Gases Of Different Densities Research Articles

Realization of a shock-tube facility to study the Richtmyer-Meshkov instability driven by a strong shock wave.

A shock-tube facility capable of generating a planar shock with the Mach number higher than 3.0 is developed for studying Richtmyer-Meshkov instability induced by a strong shock wave (referred to as strong-shock RMI). Shock enhancement is realized through the convergence of shock within a channel with the profile determined by using shock dynamics theory. The facility is designed considering the repeatability of shock generation, transition of shock profile, and effects of viscosity and flow choking. By measuring the dynamic pressure of the tube flow using pressure sensors and capturing the shock movement through the high-speed shadowing technique, the reliability and repeatability of the shock tube for generating a strong planar shock are first verified. Particular emphasis is then placed on the ability of the facility to study strong-shock RMI, for which a thin polyester film is adopted to form the initial interface separating gases of different densities. The results indicate that the shock tube is reliable for conducting strong-shock RMI experiments.

Designing Multishape Dual-Gas Initial Conditions for the Study of Hydrodynamic Instabilities

Abstract The flow resulting from the rotation of a series of thin plates that initially separate two gases of different densities is analyzed using direct numerical simulations (DNSs). The 90-deg plates' rotation forms a vorticity shear layer and a density interface in between the tips of two neighboring plates. Results of this study show that the shape of these layers strongly depends on the plate's tip-based Reynolds number that can be varied thanks to a parametrization of the plates' opening law. Different regimes are identified corresponding to single- or multimode initial interfaces, with or without the occurrence of starting vortices during the formation of the shear layer. The density interfaces resulting from this procedure are particularly well-suited to serve as initial conditions for the study of the Richtmyer–Meshkov (RM) instability-induced mixing. Results of this study also provide a description of vortex formation in stratified flows.

POD-analysis of the near field of a turbulent circular jet when mixing gases of different densities

POD-analysis of the near field of a turbulent circular jet when mixing gases of different densities

Gas Component Transport Across the Soil‐Atmosphere Interface for Gases of Different Density: Experiments and Modeling

AbstractWe investigate the influence of near‐surface wind conditions on subsurface gas transport and on soil‐atmosphere gas exchange for gases of different density. Results of a sand tank experiment are supported by a numerical investigation with a fully coupled porous medium‐free flow model, which accounts for wind turbulence. The experiment consists of a two‐dimensional bench‐scale soil tank containing homogeneous sand and an overlying wind tunnel. A point source was installed at the bottom of the tank. Gas concentrations were measured at multiple horizontal and vertical locations. Tested conditions include four wind velocities (0.2/1.0/2.0/2.7 m/s), three different gases (helium: light, nitrogen: neutral, and carbon dioxide: heavy), and two transport cases (1: steady‐state gas supply from the point source; 2: transport under decreasing concentration gradient, subsequent to termination of gas supply). The model was used to assess flow patterns and gas fluxes across the soil surface. Results demonstrate that flow and transport in the vicinity of the surface are strongly coupled to the overlying wind field. An increase in wind velocity accelerates soil‐atmosphere gas exchange. This is due to the effect of the wind profile on soil surface concentrations and due to wind‐induced advection, which causes subsurface horizontal transport. The presence of gases with pronounced density difference to air adds additional complexity to the transport through the wind‐affected soil layers. Wind impact differs between tested gases. Observed transport is multidimensional and shows that heavy as well as light gases cannot be treated as inert tracers, which applies to many gases in environmental studies.

Turbulent transition of a gaseous mixing zone induced by the Richtmyer-Meshkov instability

A new experiment analyses the process driving the fast transition towards turbulence of an impulsively accelerated then decelerated interface between gases of different densities, when the timescale of this transitional process is of the same order as that of the resulting turbulence. This transition to turbulence is revealed as the imprint of the initial condition is lost and the dynamical spectral content covers a wide range of scales compatible with a self-similar trend.

