# Advances in Vibroacoustics and Aeroacoustics of Aerospace and Automotive Systems
## 2 The Present Issue
In [20], the mechanism by which plasma actuators can control the cavity tone was explored; in particular, the amount of sound reduction ('control effect') produced by actuators of differing dimensions was measured and direct aeroacoustic simulations were performed. The simulations showed that longitudinal streamwise vortices are introduced in the incoming boundary by the actuators, and the vortices form rib structures in the cavity flow. These vortices distort and weaken the two-dimensional vortices responsible for producing the cavity tone, causing the tonal sound to be reduced. In [21], the prediction and reduction of noise from subsonic jets through the reconstruction of turbulent fields from Reynolds-averaged Navier-Stokes (RANS) calculations are addressed. This approach, known as stochastic noise generation and radiation (SNGR), reconstructs the turbulent velocity fluctuations by RANS fields and calculates the source terms of vortex sound acoustic analogy. An active fluid injection technique, based on extractions from turbine and injections of high-pressure gas into the main stream of exhausts, has been proposed and finally assessed with the aim of reducing the jet-noise through the mixing and breaking of the turbulent eddies. The SNGR method is, therefore, suitable to be used for the early design phase of jet-noise reduction technologies. In [22], an aerodynamic noise optimization was performed by a non-linear acoustics solver (NLAS) approach for acoustic calculation. With the use of Kriging surrogate model, a multi-objective optimization of the streamlined shape of high-speed trains was obtained, which takes the noise level in the far field and the drag of the whole train as the objectives. To efficiently construct the Kriging model, the cross validation approach has been adopted. In [23], a vibro-acoustic numerical and experimental analysis was carried out for the chain cover of a low powered four-cylinder four-stroke diesel engine. By applying a methodology used in the acoustic optimization of new engine components, firstly a finite element model (FEM) of the engine was defined, then a vibration analysis was performed for the whole engine (modal analysis), and finally a forced response analysis was developed for the only chain cover (separated from the overall engine). A boundary element (BE) model of the only chain cover was realized to determine the chain cover noise emission, starting from the previously calculated structural vibrations. All the information thus obtained allowed the identification of those critical areas, in terms of noise generation, in which to undertake necessary improvements. The paper [24] provides a summary of some common research works carried out by the authors concerning computational methods for the prediction of the responses in the frequency domain of general linear dissipative vibroacoustics (structural-acoustic) systems for liquid and gas in the low-frequency (LF) and medium-frequency (MF) domains, including uncertainty quantification (UQ) that plays an important role in the MF domain. An efficient reduced-order computational model (ROM) is constructed using a finite element discretization (FEM) for the structure and the internal acoustic fluid. The external acoustic fluid is treated using a symmetric boundary element method (BEM) in the frequency domain. All the required modeling aspects required for the analysis in the MF domain have been introduced, in particular the frequency-dependent damping phenomena and model uncertainties. The authors declare no conflict of interest.