In this study; two dimensional, viscous, incompressible, fully developed and turbulent flows on the surface of the cylinder placed in a channel were analyzed using numerical methods. Momentum and continuity equations were solved numerically by ANSYS FLUENT program using finite volume method. The effect of the half cylinder (d = 4.4 mm) placed on the cylinder diameter (D) 44 mm in different positions (0°, 45°, 90°, 135° and 180°) on the flow structure was examined. Streamlines and the turbulence kinetic energy contours were drawn for the Reynolds number 10 4 value depending on cylinder diameter. As a result, downstream flow characteristics of cylinders having semi-circular protrusions placed in different positions were examined and drag force coefficients (CD) were obtained. It was concluded that the protrusion on the cylinder placed at different angles changed the flow structure and controlled the separation.