Structure and parameters of the shock layer formed when a supersonic underexpanded jet interacts with a counterpropagating hypersonic flow in the transition regime

The method of molecular dynamics (the Bird system) has been used to mathematically model a planar, strongly underexpanded supersonic jet that encounters a hypersonic flow of rarefied gas. Particular attention is paid to the structure and parameters of the shock layer close to the plane of symmetry. The results of calculations are presented for currents of a monatomic gas, simulating argon, with a Mach number of the external flow of M_∞=5.48, a Mach number at the nozzle edge of M_a=1, a ratio of the density at the nozzle edge to the density of the unperturbed flow equal to 130, and various stagnation temperatures of the external flow and of the jet. The evolution of the structure and the parameters of the shock layer as the Knudsen number Kn_∞ varies from 0.02 to 0.35 is considered. The results are compared with the data calculated for the shock layer when argon flows around thermally insulated cylinders. The main features and regularities of the relaxation of the translational degrees of freedom of the gas for external and jet flows are considered. Data are presented on the form of the distribution function over velocities and its evolution as gas moves through the shock layers. Zh. Tekh. Fiz. 68, 13–18 (July 1998)     

Thermodynamic theory of the equation of state of materials

A universal derivation of the thermodynamic equations on the basis of a combined analysis of the exact relations for any material — the virial theorem, the shock adiabat, and the differential thermodynamic identity relating the thermic and caloric equations of state of matter — is given. This combination makes it possible to reduce the fundamental problem of thermodynamics to a boundary-value problem of mathematical physics. Analytic relations T _s=T(P _s,ρ _s and T _s=T(D,u) are obtained for classical systems. Zh. Tekh. Fiz. 68, 1–9 (October 1998)     

Study of an Underexpanded Plasma Jet

Supersonic jets (Mach number M = 1.4 – 2.0) of dense plasma were produced by a plasmatron in a pulsed regime. The shock structure was investigated by fast mirror cameras, by the schlieren technique and by probe and spectroscopic methods. It was found that during jet expansion the bow shock ahead the gas-plasma interface and the shock behind it are superimposed with the quasistationary shock structure which includes the Mach disk. Whereas the plasma in the discharge chamber is in LTE (n_E ≈ 5 × 10²³ m^?3, T ≈ 14000 K), the electron density of the expanding jet plasma falls to a plateau value of n_E ≈ 5 × 10¹⁹ m^?3 below the LTE limit. At this phase, the jet plasma formed a ball-like turbulent plasmoid which moves at a subsonic velocity through the air up to some milliseconds.     

Variety of LaSrMnO structures induced by growth conditions and laser irradiation

Crystallographic phase transitions in perovskite-like LaSrMnO metallic oxides are studied. The transitions are induced when internal stresses generated during film synthesis (at temperatures between 450 and 730°C) vary (decrease or increase) upon subsequent irradiation by a KrF laser emitting in the UV range. As the synthesis temperature T _s grows, the rhombohedral-to-orthorhombic phase transition occurs at 650–670°C. The resistivity is shown to be either temperature-independent, ρ(T)=const, at T<T _crit, or varies and reaches a maximum, ρ(T)=ρ_max, at the Curie temperature T _c. Optical transmission spectra taken at photon energies ℏω=0.5–2.5 eV exhibit both a high (0.8–0.9) and low (0.1–0.3) transmission coefficient t, depending on the synthesis temperature. As follows from X-ray diffraction data, the laser irradiation causes a phase transition only in LaSrMnO films grown at T _s<650°C. Phases of different size scales appear: the long-range-order orthorhombic matrix and mesoscopic-range-order rhombohedral clusters are observed in the films grown at T _s=450–550°C and the rhombohedral matrix with orthorhombic clusters, in the films grown at T _s=550–650°C.     

Pure and Sn-, Ga- and Mn-doped ZnO gas sensors working at different temperatures for formaldehyde,humidity, NH<Subscript>3</Subscript>, toluene and CO

ZnO and Sn-, Ga- and Mn-doped ZnO nanoparticles were prepared by a coprecipitation method, and characterized by scanning electron microscopy (SEM), energy dispersive spectra (EDS), X-ray diffraction (XRD) and Raman spectra. The gas sensing properties were studied using formaldehyde, relative humidity, NH₃, toluene and CO as the probes. The results show that all particles have wurtzite ZnO phase, though Sn–ZnO has a relatively smaller particle (and crystallite) size than the other three samples. Gas sensing property tests reveal that the temperature where the gas sensing maximum is gained (T _M) is changed by different dopants: Sn–ZnO and Mn–ZnO have relatively lower T _M (∼100°C lower) compared with that of pure ZnO, while Ga–ZnO has the same T _M as pure ZnO except in CO sensing. Thermoluminescence (TL) spectra were used to investigate the mechanism of T _M change. The peak positions of Ga–ZnO and ZnO are the same at 300–350°C, while that of Sn–ZnO shifts to 250–300°C, which might contribute to the same T _M of Ga–ZnO and pure ZnO and relatively lower T _M of Sn–ZnO. In the case of Mn–ZnO, the luminescence emission is evidently limited by its black color.     

