Self-excited jet: upstream modulation and multiple frequencies

The self-excited oscillation of a planar jet impinging upon a wedge can give rise to not simply a single, but as many as seven well-defined frequency components in the range of Reynolds-number (based on nozzle width and mean velocity) 250 ≤ Re ≤ 1150. All of these components are traceable to the nonlinear distortion/interaction (i.e. sum and difference) frequencies of two primary components: the most stable frequency of the jet shear layer (β); and a low-frequency modulating component ( 1/3 β). The modulating component 1/3 β arises from vortex-vortex interaction at the impingement edge. This interaction involves vortices of both like and opposite sense; the vortices of opposite sense to those of the incident shear layer arise from eruption of the viscous layer at the edge and tend to form counter-rotating vortex pairs with the incident vortices. The details of the vortex-vortex interaction pattern vary with Reynolds number; however, the pattern adjusts itself to maintain the modulating component 1/3 β. Its upstream influence strongly modulates the sensitive region of the shear layer at the jet nozzle lip. Consequently, the linear-growth region of the disturbance near the lip is dominated by the component 1/3 β, which eventually gives way to the most unstable component β further downstream.