Gases Of Different Densities Research Articles

壁面上の突起に衝突する非定常噴流の数値解析

As a fundamental study of the mixture formation process of the impinging diffusion direct injection diesel engine, characteristics of the unsteady gas jet impinging onto a projection on a wall was numerically studied.Numerical analysis was carried out on a 2-dimensional incompressible unsteady flow with two different gases of different density. The program code used was based on the SOLA scheme, which was expanded so that the density change caused by the mixing of the two different density of gases can be treated.Numerical results are able to describes the nature of the mixture formation process of the unsteady gas jet impinging onto a projection on a wall, which had been obtained through the authors' previous experimentation.

Skew plane shock wave at the interface between two polytropic gases. A heavy-light gas system

In [Inzh.-Fiz. Zh., 72, No. 2, 208–215, 216–225 (1998)] the problem of break decay that occurs when a plane skew shock wave (SSW) arrives at the interface between two polytropic gases of different density (the polytrope indices of both gases remain constant in the entire region of interaction) from the side of the less dense of them is considered in detail. The change in the stationary shock-wave configuration with an increase in the angle of incidence from 0 to π/2 is tracked. In the present work, consideration is given to the development of a stationary shock-wave configuration in the case where the SSW emerges at the interface from the side of the more dense gas. Problems associated with boundary layers and mixing of contacting gases were not considered.

Evolution of mixing zone under the influence of nonstationary compression wave

In the present study we performed an experimental investigation of the Rayleigh–Taylor instability (RTI) in a mixing zone between two gases entrapped into accelerated motion by a nonstationary compression wave. Acceleration g was ∼1.5·107 cm/s2, Atwood numbers ranged from 0.04 to 0.77. A mixing zone was formed by an oxygen-hydrogen mixture and an inert gas (Ne, Ar, Kr, Xe) or SF6. The initial gas pressure was 0.5 atm. A specific feature of our experiment is compressibility of media tested and initially continuous interface between gases of different densities. The present work is a continuation of investigations on nonlinear, transition, and early turbulent stages of the RTI.

Experiments on the Richtmyer–Meshkov instability: Wall effects and wave phenomena

Experiments examining the interaction of shock waves with an interface separating two gases of different densities are reported. Flow visualization by the schlieren method and x-ray densitometry reveals that important secondary effects are introduced by the experimental apparatus, especially at the walls of the shock tube from shock wave/boundary layer interaction below, above, and at the interface itself. These effects can impair the observation of the primary phenomenon under study and can lead to the overall deformation of the interface. In particular, the thickness of the viscous boundary layer at the interface is computed using a familiar shock tube turbulent boundary layer model and the occurrence of bifurcation of reflected waves below and above the interface is successfully predicted based on classical bifurcation arguments. The formation of wall vortical structures at the interface is explained in terms of baroclinic vorticity deposition resulting from the interaction of reflected waves with the interface distorted by the boundary layer. This mechanism of wall vortex formation can also explain observed test gas contamination in reflected shock tunnels when shock wave bifurcation is absent. In general, it is found that most of the side effects of the experimental investigation of the Richtmyer–Meshkov instability can be alleviated by performing experiments in large test sections near atmospheric initial pressure.

Development of the Richtmyer-Meshkov instability on an interface with continuous density variation

The development of the Richtmyer-Meshkov instability driven by a shock wave reflected from a rigid wall is investigated. It is shown that the perturbation amplitude growth rate depends on the nature of the shock wave refraction on the interface between gases of different density. In the case of regular-soft refraction the reduction in the growth rate of the Richtmyer-Meshkov instability associated with the continuous change in density on the interface is measured.

