A theoretical study of the features of sound propagation in a cylindrical duct with rigid walls in the presence of a flow with a thin boundary layer is presented. A uniformly suitable asymptotic ...solution of the Pridmore-Brown equation describing the propagation of sound modes in the model situation considered is obtained under the condition of a small thickness of the boundary layer. It is shown that the boundary layer can have a significant effect on the axial wave numbers of propagating sound modes, and this effect increases with increasing frequency. At the same time, for individual sound modes located near the cut-off condition, a finite-order effect may occur: a sound mode propagating in a homogeneous flow without attenuation may become attenuated in the presence of a thin boundary layer.
The ground state physical properties of the newly synthesized 312 MAX compound, Hf3AlC2 have been investigated using the first-principles density functional theory (DFT). The optimized unit cell ...parameters show good agreement with the experimental values. The calculated elastic constants and phonon dispersion confirm the mechanical and dynamical stabilities of this new compound. High bulk modulus, combined with low shear resistance and low Vickers hardness, indicates good machinability of Hf3AlC2, as expected for a metallic compound. On the other hand, significant stiffness due to large Young's modulus as well as the brittle nature according to the calculated Pugh's and Poison's ratios and Cauchy pressure are comparable to that of a ceramic. The present calculations show that Hf3AlC2 is elastically and optically anisotropic. The chemical bonding in Hf3AlC2 consists of a mixture of metallic, covalent and ionic contributions. The calculated Fermi surface contains quasi-two-dimensional topology, which indicates possible superconductivity of Hf3AlC2. The new phase Hf3AlC2 may also be a promising thermal barrier coating (TBC) material. The calculated enthalpy and entropy are found to increase with temperature above 100 K though a decrease is observed for the free energy.
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•The ground state physical properties of the newly synthesized Hf3AlC2 are calculated.•The mechanical and dynamical stabilities of Hf3AlC2 are predicted.•The new phase Hf3AlC2 may be a promising thermal barrier coating (TBC) material.•Fermi surface topology of Hf3AlC2 indicates that it is likely a superconducting phase.
The problem of sound propagation in a cylindrical duct with a uniform flow is considered with nonlinear impedance boundary conditions resulting from the dependence of the impedance of acoustic liners ...on the sound pressure level. An iterative procedure for solving this problem has been constructed, in which sound propagation is described by an asymptotic solution to the problem of the propagation of sound modes in a cylindrical duct with a uniform flow with a smoothly non-uniform impedance of the walls in the axial direction, and the nonlinear mode of operation of the liners is based on a semiempirical model of a two-layer acoustic liners. It is shown that the constructed iterative algorithm converges within the limits of applicability of the asymptotic solution and diverges beyond them. It is shown that, for the parameters with which the calculations were carried out, the nonlinear effect of the liners operation leads to an increase in sound attenuation compared to a linear solution of a similar problem, and this effect is when sound propagates along rather than against the flow.
This is an analytical solution of the two-dimensional non-isothermal mathematical model describing the change in the velocity profile of a cylindrical extrusion die. This solution is based on the ...following assumptions. The two-dimensional melt flow is asymmetric. A melt viscosity anomaly may take place. Heat generated by viscous friction is a factor affecting the melt flow. The melt flow moving towards the metering section is in a steady state. Neither mass forces nor inertia forces are present. Velocity gradients along the channel are neglected. The mathematical model was built up from the incompressibility equation, motion equations, energy equation, and the rheological equation. This model depicted a non-isothermal flow of rheological fluid moving through the cylindrical extrusion die. A diagram was drawn. It depicts the melt velocities at a die entrance in different cross-sectional views. Computer testing was performed to verify the obtained solutions and compare them with the real extrusion process. Difference between calculated and experimental data was below 14%. Results allow concluding a matching of numerical results with experimental data, and so the possibility of using a built-up model in an extrusion die design for single-screw extruders.
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A method has been developed for determining the modal composition of correlated sound modes propagating in a cylindrical duct, the peculiarity of which is that the effect of sound reflection ...from the open end of the duct is taken into account by numerical or analytical calculation of the reflection coefficients. This method was validated in the TsAGI AC-2 anechoic chamber, in which the sound field was created in the duct of a small-scale air intake model using 12 speakers at fixed frequencies; the modal composition was determined using 48 microphones installed flush with the duct walls, and synchronous measurements of the directivity of sound emission from the open end of the duct in the far field were carried out. In this case, on based on the found amplitudes of sound modes in the duct, the radiation characteristics in the far field were calculated using an analytical solution for sound emission from a semi-infinite cylindrical duct, the results of which were compared with the measurement results. It is shown that the amplitudes of sound modes found according to the developed method make it possible to reconstruct the far sound field characteristics with high accuracy in cases where the number of generated propagating modes in the duct does not exceed the number of installed microphones in the cylindrical array.
This work is devoted to a theoretical study of the features of sound propagation in a lined horseshoe-shaped duct that simulates the outer duct of an aircraft engine in the presence of a pylon. As a ...result of this study, cases were determined in which separation of variables is allowed, that is, a direct analytical solution for eigenmodes is possible, and the form of waveguide modes is obtained. The characteristic equations of this duct are found, which are satisfied by the eigenvalues, which are the azimuthal and radial wave numbers. A study of the axial wave numbers characterizing the effectiveness of the cladding was carried out for different impedance values in the presence and absence of sound-absorbing structures on the pylon. The calculation has shown that the lining of the pylon can lead to an increase in the level of noise attenuation by 2 dB.
This paper uses the finite element method (FEM) to study sound propagation in installations of the “interferometer with the flow” type in order to solve the problem of impedance eduction, taking into ...account the inhomogeneity of the flow for the two-dimensional and three-dimensional geometries of the installation duct. The propagation of sound in a duct in the presence of a plane-parallel airflow is described by a linearized Euler equation, which is solved by the FEM for the given sound frequency. A comparison of the experimental and calculation results demonstrates the effect of the calculated duct geometry on both the distribution of the sound field and the educed impedance values.
Plasma-assisted ammonia (NH3) synthesis is receiving significant attention·NH3 is globally important to fertilizer production and as a fuel. However, plasma research is, intrinsically, significantly ...cross-disciplinary and encompasses chemistry, physics, materials science and electrical engineering. A consequence is that it is uncommon for plasma researchers to be fully aware of the entirety of plasma-related research beyond a particular discipline. Here for the first time we critically review this cross-disciplinary literature to address this problemusing plasma-enabled NH3synthesis as an eminent showcase for process chemistry.A justification is that an improved understanding will be helpful to deal with the complexity of plasma processes which typically involve a set of chemical reactions withan ensemble of manyshort-lived excited, or reactive, species that vary amongst different plasmas.In this way,mechanisms for plasma-driven NH3synthesis are deciphered which helps to improve plasma reaction engineering.Modern analytical techniques are critical in deciphering these fundamentals and in this review optical emission spectroscopy (OES) is featured.
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