Abstract
The behaviour of low Reynolds number, non-Boussinesq fountains from four different nozzle geometries (one circular and three rectangular nozzles) are studied. High speed laser schlieren imaging is used to study the fountain behaviour (frequency and penetration height). Bi-orthogonal decomposition and dynamic mode decomposition (DMD) are used to understand the unsteady characteristics of fountains. The flow regimes of fountains are classified as steady, flapping, and flapping-bobbing type. The DMD technique successfully separates the bobbing oscillation from the combined flapping-bobbing oscillation of the fountain. The frequency of the flapping oscillation, and the frequency of the bobbing oscillation in the flapping-bobbing regime scales as SthFrh = C1 and \documentclass[12pt]{minimal}\begin{document}$St_h Fr_h^2 = C_2$\end{document}SthFrh2=C2, respectively, where the characteristic length scale is the smallest dimension (h) of the nozzle. The mean steady state penetration heights (Zs/h) of “forced” low Reynolds number non-Boussinesq fountains are independent of nozzle shape (circular and rectangular), and scales linearly with the Froude number.
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