# Near-Field Aeroacoustics of Spanwise Forcing on a Transonic Wing: A DNS Study
## 1 Introduction
They also provided the first description of the effects of StTw on the flow field on the wing, focusing in particular on the interaction between the control effects, the suction-side shock wave, and the turbulent boundary layer. Flow control for noise reduction is clearly a different topic, where again active and passive strategies coexist. Passive techniques for noise reduction rely, for example, on superficial roughness or on porous media coatings [14]. The primary effect of the surface treatment is the partial dissipation of acoustic energy into heat, or its reflection towards the original source [1]. Another important example of passive noise reduction is the technique adopted in the jet flows exiting from airplane engines: since the radiated noise increases with the Mach number, the solution typically exploited is to increase the so-called 'bypass ratio' of the outflow, thus reducing the speed and the temperature of the outflow, eventually lowering noise. Recently, the reduction in the total, trailing-edge noise of a NACA0012 airfoil at relatively low Reynolds and Mach numbers was attempted by placing small superficial roughness elements at the wall [15; 16]. Examples of active control techniques for noise reduction in aeronautical applications are the blowing and/or suction of the boundary layer [17] and the use of plasma actuators [18]. A third group of sound-abating technologies is made by the'reactive' noise suppression systems [1]. These rely on the generation of additional noise which, due to its amplitude and phase, interferes destructively with the noise generated by the sound source, thus reducing it locally. Also in this case, a supply of energy is required; the approach is inevitably more complicated than the former ones, as elements such as an acoustic sensor and a controller for generating the so-called \"anti-sound\" are needed. These kinds of systems have been employed successfully, for example, in combustion chambers, where they have drastically reduced noise and flame instabilities, thus improving the performance and efficiency of this component of thermal engines. Flow control for turbulent drag and noise reduction are typically two distinct research domains and are thus seldom addressed together. Hence, only a limited number of studies exists, where the effect of one kind of flow control on the goal of the other is considered. Two of the few examples are the recent work by [PERSON] et al. [14], who showed the effects of a uniform porous coating on both drag and noise reduction in a cylinder configuration, and by [PERSON] and [PERSON] [19], who performed an experiment by placing riblets on a portion of an aerodynamic profile; they concluded that the passive device could potentially reduce the airfoil self-noise in low and high frequency ranges. To the authors' knowledge, the effects of spanwise forcing for skin-friction drag reduction on the aeroacoustic field have never been considered so far in the literature. The present study intends to investigate, via direct numerical simulations (DNSs), the effects of spanwise forcing for turbulent drag reduction on the near-field pressure fluctuations around a transonic wing. Our goal is to assess the near-field changes induced by the control, and to observe whether this type of forcing, that is very effective in drag reduction, possibly yields noise increase as an undesired side effect. To this aim, the dataset produced by [PERSON] et al. [13] will be studied for the physical setup of a compressible, transonic flow around a wing slab with a shock wave. While the simulations have been carried out and described in that previous study, their analysis presented here is entirely original. The paper is structured as follows: Section 2 briefly presents the numerical methods adopted for the computations, while Section 3 shows the main modifications of the near-wing flow field ensuing from the forcing.