Experimental investigation of Richtmyer–Meshkov instability in shock tube

An experimental investigation of the Richtmyer–Meshkov instability is carried out in a shock tube. The purpose of this study is to obtain information on the growth in the thickness of the turbulent mixing zone, which is induced by the impulsive acceleration of the interface between two gases of different densities. The turbulent phase of the evolution of this instability is of interest here. The thickness of the turbulent mixing zone is inferred from two different diagnostic techniques: measurements of infrared emission of CO2 and black-and-white or color schlieren photographs. Following an assessment of the diagnostic techniques, discussions of the main experimental difficulties as the presence of membrane fragments and the disturbances induced by the wall boundary layers, are given. Comparisons of the thickness and the thickness growth rate of the turbulent mixing zones obtained in the present experiments, with both experimental and theoretical results, are made. A tentative picture of the evolution in time of the turbulent mixing zone thickness has been developed. Results fall between the predictions of the linear and the t2/3 power law theories, with a tendency toward the latter one.

Richtmyer-Meshkov mixing zone study by a multidirectional laser absorption technique.

Shock tube experiments have been undertaken in which a shock wave accelerates normally to an interface separating two gases of different densities leading to the formation of a three-dimensional mixing zone between the two gases. Assuming the mixing zone to be homogeneous and weakly dependent on the wall effects, an integrated monodirectional absorption technique had been previously carried out and average temperature and density evolutions were determined within it. But the mixing zone is really deformed by the wall effects and in consequence it is not homogeneous. Thus, to improve the diagnostic technique, the experimental setup has been modified so that the mixing zone is divided into nine identical homogeneous imaginary regions. Temperature and species concentration are determined through each region of the mixing zone by a multidirectional laser absorption technique in order to have some new information on the influence of the wall effects on the Richtmyer-Meshkov instability study in square-cross-section shock tubes. Furthermore, a better accuracy in the measurements of the mixing zone thickness is obtained from a discussion of the concentration profiles. The consequence is that previous results achieved with integrated techniques such as global Schlieren, shadograph, or monodirectional absorption methods seem to be strongly overestimated.

CO2 laser absorption measurements of temperature and density in the shock-induced Richtmyer-Meshkov mixing zone.

The aim of this paper is to discuss the interest of using spectroscopic diagnostic methods to study shock-induced Richtmyer-Meshkov mixing as a complement to other experimental techniques, numerical simulations, and theoretical studies. Using a ${\mathrm{CO}}_{2}$ laser absorption technique, the average temperatures and densities are experimentally determined through a high velocity gaseous mixing zone. This mixing is originated from a shock-induced acceleration of a plane interface, materialized by a thin plastic membrane, that initially separates two gases of different densities. Two gas combinations have been tested: ${\mathrm{CO}}_{2}$-Ar and ${\mathrm{CO}}_{2}$-He. Temperature and density are determined within the shock accelerated mixing zone from infrared absorption measurements of two characteristic vibrational-rotational lines of the ${\mathrm{CO}}_{2}$ bending mode, using a ${\mathrm{CO}}_{2}$ continuous wave laser as a diagnostic probe. The absorption coefficient is a function of the temperature and density of the ${\mathrm{CO}}_{2}$ for a known linewidth profile. Thus, temperature and density profiles within the mixing region are calculated from two measurements. For this, a ${\mathrm{CO}}_{2}$ absorption coefficient model, assuming the Voigt linewidth profile in the mixing region and taking into account the contribution of the hot bands, has been developed. It is shown that the technique is well applicable to this type of study, and that there is a strong correlation between the Atwood number and the shape of the mixing zone.

Evolution of the continuous interface between gases of different density during the passage of a shock wave

The evolution of the interface between gases of different density following the passage of a shock wave has been experimentally investigated. It is shown that replacing the discontinuous change of density on the wavy contact discontinuity by a continuous change in a layer of finite thickness leads to a reduction in the amplitude growth rate in the initial stage of development of Richtmyer—Meshkov instability. The experimentally determined values of the amplitude growth rate reduction factor are satisfactorily described by a model to be found in the literature.

Experiments on the Richtmyer–Meshkov instability: single-scale perturbations on a continuous interface

We report on experiments to measure the shock-induced growth of sinusoidal perturbations on thick interfaces separating two gases of different densities. The results show that the growth rates are reduced as the interface thickness is increased. A model that accounts for the growth rate reduction caused by the presence of a finite density gradient on the interface is proposed and good agreement is obtained with the experimental results.

