Statistics of high-speed turbulent wake boundaries.

Some statistical characteristics of high-speed turbulent wake boundaries are presented. These were obtained by measuring the detailed turbulence outline structure appearing in shadowgraphs of three cone and three sphere wakes. The data cover downstream distances from 16 to 5000 body diam; freestream Reynolds numbers Um d/vm for the six cases range from 0.25(10) to 2(10). Statistical parameters measured include standard deviation and autocorrelation function of front deviations about the mean, correlation microscale, and intermittency factor. It is found that the random boundary structure of high-speed wakes produced by axisymmetric bodies has the same correlation function and probability distribution about the mean as a low-speed, two-dimensional boundary layer. Use of a Lagrangian model for front displacements perpendicular to the wake axis enables relationships between outline statistics and wake turbulence parameters to be obtained. Estimates are made thereby of turbulence dissipation rate, Kolmogoroff and energy-containing wave numbers, and the Lagrangian time integral scale. Results from these estimates allow some comparisons with low-speed experiments and other schlieren and shadowgraph diagnostic techniques to be made.