Influence of the axisymmetric contraction ratio on free-stream turbulence.

The effect of axisymmetric contractions of a given shape and of contraction ratio c 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 in much lower than that predicted by theory. The linear theory overpredicts the decrease in the longitudinal turbulence intensity with increasing c and underpredicts the decrease in the lateral turbulence intensity with increasing c. For the given flow tunnel it is found that a contraction ratio c greater 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 11 suggest that the exit turbulence intensities are essentially independent of the Reynolds number based on the screen mesh size of screen wire diameter of the upstream screen. (A)