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, th increase is much lower than that predicted by the theory. The linear theory overpredicts the decrease in the longitudinal turbulence intensity with increasing c and under-predicts the decrease in the lateral turbulence intensity with increasing c. For the given flow tunnel, it is found that a contraction ratio c greatee than about 45 is not greatly effective in reducing longitudinal turbulence levels further; the lateral turbulent intensity continues to decrease with increasing c. 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 Reynoldsnumber based on the screem-mesh size or screen-wire diameter of the upstream screen. (A)