INFLUENCE OF THE AXISYMMETRIC CONTRACTION RATIO ON FREE-STREAM TURBULENCE.

The effect of axisymmetric contractions of a given shape and of contraction ratios c equals 11, 22, 44. 5, 64, and 100 on the free-stream turbulence of an incompressible flow has been studied experimentally with hot-wires. It is found that the longitudinal and lateral kinetic energies of turbulence increase along the contraction. The monotonic increase of the longitudinal turbulent kinetic energy with increasing c is in contrast with the linear (Batchelor-Proudman-Ribner-Tucker) theory. The variation of the lateral turbulent kinetic energy with c is in qualitative agreement, with the theory; however, the increase is much lower than that predicted by the theory. In the design of a low turbulence-level tunnel, the panacea for the reduction of the turbulence level does not lie in an indefinite increase of the contraction ratio alone. Studies with various upstream screens and a given contraction of c equals 11 suggest that the exit turbulence intensities are essentially independent of the Reynolds number based on the screen-mesh size or screen-wire diameter of the upstream screen.