Computational and Experimental Investigation of the Development of Turbulent Mixing in a Gas Layering in Passage of a Shock Wave

The experimental data and the results of direct numerical simulation of the flow developed in a constant-cross-section tube in passage of a shock wave through a three-layer gas system are presented. The three-layer systemis formed as a result ofmounting two thin films in the tube and filling the space between them with gases of different densities. The first interface (thin film) makes an angle of 45◦ with the shock front and the second interface is located in parallel to the front. The shock wave is formed at the left tube end and moves towards the first interface at the Mach number M = 2.4. The results of simulation of the problem are compared with the experimental data.

Experimental study of energy exchanges between two coupled granular gases.

We report on the energy exchanges between two granular gases of different densities coupled electromechanically by immersed blades attached to dc motors. Zeroing the energy flux between the two subsystems, we demonstrate that an immersed blade is a convenient way to assess the properties of the granular gases, provided that the dissipation in the motor is properly taken into account. In addition, when the two gases have different densities, the fluctuations of the energy flux are asymmetric, very intermittent, and with most probable zero flux. We show that, for weak coupling, the main features of the energy exchanges can be explained considering the fluctuations of the two subsystems.

Study of the formation mechanisms of forced expiratory wheezes in a healthy person when breathing gas mixtures of different density

The aim of the study is to specify the localization and formation mechanisms of forced expiratory wheezes (FEWs) in the bronchial tree. Statistical modeling of FEW spectral parameters has been performed, which were recorded above the lower sections of the lungs to the right and left, as well as over the trachea, from a sampling of 25 volunteers who breathed gases of different density. It was established that the highest amplitude FEWs formed closer to the trachea, and the lowest, in more distally located levels of the bronchial tree. The lower the density of the gas, the closer to the trachea the formation of mid-frequency FEWs occurs. For earlier high-frequency FEWs, an indirect dependence of their peak frequency on the Reynolds number was revealed. Turbulent-flow-induced forced oscillations and, in particular, vortex shedding are the probable formation mechanism for some of the FEWs.

Mathematical modeling of instabilities in the interaction of wave processes with the contact discontinuities between gases of different densities

Works performed in the field of mathematical modeling of the process of mixing of gases of different densities on the interface between them under the action of transmitted and reflected shock waves and compression and rarefaction waves are reviewed. A mathematical model of two-velocity, two-temperature gases, which has been derived from the basic principles, is proposed for this modeling. A number of examples on the interaction of the above wave processes with the interfaces in helium–xenon and helium–argon mixtures are given; the appearing Richtmyer–Meshkov instability and the distinctive features of the wave dynamics of flow of a mixture are described. The performed comparison of the time dependence of the mixing-zone breadth and other dependences has shown that the description of the phenomenon within the framework of the proposed approach is satisfactory.

Development of the Richtmyer-Meshkov instability during interaction of the diffusion mixing layer of two gases with transient and reflected shock waves

The evolution of the initially disturbed mixing layer of two gases of different densities under the action of an incident shock wave, shock waves reflected from the end face, compression and expansion waves is studied in a two-dimensional unsteady approximation on the basis of the previously formulated mathematical model of mechanics of a two-velocity two-temperature mixture of gases. Problems of wave interaction with a sinusoidally disturbed diffusion layer are solved numerically. It is shown that the calculated width of the mixing region is in good agreement with experimental data.

Development of the Richtmeyer-Meshkov instability during interaction of the mixing diffusion layer of two gases with transmitted and reflected shock waves

