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frequency analysis using a moving fluid

Question asked by rudy pereda on May 7, 2008
Latest reply on May 16, 2008 by Bill McEachern

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I used floworks to simulate the flow of steam in a pipe, across a solid non rotating object. I exported the results to Cosmos.
My question is how do you find the frequencies generated by the flowing fluid at a specific velocity/volumetric flowrate, etc.
I can find the resonance frequency for the system but have not found anything in the vibrational analysis that will tell me the frequency generated by the flowing fluid.

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Basil Gello May 7, 2008 5:38 PM
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Ruby,

try get the forces and apply them as a distributed load for frequency study.

Regards, Basil
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- rudy pereda @ Basil Gello on May 7, 2008 6:48 PM
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  Thanks Basil.
  
  After the analysis is complete will the resonance frequencies generated for this loading condition be the frequencies i am looking for, or do i have to look somwhere else for these?
  
  Rudy.
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  - Basil Gello @ rudy pereda on May 8, 2008 4:55 AM
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    Ruby,
    I hope it gives you the approximate value of a resonance frequency. And, in fact, the resonance frequency depends not rather on environment causing the oscillations but on the body internal characteristics. So that's why it must help.
    Regards, Basil
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Anthony Botting May 9, 2008 12:21 AM
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Hi Rudy: You appear to be crossing into FSI territory, or "Fluid-Solid Interaction" capability, as touted by some competitors in CAE. If the flow field is transient in nature, (such as with vortex shedding), you would have to use that approach. However, by running a time-transient analysis in FloWorks, you might be able to observe a pressure "pattern" that repeats over some period, where you can identify the pressure field as a function of time. If the body being blown by the steam is not too affected by the flow (i.e., does not bend or move back and forth as a light pole bending in the wind), then it won't affect the flow field (that's a case to use true FSI software). So, if you could extract a pressure field around the surface as a function of time, you could apply that as an excitation load using COSMOSWorks Advanced Pro, Advanced Dynamics functionality. The software will use the system's natural frequencies (from a frequency analysis), and apply your pressure load over time to give you the structural response of the system. You want the structural deflection response to be small enough, in your judgment, not to affect the flow field for this entire approach to be valid, however. They key seems to be able to extract the pressure values, by surface coordinate, at each time step in the transient FloWorks analysis, over the recognized interval. Whew! I have not done that, but you would certainly be able to publish and charge for the method you develop! Tony
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- rudy pereda @ Anthony Botting on May 9, 2008 2:01 PM
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  Thanks Anthony,
  
  Upon digging into some archived files i found that the previous engineer was in fact using the Frequency generation for vortex shedding. Since i have a very high reynolds number, we were most definatelly in the turbulent region and this becomes an excellent estimator for the frequency i was looking for.
  Your other suggestion sounds interesting and if i ever get any free time i may look into that.
  
  Rudy
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Bill McEachern May 16, 2008 8:17 PM
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run a transient analysis and count the shedding frequency. I have done this for flow over a cylinder and it pretty much nails it relative to a hand calc using Strohal number.
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frequency analysis using a moving fluid

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