Journal of Applied Mechanics and Technical Physics最新文献

This paper presents a theoretical study of the effect exerted on the stress-strain state in a shell mold by the contact angle between the support filler (SF) surface and the shell mold at which the spherical shell mold is not destroyed by the temperature stresses arising in it. We formulate the problem of optimization of the resistance of the spherical shell mold as a function of the contact angle of its support filler while the solidifying spherical casting within it cools down. The problem is solved using Navier equations, a heat equation, and a numerical method. The numerical scheme and the algorithm developed for solving the problem are given. It is shown that the crack resistance of the ceramic shell mold is determined by the normal stress value. The resulting resistance of the spherical ceramic shell mold is analyzed with account for the dependence of the shear modulus of the mold material on the support filler temperature.

The formation of streamwise and transverse vortices in a subsonic air jet and in an air jet with the addition of helium due to the natural instability of the flow is shown using a high-resolution direct shadow method.

Solutions to a problem of a solid circular rod made of orthotropic creep material and subjected to torsion by a constant torque are generalized to the case of an annular rod. Calculations are carried out using the Bhatnagar–Gupta method, the method based on the principle of minimum additional dissipation, and the finite element method. It is shown that the characteristic parameter method can be used to estimate a stress-strain state. The resulting analytical dependences of the twist angle rate on time at the steady-state stage of creep can be used to determine the shear parameters of the orthotropic Hill potential in torsion experiments or to refine them if these parameters were obtained via other approaches.

The direction of propagation of a straight-line plane crack in structurally inhomogeneous (grainy) materials under the combined effect of loading corresponding to fracture modes I and II is studied. The theoretical curve of the material strength or the Coulomb–Mohr curve type is assumed to be known. Based on the Neuber–Novozhilov force (integral) criterion relations are derived, which allow one to determine the angles of kinking (branching) of the crack path in the case of an arbitrary generalized stress state. Asymptotic presentations of the stress components in the vicinity of the crack tip take into account nonsingular terms ((T)-stresses). It is found that the crack can develop: 1) normal to the maximum stress direction if there are no shear stresses near the crack tip (Erdogan–Sih hypothesis) in the case of brittle fracture; 2) along the maximum shear direction if there are no normal stresses near the crack tip in the case of viscous fracture (in this case, a dislocation is emitted); 3) along a certain direction corresponding to a mixed stress state in the case of quasi-brittle or quasi-viscous fracture. The crack propagation direction depends on the ratio of the stress intensity factors for fracture modes I and II, sign of (T)-stresses, and shape of the theoretical curve of strength on the plane of the critical states.

Kelvin–Voigt equations for inhomogeneous fluids with a singular right side are studied. A singular term that approximates the Dirac delta function on an initially infinitely thin layer is introduced into the right side of a mass balance equation. This singular term is similar to the relaxation term used to describe nonequilibrium processes in hydrodynamics. In an extreme case, when a small parameter, namely, the characteristic size of the initial layer, tends to zero, the density and velocity at the initial time change abruptly.

A plane problem that describes the motion of a two-dimensional cavity located in a heavy ideal incompressible fluid is considered. Profiles of the free boundary are constructed by means of semi-analytical methods, and the motion of virtual singular points of the solution is studied.

This paper presents the results of an experimental study of the flow structure in a hemicylindrical dimple located on one of the walls of a rectangular channel with a height (H = 0.02) m and a length-to-width ratio of 7.5. A dimple with a width (D = 0.0158) m and a length (L{text{/}}D = 6.65) calibers could be oriented at different angles to the longitudinal axis of the channel ((varphi = 0)–90°). In the experiments, the pressure in the median sections along and across the dimple, the velocity components, and their fluctuations in the longitudinal and transverse directions were measured. In the experiments, the Reynolds number based on the flow-rate-averaged velocity and the hydraulic diameter of the channel was constant and equal to ({text{R}}{{{text{e}}}_{{{text{ch}}}}} = 3.88 times {{10}^{4}}). The pressure distributions on the dimple wall both in the transverse direction and along its length were found to depend significantly on its inclination angle to the channel axis. At the dimple inlet where the flow enters, a zone of strong rarefaction was formed. The length of this zone along the dimple did not exceed one caliber, and outside this zone, the pressure coefficient remained practically unchanged up to the dimple outlet, where there was a sharp increase in pressure due to stagnation. The greatest rarefaction in the transverse direction relative to the dimple occurred at an inclination angle (varphi = 45^circ ). The flow structure in different sections along the dimple length was studied. The maximum velocity of the circulation flow in the hemicylindrical dimple was observed at its inlet. Downstream along the dimple, the intensity of the vortex flow of the gas significantly decreased, and in the case of shallow dimples ((Delta {text{/}}D = 0.22)), the flow became unseparated.

A numerical finite-volume method of solving averaged Navier–Stokes equations closed by a one-parameter Spalart–Allmaras turbulence model is discussed. The proposed method is implemented within the framework of the HyCFS numerical code and is verified through comparisons of the HyCFS numerical solutions with those predicted by the CFL3D and FUN3D numerical codes. Three verification cases are considered: flow past a flat plate, bump-in-channel flow, and coflowing jet. It is shown that the numerical solutions obtained are in good agreement with each other. The numerical data on the flow velocity in the mixing layer are compared with experimental results.

Molecular dynamics (MD) simulations have been performed to study the uniaxial tensile responses of nanopolycrystalline niobium. Models with different grain sizes were established by using the Voronoi algorithm, and the effects of grain size and system temperature on the mechanical properties of polycrystalline niobium were investigated. The results indicate that grain size has a significant impact on deformation mechanism of nanopolycrystalline niobium. During the deformation process, the number of atoms at grain boundaries rises significantly, while dislocation density gradually decreases. Young’s modulus and yield stress reduced with reduction of grain size, which accords with inverse Hall–Patch formula. Specimens with smaller grain size have more grain boundaries and a larger proportion of chaotic atoms on grain boundaries, which leads to a decrease in mechanical properties. Young’s modulus and yield strength show an inverse relation with increase in system temperature, which is due to the higher temperature enlarge the number of disordered atoms at grain boundaries.

A setup for experimental studies of the structure, shape, and size of the gas–liquid interface under ultrasonic exposure and forced aeration has been developed. It has been found that ultrasonic exposure leads to an about 1.5-fold increase in interfacial area during aeration. The ultrasound intensity has been shown to have an optimal value that provides a maximum increase in interfacial area per unit ultrasonic energy input.