Investigation on tones due to self-excited oscillation within leading-edge slat cove at different angles of attack: Frequency and intensity

Abstract

In an effort to understand the tonal noise characteristics of high-lift configurations, aero-acoustic experiments are conducted on a two-dimensional, three-element high-lift configuration model, with a stowed flap. As a less expensive aid to experimentation, the numerical simulation method is used to obtain flow parameters possibly related to tones. Experimental results show that the low-mid-frequency band tonal noise generated by the self-excited oscillation in the slat cove is the main far-field noise source. Based on the local flow field information obtained by numerical simulations, an improved empirical formula, as proposed by Terrocal, is adopted to predict the characteristic frequencies of self-excited oscillation within the slat cove, which agrees well with the tonal frequencies observed by wind tunnel testing at varying angles of attack. The intensity of these low to mid frequency band tones is also found to be dependent on the angle of attack, i.e., the primary tone of the self-excited oscillation switches from the second mode to the third mode, with increasing angles of attack. By investigating the local flow around the slat cove, the ratio of the shear layer length over the shear layer momentum thickness is determined, i.e., Lv/θ plays an important role in this transition of tonal intensity. As the angle of attack increases, both Lv and θ monotonous decrease, whereas Lv/θ increases and gives rise to a switch in the main characteristic frequency of the self-excited oscillation and a clear reduction in the overall sound pressure level.