Studies of the Turbulence Field in a Three-Dimensional Wall-Jet

Paper i proceeding, 1992

Today, the interest in the modelling of turbulence is directed towards complex, three-dimensional and anisotropic flow fields. Successful models are usually based on the transport equations for the Reynolds stresses and terms containing derivatives of these stresses must be modelled. Direct simulations together with well-defined, simple and fundamental experiments, in which the gradients of the turbulent quantities are determined, yield a good base for the improvement of this turbulence modelling. Such a fundamental and simple flow case is the three-dimensional turbulent wall-jet (3DTWJ), where an interaction between a wall boundary layer and a free shear layer forms the anisotropy as well as the inhomogeneous character of the flow field. According to Launder and Rodi (1981) the first experiment on a 3DTWJ was carried out by Viets and Sforza (1970), and until today very few experimental studies on 3DWTJ have been reported. However, quite recently Matsuda et al. (1990) have published a work on coherent structures in a 3DTWJ. The purpose of the present work was to study the turbulence field of a three-dimensional wall-jet without an outer disturbing flow field. This was accomplished using a wall-jet rig which consisted of a fan, a settling chamber and a circular outlet located so that the flow is blowing tangentially along a horizontal flat plate. To define the boundary conditions a vertical wall is positioned perpendicular to the outlet and the flat plate. In an introductory part of this work, the shape of the jet outlet and the influence of the vertical wall were studied, see Löfdahl et al. (1991). From this study, a circular outlet and a vertical wall, of approximately the same size as the flat horizontal plate on which the three-dimensional wall-jet (3DTWJ) was formed, were chosen. Smoke visualisations as well as hot-wire measurements have been used in the investigation of the flow field. All measurements were carried out using hot-wire technique, using standard probes and conventional methods for the determination and the mean velocities and turbulent quantities, see Löfdahl (1991). In the present measurements Reynolds numbers based on the outlet diameter, d, in the range of 10000 through 40000 were studied. The extension of the measurements in the flow direction was in the range of x/d=33 through x/d=113, (x-coordinate in the flow direction) and an apex angle interval of 40 degrees in steps of 10 degrees. Profiles of the mean velocities, Reynolds stresses and triple correlations were determined along each “streamline”. In Figure 1 the turbulence intensity and the shear stress distribution, respectively, are shown at different locations along the centre line. The turbulence intensity shows the tendency of the expected two maxima, while in the shear stress only one positive maximum in the outer portion of the jet is captured due to the probe size. As an example of the energy redistribution in the 3DTWJ, the Reynolds stresses at location x/d=47 and x/d=87 are shown in Figure 2. A clear reduction in the fluctuating velocity normal to the wall can be noted as the wall jet is spread in the downstream direction. In the full paper all measured mean velocities, Reynolds stresses and triple correlations are shown together with studies of the turbulent structure at the centre line using the quadrant method.