UNSTEADY FLUID FORCE ACTING ON OSCILLATING SQUARE-SECTION CYLINDER

The difference between experimentally measured lift forces under dynamic condition and quasi-steady condition is termed as an unsteady force, in this technical note. Analysis shows the magnitude of the unsteady force increases with the maximum angle of incidence, alpha(max) encountered by the square cylinder during a cycle of transverse oscillation in a fairly anticipated manner; the phase between the unsteady force and the cylinder displacement only deviates significantly from zero over the range of 8 degrees less than or equal to alpha(max) less than or equal to 16 degrees. In this (alpha(max)) range the unsteady force contributes strongly to the driving force for transverse galloping and cannot be neglected. An attempt had been made to incorporate the unsteady force term into the quasi-steady theory; preliminary results show this leads to an improvement in the agreement between theory and other independent experimental measurements.