Falling Water Films over a Ring Attached to a Vertical Cylinder : Wave Characteristics Measured by a Capacitance Method

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This experimental study concerns falling water films on a vertical cylinder to which a ring with a square cross section of 1.5 mm×1.5 mm is attached. Using the electric capacitance technique, some characteristics of surface waves of water film were measured near the ring under the same conditions as in the previous experiment in which the film thicknesses were measured by a needle contact method. When the ring was in the non-equilibrium flow region, the predominant frequency of waves fmax increased just downstream of the ring and then gradually decreased. When the ring was in the equilibrium flow with disturbance waves, little difference was found in fmax upstream and downstream of the ring. Wave velocity was not considerably influenced by flowing over the ring. The effect of the probe diameter on accuracy of the capacitance technique was theoretically discussed by assuming two-dimensional sinusoidal waves.