A combined theoretical and numerical analysis of an experiment devoted to the excitation of Görtler vortices by localized stationary or vibrating surface nonuniformities in a boundary layer over a ...concave surface is performed. A numerical model of generation of small-amplitude disturbances and their downstream propagation based on parabolic equations is developed. In the framework of this model, the optimal and the modal parts of excited disturbance are defined as solutions of initial-value problems with initial values being, respectively, the optimal disturbance and the leading local mode at the location of the source. It is shown that a representation of excited disturbance as a sum of the optimal part and a remainder makes it possible to describe its generation and downstream propagation, as well as to predict satisfactorily the corresponding receptivity coefficient. In contrast, the representation based on the modal part provides only coarse information about excitation and propagation of disturbance in the range of parameters under investigation. However, it is found that the receptivity coefficients estimated using the modal parts can be reinterpreted to preserve their practical significance. A corresponding procedure was developed. The theoretical and experimental receptivity coefficients are estimated and compared. It is found that the receptivity magnitudes grow significantly with the disturbance frequency. Variation of the span-wise scale of the nonuniformities affects weakly the receptivity characteristics at zero frequency. However, at high frequencies, the efficiency of excitation of Görtler vortices depends substantially on the span-wise scale.
We report the first search for dark sectors performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super ...Proton Synchrotron with a momentum of 160 GeV / c are directed to an active target. The signal signature consists of a single scattered muon with momentum < 80 GeV / c in the final state, accompanied by missing energy, i.e., no detectable activity in the downstream calorimeters. For a total dataset of ( 1.98 ± 0.02 ) × 10 10 muons on target, no event is observed in the expected signal region. This allows us to set new limits on the remaining ( m Z ′ , g Z ′ ) parameter space of a new Z ′ ( L μ − L τ ) vector boson which could explain the muon ( g − 2 ) μ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal dark matter relic abundance. Our results pave the way to explore dark sectors and light dark matter with muon beams in a unique and complementary way to other experiments. Published by the American Physical Society 2024
We suggest a methodology of assessing cyclic loading–induced damage in austenitic steel by combining the ultrasonic and magnetic monitoring. Cyclic loading increases the martensitic phase volume, ...thus enhancing the plastic deformation–and resulting damage–in the austenitic phase having lower strength and stiffness. The damage in the austenitic phase can be detected through wavespeeds changes. The latter, however, are small thus requiring high accuracy of wavespeeds measurements. These requirements are satisfied by focusing on ratios of the longitudinal and shear wavespeeds (that do not depend on imprecisely known parameters such as sheet thickness). In order to separate changes due to damage from the ones due to growth of the martensitic phase, we combine the acoustics- and magnetic data. Preliminary results on comparison with available experimental data are encouraging.
The present experimental study is devoted to examination of the vortex receptivity mechanism associated with excitation of unsteady cross-flow (CF) waves due to scattering of unsteady free-stream ...vortices on localized steady surface non-uniformities (roughness). The measurements are carried out in a low-turbulence wind tunnel by means of a hot-wire anemometer in a boundary layer developing over a
$25\textdegree $
swept-wing model. The harmonic-in-time free-stream vortices were excited by a thin vibrating wire located upstream of the experimental-model leading edge and represented a kind of small-amplitude von Kármán vortex street with spanwise orientation of the generated instantaneous vorticity vectors. The controlled roughness elements (the so-called ‘phased roughness’) were placed on the model surface. This roughness had a special shape, which provided excitation of CF-waves having basically some predetermined (required) spanwise wavenumbers. The linearity of the stability and receptivity mechanisms under study was checked accurately by means of variation of both the free-stream-vortex amplitude and the surface roughness height. These experiments were directed to obtaining the amplitudes and phases of the vortex-roughness receptivity coefficients for a number of vortex disturbance frequencies. The vortex street position with respect to the model surface (the vortex offset parameter) was also varied. The receptivity characteristics obtained experimentally in Fourier space are independent of the particular roughness shape, and can be used for validation of receptivity theories.
This paper is devoted to an experimental investigation of receptivity of a laminar swept-wing boundary layer due to scattering of free-stream vortices on localized (in the streamwise direction) ...surface vibrations. The experiments were conducted under completely controlled disturbance conditions by means of a hot-wire anemometer on a model of a swept wing with a sweep angle of 25°. Both the free-stream vortices and the surface vibrations were generated by disturbance sources; their frequency–wavenumber spectra were measured thoroughly. The free-stream vorticity vectors were directed perpendicular to the incident-flow velocity vector and parallel to the swept-wing-model surface. The linearity of the receptivity mechanism under investigation (in a sense that the corresponding receptivity coefficients are independent of the disturbances amplitudes) has been checked carefully. The main goal of this experiment was to estimate the vibration-vortex receptivity coefficients as functions of the disturbance frequency, spanwise wavenumber and vortex offset parameter. This goal has been attained. Being defined in Fourier space, the obtained receptivity coefficients are independent of the specific surface vibration shape and can be used for verification of various receptivity theories.
The problem of calculating systems with one-sided connections is a rather complicated one and a lot of attention and interest has been paid to it. The theory of calculating structures with one-sided ...elastic and inelastic bonds is connected with general problems of calculating beams, frames, plates contacting with one-sided bases of different types, with calculating span structures, bridges, tunnels and underground structures.
The problem of multiple cracks originating at free surface (surface damage) in an anisotropic material is considered. We focus on the effect of material anisotropy on the mechanics of crack ...interactions – in particular, on the suppressing effect of interactions on crack nucleation and growth under the tensile loading (stress shielding). Importantly, this effect may change to the opposite one of enhancement, under the shear loading. These effects strongly depend on the material anisotropy. Calculations were performed for the orthotropic materials and the effect of anisotropy was examined by case studies of specific configurations that included comparison with the case of the isotropic material.
Interaction of a single rectilinear crack with the free boundary of a finite-dimension anisotropic elastic plate is considered. It is shown that as the crack tip approaches the plate boundary, the ...influence of elastic anisotropy of material on stress intensity factors is enhanced. The influence of the degree of anisotropy of the elastic properties of material and crack orientation (fracture mode) on this phenomenon is considered.
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