FLEET Velocimetry for Aerodynamics

Long-lasting emission from femtosecond excitation of nitrogen-based flows shows promise as a useful mechanism for a molecular tagging velocimetry instrument. The technique, known as femtosecond laser electronic excitation tagging (FLEET), was invented at Princeton a decade ago and has quickly been adopted and used in a variety of high-speed ground test flow facilities. The short temporal scales offered by femtosecond amplifiers permit nonresonant multiphoton excitation, dissociation, and weak ionization of a gaseous medium near the beam's focus without the generation of a laser spark observed with nanosecond systems. Gated, intensified imaging of the resulting emission enables the tracking of tagged molecules, thereby measuring one to three components of velocity. Effects of local heating and acoustic disturbances can be mitigated with the selection of a shorter-wavelength excitation source. This review surveys the development of FLEET over the decade since its inception, as it has been implemented in several test facilities to make accurate, precise, and seedless velocimetry measurements for studying complex high-speed flows.

Функциональные возможности электромагнитно-акустических преобразований в токовом режиме в металлическом расплаве

The paper deals with a symmetric problem on the base of physically substantiated estimates of the processes of electromagnetic-acoustic transformations (EMAT) of energy during the flow of an electric current through a melt, the key parameters of the open problem of the system "Power source parameters – Parameters of the magnetic field and magnetic pressure of the skin layer – Parameters of acoustic disturbances". It was shown that the key parameter when formulating the EMAT problem in technological applications is the geometry of the container with the object of processing and the material of the form. And when solving the problem, they are the parameter of the skin layer and the time dependence of the discharge current. It was established that a part of energy during the formation of the magnetic pressure in the skin layer from the amount of the energy stored in the capacitor bank of the pulse current generator is on the order of 10-4–10-2. The value of this part depends on the period of the discharge current and is proportional to the T1/2. When acoustic disturbances propagate in a melt, the main share of energy losses is determined by the difference in the acoustic stiffness of the melt and the shape of materials. The frequency spectra of the pressure of sound waves at the parameters selected for the analysis can cover the range of up to hundreds kHz, which is a good reason for the realization of resonance effects and the active formation of dissipative structures. Attention is focused on the fact that EMAT effects are manifested in the melt not only under the influence of an acoustic field, but also under that of an electromagnetic one in the skin layer. They are separated in time, but the acoustic field can occupy the entire volume of the melt and its effect is longer in time.

Detection of unmanned aerial vehicle using radio wave scatter on acoustic disturbances of the environment created by aircraft

Unmanned aerial vehicles (UAVs) have recently become widespread, because they are capable of performing a wide range of functions useful for mankind. At the same time, UAVs are a source of potential threats in a number of areas of human activity, namely, military, economic, and everyday life. Therefore, an urgent scientific and technical problem of detecting and observing UAVs has been formed recently to prevent them from performing unauthorized actions. The main means of UAV surveillance are radar (both active and passive), optical, infrared, acoustic stations, as well as complex systems in which joint processing of information obtained using these information channels is carried out. However, in general, the scientific and technical problem of monitoring UAVs, especially small UAVs, remains unresolved: the efficiency of UAV detection using all these methods remains insufficient, and the needs of practice are far from being fully satisfied with the available means. This article is devoted to the analysis of currently known scientific and practical results aimed to assess the possibility of detecting UAVs by radio signals scattered by acoustic disturbances of the environment created by UAVs, and to formulate urgent scientific and technical problems in this aria of knowledge.

Development of tomography–transmission electron microscope (TEM) specimen holder stable under acoustic disturbances

This paper focuses on the development of tomography—transmission electron microscope (TEM) specimen holder stable under environment effect that allows atomic resolution. The successful holder must be dynamically stable for accuracy and image processes to obtain an atomic resolution, with a minimum controllable drift of the sample position. Different strategies to reduce the effect of acoustic disturbances are investigated. The approach to the problem has been two-fold, numerical and experimental. The effect of mechanical and acoustic noise is analyzed. Finite element results match very well previous experimental results and observations. Theoretical analysis showed that air pressure fluctuations have a significant impact on microscopes with side entry goniometers, especially when the exciting frequency matches a vibration mode of the sample holder. For example, finite element analysis (FEA) predicts that the tip deflections are 4.5 Å and 0.09 Å under air pressure excitation of 64 and 40 dB respectively. Utilizing a sandwiched constrained damping shell layer made of viscoelastic material that partially covers the inner part of TEM holder body successfully decreased the vibration. Finite element simulations predict that a shell layer of viscoelastic material with a thickness equal to the 1/10 of the body holder diameter reduces the vibrations by 30%. The viscoelastic layer shell thickness, loss factor, and elastic modulus have a strong effect on the damping behavior and the optimal combination should be determined.

Acoustics of impacts as a factor of ensuring mutational and evolutionary transformations of the environment

Within the framework of the conceptual approach, taking into account the previously unknown, first identified by the authors features of the conditions of the internal energy equilibrium of material media (between their different physical nature energy states), the real contribution of acoustic disturbances experienced by media in the conditions of existence into ensuring of their and of the material world as a whole mutational and evolutionary transformations is taken into account and confirmed.

Design of metacontinua in the aeroacoustic spacetime

The effect of background flows on the response of acoustic metamaterials is a key aspect that prevented the full disclosure of their potential in those applications where an aerodynamic velocity field strongly influences the propagation of acoustic disturbances. Indeed, the classic approaches for metamaterial design do not consider the aeroacoustic interaction, and the resulting metamaterials cannot preserve their response when operating in flows. So far, only few authors have addressed the problem, mostly focusing on understanding the phenomenon or identifying corrective techniques with limited usability in practical applications. The present study proposes a general method for the modification of the mechanical properties of acoustic metacontinua to preserve their response in presence of a background flow. The method is based on the application of spacetime coordinate transformations exploiting the spacetime formal invariance of the generalised d’Alembertian. This methodology applies to the equation governing the propagation of acoustic disturbances in a metamaterial having arbitrary constitutive equations independently on the method used for its original design. The approach is validated through numerical simulations, using as a benchmark the problem of the acoustic cloaking of a cylinder impinged by a perturbation generated by an isotropic point source within a flowing medium. Numerical results are obtained for an asymptotic Mach number $$M_\infty \le 0.35$$ M ∞ ≤ 0.35 .