Abstract: An apparatus (10) for modulating fluid flow which comprises a first flow path (13) and a second flow path. The second flow path is transverse to the first flow path (13), and has a fluid inlet (31) which opens onto the first flow path (13). The fluid inlet (31) is such that a portion of fluid flowing along the first flow path (13) is diverted into the second flow path. The position of the fluid inlet (31) is set in relation to the direction of fluid flow in the first flow path (13) and in relation to the extent of fluid to be diverted from the first flow path (13) into the second flow path.