# Aeroacoustic and Vibroacoustic Advancement in Aerospace and Automotive Systems
## 1 Introduction  
This Special Issue follows a previous one [1] on a similar topic, with the aim to keep on providing updates on state of the art research with reference to aeroacoustics and vibroacoustics in aerospace and automotive systems. In the following, a brief summary on the content of the papers included in this special issue, clustered based on a thematic criterion, starting with the spacecraft vehicles and satellites, then moving to the automotive field, followed by the aeronautic field, the industry components for conditioning and, finally, the railway transport system. In the section on spacecraft vehicles and satellites, the first paper is concerned with the vibration attenuation of meteorological satellites in the presence of periodic disturbances, leveraging on an optimal design of constant compensations against known-law periodic disturbances [2]. The methodology proposed is applied to a flexible spacecraft actuated by constant control torque in the presence of sustained periodic disturbances. The effectiveness of the optimal compensation torque is provided and compared with results of other selections in the frequency and amplitude ratio domain. Numerical simulation results and experimental results clearly demonstrate the good performance of proposed criteria. In a second paper [3], a frequency weighting matrix beamforming algorithm is proposed to localize air leakages caused by clashes between space debris and spacecraft in orbit. The elastic Lamb waves that are caused by leakages are acquired by an 'L' shaped sensor array consisting of eightacoustic emission sensors. The angle of a leak can be obtained through the superposition of different time delays, and the intersection of two angles can be used to find the location of the leak. Moving to the automotive field, again a couple of papers can be retrieved. The first analyses the influence of crankshaft torsional frequencies on its rotational speed behavior [4], with the aim to reduce the vibrations at the engine supports. In particular, a multibody calculation methodology has been applied to the vibration analysis of a 4-cylinder, 4-stroke, turbocharged diesel engine, with a simulation-driven study of the angular speed variation of a crankshaft. Time-dependent simulation results, evaluated at the engine supports, are condensed to a vibration index and compared with experimental results, obtaining satisfactory outcomes. The modal analysis also considers the damping aspects and has been conducted using a multibody model created with the software AVL/EXCITE. A second one is concerned with the vibration behavior of a 4-cylinder, 4-stroke, petrol engine as simulated by the finite element method (FEM) [5]. A reduced modelling strategy based on the component mode synthesis (CMS) is adopted to reduce the size of the full FEM model of the engine. Frequency response function (FRF) analyses are used to identify the resonant frequencies and corresponding modes of the different FEM models and the obtained results are compared with experimental data to get the model validation. Subsequently, modal-based frequency forced response analyses have been performed to consider the loads acting during the real operating conditions of the engine. Finally, the impact on vibrations at the mounts, produced by an additional bracket connecting the engine block and gearbox, is investigated. Both the full and reduced FEM model reproduce with high accuracy the vibration response at the engine mounts, but the reduced modelling strategy requires significantly shorter runtimes. Considering then the aeronautic field, three papers are included in the Special Issue. In one of them, an analytical model to predict the performance of the plasma synthetic jet actuator (PSJA) array, coupled with the multichannel discharge model and PSJA aerodynamic model, is put forward [6]. Such a model can be used to improve the aeroacoustic performances of turbofans by active flow control.