M.Sc Thesis


M.Sc StudentHilla Tor
SubjectLaminar flow over a backward-facing step with a wavy-
surface bottom
DepartmentDepartment of Aerospace Engineering
Supervisors Full Professors Cohen Jacob
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Abstract

The phenomena of separation and reattachment of flows are common in general engineering and in aviation in particular. Flow over a backward facing step is a canonical example of separation. In previous studies it was found that for laminar flow the distance to the reattachment location increases with the Reynolds number. In the current study, the laminar flow over a backward facing step subjected to a wavy surface is studied in order to examine its effects on the flow field, the drag force acting on the surface, the flow stability and the location of the reattachment point. It is hoped that adding a wavy surface will provide a better control of the separation region and thereby the drag force. In other words, by selecting the appropriate wavy surface parameters, the instability associated with infinitesimal disturbances can be controlled and consequently the laminar to turbulence transition can be delayed. For this purpose, a two-dimensional numerical simulation, using commercial CFD software, ANSYS FLUENT, has been employed. The stability of the simulated flow with respect to infinitesimal two-dimensional disturbances has been carried out by using a numerical Spectral Method. For the laminar flow and given Reynolds numbers, mapping of the stability of the wavy disturbances having various frequencies and motion amplitudes has been carried out.

The results of the study show that the flow field varies with time throughout the wall motion cycle and several internal vortices are observed within the bubble region downstream of the backward facing step. In comparison, only a single vortex is observed when the backward facing step is connected to a rigid surface. In addition, Kelvin-Helmholtz vortices are formed in the shear layer downstream of the backward facing step. Furthermore, in comparison to a rigid surface, the viscous drag force increases for all the examined cases and the extent of the increase depends on the selected parameters of the wavy surface. The stability investigation of the separation bubble region with respect to small disturbances show, that by selecting appropriate parameters for the wavy surface the flow can become more stable and there is an associated optimal frequency.  Finally, it has been found that the maximum stability range, the minimum bubble size and minimum increase of the viscous drag force in comparison with the rigid surface are achieved when the frequency is set to be close to ω0, corresponding to the frequency associated with the neutral wave (αn), based on the stability of the velocity profile in the center of the reattachment bubble for the rigid surface case.