Torsional Flutter of Rectangular Prisms

Prismatic bar models tested are of rectangular cross section and are provided with different side ratios (depth/height) of 1, 2, 4, and 5. The position of axis of rotation of the model, which is spring-mounted so as to oscillate in torsion, is varied along one of the center lines of the cross section to see the axis position effects on torsional flutter. The validity of the C_Mα criterion for torsional flutter is examined by finding a possible correlation between the onset of flutter and the sign of C_Mα (steady aerodynamic torsional moment derivative) at each axis position. Theoretical analyses of unsteady aerodynamic torsional moment acting on a bluff structure oscillating in torsion reveal that it has two constituents, one arising from the fluid memory effect, and other from the quasi-steady flow effect. It is indicated that the C_Mα criterion for torsional flutter is applicable to cases where the fluid memory effect is predominant. Further analysis comprising the measurements of the wake velocity variations and the flow visualization results emphasizes the significant role played by the fluid memory effect in the torsional flutter of structures.