Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Engineering Science and Mechanics


An experimental and theoretical study of the spontaneous oscillations observed when a very viscous fluid flows from an orifice vertically against a flat surface was carried out. Silicone oils of very high viscosities were the main working fluids;Both plane and axisymmetric jets were studied experimentally. The main theoretical considerations, such as the basic stable jet shape and the equations governing the response of the jet to external disturbances were, however, carried out only for the one-dimensional, axisymmetric jet;The "moving threadline" equation is shown to also govern the oscillations of the fluid column and it is suggested that certain conclusions reached in the study of the "moving threadline" have direct application to the problem of fluid buckling;Results are presented in non-dimensional form for the "buckling height", the plate-orifice distance at which the spontaneous oscillations first occur and the behavior of these oscillations as functions of geometrical, fluid and flow properties for both axisymmetric and plane jets;Analytical solutions for the equations governing the jet radius along the stable jet for the "viscous-gravity" jet are provided. Based on the one-dimensional assumptions and continuity considerations, the velocity along the jet can easily be determined from these solutions;The analytic solutions for the "viscous-gravity" jet profile are compared with the experimentally determined jet profile and with the numerically obtained solutions of the complete one-dimensional equations (including inertia and surface tension);The existence of a "jump" or discontinuity in the oscillations of the jet is postulated and a one-dimensional model of the unstable jet, based on the concept of a discontinuity, is used to show that there may be a net energy loss across the discontinuity;Finally, it is suggested that other fluid stability problems might be governed by the same mechanisms responsible for fluid buckling, and a hypothesis for the determination of the regions of instability in fluids based on a one-dimensional linearized approach is offered.



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Joseph Odartey Cruickshank



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228 pages