Acoustic behavior of the VEGA launch pad environment

[EN] The acoustic pressure levels experienced by the spacecraft and launchers during the lift-off is due among other factor by the reflection of the sound waves on the launch pad. The acoustic load distribution in the area of the launcher depends on the geometric, mechanical and acoustic characteristics of the ground facilities. This work is intended to study the acoustic environment of the launch pad. A numerical and experimental investigation is developed in order to study in the linear regime the acoustic behaviour of a subscale model of the VEGA's launch pad. The acoustic measurements are performed in an anechoic chamber using an electroacoustic source that emits incoherent noise, mimicking the real acoustic source. The acoustic pressure field is measured at different positions in front of the launch pad mock-up, in the area where the acoustic waves are reflected. Among the future perspectives of this work is to study and develop new methods for the mitigation of the sound pressure levels.Authors acknowledge the support of the European Space Agency under contract ¿Sonic Crystals For Noise Reduction At The Launch Pad¿ ESA ITT 1-7094 (ITI) and the 441-2015 Co-Sponsered PhD ¿Acoustic Reduction Methods for the Launch Pad¿. The work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through project FIS2015-65998-C2-2. Authors aknowledge Dhéric Mutel, Cyril Bernard and Clément Jost for their contribution to this workPicó Vila, R.; Herrero-Durá, I.; Sánchez Morcillo, VJ.; Salmerón-Contreras, LJ.; García-Raffi, LM. (2016). Acoustic behavior of the VEGA launch pad environment. Universidade do Porto. 1-6. http://hdl.handle.net/10251/181097S1

Solitary waves in nonlinear phononic crystals

[EN] We discuss two possible regimes of solitary wave formation in acoustic layered media. In the weakly dispersive limit, KdV-type solitons are formed, consisting of broad pulses with a width much larger than the lattice periodicity. Such KdV solitons are shown to exist even far from the weakly dispersive conditions. On the other hand, in the strongly dispersive regime, gap acoustic solitons are demonstrated. They are formed by a fast carrier wave inside the band-gap of the structure, near the Bragg frequency (whose propagation is not allowed in the case of linear waves), modulated by a wide envelope, whose width lies inside the gap. Gap solitons propagate slower than linear waves, or can be even reach a stationary non-propagating state within the medium. The parameters for a realistic acoustic medium supporting both types of solitary waves are discussedThe work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through project FIS2015-65998-C2-2.Mehrem, A.; Picó Vila, R.; Sánchez Morcillo, VJ.; García-Raffi, LM.; Salmerón-Contreras, LJ.; Jimenez, N.; Staliunas, K. (2016). Solitary waves in nonlinear phononic crystals. Universidade do Porto. 1-7. http://hdl.handle.net/10251/183355S1

Reflection of sound by Sonic Crystals: an application to the aerospace engineering

[EN] From the acoustical point of view one of the most extreme events is the lift-off of a rocket. In such events, an enormous amount of energy is liberated in the form of acoustic waves that are reflected in the launch pad, coming back over the rocket and affecting both the rocket and the load contained in the fairing. Here we propose a possible solution to reduce the sound pressure level in the area of the spacecraft-launcher: placing structures based on Sonic Crystals (SCs) at the launch pad to control waves reflecting on it. In this work preliminary reults, in linear regime and without considering dissipation, about the use of SCs to control the reflected waves in a broadband range of frequencies are presented. This proof of concept is experimentally tested in a sub-scale system, that works at ultrasonic frequencies in water. Different types of SCs and different geometries of the reflecting backing are tested. In particular, geometries that mimic that of the VEGA's launch pad of the European Space Agency (ESA).Authors acknowledge the support of the European Space Agency under contract "Sonic Crystals For Noise Reduction At The Launch Pad" ESA ITT 1-7094 (ITI) and the 441-2015 Co-Sponsored PhD "Acoustic Reduction Methods for the Launch Pad". The work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through project FIS2015-65998-C2-2García-Raffi, LM.; Salmerón-Contreras, LJ.; Herrero-Durá, I.; Picó Vila, R.; Redondo, J.; Sánchez Morcillo, VJ.; Cebrecos, A.... (2016). Reflection of sound by Sonic Crystals: an application to the aerospace engineering. Universidade do Porto. 1-10. http://hdl.handle.net/10251/181078S11

Nonlinear dispersive waves in repulsive lattices

[EN] The propagation of nonlinear waves in a lattice of repelling particles is studied theoretically and experimentally. A simple experimental setup is proposed, consisting of an array of coupled magnetic dipoles. By driving harmonically the lattice at one boundary, we excite propagating waves and demonstrate different regimes of mode conversion into higher harmonics, strongly influenced by dispersion and discreteness. The phenomenon of acoustic dilatation of the chain is also predicted and discussed. The results are compared with the theoretical predictions of $\alpha$-FPU equation, describing a chain of masses connected by nonlinear quadratic springs and numerical simulations. The results can be extrapolated to other systems described by this equation.The work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through Project No. FIS2015- 65998-C2-2 and by Project No. AICO/2016/060 by Conselleria de Educacion, Investigacion, Cultura y Deporte de la Generalitat Valenciana. L.J.S.-C. gratefully acknowledge the support of PAID-01-14 at Universitat Politscnica de Valsncia. A. M. gratefully acknowledge to Generalitat Valenciana (Santiago Grisolia program).Mehrem, A.; Jimenez, N.; Salmerón-Contreras, LJ.; García-Andrés, FX.; García-Raffi, LM.; Picó Vila, R.; Sánchez Morcillo, VJ. (2017). Nonlinear dispersive waves in repulsive lattices. Physical Review E. 96(1). https://doi.org/10.1103/PhysRevE.96.012208S00220096