Fluid Dynamics最新文献

The greater thrust-vector angles can be obtained in the axisymmetric divergent bypass dual throat nozzle (ADBDTN). Meanwhile, the axisymmetric divergent bypass dual throat nozzle also has a certain flow adaptive capability and can solve the starting problems existing in the non-vectored state. In the present paper, the results of experimental investigations on the vectoring characteristics of the axisymmetric divergent bypass dual throat nozzle are given. By comparing the structures of the flow field obtained from experiments and numerical simulations as well as the wall static pressure distributions along the flow direction and circumferential direction, it can be seen that, as the nozzle pressure ratio (NPR) increases, flow reaches a critical state near the nozzle exit, and incompletely expanded flow in the cavity continues to accelerate after flowing out of the nozzle, a diamond-shaped structure with alternating shock and expansion wave systems appears downstream of the nozzle exit, and the flow field structures in the cavity are no longer changed when NPR ≥ 6. In addition, the static pressure distributions on the upper and lower wall surfaces of the cavity of the nozzle obtained from the experiments are in good agreement with the results of the numerical simulations, and the wall static pressures in the cavity are basically symmetrically distributed at various circumferential angles.

The effect of nozzle pressure ratio (NPR) on the starting characteristics of an axisymmetric divergent dual throat nozzle is investigated numerically. The steady and unsteady numerical simulation methods are used to study the internal flow field of the nozzle and variation in the related performance parameters as a function of the nozzle pressure ratio. The results show that for change in the total pressure of the nozzle inlet flow or back pressure under the same nozzle pressure ratio, the flow field structures in the nozzle cavity remains basically the same, and there is a little difference between the unsteady numerical simulation results and the corresponding time-independent numerical simulation results. In addition, the discharge coefficient of the nozzle increases rapidly with the increase of the nozzle pressure ratio, and then changes only slightly when the nozzle pressure ratio reaches a certain value. With increase in the nozzle pressure ratio, the thrust coefficient of the nozzle will oscillate in the initial stage, then gradually decrease, and then slowly increase, and suddenly decrease near the critical nozzle pressure ratio (NPR_cr), and then gradually increase. The typical flow field structures in the cavity under the starting and non-starting conditions are presented before and after reaching the critical nozzle pressure ratio at which the thrust coefficient of the nozzle suddenly drops.

The motion of a slender body in fluid beneath an ice cover coated with wet snow is considered. It is assumed that the fluid is ideal and incompressible, and the fluid flow is potential. The ice cover is modeled by a viscoelastic floating plate, the snow cover is modeled by a viscous layer. Formulas for calculating the wave resistance, the lift force, and the trimming moment exerted on a slender body which moves unsteadily and rectilinearly in fluid beneath the ice and snow covers are analytically obtained. A numerical analysis of the results shows that the snow cover reduces the absolute values of the extrema of hydrodynamic loads. The combined influence of increase in the snow-cover thickness, decrease in the depth of body’s submergence, increase in the ice-cover thickness, and decrease in the depth of water basin on the magnitude of hydrodynamic loads is analyzed.

In order to study the variation in the lift performance as a function of the angle of attack of a hypersonic gliding vehicle under supersonic (hypersonic) conditions, the cuspidal waverider was taken as the object of this study, and the variation in the lift performance depending on the angle of attack was simulated for its design condition (M = 3.86 and H = 25 km) and hypersonic incoming flow condition (M = 8 and H = 25 km). It was also compared with the delta-wing Model 1 with the same leading-edge swept angle and Model 2 with the same spread length, respectively. The obtained results show that the zero-lift angle of attack and the critical angle of attack of the cuspidal waverider are both greater than those of the Model 1 and Model 2. The critical angle of attack increases with the free-stream Mach number for all three models. The maximum lift coefficient angle of attack on the upper surface of the cuspidal waverider decreases with increase in the Mach number, contrary to Models 1 and 2, and this relates to the degree of expansion of the free-stream flow conditions, the model layout, and the vortex structure formed on the leeward side.

