Modal Effective Mass & Participation Not Appearing in Results

[brandon.hallinan]

Hello all,

I have performed a natural frequency analysis with nonlinear materials using Autodesk Simulation Mechanical 2013.

I have looked for the modal effective mass and participation notes in the report summary and report log documents, but am unable to find the information.

Doing a CTRL+F function to search fro the specific terms does not give any matches.

Where else might I be able to find this information from my analysis?

Thank you!

[AstroJohnPE · Accepted solution]

Hi Brandon,

It could be that the nonlinear modal does not output the mass participation. I suggest that you change the analysis type to Linear > Natural Frequency and run the analysis. That should give the mass participations.

If you have any nonlinear materials in the simulation, the nonlinear modal analysis is only using the stiffness at the 0 stress-state. So you can use the 0 stress-state stiffness in the linear modal analysis, and you should see that the frequency results are very close to the same values as the "nonlinear" modal.

[brandon.hallinan]

I used this video https://www.youtube.com/watch?v=xvBsIA62I8U as a tutorial for setting up a frequency analysis on a venturi model.

At around 8 minutes into the video he assigns a frequency value of 11.8 Hz and mentions that this equates to a wind speed of 10 m/s.

I have no idea how he came up with this conversion, but I need to determine the frequency for a nominal mass flow rate through the venturi.

Any idea how he arrived at the frequency value he used in his example?

[John_Holtz]

Hi Brandon,

For a pole (or chimney), the frequency of vortex shedding is related to the wind speed by the Strouhal number. See this page on Wikipedia: https://en.wikipedia.org/wiki/Vortex_shedding

I am guessing that this is how Pat related the wind speed going around the pole to the frequency of vibration.

[brandon.hallinan]

Thank you John,

the Strouhal number helps a lot.

So for my venturi model I know the nominal mass flow rate to be 68.4 lbm/s.

Using the equation,

m(dot) = rVA

where;

r = density (air)

V = velocity (air)

A = area (inlet)

I can solve for the velocity of the air at the inlet of the venturi.

The density of air (at sea level) is 0.00004335 lb/in^3

And given the internal diameter of the venturi inlet is 41.5 inches, the area at the inlet is 1,352.652 in^2

Rearranging the equation to solve for V,

V = m(dot)/rA = (68.4 lbm/s) / ((0.00004335 lb/in^3)(1,352.652 in^2)), so my velocity at the inlet;

V = 1,166.49 in/s

Now, using the equation from the vortex shedding article;

St = (f*D) / V, where;

St = 0.2

f = frequency

D = diameter of cylinder

V = flow velocity

I can rearrange the equation to solve for f, so that;

f = (St*V) / D 

and using my known and calculated values;

f = (0.2(1166.49 in/s)) / 41.50 in = (233.298 in/s) / 41.50 in

f = 5.622 s^-1 = 5.622 Hz

Therefore, in my frequency response simulation, I should assign the 5.662 Hz and a 68.4 lb force to a node at the inlet of the venturi?

And these values should act in the X direction, as the inlet and outlet of the venturi is along the x-axis?

[John_Holtz]

Hi,

I am not sure that there is a correlation between the air blowing around a stack (St = (f*D) / V) and air blowing through a pipe or venture. I would think that the physics are different.

It has been a while since I did fluid flow calculations, but I would be surprised if the force on the venture is numerically equal to the mass flow rate. I am only saying this because I remember the calculations being more complex than "F=m".

You should explain the phenomenon that is occurring, or what you need to calculate, and perhaps someone will have an idea how to simulate it.

[brandon.hallinan]

Hi John,

for the portion of the simulation I am trying to perform,

I am trying to do a frequency response analysis on the venturi model, given that the maximum mass flow rate through the venturi is 68.4 lbm/s at a nominal temperature of -11 degrees F.

For the first part of the analysis, I did the modal analysis to generate a number of resonant frequencies of the venturi with resultant mode shapes, and so that I could use the results in the frequency response analysis. 

After that I am a bit at a loss trying to determine the force load and frequency of the force load to simulate the air mass flowing through the venturi.

What I am trying to do now is show the displacement magnitude(s) of the venturi during operation with the given mass flow rate information...