Abstract: A wind tunnel having a test section and a fan section, interconnected by two passageways to form a recirculating air flow through said sections. There is a diffuser immediately downstream of the test section, and a second diffuser immediately downstream of the fan section. Each diffuser has an intermediate flow section where the side wall extends radially outwardly in a downstream direction at a relatively sharp angle. At the upstream end of the intermediate section there is a peripheral suction slot to draw in boundary layer air, and a by-pass passageway to discharge the air tangentially in a downstream location of the diffuser. This enables a substantial reduction in air velocity in a relatively short axial length.