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oscillating inlet velocity result
Posted Dec 12, 2011, 9:03 a.m. EST 3 Replies
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Hi, i am new to COMSOL and my CFD work too. Hope to have your guidance here.
Currently I start with simulation of fluid flow within an 2D axisymmetric cylinder. I applied boundary conditions:
inlet as pressure=0,
outlet as velocity=0.2m/s with step function;
Then I plot a 1D graph to show the result of inlet velocity (by selecting a point located at cylinder inlet symmetrical axis). However the graph shows that inlet velocity is oscillating like sine waves (green line in attached picture).
May I ask is this caused by numerical problem or it is a genuine phenomena?
Or actually i should plot the inlet velocity by selecting the inlet boundary line instead of a point?
Thank you. Looking forward for your reply and help.
Currently I start with simulation of fluid flow within an 2D axisymmetric cylinder. I applied boundary conditions:
inlet as pressure=0,
outlet as velocity=0.2m/s with step function;
Then I plot a 1D graph to show the result of inlet velocity (by selecting a point located at cylinder inlet symmetrical axis). However the graph shows that inlet velocity is oscillating like sine waves (green line in attached picture).
May I ask is this caused by numerical problem or it is a genuine phenomena?
Or actually i should plot the inlet velocity by selecting the inlet boundary line instead of a point?
Thank you. Looking forward for your reply and help.
Attachments:
3 Replies Last Post Dec 12, 2011, 12:39 p.m. EST
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Posted:
1 decade ago
Abbie,
I would expect to see oscillations when one end of the tube is excited by a step function. It probably acts like a pipe and the frequency you see should some way correspond with the length of the cylinder.
Cheers
Edgar
I would expect to see oscillations when one end of the tube is excited by a step function. It probably acts like a pipe and the frequency you see should some way correspond with the length of the cylinder.
Cheers
Edgar
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Posted:
1 decade ago
Thanks so much Edgar. Would you mind to explain more why the step function would lead to oscillation?
For what i have in mind, here I input a step function at location time=0.01s to let comsol ramp up slowly from initial zero velocity to achieve 0.2m/s at outlet (to go through simulation in 1 second). What I thought is, if step function leads to oscillation, i would expect it shown within the first 0.02s of inlet velocity, however it occurs continuously within 1s of simulation. Please correct me if my concept is wrong.
Thanks a lot!
For what i have in mind, here I input a step function at location time=0.01s to let comsol ramp up slowly from initial zero velocity to achieve 0.2m/s at outlet (to go through simulation in 1 second). What I thought is, if step function leads to oscillation, i would expect it shown within the first 0.02s of inlet velocity, however it occurs continuously within 1s of simulation. Please correct me if my concept is wrong.
Thanks a lot!
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Posted:
1 decade ago
Abbie,
if the blue graph in the plot is the output velocity, then this is a step, not a ramp. If you want a continuous ramp you must set something like ramp = 0.2*t*(t<=1) + 0.2*(t>1)
This will ramp up linearly from 0 to 0.2 within a second and then remain constant.
Cheers
Edgar
if the blue graph in the plot is the output velocity, then this is a step, not a ramp. If you want a continuous ramp you must set something like ramp = 0.2*t*(t<=1) + 0.2*(t>1)
This will ramp up linearly from 0 to 0.2 within a second and then remain constant.
Cheers
Edgar