IIAn experimental investigation was made of round turbulent incompressible jets confined within a duct of varying cross section. The duct consists of a conical convergence (giving a doubling of the secondary stream velocity), a constant-diameter section, and a conical diffuser. Measurements of longitudinal mean velocity and turbulence intensity components are compared with previous measurements that used a constant-area duct with the same jet and duct initial conditions. The unattached jets undergo longitudinal straining, which reduces and then increases their rates of spread, compared with the equivalent jets in the constant-area duct, although the jets maintain similar forms of mean velocity profiles. The excess jet velocity (centerline velocity U 1 minus secondary stream velocity U2), as a function of distance downstream, is influenced by the varying area duct, but there is an initial jet length, up to where U 2 dU 2/dx O. 1 U I dU 1 /dx. which is influenced little by the longitudinal strain.
A significant difference between the cases using the constant-area and varying ducts is seen in the longitudinal turbulence intensity normalized by the local excess jet velocity. The large variations in this parameter for the varying area duct, with distance downstream, are explained by the slow response of the turbulence to variations of mean strain. The data set are initial conditions are presented in forms that are of value for the validation of computational models of free shear flows.