Experimental Study of Three-Dimensional Offset Jets and Wall Jets

An experimental study was designed to examine and document the development and structures of turbulent 3D offset jets. The generic 3D wall jets at the same Reynolds numbers was used as the basis of comparison. The experiments were performed using a high resolution particle image velocimetry technique to perform velocity measurements at three Reynolds numbers based on the jet exit diameter and velocities of 5000, 10000 and 20000 and four jet offset height ratios of 0.5, 1.0, 2.0 and 4.0. The measurements were performed in the streamwise/wall-normal plane from 0 to 120 jet exit diameters and in the streamwise/lateral plane from 10 to 80 jet exit diameters. The velocity data were analyzed using (i) mean velocities and one-point statistics such as turbulence intensities, Reynolds stresses, triple velocity products and some terms in the transport equations for the turbulence kinetic energy, (ii) two-point velocity correlations to study how the turbulence quantities are correlated as well as the length scale and angle of inclination of the hairpin-like vortex structures, and (iii) proper orthogonal decomposition to examine the energy distribution and the role of the large scale structures in the turbulence intensities and Reynolds shear stresses.

The decay of the maximum mean velocities and spread of the jet half widths became independent of Reynolds number much earlier in the generic wall jet than the offset jets. The flow development is delayed with increasing offset heights. The decay rate and wall-normal spread rate increased with the offset heights, whereas the lateral spread rate decreased with offset heights, which is consistent with previous studies.

The two-point auto-correlations and the proper orthogonal decomposition results indicate the presence of more large scale structures in the outer and self-similar regions than in the inner and developing regions. The iso-contours of the streamwise auto-correlations in the inner regions were inclined at similar angles of β = 11.2 ± 0.6 degrees, which are in good agreement with reported values in boundary layer studies. The angles decrease with increasing distance from the wall.