Behavior of a water column oscillator with a cover gas system was studied experimentally and analytically. One of the free water surfaces was sealed with the cover gas, while the other was open to the atmosphere. The cover gas was supplied with a constant rate, and it was ejected through a valve when the water level reached a certain point. As soon as the level reached another point, the valve closed and the cover gas quantity increased again. In the experiment, a bifurcation-like phenomenon was observed; the valve closed period usually had single value, while it took two values under certain conditions. In order to examine the cause of this bifurcation-like phenomenon, a numerical analysis model was developed which described the system behavior precisely. The analytical result revealed that the bifurcation-like phenomenon was not bifurcation. Very small noise superimposed on the singular characteristic of the system was found to cause the two duration values under the conditions; the duration changed abruptly by an uncontrollable slight variation of the system conditions. It is likely to be confused with bifurcation by observers who could not perceive the uncontrollable noise.

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