Thermal crisis of a vortex source in a real gas flowing into vacuum with heat supply under constant pressure

Thermal crisis of a vortex source at energy input under constant pressure is investigated. The energy release intensities per unit mass and per unit volume are analyzed for the case of the vortex source flow into vacuum (regime I). Analytical solutions for gasdynamic parameters and an algorithm for constructing the heat release intensity function for different variants of vortex sources are studied. Limitations on the extension of a constant-pressure interval are determined, and the dependences of the energy input area critical coordinates, critical temperature, and energy input parameters on the mass flow, gas circulation, and initial coordinate of the energy input area are obtained. A small correction to the gas specific heat with rising temperature and a significant correction to the specific heat at a stagnation temperature of 1000 K are considered.     

Thermal crisis of a vortex source

The effect of circulation in the field of a bulk vortex source on thermal crisis (flow choking induced by energy supply in a layer in accordance with a known law) is studied. Substantial changes in the value of energy supply parameter and slight variations in the coordinate of the critical cross section in which the velocity of sound is attained are revealed, and the dependence of these parameters on the location and width of the heat-supply region is noted. The possibility of transition to a supersonic flow when the heat release region is near the minimal cross section of the vortex source is analyzed. The difference between the cases of polyatomic and monatomic gases is demonstrated. Distinguishing features of the vortex sink are considered.     

Vortex source flowing into vacuum under thermal crisis

Thermal crisis of a stationary vortex source flowing to vacuum is considered for air on the basis of the model of a diatomic gas with variable heat capacities due to the excitation of vibrational degrees of freedom of molecules. The versions with different heat supply laws are compared. The effect of the size of the heat-release region (from close-to-zero value to that exceeding the minimal radius of the vortex source by tens of times) as well as the effect of the circulation of the flow on the critical parameters determining thermal crisis are considered. A qualitative difference from the thermal crisis in a perfect (ideal) gas with constant heat capacities is demonstrated.     

Estimation of the detonation cell size in gases

A simple method to calculate the parameters of a shock wave in a space between the shock wave front and the Chapman-Jouguet plane is considered. Solving a velocity equation, one can calculate the pressure, density, and temperature of the gas, as well as determine the size of a detonation region in a one-dimensional approximation. The dependences of the detonation region size on input parameters are derived. From these dependences, one can estimate the run of the same curves in the real situation.     

Vortex sink under conditions of a thermal choking with regard to the real properties of gases

Changes in the formulation of the problem for a vortex source and a vortex sink upon taking into account the change in the heat capacity and the adiabatic exponent for a diatomic gas (for the example of air) in response to an increase in the temperature from 300 K to a few thousands of Kelvin are discussed. A thermal choking is studied for a vortex sink, and critical values of the energy parameter are calculated. It is shown that the minimal radius of the vortex sink decreases upon a heat release. Similarity parameters including the dimensionless circulation (or mass flow), the energy parameter, and the position and thickness of the heat-release region are varied. Errors of the gas model that assumes constant heat capacities and a constant adiabatic exponent are estimated.     

Impact of the external energy supply to the shock layer region on the shock wave parameters

A system of equations describing supersonic gas flow in the presence of a heat source near the shock front is obtained. Relations between the gas parameters in disturbed and undisturbed regions, which generalize the classical Hugoniot-Rankine relations, are derived. Formulas for calculation of the flow parameters in the presence of an energy supply to the shock layer region are presented. It is demonstrated that there exists a critical intensity of energy supply at which the system of equations of the conservation laws for the gas parameters on both sides of the shock layer possesses no stationary solution.     

Effect of external power supply to a shock layer on the shock wave structure

A physical model that describes the structure of a 1D shock wave in a gas containing a moving heat source is put forward. A stationary equation for the profile of a shock wave in a gas with an arbitrary-shape heat source that is at rest relative to this wave is derived. Analytical solutions to this equation make it possible to analyze the flow pattern in the case of external power supply.     

