**AUTHORS:**Hayder Azeez Diabil, Xin Kai Li, Ibrahim Elrayah Abdalla

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**ABSTRACT:**
The current study sheds a light on two fundamental aspects of a transitional separated-reattached flow induced over a two-dimensional blunt flat plate and three-dimensional square cylinder employing large eddy simulation conducted with Open FOAM CFD code. These aspects are different vortices shedding frequency modes and large scale structures and their development. The current paper is the first study to investigate a transitional separated-reattached flow in a three-dimensional square cylinder and compare between transition aspects of this case and that in a two-dimensional flat plate. It is not clear whether all transitional separated-reattached flows have low frequency shear layer flapping and selective high shedding frequency. This issue is addressed. The current LES results show that the characteristic shedding frequency value for the square cylinder is different from that in the flat plate. Coherent structures and their development are visualized at different stages of transition for both geometers. In the square cylinder, Kelvin-Helmholtz rolls are twisting around this geometry and evolve topologically to form hairpin structures. In the flat plate, Kelvin-Helmholtz rolls stay flat and hairpin structures formed by a braking down process

**KEYWORDS:**
Separated-reattached flow, Transition, Shedding frequency modes, Coherent structures, Flow visualization, Large eddy simulation

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