Influence of cross-sectional configuration on the synchronization of Kármán vortex shedding with the cylinder oscillation

The influence of cross-sectional configuration of a cylindrical body on the lock-in phenomenon of Karman vortex shedding was investigated using a mechanical oscillator for cross-flow oscillation of the cylinder. A circular, a semi-circular and a triangular cylinder with an equal height were used as test cylinders to see the effect of the movement of the separation point. The lock-in criteria accounting for the spanwise coherency of the Karman vortex were discussed based on the experimental data under a fixed Reynolds number of around 3500. The lock-in region on the plane of non-dimensional cylinder frequency and the non-dimensional amplitude was almost the same for all of the cylinders in spite of differences in the range of the separation point movement. The minimum value of non-dimensional threshold amplitude for lock-in was much smaller than the value of 0.05 which was reported so far. Results obtained in this work imply that the movement of the separation point is a result of the lock-in phenomenon, rather than an essential cause

Suppression of Kármán vortex excitation of a circular cylinder by a second cylinder set downstream in cruciform arrangement

A new technique for suppressing the Kármán vortex excitation of an elastically supported cylinder placed in an otherwise uniform flow is presented in this paper. By placing an another cylinder downstream of it in a cruciform arrangement with a gap s between them, the oscillation of the upstream cylinder can be virtually eliminated in the range of s/d1 < 0.4, where d1 is the diameter of the upstream cylinder. Compared with conventional techniques, this possesses the following advantages: i) it is unnecessary to change the shape of the oscillating body or remodel its supporting structure, and ii) the flow approaching the upstream body is practically undisturbed

Development of robust velocimeter for natural water flow monitoring

The ring-velocimeter coupled with a hot wire/film probe was developed and has been applied to wind and water tunnel experiments in Fluids Engineering Laboratory of Nagaoka University of Technology. In this study, the hot-wire/film probe is replaced by a cantilever attached by a strain gauge to detect the drag acting on the ring. The vortex shedding frequency from the ring is determined from the drag fluctuation by applying the spectrum analysis, and the flow velocity in turn since it is proportional with the vortex shedding frequency. This technique for flow velocity measurement is robust in the sense that it is strong against the noise or decay of the detected signal since the dominant frequency is insensitive to such disturbances, and that the detecting probe is strong against the contaminants or particles/objects carried by the fluid. These advantages, together with its simple and cheap characteristics, make it possible to apply to natural water flow with severe conditions