates (r) but can someone confirm this?
Thanks,
Sirisha
3 Replies Last Post Oct 30, 2011, 8:38 a.m. EDT
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Posted:
1 decade ago
Hi
1) I would propose to check if you cannot use the predefined laminar inflow BC.
2) r and R are referring to the spatial, respectively the Material frame coordinate. R is ok for an initial condition, for a case like yours r is proably better but it makes the problem non-linear as you are dependent on the solution.
Now H is the radius of your pipe (at rest so R/H is the normalised radial distance [0-1]) I suspect you have to calculate "hvar" (note: "h" is already used by COMSOL so I cali it "hvar") by an integration on the spatial frame of the boundary length.
If you are in 2D you can use the "edge = boundary" variable "s" that I believe is referred to the spatial frame, to be checked). In anycase I would try 1) above first ;)
--
Good luck
Ivar
1) I would propose to check if you cannot use the predefined laminar inflow BC.
2) r and R are referring to the spatial, respectively the Material frame coordinate. R is ok for an initial condition, for a case like yours r is proably better but it makes the problem non-linear as you are dependent on the solution.
Now H is the radius of your pipe (at rest so R/H is the normalised radial distance [0-1]) I suspect you have to calculate "hvar" (note: "h" is already used by COMSOL so I cali it "hvar") by an integration on the spatial frame of the boundary length.
If you are in 2D you can use the "edge = boundary" variable "s" that I believe is referred to the spatial frame, to be checked). In anycase I would try 1) above first ;)
--
Good luck
Ivar
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Posted:
1 decade ago
Ivar,
Can you please explain some more on "I would propose to check if you cannot use the predefined laminar inflow BC."
Are you asking me to check if u=const*(1-(r/H)^2) can be applied to an expanding pipe boundary?
If yes, then based on the theory of the equation, it is derived by considering a rigid pipe boundary.
However, I have seen journal papers where this boundary condition has been applied sucessfully even to pipes with expanding boundaries. I believe it needs some adjustment and thats what I am trying to see if I can do. I will also look at the integration idea you suggested. Although I have no clue how to do it!
Can the "s" variable be used only in 2D? I came across it but I don't understand it fully.
Thanks,
Sirisha
Can you please explain some more on "I would propose to check if you cannot use the predefined laminar inflow BC."
Are you asking me to check if u=const*(1-(r/H)^2) can be applied to an expanding pipe boundary?
If yes, then based on the theory of the equation, it is derived by considering a rigid pipe boundary.
However, I have seen journal papers where this boundary condition has been applied sucessfully even to pipes with expanding boundaries. I believe it needs some adjustment and thats what I am trying to see if I can do. I will also look at the integration idea you suggested. Although I have no clue how to do it!
Can the "s" variable be used only in 2D? I came across it but I don't understand it fully.
Thanks,
Sirisha
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Posted:
1 decade ago
Hi
have you checked (see capture image below) laminar inflow.
For the "r" and "R", if you have structural/solid involved, COMSOL normally splits the reference frame and the spatial frame (theone deformed) and you refer to the Reference frame wit upper case X,Y,Z,R and to the spatial with lower case x,y,z,r. In SPF alone the two frames overlap and you can ony use lower case, with FSI you have upper and lower case because of the presence of the "solid" part.
To find out, try adding a Definition Coordinate System, and hit the Reference Frame field, you will see the four options, and their coordinate names, all lower case, lower and upper, or even lower, upper and XmYm... and Xg,Yg ... for a distinct mesh and respectively Geometrical frame.
"s" is defined in 2D along an edge, and goes from 0 to 1 along the direction of the arrow (in edge mode), in 3D there are s1 and s2 but I do not beleive they are full ynormalised always to 0-1 so it's anyhow tricky to define laues with these. But handy and easy in 2D and 2D-axi
--
Good luck
Ivar
have you checked (see capture image below) laminar inflow.
For the "r" and "R", if you have structural/solid involved, COMSOL normally splits the reference frame and the spatial frame (theone deformed) and you refer to the Reference frame wit upper case X,Y,Z,R and to the spatial with lower case x,y,z,r. In SPF alone the two frames overlap and you can ony use lower case, with FSI you have upper and lower case because of the presence of the "solid" part.
To find out, try adding a Definition Coordinate System, and hit the Reference Frame field, you will see the four options, and their coordinate names, all lower case, lower and upper, or even lower, upper and XmYm... and Xg,Yg ... for a distinct mesh and respectively Geometrical frame.
"s" is defined in 2D along an edge, and goes from 0 to 1 along the direction of the arrow (in edge mode), in 3D there are s1 and s2 but I do not beleive they are full ynormalised always to 0-1 so it's anyhow tricky to define laues with these. But handy and easy in 2D and 2D-axi
--
Good luck
Ivar
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