A fluid system comprising a surface that is fluid washed in use by a fluid flow travelling substantially parallel to the surface is disclosed. The system has a first port through the surface and a second port through the surface, the first and second ports having respective first and second port inlets that are substantially flush with the fluid washed surface. The first and second port inlets are stacked in a direction parallel to the normal flow of fluid over the fluid washed surface such that the first port is upstream of the second port and fluid travelling closer to the fluid-washed surface entering into the first port tends to entrain fluid travelling further from the fluid-washed surface for entry into the second port.