Nature of the source of vortex sound flowing around a cylindrical profile

This paper is devoted to refining the nature of a vortex sound source and validly estimating the parameters of the region of source origination in a wake behind a cylindrical profile depending on the incoming flow velocity and profile diameter. Based on experimental measurements of the rms values of pressure pulsations on the surface and in the wake behind the profile and hydrodynamic laws for 2D fluid flows, the position of the origin of the vortex street in the wake and the size of the region where the street is still irregular are estimated. In this region, the street dimensions and pressure pulsation amplitudes change with distance from the profile. It is found that the maximum of the pressure pulsation in the wake approaches the profile surface in the range of Reynolds numbers (4.7 × 10³?1.5 × 10⁴); the amplitude of pressure pulsations on the profile and vortex sound intensity also increase. Based on the relationship between the source’s position and size and the width of the vortex street, as well as taking into account the decay of vortex circulation in the street with increasing distance to the profile, it is shown that the distance from the source to the surface of the profile should not exceed two gages. It is shown that an obstacle in the wake in the region of its irregularity causes a decrease in pressure pulsations on the profile and attenuation of emitted sound. Sound emission ceases completely when the obstacle comes in direct contact with the region of origination of the vortex street. Theoretical estimates satisfactorily agree with the measurement results.     

Combined energy–power action on a source in the constant Mach number regime with the given external force

Combined action on a source that flows into a submerged area or vacuum in the constant Mach number regime has been studied. The action by an external force has been defined with a constant distribution function (the force is given per unit volume) and with a distribution function proportional to the gas density (the force is given per unit mass). The investigations have been carried out for cylindrical and spherical sources. Similarity and differences, advantages and drawbacks of the above-mentioned cases and variants have been analyzed. It has been shown that the enthalpy increases significantly in subsonic flow (for the Mach number smaller than unity) by several times in the cylindrical source and by more than an order of magnitude in the spherical source. The total enthalpy increment increases with the length of the action zone or with the coordinate of the closing section.     

On the possibility of controlling transonic profile flow with energy deposition by means of a plasma-sheet nanosecond discharge

A way of effectively affecting the gasdynamic structures of a transonic flow over a surface by means of instantaneous local directed energy deposition into a near-surface layer is proposed. Experimental investigations into the influence of a pulsed high-current nanosecond surface discharge of the “plasma sheet” type on gas fast flow with a shock wave near the surface are carried out. The self-localization of energy deposition into a low-pressure region in front of the shock wave is described. Based on this effect, a facility for automated energy deposition into a dynamic region bounded by the moving shock front can be designed. The limiting value of the specific energy deposition on the surface in front of the shock wave is found. With the help of the direct-shadow method, an unsteady quasi-two-dimensional discontinuous flow arising when a plasma sheet is initiated on the wall in a flow with a plane shock wave is studied. By numerically solving the two-dimensional nonstationary equations of gas dynamics, the influence of the energy of a pulsed nanosecond discharge, which is applied in the frequency regime, on the aerodynamic characteristics of a high-lift profile is investigated. It is ascertained that the energy delivered to the gas before the closing shock wave in a local supersonic region that is located in the neighborhood of the profile contour in zones extended along the profile considerably decreases the wave drag of the profile.     

激波作用下SF6气泡界面演化和射流发展的数值模拟

 数值研究了平面激波冲击氮气环境中SF₆气泡界面的Richtmyer-Meshkov不稳定性,重点关注其中的激波聚焦及射流的产生和发展过程。在入射激波马赫数为1.23的情况下,给出了压力、密度、数值纹影和涡量等物理量的演化图像,定量分析了流场中压力最大值、密度最大值、射流速度、环量和斜压力矩随时间的变化关系。计算结果表明,平面激波冲击SF₆气泡过程有很强的聚能效应,在气泡内部靠近下游极点处发生激波近似理想聚焦和点爆炸现象,直接导致出现二次波系以及向下游运动的细长射流结构。相比入射激波,二次波系产生斜压力矩和涡量的能力要弱得多。     

激波管中非平衡可凝结流的流动分析

理论推导激波管中相变加热的临界加热量,得到一个显式计算临界加热量公式.该式可广泛用于湿蒸汽或纯蒸汽相变加热流动计算.结果表明,考虑相变加热的临界加热量要大于外部加热的临界加热量;热壅塞时会产生一道向上游传播的凝结激波,且凝结激波位置在液相质量分率开始增长点之前.采用AUSM格式数值求解激波管中湿空气的凝结流动,理论预测与数值计算吻合良好.     

