Band-pass filtering analysis on low-frequency tonal noise generated from a Krueger storage cavity

With the aim of investigating noise characteristics of tonal noise generated from a Krueger storage cavity, aero-acoustic experiments on a two-dimensional, Krueger flap configuration with an open or closed gap were conducted in the D5 aero-acoustic wind tunnel at Beihang University. The angles of attack are from 4° to 8°, and the free-stream velocities are from 45 m/s to 60 m/s. To study further the noise temporal characteristics and generation mechanism, experimental results were analyzed using a band-pass filtering method. The results show that low-frequency noise generated by the Krueger storage flow is dominant when the Krueger flap is deployed without a gap. For the Krueger configuration with a closed gap, low-frequency multiple tones observed at a free-stream velocity above 45 m/s are generated by self-excited oscillation within the storage cavity. The frequencies corresponding to these tonal noises have an octave relationship and satisfy the Strouhal number related scaling law. Filtering results reveal further that these low-frequency tones are intermittently and alternately excited in time and satisfy the oscillation mode switching mechanism.