Influence of the line stiffness on the critical properties of 2d domain walls and polymer systems

A recently developed efficient Monte-Carlo method is used to calculate the critical equilibrium properties of a 2-dimensional system of thermal loops (loop gas) in dependence of the line stiffness energys. With increasing s the critical temperatureT _c (defining an Ising-like behaviour fors<1)decreases monotonically toT _c =0 ats=1 (in units of the line energy). Fors>1,T _c increases monotonically withs and defines anon-universal critical behaviour. The critical line is calculated in a phase diagram (i) as aT _c -versus-s plot showing a dipT _c =0 ats=1 and (ii) in a concentration (c)-versus-s diagram, describing, alternatively, a dilute system of rough polymers. In the latter diagram the critical concentration decreases monotonically withs fors<1 and increases withs fors>1.     

PSP visualization studies on a convergent nozzle with an

An in-house Pressure Sensitive Paint (PSP) formulation has been developed at the Aero-Physics Laboratory at the University of Manchester. The PSP uses Bathophenanthroline Ruthenium as the luminophore molecule and is incorporated in a sol-gel matrix. Excitation occurs at400–500 nm and emission at550–650 nm. The Stern-Volmer plot of the PSP reveals small temperature dependence, which has always been an intrinsic drawback of PSPs. As a baseline experiment the PSP has been applied to examine the side-wall pressure field of the flow through a convergent nozzle with an ejector, at fully expanded Mach numbers in the rangeM _j =0.52–1.36. Simultaneous static pressure measurements were also conducted to ascertain the accuracy of the PSP results. The paint has demonstrated satisfactory capabilities in not only measuring static pressures but also in visualizing key physical elements of the flow, such as the location of the expansion and oblique shock waves present in such flows.     

String quantization in accelerated frames and black holes

We quantize a closed bosonic string in a light-cone gauge in Rindler (uniformly accelerated) space-time and apply it to the Schwarzschild-Kruskal manifold. Inertial and accelerated particle states of the string associated to positive frequency modes with respect to the inertial and Rindler times respectively, are defined. There is a stretching effect of the string due to the presence of an event horizon. We explicitly solve the dynamical constraints leaving as physical degrees of freedom only those transverse to the acceleration. Different mass formulae are introduced depending on whether the centre of mass of the string has uniform speed or uniform acceleration. The expectation value of the Rindler (Schwarzschild) number-mode operator in the string around state (tachyon) results equal to a thermal spectrum at the Hawking-Unruh temperature T_s=α/2π (∼ M_Pl(M_Pl/M)^1/(D−3), where M is the black hole mass). We find T₀=M′/2π where M′ is the accelerated ground state string mass and T₀ the temperature T_s in dimensionless frequency units. Correlation functions of string coordinates and vertex operators and their Fourier transforms in accelerated time (string response functions) are computed and their thermal properties analyzed.     

On the ferromagnetism of liquid gold-cobalt alloys

The magnetizationM of the eutectic AuCo alloy was measured in various fields above the melting pointT _s. There is strong evidence for spontaneous magnetization in the temperature range some 17 degrees aboveT _s.T _s was examined by thermal analysis during the measurement ofM.     

Determination of heat losses and their influence on the performance characteristics of high-enthalpy hot-shot tubes

An analysis of the losses of heat into the walls of settling chamber in a hypersonic hot-shot tube has been performed. Tests without diaphragm rupture showed that the fall of settling-chamber pressure during the operating flow regime in the tube was the consequence of the transfer of heat from working body to wall; this has allowed us to evaluate the heat-transfer coefficient α and the inner-surface temperature of the wall T _w. An empirical formula relating the coefficient α with the pressure and working-body temperature in the settling chamber in the range of pressures and temperatures 160 to 540 bar and 700 to 3400 K was obtained. Using the gained dependences of α and T _w on pressure and temperature, we have developed a physical model for calculating the working-body characteristics in the tube with allowance for enthalpy losses. We found that by the hundredth millisecond of the operating regime the disregard, in such calculations, of the wall heat flux in the first settling chamber resulted in overestimation of the stagnation temperature in the test section in comparison with similar calculations made without allowance for the heat losses by 6–18 % in terms of the full-scale temperature for aircraft flight in Mach number range 5 to 8. The developed calculation procedure has been tested in experiments without diaphragm rupture.     

