a first duct section arranged to receive a liquid flow therethrough, the first duct section defining a first direction through the first duct section from a liquid inlet end to a liquid outlet end;

a second duct section defining a second direction through the second duct section from a liquid inlet end to a liquid outlet end, the second duct section comprising a vortex generator surface upstream of the first duct section, the vortex generator surface comprising a plurality of projections laterally aligned and each extending in only one dimension along a circumference of the second duct section, wherein the vortex generator surface is arranged to induce vortices in the liquid flow downstream through the first duct section; and

a rotor housed in one of the first or second duct sections.