Formation of high-energy electron beams in gases of different densities

An investigation was made of the formation of beams of fast electrons in different gases at pressures of from 0.01 to 100 kPa. Plots were made of the dependences of the electron beam currents Ie on the gas pressure p for different electric field strengths E. The dependences Ie=f(p) for air were found to intersect the similar dependences for other gases (helium, nitrogen, neon, and argon) at a pressure of p=10.6 kPa and for E=2.3·105 V·cm−1. This fact is explained by the influence of the oxygen ions and atoms on the electron beam formation process. Another experimental result, the appearance of a minimum in the dependences Ie=f(p) for all gases, is explained by defocusing of the electron beam, the appearance of a reverse current, and magnetic neutralization of the beam. Electron beams were obtained having a pulse duration of 15–20 nsec and a current of 105-106 A/m2 per unit cathode area.

Simultaneous Measurement of Concentration and Velocity by Applying Hot-Wire Techniques. 3rd Report. Statistical Investigation Aided by a Microcomputer.

In the turbulent mixing regions which were formed by issuing gases of different densities into ambient air, the instantaneous values of concentration and velocity are derived by a set of probes consisting of an ordinary hot-wire and a probe for concentration measurement. The signals of the two probes are input into a microcomputer, and the calculations are carried out immediately. The instantaneous measuremests of concentration and velocity make possible statistical investigation into the turbulent mixing region. The present paper provides some illustrated instances on the power spectra. The integral length scales for concentration and velocity are estimated by the autocorrelation coefficients which are obtained by the instantaneous values. The comparison between the scales for concentration and velocity shows that the length scale for concentration is much larger than that for velocity. The accuracy of this method of measurement is discussed.

High-frequency ventilation in dogs with three gases of different densities

Dogs were ventilated with a high-frequency oscillation device varying the frequency (5-15 Hz), the tidal volume (25-100 ml), and the resident gas (He, N2, SF6). Tidal volume was measured with a body plethysmograph. Blood gases were measured after a quasi-steady state was established. The kinematic viscosity of the breathing gas mixture, which changed by 1,700%, was found to have little effect on arterial PO2 and PCO2. The results are consistent with findings in a branched model that consisted of tubes with a diameter of 1 cm and with the theory of Taylor-type diffusion in turbulent flow. In addition, experiments were performed reducing and increasing the equipment dead space. This resulted in changes of PO2 and PCO2 that were appreciably less than those resulting from variations of tidal volume of the same magnitude.

Growth induced by multiple shock waves normally incident on plane gaseous interfaces

Experiments to measure the visual thickness of the mixing region produced by shock waves incident on continous and discontinous interfaces between gases of different densities have been carried out in a new vertical shock tube. The generation of vortical structures by shock-wave boundary layer interaction at the interface is observed and the need for experimental methods to distinguish the effects of resulting jets on the walls and windows of the shock tube is emphasized. Initially thick interfaces, smoothed by molecular diffusion, exhibit growth after being perturbed by acoustic noise induced during wave reverberation. Thin interfaces, whose perturbations are introduced by the rupture of the supporting plastic membrane, have a more rapid initial growth but the late time growth is comparable to that of the thicker interfaces. In both cases the rate of increase of the thickness of the interfaces is an order of magnitude less than reported by previous investigators.

Preferential axial flow during high-frequency oscillations: effects of gas density

Allen et al. (J. Clin. Invest. 76: 620-629, 1985) reported that regional phasic lung distension during high-frequency oscillations (HFO) is substantially and systemically heterogeneous when both frequency (f) and tidal volume (VT) are large. They hypothesized that this phenomenon was attributable to central airway geometry and preferential axial flow induced therein by the momentum flux of the inspiratory gas stream. According to that hypothesis, the observed distribution of phasic lung distension would depend on the ratio VT/VD* (where VD* is an index of anatomic dead space), independent of gas density (rho), when f is scaled in proportion to lung resonant frequency, fo. To test this hypothesis, we used the methods of Allen et al. (ibid.) to study six excised dog lungs during HFO (f = 2-32 Hz; VT = 5-80 ml) using gases of different densities. Alveolar pressure excursions (PA) were measured as rho spanned a 12-fold range using He, air, and SF6. The apex-to-base and right-to-left ratios of PA were used as indexes of regional heterogeneity of phasic lung distension. For each gas at low f, distension of the lung base was favored slightly independent of VT, but at higher f distension of the lung apex was favored when VT was small, whereas distension of the lung base was favored when VT was large. In addition, we observed substantial right-to-left differences in apical lobes during oscillation at high f not seen before.(ABSTRACT TRUNCATED AT 250 WORDS)

