The simplest solution is the installation of a circular baffle plate with a diameter of four times the pipe diameter directly over the outlet pipe at a distance of two diameters above the pipe opening. See the figure on the right. This results in an apparent opening which is eight times the original opening and a corresponding reduction in flow velocity as the liquid moves under the baffle. Of course the velocity increases under the baffle plate, but then it is protected from the surface air.
If the baffle plate is supported by four to six vertical plates forming a star to a diameter of two times the pipe diameter, they will further reduce the start of a vortex.
For more information on this subject see ANSI/HI 9.8 - Pump Intake Design.