On Turbulent Wall Jets

Sammanfattning: An experimental investigation of the two- and three-dimensional turbulent wall jets has been conducted. The measurements have been performed using different hot-wire techniques in large scale wall jet facilities, and the measurements were focused on the fully-developed region of the flow.

The two-dimensional wall jet is found to be self-preserving, and the streamwise development of the maximum mean velocity and the half-width is independent of the inlet Reynolds number when the viscosity and inlet momentum flux are used as scaling parameters. The near wall region is characterised by a short universal mean velocity profile and a high turbulence level caused by the influence of the outer region.

The lateral spreading rate of the three-dimensional wall jet is found to be five times larger than the normal spreading rate, and this is connected to a secondary mean motion in the lateral plane which shows a negative normal mean velocity and a sharp twist of the velocity vector away from the symmetry plane near the wall. The turbulent kinetic energy budget indicates a transport of turbulent energy by advection and turbulent diffusion towards the wall region, where the turbulence level is enhanced and the near wall peak in the streamwise turbulence intensity is attenuated.

A similarity analysis of the equations governing the 2DWJ show that the shear stress in the outer region scales with the friction velocity instead of the commonly used maximum mean velocity, and thus the outer region is governed by two velocity scales. The similarity analysis also shows that the growth of the wall jet (i.e. the half-width) is not linear but depends on the local Reynolds number. The velocity profiles in the overlap region are not logarithmic (as commonly believed) but show a power law behaviour. The similarity theory is in good agreement with experimental observations.

Pulsed hot-wire anemometry measurements (PHWA) show good agreement with LDA data in the outer region of the 2DWJ, and these techniques give a significantly higher turbulence level (e.g. the shear stress) than the HWA. This difference can be explained by the high local turbulence levels, which are so high that local flow reversals occur. Even larger differences between PHWA and HWA are found in the 3DWJ because the local turbulence levels are higher and negative velocity samples are more frequent in the 3DWJ than the 2DWJ.