TURBULENT-FLOW CHARACTERISTICS IN AN S-SHAPED DIFFUSING DUCT

Velocity characteristics of the flow in a round cross-sectional S-shap ed diffusing duct with an offset of 0.3 of the length (axial) of the d uct were measured, using a laser Doppler anemometry and refractive ind ex matching technique, with two inlet boundary layer thicknesses, corr esponding to 0.5 and 0.2 of the inlet duct diameter at a fixed Reynold s number of 4 x 10(5). The duct had inlet and exit diameters of 48 mm and 56.8 mm respectively, a length of 170.4 mm and an offset between t he centre-line of the inlet and exit of 51.1 mm and the dimensions cor responded to a scaled model of the Royal Aerospace Establishment 2129 duct used for jet aircraft intakes. The previously speculated separati on regions were confirmed and found to be larger with the thinner inle t boundary layer and, as a consequence, the exit mean f/ow distributio n consisted of a large pair of contra-rotating vortices occupying almo st half of the cross-section with maximum secondary mean velocity 20% of the inlet bulk velocity, compared to a third of the cross-section i n the thicker inlet boundary layer and with maximum secondary mean vel ocity 15% of the inlet bulk velocity.