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Modeling flow through a thin channel
Posted 24.06.2012, 05:52 GMT-4 Fluid & Heat, Mesh Version 4.2 2 Replies
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Hi,
I'm trying to model water flow through a 2D thin channel (8 cm X 2 mm). This is part of my PhD research.
The inlet pressure is 1.5 bar, outlet pressure is 1 bar.
The walls have a no slip conditions. I'm using laminar flow, incompressible fluid at steady state conditions.
although this problem is supposed to be simple, the results show high velocities (about 700 m/s) and they change as I refine the mesh.
Can you help me define my parametes or explain what I'm doing wrong ?
I tried this on ver 3.5 and 4.2.
Best,
Avner
I'm trying to model water flow through a 2D thin channel (8 cm X 2 mm). This is part of my PhD research.
The inlet pressure is 1.5 bar, outlet pressure is 1 bar.
The walls have a no slip conditions. I'm using laminar flow, incompressible fluid at steady state conditions.
although this problem is supposed to be simple, the results show high velocities (about 700 m/s) and they change as I refine the mesh.
Can you help me define my parametes or explain what I'm doing wrong ?
I tried this on ver 3.5 and 4.2.
Best,
Avner
2 Replies Last Post 20.08.2012, 18:53 GMT-4
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Posted:
1 decade ago
Hi
I suppose that with a 0.5bar = 50[kPa] pressure across a 80 mm long path, even with 2 mm height and ambient temperature water you would get quite a high speed, as that is quite some pressure for such a small device.
So for me you are no longer in laminar flow conditions, but far into turbulent flow, hence need a much finer mesh.
I would propose to ramp up the velocity with a continuation sweep in your stationary solver, with a p=0 outlet pressure, and laminar inflow, and go from 1 to perhaps 100 m/s and check your Reynolds cell number and the pressure and velocities as you increase the inlet condition.
Then find the transition values for turbulent flow.
By the way 1.5 bar in and 1 bar out is equivalent to 0.5 bar in and 0 bar out + a 1bar static pressure used by default by COMSOL for the water properties
Still what puzzles me a little: in 2D you are talking about flow between two plates (default per meter depth) how is our depth defined, as well your pressure w.r.t. this open Z value ?
for a laminar flow you can also use the Poiseuille formula to estimate the pressure drop, to validate your results (and perhaps also your input specifications ;)
And do not forget if you choose the laminar inflow with inlet pressure node setting, the pressure drop includes the extra "inflow" length, so the remaining pressure in your model part is very roughly linear to the length ratio
--
Good luck
Ivar
I suppose that with a 0.5bar = 50[kPa] pressure across a 80 mm long path, even with 2 mm height and ambient temperature water you would get quite a high speed, as that is quite some pressure for such a small device.
So for me you are no longer in laminar flow conditions, but far into turbulent flow, hence need a much finer mesh.
I would propose to ramp up the velocity with a continuation sweep in your stationary solver, with a p=0 outlet pressure, and laminar inflow, and go from 1 to perhaps 100 m/s and check your Reynolds cell number and the pressure and velocities as you increase the inlet condition.
Then find the transition values for turbulent flow.
By the way 1.5 bar in and 1 bar out is equivalent to 0.5 bar in and 0 bar out + a 1bar static pressure used by default by COMSOL for the water properties
Still what puzzles me a little: in 2D you are talking about flow between two plates (default per meter depth) how is our depth defined, as well your pressure w.r.t. this open Z value ?
for a laminar flow you can also use the Poiseuille formula to estimate the pressure drop, to validate your results (and perhaps also your input specifications ;)
And do not forget if you choose the laminar inflow with inlet pressure node setting, the pressure drop includes the extra "inflow" length, so the remaining pressure in your model part is very roughly linear to the length ratio
--
Good luck
Ivar
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Posted:
1 decade ago
Hello:
I am modelling a similar situation (model attached), but the solution just don't converges. The idea of this model is to see how the fluid behaves as it is passed through a very small orifice.
I can't determine what is causing the error, but I've stumple upon 2 diferent error messages, could you please explain me what is causing them or what am I doing incorrectly?
1)Failed to find consistent initial values.Last time step is not converged.
2) May have reached a singularity. Last time step is not converged.
I have read that singularities arise around r=0 while using 2D-AXI, how can this be avoided ?
I am modelling a similar situation (model attached), but the solution just don't converges. The idea of this model is to see how the fluid behaves as it is passed through a very small orifice.
I can't determine what is causing the error, but I've stumple upon 2 diferent error messages, could you please explain me what is causing them or what am I doing incorrectly?
1)Failed to find consistent initial values.Last time step is not converged.
2) May have reached a singularity. Last time step is not converged.
I have read that singularities arise around r=0 while using 2D-AXI, how can this be avoided ?
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