Anisotropy in a Turbulent Jet Near a Free Surface

Laboratory measurements of the turbulent flow in a plane jet issuing at the free surface are reported and analyzed. Single point hot-film data were obtained for mean velocities and turbulent fluctuations of velocity and shear stress in the developing jet to X₁/b₀ = 240 with an emphasis on the near surface turbulent behavior. Anisotropy distributions indicate two layers within the jet. In a surface-influenced layer of thickness comparable to 1.5 times the local jet half-width, the vertical anisotropy is constant and the primary net energy transfer is from the streamwise component of turbulent kinetic energy to the spanwise component. In a near-surface layer of about 1/4 the local half-width, the vertical fluctuations decrease rapidly with a concomitant redistribution of energy to the horizontal components. Analysis of one-dimensional spectra reveal integral scales that are consistent with the anisotropy data and in qualitative agreement with analyses and experiments for isotropic turbulence near moving walls and free surfaces but differs by exhibiting a wider range of surface-influenced turbulent length scales.