The oscillatory free-surface displacement in an orifice periodically driven at the inlet is studied theoretically and experimentally. A potential flow analysis is applied using two different sidewall boundary conditions: no flow through the wall, and no slip at the wall. Both analyses result in surface shapes defined by Bessel functions, but with significantly different frequencies associated with the discrete modal shapes. Experimental results for two orifice diameters, 1180 µm and 794 µm, verify the general shape of the surface displacement but show a significant downward shift of the measured modal frequencies from the predicted values. This shift is shown to be a function of mode number and is thought to be a consequence of orifice edge effects.

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