Observation of photoacoustic Smith-Purcell radiation

Relativistic electrons moving over a periodic metal grating can lead to an intriguing emission of light, known as Smith-Purcell radiation (SPR), the precursor of the free-electron laser. During the radiation process, the speed of light plays a critical role in determining the emitted angle and frequency spectrum. Inspired by the photonic SPR, here we experimentally demonstrate a photoacoustic version of the Smith-Purcell effect using laser-induced surface shock waves generation. We observe similar acoustic radiation pattern and their associated frequency spectrum in the far-field, perfectly predicted by a universal theory working for both the photonic and acoustic SPR. Moreover, our numerical studies reveal non-constant frequency components due to the supersonic traveling of the shock waves in the near field, greatly contrasting its photonic counterpart. This scheme extends the SPR into the acoustic domain by levitating the wave’s speed limit, paves the way towards coherent acoustic wave generation and microstructure metrology.

Problem of slip definition in driving systems

The pneumatic tire slip phenomena in vehicle driving system have been investigated for 100 years. Many models describe the tire –road force generation base on it, but in literature we can find various definition of slip. In this paper authors present the most known model of force generation and slip definitions and discuss consequences of choice of them. The model of force generation which is the combination of tire to road friction models and force carrying by deformable running elements which are getting and losing contact with road seems to be very simple and intuitive for process explanation. The implementation of the tire deformation model is the base for many models but is not easy to implement it into dynamic computer modelling of process. So for many years the most common models base on slip function. In paper the concept model is based on deformation introduced and its carry out balance is presented. This model has been adapted to other friction gears like belt gear or friction wheels modelling. The deformation model appears to be quite universal and developable to the energy efficiency analyses or acoustic wave generation.

Acoustic wave generation in collapsing massive stars with convective shells

ABSTRACT The convection that takes place in the innermost shells of massive stars plays an important role in the formation of core-collapse supernova explosions. Upon encountering the supernova shock, additional turbulence is generated, amplifying the explosion. In this work, we study how the convective perturbations evolve during the stellar collapse. Our main aim is to establish their physical properties right before they reach the supernova shock. To this end, we solve the linearized hydrodynamics equations perturbed on a stationary background flow. The latter is approximated by the spherical transonic Bondi accretion, while the convective perturbations are modelled as a combination of entropy and vorticity waves. We follow their evolution from large radii, where convective shells are initially located, down to small radii, where they are expected to encounter the accretion shock above the proto-neutron star. Considering typical vorticity perturbations with a Mach number ∼0.1 and entropy perturbations with magnitude ∼0.05kb/baryon, we find that the advection of these perturbations down to the shock generates acoustic waves with a relative amplitude $\delta {\rm p}/\gamma {\rm p} \lesssim 10{{\ \rm per\ cent}}$, in agreement with published numerical simulations. The velocity perturbations consist of contributions from acoustic and vorticity waves with values reaching ${\sim}10{{\ \rm per\ cent}}$ of the sound speed ahead of the shock. The perturbation amplitudes decrease with increasing ℓ and initial radii of the convective shells.

Influence of duct parameters on the acoustic wave generation

Purpose This paper aims to determine the influence of geometrical features of the channel on the acoustic wave generation in a ducted cavity. The analysis is focussed on the effects of the change in the entrance length upstream the cavity and the height. The study is supposed to investigate boundary layer and acoustic wave parameters, and an attempt will be made to determine the correlation between the geometrical dimension and those parameters. Design/methodology/approach Analysis is conducted with the aim of a computational fluid dynamics (CFD) tool and selected results are validated with experimental investigations. The influence of grid resolution and time discretization is analysed. Four different entrance lengths and height are investigated. Qualitative and quantitative comparison between cases is presented. Findings The investigations prove the small influence of the entrance length on acoustic wave generation, but channel height due to wave reflection and interference inside of the cavity has a significant impact on wave frequency and sound pressure level. Channel height has also impact on generation and shape of the vortex created in the cavity inlet. Originality/value The paper extends the knowledge of phenomena taking place in the ducted cavities. Results obtained from these investigations will be useful in designing new cooling techniques and in noise reduction. The CFD analysis makes it possible to determine the correlations between channel dimensions and SPL function and frequency of sound waves.

Electromagnetic–Acoustic Sensing for Biomedical Applications

This paper reviews the theories and applications of electromagnetic–acoustic (EMA) techniques (covering light-induced photoacoustic, microwave-induced thermoacoustic, magnetic-modulated thermoacoustic, and X-ray-induced thermoacoustic) belonging to the more general area of electromagnetic (EM) hybrid techniques. The theories cover excitation of high-power EM field (laser, microwave, magnetic field, and X-ray) and subsequent acoustic wave generation. The applications of EMA methods include structural imaging, blood flowmetry, thermometry, dosimetry for radiation therapy, hemoglobin oxygen saturation (SO2) sensing, fingerprint imaging and sensing, glucose sensing, pH sensing, etc. Several other EM-related acoustic methods, including magnetoacoustic, magnetomotive ultrasound, and magnetomotive photoacoustic are also described. It is believed that EMA has great potential in both pre-clinical research and medical practice.