Effect of a contraction on turbulence. Part 1: Experiment

Measurements are reported of grid-generated turbulence subjected to a 11.4 : 1 contraction. Three different upstream positions of the grid relative to the contraction were used. Particular effort was made to remove the effect of background disturbances on the measurements. This was especially important for the streamwise fluctuations which decrease through the contraction (due to negative production) while background disturbances are strongly amplified by it. The corrected measurements were used to compute the budget of the turbulence kinetic energy, both in the contraction and in the duct preceding it. Over most of the contraction, the energy balance is almost entirely between convection of turbulence energy by the mean flow and the energy production terms. Since, however, the contraction promotes a trend toward isotropy initially, then away from it, there is not only a point at which the kinetic energy is a minimum, but also a nearby crossing point where the production terms are identically zero. Hence over a small region, the leading terms in the energy balance are nearly zero. The theoretical implications of these experimental results are considered in Part 2.