Low-Speed Three-Dimensional Contraction Design

Extract

The problem of designing a three-dimensional contraction arose recently in the course of the projected construction of a new low-speed (200 ft. /sec), closed-return wind-tunnel at the Northampton College of Advanced Technology. A three-dimensional contraction was required of contraction ratio 7·4:1 to provide smooth low turbulence conversion to the working section speed within an axial length of 126 inches. The inlet and outlet (working-section) dimensions are shown in Fig. 1 and the sections are similar. It was decided to design the contraction on the basis of an equivalent axi-symmetric contraction whose inlet and outlet circular areas are equal to those of the corresponding irregular octagons and then adapt the derived contraction wall profile to the actual sectional contours. The resulting equivalent inlet and outlet radii are then 55·4 in. and 20·4 in. respectively. The problem was thus reduced to the design of an axi-symmetric contraction of given area ratio within a specified axial length, with favourable velocity characteristics for low turbulence flow in the working section. This latter specification implies the absence of adverse pressure gradients on the walls near the working section and a small percentual difference between axial and wall velocities at the inlet to the working section.