In this study, characteristics of phase lag between displacement of forcibly oscillating test body and velocity fluctuation caused by vortex formation behind the test body were investigated. A right isosceles triangular cross-sectioned prism was used as the test body. The base of the isosceles triangle is 100mm and span-wise length of the prism is 300mm. The test body was rotationally oscillated at seven different frequencies from 0.205Hz to 0.235Hz in 0.005Hz interval providing the natural vortex shedding frequency is 0.220Hz. As the results, the behavior of the phase lag is summarized as followings. i) In general sense, it is well known that the phase lag is shifted by π within the lock-in region. Besides that, intermittent slip of phase lag was observed. ii) The velocity fluctuation does not synchronize perfectly with the forced oscillation even when the frequency of the forced oscillation is set to natural vortex shedding frequency of the test body. iii) For the case of forced oscillation frequency is set to the natural vortex shedding frequency, synchronization rate of the velocity fluctuation with the oscillation was found to be 80% at the maximum.
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