Development of instability in the region of mixing of fast-moving gases of different densities is a topical problem for energy-commuting facilities. Conventionally, the mixing layer is considered as a surface of the continuity break, i.e., as the contact break. The interaction of the shock wave with a perturbed contact discontinuity generates the Richtmeyer‐Meshkov instability. At the final stage, in the region of the initial contact break, the region of turbulent mixing separating the flows of compressed gases is formed. Numerous works on numerical modeling of the development of the Richtmeyer‐Meshkov instability based on the Euler equations did not take into account the influence of processes of mutual penetration of gases. Therefore, it is of interest to investigate this problem based on the equations of the double-speed two-temperature gas mixture, where each component has its own velocity and temperature. This approach allows one to describe the development of instability at various stages allowing for mutual penetration of gases. The necessity of applying models of multicomponent systems to describe the destruction of the contact boundary and formation of the region of the mixture was, e.g., pointed out in [1]. In [2], a semiempirical model of turbulent mixing of a multicomponent medium based on the use of its own velocity for each component is constructed. In this model, it is assumed that turbulent mixing appears instantly. Below, based on the equations of the double-speed two-temperature gas dynamics of the mixtures, we investigate the processes at the initial stages of mixing. We previously [4] investigated the interaction of the mixing layer with the incident shock wave and compression wave [5]. In this work, on the basis of equations of the doublespeed two-temperature gas dynamics of the mixtures [3], we investigate the processes of development of the Richtmeyer‐Meshkov instability during the interaction of the incident shock wave with the perturbed mixing layer allowing for the further repeated interaction of the layer with the waves reflected from the end. The initial sinusoidally perturbed diffusion layer of mixing was formed on the basis of the solution suggested in [4] with the perturbation amplitude a 0 , the perturbation wavelength λ , and the initial layer width δ 0 . The interaction of the mixing layer with the incident shock wave, which transfers from the light gas into the heavy gas and from the heavy gas into the light gas, is considered. The parameters of the mixture in the layer are described by the equations of the double-speed two-temperature gas dynamics of the mixtures [3], which transform to the Euler equations for pure gas out of the layer. The method of calculation of the starting set of equations is described in [4, 5]. The shock wave moved from upward to downward. At the upper boundary, the condition that derivatives were equal to zero was given, and the lower boundary is a solid wall. Symmetry conditions were given at the side boundaries. We considered the incidence of the shock wave on the mixing layer with the reflection angle in a onedimensional approximation in the absence of perturbations in the initial mixing layer. The results of these investigations are in complete agreement with the conclusions of [6].

Cyclotron mass of two-dimensional holes in (100) oriented GaAs∕AlGaAs heterostructures

Microwave cyclotron resonance of low-density high-mobility two-dimensional hole gases of different densities in (100) oriented GaAs∕AlGaAs heterostructures has been measured at 4.2K. The measurements show that the hole effective mass depends strongly on the confining potential. For holes in a 20nm quantum well, the cyclotron mass decreases from 0.48me at density p=6.9×1010∕cm2 to 0.29me at p=0.8×1010∕cm2. The cyclotron mass for holes confined at a heterojunction is fairly insensitive to the density and has a value of approximately 0.5me.

Large-eddy simulation of film cooling flows at density gradients

The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine blade film cooling. A cooling fluid is injected through an inclined pipe at α = 30° into a turbulent boundary layer at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas mixture. That is, gases of different density are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The comparison of the numerical findings with experimental velocity data from two-component particle-image velocimetry (PIV) measurements shows an excellent agreement. The results evidence the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio of the jet and crossflow fluid has to be considered.

Large-eddy Simulation of Film Cooling Flows with Variable Density Jets

The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine blade film cooling. A cooling fluid is injected from an inclined pipe at α=30° into a turbulent boundary layer profile at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas mixture. A variation of the density ratio is simulated by the heat-mass transfer analogy, i.e., gases of different density are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The numerical results and experimental velocity data measured using two-component particle-image velocimetry (PIV) are in excellent agreement. The results show the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio between the fluids and the velocity ratio have to be considered.

Development of the Rayleigh–Taylor instability due to interaction of a diffusion mixing layer of two gases with compression waves

A mathematical model of mechanics of a two-velocity and two-temperature mixture of gases is developed. Based on this model, the evolution of the mixing layer of two gases of different densities, which are accelerated by a compression wave, is considered by methods of numerical simulation. A solution of an initial-boundary problem is obtained in a one-dimensional approximation. This solution describes the formation of a diffusion layer between the two gases. The problem of interaction of this layer with the compression wave, the heavy medium being accelerated by the light medium, is solved numerically. Problems of instability development in a sine-perturbed mixing layer accelerated by a compression wave are solved numerically in a two-dimensional unsteady formulation. The calculated width of the mixing region is in reasonable agreement with experimental data.

Exact solutions for transient mixing of two gases of different densities

This Brief Communication presents a number of exact solutions describing the transient mixing of two gases of different molecular weights. Descriptions are given for both the concentration field and the associated induced motion in one-dimensional spherical, cylindrical, and planar configurations, including mixing layers, pockets, coflow jets, and concentrated mass sources.

