A device (10) includes an upstream tubular portion (12) from which extends a downstream constriction (14). The downstream constriction (14) is smaller in cross-sectional area than the upstream tubular portion (12). Velocity of fluid flowing through the downstream constriction (14) is greater than velocity of the fluid flowing in the upstream tubular portion (12). A permeable downstream portion (20) is distal to the upstream tubular portion (12). The permeable downstream portion (20) is formed with pores (22) so that the downstream portion (20) is more permeable for fluid flow laterally therethrough than the upstream tubular portion (12).
