Generating Mechanism of Wake Galloping and its Suppression

2006 Fiscal Year Final Research Report Summary

• Project/Area Number: 16560425
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Structural engineering/Earthquake engineering/Maintenance management engineering
• Research Institution: Kochi University of Technology
• Principal Investigator: FUJISAWA Nobumitsu Kochi University of Technology, Faculty of Engineering, Professor, 工学部, 教授 (80299388)
• Project Period (FY): 2004 – 2006
• Keywords: wake galloping / pressure distribution / unsteady lift force / gap flow / aerodynamic damping / helical wire

Research Abstract

The vibration test was conducted in the wind tunnel, using the model of tandem circular cylinders, in which only the leeward cylinder is supported elastically. The test result reveals that the response with the relatively small amplitude is dominated by the reduced velocity as with the typical aero-elastic vibrations. However, the vibration with large amplitude, which occurs after the excitation due to external force, appears to be dominated by the wind speed itself. The measurement of the pressure distribution shows that the lift-force on the leeward cylinder is characterized both the S-shape lift due to the pressure field in the wake of windward cylinder and the peaky lift due to the switching to the gap-flow. The aerodynamic damping is calculated from the unsteady lift. The result shows that the response with the small amplitude is caused by the negative damping and the negative damping is generated by the delay of the negative lift. On the other hand, the response with the large am plitude occurs under the positive damping. The detail of this phenomenon has not been clarified yet.
The effectiveness of the helical wire as a counter device to suppress the wake galloping is also examined. The test reveals that the vibration with small amplitude can be suppressed easily by the helical wire in all tested cases. However, the vibration with the large amplitude is suppressed only in limited cases. In the scope of the test, the wire with the winding pitch of larger than 9D shows the high-damping effect when the wires are aligned at the same position on the two cylinders. On the other hand, if the wire alignment is staggered, the violent vibration occurs in all tested cases. To investigate the effect of the wire alignment, the cylinders equipped with the wires parallel to the cylinder axis are tested, utilizing the concept of the strip theory. The test results shows that the staggered wire is more effective to suppress the vibration in contrast with the case of the helical wire.
Though the helical wire seems to be effective as the counter device for the suppression of the wake galloping, further studies are required to establish the design method for the practical use.