A plane wall jet is a flow configuration obtained by injecting fluid along a wall at a velocity higher than in the ambient flow. It has features of both a boundary layer and a free shear layer. Main applications are turbine blade cooling and air-foils in high-lift configurations. In the former case it is desirable to prevent the cooling fluid from mixing with the ambient flow in order to sustain the protective layer as far downstream as possible. In the latter case the stimulation of mixing of the wall jet with the ambient flow is desirable in order to supply momentum to a boundary layer threatened, for example, by separation.
[Zhou, Heine & Wygnanski (1996)] were able to reduce the skin friction and to increase the spreading in a wall jet by forcing it acoustically. They showed, that the exchange of energy and momentum of the wall jet with its surrounding fluid depends primarily on the large, coherent structures. It is thus important to prevent or to strengthen the formation of these large structures in order to control the mixing of the wall jet with the ambient flow.
Excitation of instabilities is an effective means for flow control. Especially transitional flows are sensitive to the control by excitation of instabilities. A wall jet has two regions which are subject to instabilities; The ``boundary-layer'' region, ranging from the wall up to the point of the local maximum velocity, and the ''shear-layer'' region, ranging from the point of the local maximum velocity to the ambient flow.
[Bajura & Catalano (1975)] investigated the transition of a two-dimensional plane wall jet at
Rej
600. They found that the Kelvin-Helmholtz instability of the shear layer was responsible for the transition of the whole wall jet. The stages of natural transition were described as the formation of discrete shear-layer vortices, coalescence of adjacent vortices, eruption of these vortices into the ambient fluid and the dispersion of these vortex-patterns into three-dimensional turbulent motion.
In a recent experimental investigation of transition in a wall jet at Rej = 1450, [Gogineni & Shih (1997)] found dipolar structures. The dipols were formed by eddies originating from the shear layer and the boundary layer. The dipols detached from the wall, inducing local reverse flow. These findings are, however, in sharp contrast to the results of [Bajura & Catalano (1975)], who did not observe flow reversal.
Therefore, we study transition in a wall jet in section 3.1.
[Tong & Warhaft (1994)] placed a fine circular ring close to the exit of an axisymmetric jet. They achieved a reduction of the spreading rate and of the turbulence intensity. Projected to a plane wall jet, a thin wire placed behind the wall-jet nozzle should enhance the effectivity for turbine-blade cooling. The effect of such a wire on the wall jet is studied in section 3.2.
[Vandsburger & Ding (1995)] used an oscillating wire to maximise the spreading of a free shear layer. The wire was placed in the shear layer and performed self-excited oscillations. The oscillating wire lead to the formation of large vortical structures. Projected to a plane wall jet, this manipulation should enhance the effectivity for high-lift air foils or flaps. The effects of an oscillating wire are therefore studied in section 3.3.
The present investigation focuses on the change of the turbulent structures in the vicinity of the wall jet nozzle. It should be noted, however, that although this paper is mainly about transition in a wall jet, the Reynolds number was sufficiently high to ensure a well developed turbulent wall jet at streamwise distances
x/b
40, where the position x in the streamwise direction is normalised with the slot width b of the wall-jet nozzle. A more detailed comparison of the turbulence characteristics in the self-similar region with recent investigations, i.e. [Wygnanski, Katz & Horev (1992)], [Abrahamsson, Johansson & Löfdahl (1994)] and [Eriksson, Karlsson & Persson (1998)], is presented in [Schober & Fernholz (1999)].