top of page

Self-Excited Vibration Behavior

station.jpg

  • During the "Structural Vibration Measurement Service"  carried out at a natural gas transfer station, it was observed that the lightning rod poles in the field were vibrating.

  • During interviews with field personnel, it was learned that the poles swayed quite a lot at certain wind speeds, and some pole tips were even damaged as a result of this shaking.

Physical Background of the Case:

  • The existence of an oscillation situation occurring at certain wind speeds indicates that the problem occurs as a result of the fluid / object relationship.

  • A rod under the influence of laminar flow, "Vortex Eddy currents" are formed with a frequency proportional to the type of fluid (Strouhal number - St), the flow velocity (U) and the rod diameter (L).

Vortex-street-animation.gif
formula.png
rod_edited.jpg

  • If this vortex frequency coincides with the structural natural frequencies of the rod / structure / object, a situation called "Self-Excited Vibration" occurs. The vortex continuously feeds energy to the system, causing the oscillation to continue and the structure / object to suffer fatigue damage after a certain period of time.

0.25X Impacts

grafik_tr.png

Mast Height [m]

0.6Hz

1st mode drive

region

2Hz

2nd mode drive

region

10km/h wind speed

15km/h wind speed

Nodal

Point

Vortex Frequency vs Height

Vortex Frequency [Hz]

0.6Hz

1st mode

2Hz

2nd mode

mod1.gif

0.6Hz

1st mode

mod2.gif

2Hz

2nd mode

  • At low wind speeds, the formation of the 1st mode at a frequency of 0.6 Hz is observed due to the vortex excitation in the lower region of the pole, while at high wind speeds, the formation of the 2nd mode at a frequency of 2 Hz is observed due to the vortex excitation in the upper/middle region of the pole.

  • In order to avoid structural damage due to fatigue, it is recommended that the pole be fixed to the ground with steel ropes.

Conclusion:

  • With structural vibration measurements, vibrations occurring in large static structures (steel construction / reinforced concrete buildings, bridges, towers, poles, etc.) as a result of natural driving mechanisms such as wind or human-induced driving mechanisms such as traffic / industrial work should be compared with ISO 4866, ISO 2631 and DIN 4150 standards and evaluated according to structural damage and / or human comfort conditions.

  • The vibrations occurring in these structures can be continuously monitored live with the "Structural Health Monitoring System".

  • If the amplitude limits are exceeded, detailed structural vibration measurement methods (modal testing, ODS, OMA, Motion Amplification, etc.) should be applied by an expert team and corrective actions should be determined.

bottom of page