Measurement of gas flow parameters in the vicinity of bodies with the front stalling zone

Problems associated with the development of a new method for the diagnostics of gas flows, which is suitable for ballistic experiments, are considered. The method is based of shock wave sensing of the flow under investigation. Various methods are proposed for introducing sensing perturbations into the flow. These methods are used for measuring the parameters of a gas streamlining flying vehicles with the front stalling zone.     

Acceleration of the Flow and Increase in the Enthalpy in an Ionized Gas Source by an Electric Field in the Constant Mach Number Regime

The possibility of passing from some characteristic physical parameters (stagnation enthalpy, temperature, pressure, density, etc.) to other preset parameters is investigated within the problem of control over a radial source (vortex) in the regime of the Mach number maintained constant by energy input and an external force (radial electric field in the given case). The variations of the total enthalpy upon the variation of the energy- and force-similarity parameters are demonstrated in the case when the force per unit mass and per unit volume is specified for identical convection current and conduction current, as well as for a conducting current prevailing over the convection current, and vice versa. In the limit of a negligibly small convection current or a conduction current, a transition to analytic solutions is demonstrated for the case of a purely energy action and a purely force action. Analytic dependences of the velocity increment (kinetic energy), temperature, and total enthalpy on the intensities of the external force and energy input, on the Mach number, and on the length of the zone of action are obtained.     

Diffraction of a two-shock configuration by a concave cylindrical surface

Diffraction of a two-shock configuration by a concave cylindrical surface with a continuously varying angle of diffraction in a perfect (inviscid and non-heat-conducting) gas is numerically investigated. Peculiarities of flow formation in the perturbed region due to a large initial angle of diffraction are revealed. The reasons for the appearance of the reverse flow over the cylindrical surface, which causes a vortex, are found. The influence of the vortex on the behavior of the loose end of the TU layer (slipstream) is studied. It is shown that the structure of the loose end of the TU layer is determined by vortex-related mixing in the gas. The nonstationary interaction of the diffracting shock wave and a sublayer shock with a concave cylindrical wall and the symmetry plane, which is accompanied by a change in the type of reflection, is examined. Published in Zhurnal Tekhnicheskoĭ Fiziki, 2007, Vol. 77, No. 10, pp. 24–33. The article was translated by the authors.     

Calculation of the drag and heat transfer from a sphere in the gas flow in a cylindrical channel

A numerical experiment on the simulation of heat transfer from a sphere to a gas flow in a cylindrical channel in the Stokes and transient flow regimes has been described. Radial and axial profiles of the gas temperature and the dependences of drag coefficient C_d of the body and Nusselt number Nu on Reynolds number Re have been calculated and analyzed. The problem of the influence of the early drag crisis for a sphere on its heat transfer to the gas flow has been considered. The estimation of this phenomenon has shown that the early drag crisis of the sphere in a strongly turbulent flow causes a reduction in heat transfer from the sphere to the gas by three to six times (in approximately the same proportion as for its drag coefficient).     

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.     

Shock waves and formation of carbon dioxide hydrate at an increased pressure in the gas-liquid medium

The processes of breaking, solution, and formation of hydrates behind a shock wave of moderate amplitude were studied experimentally in water with carbon dioxide bubbles under different initial static pressures. It is shown that an increase in the static pressure in a gas-liquid medium leads to reduction of critical relative amplitude of the shock wave, corresponding to starting development of Kelvin — Helmholtz instability and bubble splitting into small gas inclusions behind the shock wave front. It is shown that the rates of carbon dioxide solution and hydrate formation behind the shock wave front are close by the value; their dependences on medium and wave parameters are determined. Calculations by the model of gas hydration behind the shock wave are presented. The work was financially supported by the Russian Foundation for Basic Research (grants Nos. 06-01-00142 and 06-08-00657).     

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.     

Investigation of the interaction of a shock wave with the zone of a pulsed surface discharge in a rectangular channel

This study concerns the effect of the zone of a plane surface energy deposition on a gas-dynamic flow with a shock wave of M = 2.3–2.7 in a channel with a rectangular cross section. The source of the pulsed energy is a distributed sliding nanosecond discharge that develops in an approximately 1-mm-thick layer on a surface of 100 × 30 mm². The results of a 3D numerical simulation of the problem on the basis of the Navier-Stokes equations for a compressed gas are presented and compared with the experimental shadow images.