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How to define "step velocity"?

Posted 10 sept. 2018, 03:01 UTC−4 4 Replies

Krupali
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Hello to all,

Could you please help me to how can I define "step velocity" as in for first 0.5 sec velocity is zero then for next 2 second it is 1 m/s and again zero for next 0.5 sec.?

4 Replies Last Post 10 sept. 2018, 13:43 UTC−4
Mickaël Barsive COMSOL Employee

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Posted: 6 years ago 10 sept. 2018, 05:21 UTC−4

Hello,

You can use a rectangle function in "Definitions">"Functions">"Rectangle".

In your case, set "lower limit" at 0.5, "upper limit" at 2.5 then you can plot the graphic to check your function. The next step is to use the function in the physics interface: in your "Inlet" node, you can use the following syntax: rect1(t[1/s]) rect1 is the function name by default and [1/s] is used for homogeneous units.

I hope this helps !

Mickaël

Hello, You can use a rectangle function in "Definitions">"Functions">"Rectangle". In your case, set "lower limit" at 0.5, "upper limit" at 2.5 then you can plot the graphic to check your function. The next step is to use the function in the physics interface: in your "Inlet" node, you can use the following syntax: rect1(t[1/s]) rect1 is the function name by default and [1/s] is used for homogeneous units. I hope this helps ! Mickaël
Krupali
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Posted: 6 years ago 10 sept. 2018, 11:12 UTC−4

Hello,

You can use a rectangle function in "Definitions">"Functions">"Rectangle".

In your case, set "lower limit" at 0.5, "upper limit" at 2.5 then you can plot the graphic to check your function. The next step is to use the function in the physics interface: in your "Inlet" node, you can use the following syntax: rect1(t[1/s]) rect1 is the function name by default and [1/s] is used for homogeneous units.

I hope this helps !

Mickaël

Thank you very much for your help.

>Hello, > >You can use a rectangle function in "Definitions">"Functions">"Rectangle". > >In your case, set "lower limit" at 0.5, "upper limit" at 2.5 >then you can plot the graphic to check your function. The next step is to use the function in the physics interface: in your "Inlet" node, you can use the following syntax: rect1(t[1/s]) >rect1 is the function name by default and [1/s] is used for homogeneous units. > >I hope this helps ! > >Mickaël Thank you very much for your help.
Edgar J. Kaiser Certified Consultant
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Posted: 6 years ago 10 sept. 2018, 11:39 UTC−4

Hi,

rectangular velocity steps will probably stop a time dependent solver as a velocity step implies infinite acceleration and thus is unphysical. You may need to smooth the steps. This can be done with e.g. ramp functions. Take care that the time stepping properly resolves the ramp.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Hi, rectangular velocity steps will probably stop a time dependent solver as a velocity step implies infinite acceleration and thus is unphysical. You may need to smooth the steps. This can be done with e.g. ramp functions. Take care that the time stepping properly resolves the ramp. Cheers Edgar
Jeff Hiller COMSOL Employee

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Posted: 6 years ago 10 sept. 2018, 13:43 UTC−4

Hi Edgar,

The Rectangle function includes (adjustable) smoothing - See GUI.

Jeff

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Jeff Hiller
Hi Edgar, The Rectangle function includes (adjustable) smoothing - See GUI. Jeff

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