Experimental investigation was carried out on a swirl motion of a bottom blown bubbling jet in the presence of a top oil layer. Water was used as the lower liquid and silicone oil was used as the top oil. The kinematic viscosity of the silicone oil was changed over a wide range. The occurrence region of the swirl motion became narrow as the kinematic viscosity of the silicone oil, ν
o, increased. Silicone oil did not affect the period of swirl motion,
Ts. As ν
o increased, the starting time of the swirl motion,
Ts,s, increased, whereas the amplitude,
A, and the damping time of the swirl motion,
Ts,d, decreased. An apparent kinematic viscosity, ν
app, was introduced for correlating
A,
Ts,s and
Ts,d. Empirical equations were proposed for these quantities as functions of ν
app.
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