Two feedback paths for a jet-slot oscillator

The coupling between a subsonic jet-slot oscillator (usually referred to as a slot-tone) and acoustical resonances of the flow supply duct is investigated experimentally. The aspect ratio of the jet height H to the slot stand-off distance L is varied in the range 0⩽L/H⩽8. The Reynolds (Re) number based on the jet height is varied in the range 0⩽Re⩽2×104. The acoustical pressure measured at the back of the slot-plate at a distance of 75 mm from the slot edge reaches whistling values to the order 10−2 of the stagnation pressure (or 115 dB SPL). Two types of oscillation behavior have been observed depending mainly on Re. For lower Re values, a hydrodynamic oscillator (edge-tone like) is generated, while for Re⩾104, the oscillator is coupled to the resonances of the flow supply duct. This resonant duct creates an indirect feedback path (organ-pipe like) that controls the tone frequency and reinforces their amplitude. An important result is that this lock-in phenomenon involves very high-order nonplanar acoustical modes (up to the 32nd). The introduction of damping foam in the flow supply duct induces a decrease of the emitted levels reaching 25 dB for the emitted levels, but the oscillation behavior remains dependent on the duct resonance. This indicates that purely hydrodynamical behavior cannot be studied without a special design of the flow supply.