 Welcome to this video. Today we are going to discuss the basic sources of noise for mechanical systems. We have learned so far that sound is a pressure perturbation that is propagated in waveform, and that the concept of the light-old sensor gives the opportunity to explore all possible ways to produce sources of sound. This can happen with the presence of turbulent stresses with non-neumatropic effects or with viscous stresses. We have also seen that noise is defined in terms of exposure levels. Exposure levels depend on the strength of the source and on its distance to the receiver. A commonly assumed value for pain is typically 120 decibels, although a continuous strong exposure above 75 decibels is already problematic. Aircraft noise predictions have been developing through the years by certifying the level of max exposure and by imposing new reduction targets for the next decade. In particular, about 10-15 standard dB reduction is required in the next decade for all topologies of aircraft. This requires by 2050 to have aircraft that are 65% quieter and with a lower exposure impact in the surrounding of an airport. Well, we could think that flying less and with less passengers will solve our problems. However, this also makes flying less profitable. A much more beneficial way is obtained by considering the noise requirements in the aerodynamic design phase. In particular, the ACARA advisory board is trying to enforce a collaborative effort in the reduction of noise and fuel emissions for the benefit of the entire population. Such an effort requires a net reduction of CO2, NOx and perceived noise of more than 50% per item. This requires knowing where the aircraft sources of noise are coming from. By recalling what we have learned in a flying machine, five important noises sources can be found. From the component side, we can find jet noise, airframe noise, propeller noise, helicopter noise and sonic boom noise. Airframe noise includes the noise produced by all those flow structures which are generated by non-ideal aerodynamic surfaces as the wings, the landing gears, fuselages, etc. In particular, every form of aerodynamic separation as more COVID cavities in an aircraft are a source of non-negligible Reynolds stresses and unsteady force variation. Thus, sources of noise. Sonic booms are particularly noisy since derived by local non-isentropic flow conditions and they are known because of the peculiar Mac condensation footprint of some aircraft such as the one in the picture. Only military aircraft can nowadays fly a Mac above one over the oceans and supersonic flights over land are not allowed since sonic booms propagate quite significantly with the distance. Propeller noise is instead generated by several components. The noise is characterized by the fine harmonics due to the rotation of the blade. In particular, the tonal component due to the periodic flow variation induced by the rotating blade force is called loading noise. While the parting effect of the shape through the medium is the reason of another monopole contribution, the thickness noise. The most known component of propeller noise is the tonal one that is characterized by the consecutive harmonics together with a relatively lower background of propellant contribution. In these graphs, you can appreciate how strong the harmonics are with respect to the lower background. If more complex configurations are used, the helical vortex system or the separate blades can impinge in the downstream rotor creating additional sources of noise. This results in a combination of different components that can change the relativity pattern of the entire system. Engine noise is instead characterized by its constituting components including the fan, the compressor, the combustor, the turbine and exhaust. With the current tendency to increase the size of the engine while reducing the mass flow through it, the contribution of the fan grows with respect to the other ones. Helicopter noise is instead characterized by the interaction between the vertical structures that are generated by the blades and the supporting structures. In particular conditions, the blade can also interact with the wakes from the previous ones, creating a particular phenomenon called BVI, blade vortex interaction. The acoustic footprint of such a system is quite complex and rarely can be addressed without using the most advanced tools from computational fluid dynamics. An additional interest in test cases is represented by another bladed machine, the wind turbine. In particular, although the correlation between noise exposure and alpha effect is not really evident, the annoyance created by such machines is quite known. Wind turbine noise is generally determined by air force F noise. Six components are the ones that are generally referred to for the creation of such noise. We'll discuss this later. However, have a look for yourself at the different mechanisms here. Organized by the characteristics of the edge, leading our training and by the characteristics of the boundary layer, turbo and lamina. It's quite interesting to see that due to the increased size of aircraft and wind turbines, similar flow regimes and noise sources can be found between the two different machines, with important consequences on the aerodynamic and acoustic design. This concludes our video. I hope you enjoyed it and see you next time.