Graduation date: 2001The characteristics of the free surface in an orifice driven by a periodic\ud forcing function have been studied both theoretically and experimentally. In the\ud theoretical study, two wall boundary condition cases were examined: 1) the radial\ud velocity at the wall of the orifice is zero; and 2) the axial velocity at the wall of the\ud orifice is zero. The result for both boundary conditions is a modal solution where\ud the predicted surface shape as a function of orifice radius and frequency are Bessel\ud functions. The modal frequencies and annular peak locations are also predicted, as\ud a function of orifice radius, for both boundary condition cases. The modal\ud frequencies and annular peak locations differ between the boundary condition\ud cases. The experimental results show: i) the axial velocity boundary condition\ud matches the general behavior of the surface at the wall better than the radial\ud velocity boundary condition case; ii) that the surface shapes generated have the\ud same characteristics as the Bessel function; and iii) near the centerline, the surface\ud shape is well modeled by the Bessel function. The experimental results also show\ud there is a significant shift downward of the observed modal frequencies from the\ud predicted frequencies. A correlation was developed to predict the measured modal\ud frequencies for the axial velocity boundary condition case based on the theoretical\ud predicted frequencies
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