Jets instabilities producing the slot-tone

The nature of the instability governing the self-sustained tones produced by a low Mach number free plane jet impinging on a slotted plate, known as slot-tone, is identified experimentally. The experimental apparatus permits to control the Reynolds number and the distance from the jet outlet to the plate. For a Reynolds number, the shear-layer natural frequency and the jet column mode frequency of the free jet without the obstacle are estimated and compared to the minimal and maximal frequencies of the tones (for any position of the plate). The frequency of both instabilities defines the frequency domain of the self sustained tones. The lowest frequencies of the tones, obtained when the plate is placed downstream the end of the potential core, are the result of the amplification of the column mode frequency. On the other hand, the maximal frequency of the tones, obtained when the plate is situated near the jet outlet, is close to the natural shear-layer frequency. Moreover, when the tones frequency is higher than the natural shear-layer frequency, the sound pressure level produced is very inferior to the maximal sound pressure level measured at the same Reynolds number. This confirms the prominent role of the shear-layer instability in the self-sustained tones process for the highest frequencies. Tones with a frequency lower than the jet column mode frequency are rarely observed.