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Time dependent fluid problem
Posted Jan 9, 2014, 4:04 p.m. EST Fluid & Heat Version 4.3a 2 Replies
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Hi
I am trying to model a time dependent flow of a newtonian fluid in 2D. The geometry I'm using has sliding walls which cause a flow to be established. The aim of using a time dependent simulation is to gain a qualitative idea of the fluctions of the flow established. I am new to COMSOL and was just following a tutorial in which a step function is used to smoothly ramp up the velocity. I have used this to smoothly increase the speed of the walls to a predefined value and this solves fine.
My question is whether a step function is required or if a time dependent simulation can be done with the walls moving at the predefined value from the begining of the simulation and what difference this will make.
This is rather open ended so any input is appreciated
Cheers,
Finnian
I am trying to model a time dependent flow of a newtonian fluid in 2D. The geometry I'm using has sliding walls which cause a flow to be established. The aim of using a time dependent simulation is to gain a qualitative idea of the fluctions of the flow established. I am new to COMSOL and was just following a tutorial in which a step function is used to smoothly ramp up the velocity. I have used this to smoothly increase the speed of the walls to a predefined value and this solves fine.
My question is whether a step function is required or if a time dependent simulation can be done with the walls moving at the predefined value from the begining of the simulation and what difference this will make.
This is rather open ended so any input is appreciated
Cheers,
Finnian
2 Replies Last Post Jan 12, 2014, 3:30 p.m. EST
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Posted:
1 decade ago
Hi Finnian,
it is, in principle, possible to start your simulation with moving walls starting at predefined velocity values. However, you may end up with convergence problems; from a numerical point of view, a predefined wall velocity is equal to an infinite wall acceleration at t=0 (if the initial fluid velocity is zero).
The step function is used to increase the velocity smoothly to avoid numerical complications. In fact, finite wall acceleration is also the physically correct way to handle these questions.
An alternative approach would be to start with different initial conditions for the fluid velocity, e.g. a parabolic profile. In this way, you can avoid discontinuities at the initialization.
Best Regards
Phillip
it is, in principle, possible to start your simulation with moving walls starting at predefined velocity values. However, you may end up with convergence problems; from a numerical point of view, a predefined wall velocity is equal to an infinite wall acceleration at t=0 (if the initial fluid velocity is zero).
The step function is used to increase the velocity smoothly to avoid numerical complications. In fact, finite wall acceleration is also the physically correct way to handle these questions.
An alternative approach would be to start with different initial conditions for the fluid velocity, e.g. a parabolic profile. In this way, you can avoid discontinuities at the initialization.
Best Regards
Phillip
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Posted:
1 decade ago
HI Phillip,
Thanks that was a useful and very informative answer.
Cheers,
Finnian
Thanks that was a useful and very informative answer.
Cheers,
Finnian