pipe concentric with the first pipe to impart tangential momentum to the flow. The tangential flow was injected into the axial stream at a distance 39 cm upstream from the jet exit through four 1.0 cm ×0.5 cm tangential slots. This method of swirl generation is simple, has no moving parts, and provides independent control of axial and tangential momenta.
The experiments were conducted in the 1.07-m octagonal, open test-section, return-circuit wind tunnel of the Iowa Institute of Hydraulic Research. Figure 2 shows the wind tunnel and model arrangement along with the coordinate system used to report the data. The uniformity of mean velocity and turbulence intensity in the tunnel was investigated by Hyun (3), who reported a mean-flow uniformity better than 0.25% and a turbulence intensity of 0.5% in the test section. The freestream velocity U o was set at 16.5 m/s, resulting in a Reynolds number based on body length (Re=UoL/v) of 1.58×106, where v is the kinematic viscosity of air. The jet velocity was adjusted to realize the self-propelled condition, i.e., such that the axial momentum of the jet was just equal to the momentum loss due to the body drag. This condition was achieved with a maximum axial velocity at the jet exit equal to twice the freestream velocity, i.e., Uj=2Uo. The tangential momentum was adjusted such that it matched that of the propeller employed by Hyun and Patel (1). The maximum tangential velocity at jet exit was then Wmj= 0.95Uo. These operating conditions translate into a swirl number,
based on jet radius (D/2), of 0.34, where and are, respectively, the axial fluxes of axial and tangential momenta of the swirling jet.
As shown in Figure 1, the model was mounted with a part of it extending into the tunnel contraction to maximize the axial length over which the wake could be studied. This enabled measurements in the axial direction up x/D=19.531, or x/L=0.531, where x is measured from the jet exit. It will be seen later that this distance was just sufficient to establish the asymptotic state of the wake. In the radial direction, measurements were made up to r/R=4.5 to recover the freestream conditions.
As this flow is, in principle, steady and rotationally symmetric, its description requires measurements along a single radial line at each axial position. However, measurements were taken across the (vertical) diameter to monitor flow symmetry. The measurements were made with a triple-sensor hot-wire probe and a five-hole Pitot probe. The latter was used to determine the mean flow direction so that proper yaw and pitch angles for the hot-wire probe could be selected. It also provided redundant data for the mean-velocity components. The probes were traversed in the vertical direction by a simple computer-controlled mechanism. Detailed description of the experimental equipment, instrumentation, and measurement procedures can be found in Sirviente (2), along with an analysis of the uncertainty in the data. There it is shown that the uncertainties of the mean velocity components measured with the hot-wire were less than 0.02Uo and those in flow directionality were 1.5 degrees. Uncertainties of the axial Reynolds stress, and the shear stress, were estimated to be 10%, while those of the remaining stresses were 20%. Measurements with the five-hole Pitot probe had uncertainties of 0.02Uo in velocity magnitude and ±1.5 degrees in flow direction. The software used to control the experiments, and acquire and process the data, is described by Walter (4).
In the following, the data are presented in cylindrical polar coordinates (x,r,θ) where x is measured along the body axis from the base (jet exit). The mean and fluctuating velocity components in these directions are (U,V,W) and (u,v,w),