The compressible turbulent shear layer: an experimental study

The growth rate and turbulent structure of the compressible, plane shear layer are investigated experimentally in a novel facility. In this facility, it is possible to flow similar or dissimilar gases of different densities and to select different Mach numbers for each stream. Ten combinations of gases and Mach numbers are studied in which the free-stream Mach numbers range from 0.2 to 4. Schlieren photography of 20-ns exposure time reveals very low spreading rates and large-scale structures. The growth of the turbulent region is defined by means of Pitot-pressure profiles measured at several streamwise locations. A compressibility-effect parameter is defined that correlates and unifies the experimental results. It is the Mach number in a coordinate system convecting with the velocity of the dominant waves and structures of the shear layer, called here the convective Mach number. It happens to have nearly the same value for each stream. In the current experiments, it ranges from 0 to 1.9. The correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases rapidly with increasing convective Mach number, reaching an asymptotic vaue of about 0.2 for supersonic convective Mach numbers.

Nasal conductance and effective airway diameter.

1. Transnasal pressure (delta p) in Pa and gas flow in cm3 s-1 were measured in men breathing gases of different density, assuming flow through each nostril (f) to be half the measured total flow. Measurements were also made in children with allergic rhinitis before and after nasal antigen challenge. 2. Flow always showed evidence of turbulence when transnasal pressure exceeded 40-80 Pa breathing air and Reynolds number exceeded 2400 (1800 in the presence of nasal obstruction). 3. Changes in effective mean nasal airway diameter (D) after nasal challenge can be determined at points of similar pressure from the relation log (D2/D1) = 0.368 log (f2/f1) when flow is turbulent. 4. Absolute estimates of airway diameter in cm can be obtained from the relation 4.75 log D = 1.75 log f--log delta p+log L--4.756 breathing air when flow is turbulent (since the relation between pressure and flow approximates to that found in a long cylinder) and these estimates are only marginally affected by differing assumptions about nasal airway length (L). 5. Because pressure and flow are non-linearly related, and changing nasal dimensions have a profound effect on the flow at which turbulence occurs, it is suggested that measures of nasal conductance at delta p greater than or equal to 0.1 kPa are preferable to the more conventional measures of nasal resistance at a specified flow rate.

Design of submerged turbulent jets of gases of different densities

We present results of a theoretical and experimental investigation of submerged gas jets in the range of density variations (ρj/ρe=0.05–10).

The compliance curve for the flow limiting segments of the airway. II. Experiments with human subjects.

Maximum effort flow-static recoil curves were obtained in 5 healthy subjects breathing air, He/O2, and SF6/O2 mixtures. In 4 of them maximum effort flows corresponded to really maximal flows and their curves were transformed into compliance curves for the flow limiting segments of the airway and analyzed from the point of view of a previously presented lung model (Pedersen and Nielsen 1976). The results showed, that viscosity dependent pressure losses from the alveoli to the flow limiting segments were minimal for air and SF6/O2, but not for He/O2. When viscosity dependent pressure lossess could be negleted, then expiration of gases of different densities gave almost identical compliance curves for the flow limiting segments. This supported the applicability of the model. The calculated compliance curves for the flow limiting segments were compared with data from the literature, and the findings indicated that flow limitation during expirations with just maximal flows throughout began in the extrapulmonary airways and moved upstream during the expiration.

Turbulent Mixing by Gases of Different Densities

Turbulent Mixing by Gases of Different Densities