Modulation of wake flow and aerodynamic behaviors around a square cylinder using an upstream control bar

•Control bar and square bluff-body to modulate the surface-flow patterns.•The surface-oil visualization was utilized to delineate the surface-flow structures.•The surface oil-flow patterns were classified into four characteristic surface-flow modes.•The wake–vortex behaviors were adopted to measure the downstream vortex-shedding frequency.•In the leading-edge bubble mode, CD has the lowest and CL has the highest value at θ=0°.