Magnetic Phase Transitions in CePtSn

CePtSn, crystallizing in the orthorhombic TiNiSi-type structure, exhibits two antiferromagnetic transitions at T _N=7.8 K and T _M=5.2 K. Low-temperature X-ray diffraction study has been done to investigate changes in lattice parameters connected with these magnetic phase transitions. Specific-heat data in the same temperature region are also presented. Magnetization isotherms at T>T _N up to 14 T have been measured and the obtained results are compared with theoretical calculations based on a microscopic model Hamiltonian.     

Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni_2+x Mn_1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T _M and magnetic T _C (Curie point) phase transitions to converge. In the region where T _C =T _M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)     

Characteristics of dual-combustion ramjet

The authors discuss a possibility to use a diverging dual-combustion chamber as applied to high-supersonic boost ramjets operating at flight Mach numbers up to M_f = 8–10. Due to diverging, this chamber allows beginning the ramjet operation from flight Mach numbers M_{f ini} = 2–3. The diverging combustion chamber is characterized by a ratio of its exit cross-sectional area relative to the cross-sectional area of air-intake throat. This expansion area ratio is determined at M_f = M_{f ini}, but it should be the same at all flight Mach numbers M_f ? M_{f ini}, and depends on two factors: the location of a normal shock in the air-intake throat and the condition of reaching the critical velocity at the chamber exit. The dual-combustion chamber provides heat supply in its alone channel first to the subsonic flow and then, along with acceleration of the flying vehicle, to the supersonic flow, which is bound with a decrease in relative heating of working gas. Calculations of characteristics of an exemplified dual-combustion ramjet considered with a twodimensional air-intake were performed in the range of M_f = 3–7.     

Stationary Propagation of Coupled Nonlinear High Frequency Waves and Ion-Acoustic Waves in a Plasma

Stationary solutions of the coupled equations for high frequency transverse waves in a plasma and for the low frequency ion motion (T_e?T_i) are investigated numerically. The use of the nonlinear hydrodynamic equations instead of the linear wave equation for ion acoustic waves allows to look for solutions without restrictions of the Mach number M = V/c_s (V group velocity, c_s ion acoustic velocity) and the ratio ω/ω_pe (ω frequency of the HF-field, ω_pe electron plasma frequency at the undisturbed region). In particular, supersonic soliton-like solutions with n/n_o > 1 were found. Dispersion effects due to charge separation are not included.     

Experimental study of the evolution of periodic controlled disturbances in a hypersonic viscous shock layer on a flat plate

Characteristics of the fields of mean density and density fluctuations measured with introduction of periodic disturbances into a hypersonic viscous boundary layer on a flat plate are presented. The experiments are performed for a flow Mach number M_∞ = 21, Reynolds number per meter Re_1∞ = 6·10⁵ m⁻¹, and temperature factor of the surface T _w /T ₀ = 0.26. The disturbances are introduced into the shock layer by an oblique gasdynamic whistle. The work was financially supported by the Russian Foundation for Basic Research (Grants Nos. 04-01-00474 and 05-08-33436).     

Characteristic features of the temperature dependence of the surface impedance of YBa2Cu3O6.95 single crystals

The real and imaginary parts of the surface impedance Z _s =R _s +iX _s of YBa₂Cu₃O_6.95 single crystals are measured at a frequency ω/π=9.4 GHz. The quantities R _s (T) and X _s (T) are linear functions of temperature for T<0.3T _c (T _c =93.5 K). A maximum of R _s (T) and a plateau of X _s (T) are observed in the interval 35<T<65 K. Our experimental data, just as all recent measurements of R _s (T) in YBa₂Cu₃O_6.95 single crystals in the temperature range 0<T<1.3T _c , are described well in a two-fluid model which assumes electron-phonon scattering of quasiparticles. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 893–898 (25 June 1997)     

Specific heat of Nickel-Iron and Nickel-Cobalt alloys between 600° K and 1500° K

The -specific heat anomalies of ferromagnetic fcc Ni–Co and Ni–Fe alloys were investigated near the Curie temperatures. The magnetic contribution to the specific heat was found to be logarithmically divergent [c _M=log|T/T _c–1| ^A ] above the Curie temperature. The exponentA was composition dependent. Entropy and energy values associated with magnetic transions were determined experimentally and compared with theoretical predictions of the Heisenberg and Ising localized electron models obtained from series expansion calculations.     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Shock wave propagation in gases under subatmospheric pressure

In the present paper the results of the experiments on the shock wave dynamics under subatmospheric pressure in neutral gases are presented. The characteristics of spherical and plane configuration shock wave excitation and propagation are studied in the pressure region 1 torr<p<760 torr. It is shown that whenp=3 torr it is still possible to fix successfully the shock wave appearance and propagation in various neutral gases. The pressure dependence of the shock wave propagation velocity and amplitude is determined experimentally. It is shown that when the pressure decreases the shock wave amplitude decreases and an increase of the Mach number takes place. In the case of plane shock wave the Mach number reaches the valueM=5.2 under the pressurep=3 torr.