Flow around a circular cylinder with slit

Abstract

Characteristics of flow around a modified circular cylinder are experimentally investigated in the present study. The experimental campaign was performed in a wind tunnel at the Reynolds number of Re=2.67×104, based on the cylinder diameter D and the incoming airflow speed. The cylindrical test model is modified with a slit parallel to the incoming airflow to create a flow communicating channel between the windward and leeward stagnation points. The slit width S changes from 0.05D to 0.15D with an increment of 0.025D. Pressure distributions on the cylinder surface were measured to estimate the aerodynamic forces acting on the test model and a particle image velocimetry (PIV) system was employed to quantify the wake flow patterns of the natural and modified cylindrical test models. Experiment results reveal that a slit contributes to reducing the drag and suppressing the fluctuating amplitude of the dynamic wind loads acting on the test model. The changing trend in drag reduction and lift suppression with the increase of slit width are also discussed based on the surface pressure and PIV measurement results. The PIV measurement results demonstrate clearly that the slit generates a self-issuing jet into the cylinder wake and the passive jet is effective in manipulating the wake vortex shedding process from the circular cylinder. As the jet vortices being shifted downstream, they help to detach the shear layers rolled up from both sides of the circular cylinder. Because of this dynamic interaction process, the antisymmetric pattern of the wake vortex shedding from a natural cylinder is found to be converted to a bistable mode flow behind the slotted cylinders. As a result, the dimensionless vortex shedding frequency is observed to be switched to a very low level and a flip-flop phenomenon is found. A linear stability analysis is then performed to suggest that the intrinsic nature of the cylinder wake flow is greatly modified with the implementation of a slit.