Fluid Dynamics最新文献

The two-dimensional electrogasdynamic problem of anomalous glow discharge on the surface of a sharp plate in supersonic flow of a perfect gas is solved using the system of Navier–Stokes equations to describe thermogasdynamic processes in the boundary layer and the two-temperature two-fluid diffusion-drift model of gas-discharge plasma to determine the electrodynamic structure of the discharge. The near-electrode regions of space charge and the external electrical circuit consisting of a power source and an ohmic resistance are taken into account. The influence of the magnetic field which is transverse to gas flow and has the induction of up to 0.03 T on the structure of boundary layer and glow discharge is studied. The electrogasdynamic structure of anomalous near-surface discharges is studied numerically over a wide range of gas flow velocities (M = 5–20), the free-stream pressures (p = 0.6–5 Torr), the electrode voltages, and the electric currents through the discharges. The electrodynamic structure of the gas-plasma flow near the electrodes and the effect of the glow discharge on the pressure and temperature distributions along the surface of the plate are also studied.

The problem of constructing the transverse contour of a conical body having the minimum wave drag in the range of supersonic velocities provided that the length and the volume are preserved is considered. A cone is taken as the initial body, an assumption about locality of the relation between variations in the geometric parameters and the pressure on the surface is made, and the quadratic approximation of this relation is used. The found solution is compared with the results obtained within the framework of the Newton model. These solutions are proposed to combine being based on the assumption of the power-law relation between the radius and the derivative of radius with respect to the angular coordinate. In this case, a class of contours in which half of the cycle consists of the element with monotonic variation in the radius and arc of the circle is distinguished. These contours can be described by specifying a single geometric parameter, namely, the exponent. Using the inviscid perfect gas model, direct numerical optimization of the shape of transverse contour is carried out and the possibility of reducing the wave drag as compared to the star-shaped bodies with plane faces is demonstrated.

The possibility of estimating the ionization parameters of high-temperature gas mixtures formed as a result of combustion processes is considered on the basis of the current–voltage characteristics measured using electrodes that generate an external electric field in the media under consideration.

The possible application of the results of numerical modeling in developing an approximate phenomenological mathematical aerodynamic model applicable in solving the problems of dynamics is studied with reference to the example of the unsteady flow past the NACA 0015 airfoil oscillating in the angle of attack at different frequencies, amplitudes, and mean angles of attack. For this purpose, the Reynolds equations are solved in both steady and unsteady formulations, together with the k–ω SST turbulence model. The results of the calculations are validated by means of comparing them with the experimental data. The model of the normal force and the longitudinal moment formulated within the framework of an approach introducing an internal dynamic variable is identified according to the data of calculations. The results of the modeling are compared with the numerical and experimental data. The comparison with the conventional approach to the modeling based on the linear unsteady model with the use of dynamic derivatives is also carried out.

The diffusion stability of a single cavitation bubble in a spherical liquid cell surrounded by an infinite elastic solid is considered. The time-periodic pressure in the solid far away from the liquid cell is used as an external driving, which initiates bubble oscillations along with the gas diffusion process in the bubble-in-cell system. The work is based on the engineering approximation according to which the bubble growth/reduction is considered on average, assuming that during the period of the external driving the mass of gas in the bubble does not noticeably change. This theory predicts the existence of stably oscillating bubbles in confined liquid undergoing an external driving force. Three possible diffusion regimes are revealed: 1) total bubble dissolution, 2) partial bubble dissolution, and 3) partial bubble growth, where the last two regimes provide the diffusion stability in the bubble-in-cell system. The parametric study of the influence of the gas concentration dissolved in the liquid on the resulting stable bubble size is conducted. The obtained results are compared with the results for the case of the stable bubble oscillations in the pressure sound field in a bulk (infinite) liquid. The theoretical findings of the present study can be used for improvement of the modern applications of ultrasound technology.

The problem of standing waves in a circular cylindrical vessel with an elevation on the bottom is formulated and numerically solved in the long wave approximation using an accelerated convergence algorithm. As a result of the calculations, the natural frequency of the fundamental wave mode is determined with a high accuracy. To compare the theoretical results, new experimental data on the excitation of standing surface gravity waves in a circular cylindrical vessel with parabolic and conical elevations at the bottom are presented. It is shown that the calculated and measured natural frequencies of the fundamental wave mode in vessels with the profiled bottom coincide between themselves.

The results of numerical study of supersonic flow in a channel with cavity are given. The calculated oscillation spectra are analyzed using the fast Fourier transform. Two types of oscillatory modes can be distinguished in the resulting periodic self-oscillatory regime. The first type of the modes corresponds to acoustic vibrations caused by the passage of sound waves along the cavity and calculated using the modified Rossiter formula. The second type of the modes corresponds to the frequencies of flow-rate oscillations caused by mass transfer between the cavity and the external flow. It is shown that the flow structure is modified when fuel is supplied in front of the cavity. Active combustion occurs in the layer of mixing fuel and oxygen from air. The flow pattern demonstrates the onset of Kelvin–Helmholtz instability on the interface between the main flow and the reacted gas. It is shown that an increase in the supplied fuel pressure leads to a decrease in the oscillation frequency and an increase in the characteristic size of oscillations.

This paper outlines a methodology for assessing the integral characteristics of a generic scramjet with an integrated propulsion system (PS). The specific impulse and heat flows in the PS for X-43 and X-51 scramjets are calculated. The results obtained are in good agreement with the estimates of other authors (in the speed range 5 < M < 10), as well as with the known graphs of the specific impulse dependence for various types of engines depending on the flight Mach number and the type of fuel used.

The possibilities of igniting a combustible mixture in a high-speed flow using a microwave streamer discharge are considered. The results of experimental studies of the streamer discharge structure at various pressures are presented. To quantitatively characterize the intensity of combustion of fuel ignited by a microwave discharge, pressure and temperature measurements are used in various sections of the jet in the discharge wake. The results obtained demonstrate the possibility of igniting a model combustible mixture when propane or its mixture with air is sup-plied to the discharge region. The main physical mechanisms responsible for heating the discharge plasma to the ignition temperature of the combustible mixture and their characteristic time scales are revealed. The change in the dimensions of the combustion zone with a decrease in the initial proportion of propane in the mixture is discussed. The minimum duration and minimum level of microwave radiation required to ignite a model combustible mixture under various conditions are determined.

Stagnation pressure is usually increased by so-called machine methods—various compressors that operate by supplying mechanical energy to the flow. This paper considers a method for increasing the stagnation pressure based on the thermal influence on the flow (heat removal). The influence of various factors on the degree of the increase in stagnation pressure for a channel flow under only thermal influence is studied. This paper considers various flow cooling methods. It is shown that evaporative cooling is the most effective in terms of increasing the stagnation pressure. A review of publications on the use of evaporative cooling to increase stagnation pressure is provided. Based on a one-dimensional model of an evaporative cooling device, it is shown that it is possible to increase stagnation pressure by a factor of 1.25 at supersonic inlet velocities.