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Retrive Real & Imag Freq for All the Modes in a DAMP Fre
 
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joel.thompson1
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Joined: 26 Jun 2014
Posts: 2

PostPosted: Thu Jun 26, 2014 6:48 pm  Reply with quote

Hi All,

I am executing a modal Damp analysis to find the critical speed and its frequencies. I have 46 speeds for which I run the damp analysis. For each speed I have 20 modes and the Ansys help document gives the command to get the frequencies for mode as *get,a1,MODE,%mode no%,FREQ,IMAG

Kindly help me to know how to use this command or any other command to retrieve the frequencies for other speeds (loadsteps). Ansys manual tells that for spectrum analysis we can use the Item2 & It2num.

Thanks,
Joel Thompson
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jose.galan
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Joined: 21 Oct 2008
Posts: 140

PostPosted: Fri Jun 27, 2014 4:10 am  Reply with quote

Dear Mr. Thompson,

If you look at the help of the *get command, right below Table 176 (*GET
solution items, Entity=MODE), you will see that the command that you
mention only retrieves the values correspong to the first load step. In
your case, where you are calculating the modes at different rotating
velocities (multistep modal), you should use Entity=CAMP (for Campbell
diagram). That is table 178.

Best regards,

Jose M. Galan

Constr. Eng. Dept.

Univ. Sevilla

Spain

El 27/06/2014 03:48, joel.thompson1 escribió:

Quote:
Hi All,

I am executing a modal Damp analysis to find the critical speed and its frequencies. I have 46 speeds for which I run the damp analysis. For each speed I have 20 modes and the Ansys help document gives the command to get the frequencies for mode as *get,a1,MODE,%mode no%,FREQ,IMAG

Kindly help me to know how to use this command or any other command to retrieve the frequencies for other speeds (loadsteps). Ansys manual tells that for spectrum analysis we can use the Item2 & It2num.

Thanks,
Joel Thompson

------------------------
Joel Thompson
Tata Consultancy Services
Japan

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joel.thompson1
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Joined: 26 Jun 2014
Posts: 2

PostPosted: Wed Jul 02, 2014 6:48 pm  Reply with quote

Thank you Jose-san,

I understand and have performed the Step2 analysis and executed CAMP command to retrieve the frequencies.

I am facing another problem, I am using element 273 and modeling the Blade as Mass elements. For Lateral vibration ANSYS requires the mass element to be in the axis of rotation. The mass element is connected to the original location with CERIG. I have arrested the 3 Rotational DOF for mass element to solve the lateral vibration. Without proper DOF arrest in mass element I am not able to complete the analysis. My question is whether arresting 3 Rotation DOF for mass elememnt is correct ? Any advice will be a great help for me.

Thank you again,
Joel
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jose.galan
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Joined: 21 Oct 2008
Posts: 140

PostPosted: Thu Jul 03, 2014 2:20 am  Reply with quote

Dear Mr. Thompson,

I think that your main question is if a mass without rotary inertia is a
good mathematical model of the real behaviour of your equipment. It
depends.

It can be a good approximation when the rotary inertia about the center
of gravity of the element is small compared with the mass*d^2, where d
is the distance to the axis of rotation (Steiner's theorem).

I have used this approximation to model the masses of the secondary
equipment in the dynamic analysis of foundations of large compressors.

Once that you decide that a mass without rotary inertia is acceptable,
there is a better way to model it in Ansys. For point-mass element
MASS21, you can define several rotary inertia options by KEYOPT(3).

"KEYOPT(3)
Rotary inertia options:

0 -- 3-D mass with rotary inertia

2 -- 3-D mass without rotary inertia

3 -- 2-D mass with rotary inertia

4 -- 2-D mass without rotary inertia "

If you want to have a 3d mass without rotary inertia, you can do so by
setting keyopt(3)=2.

et,1,mass21,,,2

In this case, the mass21 would not have rotational d.o.f.. You would not
need the restrains that you mentioned in your email.

I have some other questions regarding your post.

Element SOLID273 is a general axisymmetric solid element, but the mass21
is not. I am sure that you have considered this carefully. Are you
modelling the whole set of blades with one mass21 element? How many
Fourier nodes in the circunferential direction are you using for the
solid273? This element has several nodes in the theta direction, that
are generated automatically. To what node are you attaching the mass
with the CERIG command?

I do not understand the sentence "For Lateral vibration ANSYS requires
the mass element to be in the axis of rotation". Why should they be in
the axis of rotation only? Do you get some error message from ansys if
you put the mass element in other arbitrary location? Aren't they
allowed to be freely located on the master plane?

I do not understand this sentence: "The mass element is connected to the
original location with CERIG." When I have used point masses, I have
placed them in the center of gravity of the equipment, and I have
connected them to the structure by links or cerig. I understand that you
are not doing this. You are placing the mass on some other spot (on the
axis of symmetry) and then you connect that point with the original
location. Could you please explain this better?

Best regards,

Jose M. Galan

Constr. Eng. Dept.

Univ. Sevilla

Spain


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christopher.wright
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Joined: 17 Jun 2009
Posts: 927

PostPosted: Thu Jul 03, 2014 7:43 pm  Reply with quote

On Jul 3, 2014, at 4:20 AM, mfernan@us.es wrote:

Quote:
It can be a good approximation when the rotary inertia about the center
of gravity of the element is small compared with the mass*d^2,
He hasn't mentioned a thing about how he's modeled the stiffness. It won't be such a good approximation if he doesn't get the stiffness right. The way he's described it he won't get any torsional modes. Torsional stiffness is an important factor in flutter, as is rotational inertia.

Quote:
I have used this approximation to model the masses of the secondary
equipment in the dynamic analysis of foundations of large compressors.
Sounds like two different things. The approximation is common for seismic analysis as well, provided the secondary equipment is much less massive that the primary structure and doesn't contribute significant stiffness to the structure. Thompson's problem involves all the mass of the blade, not just minor contributions of attached equipment.


Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/



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jose.galan
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Joined: 21 Oct 2008
Posts: 140

PostPosted: Fri Jul 04, 2014 2:21 am  Reply with quote

I agree with both comments.

Jose M. Galan

Univ. Sevilla

Spain

El 04/07/2014 04:43, Christopher Wright escribió:

Quote:
On Jul 3, 2014, at 4:20 AM, mfernan@us.eswrote:

Quote:
It can be a good approximation when the rotary inertia about the center of gravity of the element is small compared with the mass*d^2,

He hasn't mentioned a thing about how he's modeled the stiffness. It won't be such a good approximation if he doesn't get the stiffness right. The way he's described it he won't get any torsional modes. Torsional stiffness is an important factor in flutter, as is rotational inertia.

Quote:
I have used this approximation to model the masses of the secondary equipment in the dynamic analysis of foundations of large compressors.

Sounds like two different things. The approximation is common for seismic analysis as well, provided the secondary equipment is much less massive that the primary structure and doesn't contribute significant stiffness to the structure. Thompson's problem involves all the mass of the blade, not just minor contributions of attached equipment.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/ [1]


Links:
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[1] http://www.skypoint.com/members/chrisw/
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