Hello Stressers,

Why does the force for FIV as an input to harmonic analysis is perpendicular to flow? Does a two phase flow system same treatment also because the worst scenario is a slug regime in which the force is parallel to flow..

Many thanks..

Cheers!!!

In the two-phase flow scenario you describe, the loads are in the direction of flow because the forces are momentum loads applied to the ends of the pipe at each change in direction of flow, as with a fire hose spraying water at a wall.
Flow-induced vibration is a different phenomenon caused by localized variations in the flow/pressure profile which can cause lateral forces.
I find this easiest to visualize in the case of vortex shedding from wind flowing around a pipe or vessel. This is a flow-induced vibration, although outside instead of inside the pipe. When you see traffic lights moving in the wind, it's primarily perpendicular to the wind direction, either up and down for horizontal light standards, or laterally for vertical ones; i.e. not in the direction of the wind flow, but perpendicular to it. Now try to apply this mental picture to a reducer, tee or orifice plate. Same principle, but more difficult to visualize.
These flow induced loads are usually small, but because we don't always restrain our pipe laterally (to the extent we do vertically), even small loads applied in the right excitation frequency range can cause a system to move around unexpectedly.

Thanks Mr. Jim for that enlightenment. Anyway once we assign the forcing loads at a certain frequency in the harmonic analysis processor we can see the response by comparing it to what is reported in field measured vibrations at a certain dominant frequency. Then we can assign properly the direction of forcing load to match the response in the field by virtue of resonance.

Cheers!!!

I have now fully understood the Flow Induced Vibration at the flow discontinuity such as restriction orifice, etc. Upon exit of the restriction orifice vortices will be formed depending on the velocity and this will oscillate at a certain frequency (is this the Strouhal number?). If the frequency of oscillation perpendicular to the pipe flow matches the natural frequency of the pipe then resonance will occur and even small oscillation forces will be amplified to a certain extent and might cause fatigue failure.

Any other opinion is highly appreciated.

Cheers!!!