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02-07-2008 11:31 AM

Hello,

I'm performing a modal freq response analysis applying a unit acceleration load

along x on a node attached to other 4 nodes by a RBE2. Amplitude is constant between

20/2000 Hz. When I plot the output in format amplitude/phase on output nodes, amplitude

is always negative for some nodes and always positive for other nodes (curves symmetric

wrt y=0 axis). Shouldn't the amplitude be always positive? Or am I wrong?

Thanks for help.

Marco

2 REPLIES

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02-07-2008 12:02 PM

Marco,

I would expect to see that the phase of the response changes with frequency. At

some frequencies it will be in-phase with the input (postive) and other frequencies

it will be out-of-phase (negative). It will change phase at the frequencies corresponding

to modes.

Other than the input point and points close by, I would not expect the response

to be constant phase for all frequencies.

Mark

"Marco"

>

>Hello,

>I'm performing a modal freq response analysis applying a unit acceleration

>load

>along x on a node attached to other 4 nodes by a RBE2. Amplitude is constant

>between

>20/2000 Hz. When I plot the output in format amplitude/phase on output nodes,

>amplitude

>is always negative for some nodes and always positive for other nodes (curves

>symmetric

>wrt y=0 axis). Shouldn't the amplitude be always positive? Or am I wrong?

>

>Thanks for help.

>

>Marco

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02-07-2008 04:39 PM

Mark,

yes phase is correctly not constant with frequency. The problema is that the amplitude

(which is not constant either) is always < 0 for some nodes...why? isn't it positive

by definition? Examples in FEMAP help always give positive amplitudes in freq response

analyses.

Marco

"Mark Donley"

>

>Marco,

>

>I would expect to see that the phase of the response changes with frequency.

>At

>some frequencies it will be in-phase with the input (postive) and other frequencies

>it will be out-of-phase (negative). It will change phase at the frequencies

>corresponding

>to modes.

>

>Other than the input point and points close by, I would not expect the response

>to be constant phase for all frequencies.

>

>Mark

>

>

>

>

>

>

>

>

>

>

>"Marco"

>>

>>Hello,

>>I'm performing a modal freq response analysis applying a unit acceleration

>>load

>>along x on a node attached to other 4 nodes by a RBE2. Amplitude is constant

>>between

>>20/2000 Hz. When I plot the output in format amplitude/phase on output nodes,

>>amplitude

>>is always negative for some nodes and always positive for other nodes (curves

>>symmetric

>>wrt y=0 axis). Shouldn't the amplitude be always positive? Or am I wrong?

>>

>>Thanks for help.

>>

>>Marco

>

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