Learning Forum

[kamalkrishna]

I want to write Hpq for a system by using above formula,please tell me how to extract mass normalized mode shape vector from ansys workbench.I know how to extract from Mode shape (total deformation and directional deformation).But there is no phase angle involved there.Thank you for your help.

 

[dlooman]

In a mode-sup harmonic the complex mode-coefficients are written to file.mcf.  Would that be a way to get the result you are describing?

[kamalkrishna]

Thank you Dave for your quick response.I understood that response of a forced frequeny for a struture written as summation of multiplication of mode coefficients and eigen vector of modes.I think these mode coeffients are not unique and they depend on load.But i need Eigen Vector of mode in Modal analysis.

[dlooman]

An undamped mode shape is not complex.  There's no phase angle issue.  You said initially that you knew how to extract the mode shape displacements from Mechanical.  That should be all you need.  The contribution of a single mode to a harmonic result is its complex mode coefficient times its mode shape. 

[kamalkrishna]

I tried Damped modal analysis,now i am getting two new options other than undamped modal analysis in total deformation they are Amplitude and Sweeping phase.I did not understood what these terms will give,I want to export this complex mode shape from selected nodes.I am trying in several ways but i am not getting any progress.

[dlooman]

With a damped solver you have a real and imaginary mode shape.  Typically the imaginary mode shape is almost zero.  To display the imaginary mode shape specify a phase angle of 90 deg.  In rotordynamics the imaginary mode shape can be as large as the real mode shape.  That produces the whirling motion you see in a spinning body.  

[kamalkrishna]

Thank you dave,The mode shapes displayed in Ansys modal analysis (total deformation),are they mass normalized eigen vectors?.Because when we study theory about theoretical modal analysis,the magnitudes  of each element of mass normalized shape vector will be less than 1.If we see magnitude in total deformation it will be greater than 1 also.I know the concept normalizing maximum deformation values to 1.Which type will give me mass normalized eigen vector.I need to write a H(w) as shown in first question.

[dlooman]

Modes must be mass normalized {phi'}[M]{phi} = 1.0 to be used in a mode-superposition analysis and that is default.  Sometimes customers aren't going to be doing a mode-superposition and there's an option to scale the mode so that the maximum dof value is 1.0.  Mass scaled modal displacements can be greater than 1.0  If [M] is very small, {phi} must be large to make {phi'}[M]{phi} = 1.0.  Note also that even with unit scaling, total deformation can be greater than 1.0 because it is the SRSS of three dof, all of which could be 1.0 (probably not though.)

[kamalkrishna]

Thank you dave for quick response.Now i want know that how can be these mass normalized eigen vectors can be extracted.{phi'}[M]{phi} = 1.0 .I want these {phi} values for nodes.So that i can write H(w).

[dlooman]

Just right click on a modal result and select "Export."

[kamalkrishna]

Mode shapes in ansys modal analysis will be shown as total deformation.If i right click on that mode shape then export node vs total deformation (which will give node vs maximum total deformation) is that mass normalized mode shape vector??

[dlooman]

Sorry.  Didn't think of that.  You could export the UX, UY and UZ values separately or use the commands below in a commands object:

set,1,1   ! store mode 1 results 

/out,mode_1,out     ! redirect output to a file 

prnsol,u 

[kamalkrishna]

Thank you very much dave,now i am able to write H(w).