# Discussion Forum

## Questions about applying a harmonic load

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Hi, I am student who wants to simulate the anvil to see the vibrations in Frequency domain for structural mechanics. I want to apply a constant pressure in z-direction of my surface and a harmonic load which will change time to time in x-direction on the same surface. (like Fx=Acos(2pif*t)) How could I apply the harmonic load, I can't apply with a wave form, which says "an unkown variable" in the equation.

I'm glad to hear from any answers.

Zheng Sun

9 Replies Last Post Nov 28, 2022, 7:52 a.m. EST

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Posted: 2 weeks ago
Updated: 2 weeks ago

Hi Zheng,

The correct COMSOL syntax is `A*cos(2*pi*f*t)`, then assuming that `f` is a defined constant, for example.

Best regards,

Magnus

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Posted: 2 weeks ago

Hi Zheng,

The correct COMSOL syntax is `A*cos(2*pi*f*t)`, then assuming that `f` is defined constant, for example.

Best regards,

Magnus

Hi Magnus,

I use your equation, but it still has this problem "an unknown variable f". Because I want to do Frequency domain study, I want my result to change with the frequency, so I couldn't set a constant f. How could I deal with it again?

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Posted: 2 weeks ago

If you want to do a frequency domain study all you have to do is enter the phasor value that represents the load.

If there is an additional static load then you are interested in a frequency domain, prestressed study.

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Posted: 2 weeks ago

If you want to do a frequency domain study all you have to do is enter the phasor value that represents the load.

If there is an additional static load then you are interested in a frequency domain, prestressed study.

Hi Dave,

Thank you for the sugesstions!

If I want to have a harmonic load changes tme to time, how could I define the phase? change the phase to "2pif*t"? I am not sure about this.

Besides, if i need to do the frequency domain, prestressed study, i can define the static load like "16N" directly and another harmonic load with the phase change on the same surface, is it right?

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Posted: 2 weeks ago

Hi Zheng,

In a frequency-domain study, you analyze the response due to a sinusoidal (harmonic) load applied to a structure at specified frequencies. So, what you specify for the loads then is the amplitude (`A`, in your case). The time is not explicitly used.

Best regards,

Magnus

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Posted: 2 weeks ago

A frequency dependent simulation calculates the response to a sinusoidal forcing function applied for an infinite time. Calculated quantities and forcing functions are represented as phasors. OP should begin by understanding phasor representation. Phasors represent the magnitude and phase by a single complex number.

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Posted: 2 weeks ago

A frequency dependent simulation calculates the response to a sinusoidal forcing function applied for an infinite time. Calculated quantities and forcing functions are represented as phasors. OP should begin by understanding phasor representation. Phasors represent the magnitude and phase by a single complex number.

Thanks again!

Now I set static force in z-direction for "16N" and set harmonic force magnitude "79N" in x-direction and harmonic force phase "2pifreq*0.2[s]" in x-direction. I used these settings to do a frequency domain prestressed study, is it right?

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Posted: 1 week ago

A frequency dependent simulation calculates the response to a sinusoidal forcing function applied for an infinite time. Calculated quantities and forcing functions are represented as phasors. OP should begin by understanding phasor representation. Phasors represent the magnitude and phase by a single complex number.

Thanks again!

Now I set static force in z-direction for "16N" and set harmonic force magnitude "79N" in x-direction and harmonic force phase "2pifreq*0.2[s]" in x-direction. I used these settings to do a frequency domain prestressed study, is it right?

When you apply 16 N it has zero phase unless you write it as for example 16exp(isome phase in radians) or as a+ib for rectangular form for the phasor. There is no thing as harmonic force phase without an amplitude so your last input does not make sense.

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com

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Posted: 1 week ago

A frequency dependent simulation calculates the response to a sinusoidal forcing function applied for an infinite time. Calculated quantities and forcing functions are represented as phasors. OP should begin by understanding phasor representation. Phasors represent the magnitude and phase by a single complex number.

Thanks again!

Now I set static force in z-direction for "16N" and set harmonic force magnitude "79N" in x-direction and harmonic force phase "2pifreq*0.2[s]" in x-direction. I used these settings to do a frequency domain prestressed study, is it right?

When you apply 16 N it has zero phase unless you write it as for example 16exp(isome phase in radians) or as a+ib for rectangular form for the phasor. There is no thing as harmonic force phase without an amplitude so your last input does not make sense.

Thank you for your reply, However, "16N" is the clamp force that I want it to be static, and "79N" is another harmonic force applied to the same surface that I want it to be harmonic force with phasor. So my question is could it be possible to set like this and apply a frequency domain prestressed study? Or just frequency domain study?