The impact of liquid droplets on the ultracold surface affects significantly the cold start performance of internal combustion engines but the splash and spreading characteristics after impacting on the ultracold surface are not clearly understood. Therefore, droplets with various physical parameters impacting on the Al–Si alloy surface have been selected for the study under various surface temperatures (–40°C ≤ (~{{T}_{s}}~) ≤ 25°C) and droplet impact velocities (0.96 m/s ≤ (~{{V}_{0}}~) ≤ 3.52 m/s). The ultracold surface (({{T}_{s}}) = –40°C) is beneficial for corona splash, and droplets with the higher Oh number impacting on the ultracold surface easily produce corona splash as the main splash pattern. The ultracold surface assisted in enhancing the stability of the levitated lamella formation, and avoided the effects of rough surfaces, so the upper splash criterion is established to predict the transition from spreading to splash. The decreasing surface temperature reduces the maximum spreading diameter (({{{{beta }}}_{{{text{max}},{text{lt}}}}})) of low solidification point droplets (ethanol, n-propanol, and winter diesel). Based on the assumptions of qualitative temperature, the empirical correlation of the ({{{{beta }}}_{{{text{max}},{text{lt}}}}}) is created for the T_s from 25 to –40°C.

The vibrations of a floating ice cover under the action of moving disturbances of variable intensity are studied. The model of vibrations of a floating ice cover is based on the linearized fluid mechanics equations and the linear classical theory of vibrations of plates. The ice cover is considered as a thin elastic isotropic plate. The critical velocities at which the nature of the wave disturbances changes both in front of the disturbance source and behind it are determined. The critical velocities as functions of the source oscillation frequency are studied, six critical velocities being obtained. It is shown that from one to seven wave systems are formed depending on the velocity of the source and the frequency of its oscillations. The corner zones in which these waves are formed are determined. The effect of compression and tension forces on the critical velocities and the corner zones in which the waves propagate has been studied.

A simplified mathematical model of two-phase multicomponent flow in the reservoir— multistage hydraulic fractures—horizontal well system is proposed. The formulation of transport problems in the well and in hydraulic fractures is simplified based on the dimensional analysis and similarity theory. The possibility of transition to a quasi-steady-state problem of distribution of the mixture components in high-permeability hydraulic fractures is shown. The dimension of the problem in reservoir is reduced by decomposing the problem into a set of problems in independent fixed stream tubes. For numerical solution of the problem, the resulting reduction in computer time reaches two orders of magnitude and can be further reduced by using parallel computing. Accelerating the solution of the direct problem is fundamentally necessary for the possibility of solving the inverse problem of identifying the porosity and permeability properties of fractures from the results of interpretation of tracer studies.

The diffraction of plane surface and flexural-gravity waves during their normal incidence at the edge of a floating elastic semi-infinite plate in fluid of finite depth in the presence of a current with velocity shear is studied. The explicit analytical solution to this problem is constructed using the Wiener–Hopf technique. Simple exact formulas for the reflection and transmission coefficients and the energy relations are obtained. The results of numerical calculations using the obtained formulas are presented.

The two-fluid and two-temperature diffusion-drift model of gas-discharge plasma is used to study numerically the structure of the Penning discharge in a cylindrical discharge chamber at the molecular hydrogen pressure of 1 mTorr, the voltage between the electrodes of 500–1000 V, and the axial magnetic field induction of 0.001–0.2 T. Two regimes of existence of the Penning discharge are obtained in the calculations. These regimes differ qualitatively in the electrodynamic structure of the charged-particle flows of gas-discharge plasma, as well as there exist transient and extinction regimes in the weak and strong magnetic fields. The conditions under which the oscillatory motion of electron and ion flows develops in the paraxial regions are found. It is shown that the results of numerical simulation with the use of the diffusion-drift model make it possible to obtain consistent data in comparison with experiment, and at the same time to get an insight about the formation of the structure of flows of electric-discharge plasma particles. This makes it possible to explain the observed experimental data.

A criterion for estimating the instant of transition from the stage of development of the Richtmyer–Meshkov instability to developed turbulence on rough contact interfaces of layered gas systems is proposed. A number of laboratory experiments are simulated. In the first series of experiments, the Richtmyer–Meshkov instability arises on two contact interfaces of a thin gas layer after passage of a shock wave. In the experiments, a thin layer (corrugated gas curtain) is formed by pumping a heavy gas (SF₆) through a nozzle block across an air-filled shock tube. In the second series of experiments, the shock wave passes across the contact interface of two gases of different densities (air-SF₆ and He-SF₆ layerings) perturbed along a sinusoid. In this series of experiments, the end face of the tube is either connected to the atmosphere or closed by a rigid wall. Development of the Richtmyer–Meshkov instability and transition to turbulent mixing are simulated using the implicit large eddy (ILES) method by means of the MIMOZA technique. A comparison with the available experimental information is made.