# Near-Field Aeroacoustics of Spanwise Forcing on a Transonic Wing: A DNS Study
## 6 Conclusions  
In this paper, we have extended the analysis recently carried out in Ref. [13], where it was studied how spanwise forcing for turbulent skin-friction drag reduction (StTW) affects the transonic flow field surrounding a supercritical wing slab with a shock wave above the suction side. The dataset considered in this work derives from the previous study; here, we describe the near-field modifications in terms of wake characteristics and radiated sound. This work represents the first of its kind, as to our knowledge no study of the interaction between spanwise forcing for skin-friction reduction and the aeroacoustic field has been published so far. We have shown that, when flows with and without control are compared at the same angle of attack, spanwise forcing results into a small downwards displacement of the wake, as a consequence of the increased lift. On the contrary, when the control parameters of StTWs were such that skin-friction drag was actually increased, lift was diminished, and the flow velocity in the wake was further reduced. However, the percentage variation in the velocity component parallel to the free stream in the wake was quite limited, and was never found to exceed 2%. We have also shown that StTWs, which are applied on the suction side only, do bear a minimal influence on the flow on the pressure side of the airfoil, mediated by the downward displacement of the wake: in fact, when lift is increased, the small recirculation bubble that is naturally present beneath the wing becomes slightly enlarged, due to the stronger adverse pressure gradient experienced by the flow. This paper has presented the first assessment of the acoustic effects of spanwise forcing implemented via StTWs, in terms of the near-field acoustic signals extracted from a number of equally spaced probes placed on a circumference of radius \(5c\) centered on the profile. We have shown that, for cases at a constant angle of attack, noise is slightly increased at almost all angular positions as an effect of the forcing. The direction in which the largest sound intensity is perceived, which does not match the direction of the shock wave, is not altered by the application of control, and depends instead on the flow incidence only. However, in this application, StTWs are meant as a strategy to reduce the aerodynamic drag through an increase in aerodynamic efficiency of the wing. Therefore, one should compare with the flow at the reduced angle of attack that restores the original lift. When the comparison with/without StTWs is carried out at constant lift, the noise perceived by all probes is, on average, unaltered. The frequency spectrum of the pressure fluctuations is not particularly affected by the presence of this type of flow control technology. In fact, both the spectrum of the probe detecting the largest sound intensity and the directional Fourier transforms show but minimal alterations after the forcing is applied. The available evidence thus indicates that no negative interaction takes place between spanwise forcing and the aeroacoustics of the wing. This work only constitutes a preliminary study, and its results are limited. Work is ongoing to employ more advanced tools such as wavelets transforms and spatial correlations of the pressure signals in order to analyze changes in the acoustic signature around the profile. Furthermore, we are planning to investigate modifications to the far-field aeroacoustic noise. Changes away from the profile, indeed, may differ from the variation in the near-field acoustics. We also plan to consider the wall-pressure signature, which could be more sensitive to changes in the boundary conditions with respect to near-field(or far-field) acoustic signals. Finally, the possibility that StTWs could be employed as a means to alter the noise generated by the turbulent flow cannot be ruled out, provided the combination of parameters is properly tuned towards a goal that is different from turbulent skin-friction drag reduction. Conceptualization, D.G., M.Q., and S.P. ; methodology, D.G.