Estimations d'erreur a priori de la méthode de Lagrange–Galerkin pour les systèmes de type Kazhikhov–Smagulov

Kazhikhov–Smagulov type systems are a subclass of non-homogeneous, incompressible Navier–Stokes equations where density is subject to diffusion, as in mixtures of gases of different densities. An algorithm is devised for these systems, the time discretization being based on a backward-Euler scheme together with the method of characteristics, and a mixed density–velocity–pressure ( P k , P k , P k − 1 ) finite element method is used for the space discretization in R d , d = 2 , 3 . Under the constraint that k > d − 1 and Δ t = C h r , with r ∈ ] d , 2 k + 2 − d [ , we give optimal error bounds O ( Δ t + h k ) for the time step Δ t and the mesh size h. To cite this article: J. Étienne, P. Saramito, C. R. Acad. Sci. Paris, Ser. I 341 (2005).

Differences in regional wash-in and wash-out time constants for xenon-CT ventilation studies

Xenon-enhanced computed tomography (Xe-CT) has been used to measure regional ventilation by determining the wash-in (WI) and wash-out (WO) rates of stable Xe. We tested the common assumption that WI and WO rates are equal by measuring WO-WI in different anatomic lung regions of six anesthetized, supine sheep scanned using multi-detector-row computed tomography (MDCT). We further investigated the effect of tidal volume, image gating (end-expiratory EE versus end-inspiratory EI), local perfusion, and inspired Xe concentration on this phenomenon. WO time constant was greater than WI in all lung regions, with the greatest differences observed in dependent base regions. WO-WI time constant difference was greater during EE imaging, smaller tidal volumes, and with higher Xe concentrations. Regional perfusion did not correlate with WI-WO. We conclude that Xe-WI rate can be significantly different from the WO rate, and the data suggest that this effect may be due to a combination of anatomic and fluid mechanical factors such as Rayleigh-Taylor instabilities set up at interfaces between two gases of different densities.

Broadband frequency dependence of respiratory impedance in rats

Past studies in humans and other species have revealed the presence of resonances and antiresonances, i.e., minima and maxima in respiratory system impedance (Zrs), at frequencies much higher than those commonly employed in clinical applications of the forced oscillation technique (FOT). To help understand the mechanisms behind the first occurrence of antiresonance in the Zrs spectrum, the frequency response of the rat was studied by using FOT at both low and high frequencies. We measured Zrs in both Wistar and PVG/c rats using the wave tube technique, with a FOT signal ranging from 2 to 900 Hz. We then compared the high-frequency parameters, i.e., the first antiresonant frequency (far,1) and the resistive part of Zrs at that frequency [Rrs(far,1)], with parameters obtained by fitting a modified constant-phase model to low-frequency Zrs spectra. The far,1 was 570 +/- 43 (SD) Hz and 456 +/- 16 Hz in Wistar and PVG/c rats, respectively, and it did not shift with respiratory gases of different densities (air, heliox, and a mixture of SF(6)). The far,1 and Rrs(far,1) were relatively independent of methacholine-induced bronchoconstriction but changed significantly with increasing transrespiratory pressures up to 20 cmH(2)O, in the same way as airway resistance but independently of changes to tissue parameters. These results suggest that, unlike the human situation, the first antiresonance in the rat is not primarily dependent on the acoustic dimensions of the respiratory system and can be explained by interactions between compliances and inertances localized to the airways, but this most likely does not include airway wall compliance.

Evolution of the Rayleigh–Taylor instability in the mixing zone between gases of different densities in a field of variable acceleration

The interaction of the mixing zone between two gases of different densities with compression waves and shock waves has been investigated. The characteristics of the mixing zone in which the Rayleigh–Taylor instability is developing have been analyzed. The evolution of the mixing zone volume and mass during the accelerated motion has been defined. A qualitative distinction in the evolution of the mixing zone under the influence of a continuous deceleration resulting from the interaction with the reflected compression wave—shockless deceleration—is revealed as compared to deceleration that is accompanied by appearance of a shock wave moving through the mixing zone—shock-induced deceleration.