On Controlling the Flow in a Turbulent Mixing Layer Created Downstream of a "Λ" Notch

Abstract

An experimental investigation aimed at studying the three-dimensional character of a turbulent mixing layer created downstream of a “Λ” notch has been undertaken. The potential for controlling this flow was also evaluated by using spanwise uniform periodic excitation in order to assess the sensitivity of the flow to the imposed perturbations. It was noted that the “Λ” notch creates a pair of streamwise counter-rotating vortices that distort the mixing layer relative to the two streams being mixed together and force it to penetrate into the high-speed stream in the center. In the absence of excitation, the mixing layer spreads linearly at the rate of the two-dimensional mixing layer and is independent of span and initial boundary layer thickness. This affirms the validity of the boundary layer independence principle in turbulent mixing layers. When spanwise uniform excitation was applied to the flow by two small fliperons hinged to the splitter plate on both legs of the “Λ” trailing edge, the rate of spread of the mean flow varied along the span. Thus, spanwise uniform periodic excitation invalidated the boundary layer independence principle on this configuration. In contrast to the two-dimensional case where the coherent fluctuations dominated the flow over a substantial downstream distance, the incoherent fluctuations dominated the turbulence levels in the current experiment. Comparable fliperon oscillations imposed weaker coherent fluctuations on this flow due to the sweep back of the trailing edge. The effects on the mean flow due to periodic excitation was still noticeable but primarily near the plane of symmetry. When actuation was applied to a single trailing edge of the “Λ” notch, the streamwise rate of growth of the mixing layer was enhanced near the plane of symmetry and was not significantly affected at greater outboard locations. Variations in amplification rates of the imposed oscillations were attributed to the variations in momentum thicknesses at the origin of the flow. The initial momentum thickness increased with spanwise distance due to the growth of the boundary layer since the fetch along the splitter plate increased at outboard locations. Forcing both fliperons in phase promoted strong nonlinear interactions near the plane of symmetry which further enhanced the local rate of growth of the mixing layer. Results suggest a possible generation of a resonant triad caused by the interaction of synchronized oblique waves emanating from both legs of the “Λ” trailing edge. The interactions occurring between the two oblique waves are shown to be sensitive to the phase relation between them. Oscillating the fliperons at 180° out of phase with each other suppressed the rate of growth downstream of the notch center. However, at outboard regions downstream of the “Λ” notch, this type of forcing developed a higher spreading rate of the mean flow than the in-phase forcing suggesting slow changes in the phase with increasing distance from the origin and potential nonlinear interactions.