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# Eigenvalue frequencies

Creator

Hello.

Can anyone explaine why eigenvalue frequencies (429, 1073Hz) depend on mass displaysment?

I simulate pressure stabilizator. Inlet pressure is growing from 0 to 500bar. Outlet pressure stay stable at 277bar. During pressure growing mass is moving. There is a spring too (Single DOF). So I defined euigenvalue freqs. at different positions (blue line) and found big difference, i.e. 429 and 1073Hz. Why So?

7 REPLIES

# Re: Eugenvalue frequencies

Siemens Genius

Hi @vitalii

your model contains hydraulic and mechanical elements. Hence it is highly non-linear.

Only for pure linear models (hence simple mechanics) you will have the same eigenfrequencies for different linearization times.

For example in your case you have a hydraulic stiffness represented by the blue volume component.

This hydraulic stiffness is non-linear due to the dependence on pressure of the Bulk modulus.

By the way, you should use the brown HCD volume element to link with the HCD components. Only with this element you can take into account the volume variation in the computation of the hydraulic stifness.

Best wishes

# Re: Eugenvalue frequencies

Siemens Phenom

Hi vitalii,

The eigenvalue that you see is the mass 'bouncing' on the blue hydraulic volume. The volume being a hydraulic stiffness which stiffness will depend on the pressure level (via the bulk modulus of the fluid) and its restrictions.

In your scenario, the only thing which can vary consideraby with the mass position is the opening area of the spool. If you plot the fow area of bao9 it should change between your two linearization times. It should decrease quite a lot which will increase the equivalent stiffness and hence the higher frequency.

By the way, I see you have used a blue hydraulic volume (fixed volume) - if the diameter of the spool/rod are different on the piston and on the spool you should definitely use a brown hydraulic volume (from HCD library) to account for volume variation with changing spool position.

# Re: Eugenvalue frequencies

Creator

Wow! I did not know that would be so complicated.

Thanks for help.

Actually stabilizator looks more complex (see chart). Here I placed HCD chambers. Dynamic x table (red one) is for convergence between experimental flow and simulated one.

I analyze its dynamic behavior (start/shut down), frequencies and behavior inside the hole system of connected pipes with acoustic waves in it (distributive C-IR-***-C-IR - HL040).

And now I am not sure I have modeled it right (see 1D model). Are there any rough mistakes?

# Re: Eugenvalue frequencies

Siemens Phenom

Hi vitalii,

there are no rough mistakes easily visible and the structure of your model looks good 👍

Pay attention to the parameters, typically all the dead volumes of your hydraulic chamber correctly set for the initial position of the spool.

# Re: Eugenvalue frequencies

Creator

I think I handled with it.

Saved the model into the HCD library and now can use it in other projects as a standard element.

But, there is another question – how to simulate cavitation? The pressure loss is higher then the difference between inlet and outlet.

As a submodel of a spool I used Hydraulic spool with specific orifice.

# Re: Eugenvalue frequencies

Creator

Nobody works with cavitation?

# Re: Eugenvalue frequencies

Siemens Genius

Hi @vitalii

If you are interested in the cavitation in pressure drops components you can have a look at the orifice submodel BHO0014 from the HCD library which allows to model cavitating orifice.

If you have questions about it, don't hesitate to contact our technical support on www.siemens.com/gtac by opening an incident report (IR).

Best wishes