frequency of modes decrease with flow rate?

[Koether]

I am working with a fairly involved flow induced vibration test. The main component of the structure involves a high fluid (water) velocity down through (not around) an annulus. One of the first things we are seeing in the data is that the first few modes seem to move down in frequency as flow rate increases. This seems counter-intuitive, but I think what we are seeing is real. Is there some effect of velocity similar to a Coriolis force that would increase the effective mass of a system as the fluid velocity increases? The structure is stainless steel and the temperature is well controlled so there is no temp increase with flow rate.

 

[electricpete]

How about the "Lomakin" effect which creates a negative stiffness:

http://caltechbook.library.caltech.edu/22/1/chap10.htm

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[Strong]

El-Pete,

This section of your Reference "10.5 ANNULUS AT HIGH REYNOLDS NUMBERS" caught my eye.

Walt

 

[electricpete]

I was looking at 10.3 (just before Lomakin effect)

This is called the ``Bernoulli effect'' or ``inertia effect'', and can be simply explained as follows. When an eccentricity is introduced, the fluid velocities will be increased over that part of the rotor circumference where the clearance has been reduced. At Reynolds numbers much larger than unity, the Bernoulli equation is applicable, and higher velocities imply lower pressure. Therefore the pressure in the fluid is decreased where the clearance is small and, consequently, there will be a net force on the rotor in the direction of the displacement. This ``negative stiffness''[/b] (K < 0) is important in the rotordynamics of seals and impellers.

The decrease in stiffness with flow would cause reduction in resonant frequency of center structure with flow

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[electricpete]

The stiffness that decreases is the stiffness radially accross the annulus.... like a seal or bearing supporting a rotor.

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[btrueblood]

Good thought Pete. I was thinking the vibrations were in the longitudinal direction (e.g. along the pipe), and would attribute lowering of frequency to an increase in the void space caused by cavitation across a centerbody.

 

[Koether]

It seems like this effect you are referring to would only apply if you had an annulus that is close to a thin film, like in bearing. The annulus in this case is a few inches wide, with an ID close to 2 feet. The part is very stiff so the vibration shouldn't really have a noticeable effect on flow area on any one area.

I am beginning to think this might be more connected to pressure, or the pressure drop. the main resistance is located after the annulus, and pressure is controlled after that resistance so the pressure in the annulus will increase with flow rate. I realize this does point towards entrapped air at some point but they are pretty meticulous in their venting.

 

[electricpete]

I agree... if the liquid gap is 2" and the pipe is only moving a miniscule fraction of that (? 0.020?), then I wouldn't think there would be any significant effect of the kind I mentioned because change in pressure distribution due to vib would be negligible.

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[hacksaw]

How is the flow introduced into the annulus, and what is the length of the annular center piece, also its means of support,whether liquid filled, solid or vented, etc.?

When you use the term modes are you refering to the frequency content of flow induced motion?

 

[Koether]

The flow is introduced through a few inlets (in the radial direction, towards the center), the flow travels down to the bottom of the center piece which is only fixed at the top, above the inlets. The flow turn at the bottom and goes through a porous plate on the bottom of the center peice and goes up through the center, which we'll call the "core"

 

[hacksaw]

the natural frequencies of the structure are not dependent on the flow apart from the mass loading, so you are dealing with a flow instability of some sort. How do you control the inlet pressure distribution and velocity profiles in the annular region?