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An A/B/C-type gas cylinder with various concentrations of SF6 (ranging from 5% to 80% in volume fraction) in the inner cylinder is constructed to investigate the dependence of the interface evolution on the Atwood number. For negative Atwood numbers, secondary vortex pairs emerge at the downstream interface of the outer cylinder following the interaction of a high-pressure triple point with the downstream interface, while a downstream jet is formed due to the generation of a notably higher-pressure zone after the transmitted shock wave traverses the convergence point. The widths and heights of both outer and inner cylinders are analyzed to quantify the interface evolution. The mechanism behind the vorticity evolution is investigated using the vorticity transport equation. The vorticity equation is introduced to investigate the mechanism of vorticity evolution. The dilatation and baroclinic terms play a dominant role in the dynamics of vorticity production. The net circulation can be predicted by linearly summing existing circulation models. Analysis of the area and mean mass fraction histories of the outer and inner cylinders shows that more ambient gas dilutes SF6 and promotes gas mixing as the Atwood number decreases. [ABSTRACT FROM AUTHOR]

The propagation of the weak shock wave (WSW) to the tunnel exits and their radiation as micro-pressure waves (MPWs) may cause sonic booms or structural resonance of buildings, posing potential hazards to humans, animals, and buildings in the exit's environment. The characteristics of the WSW and sonic booms of a maglev train/tube coupling model were studied based on the two-dimensional axisymmetric unsteady Reynolds average Navier–Stokes turbulence model. In the later stage of a MPW, the formation mechanism, geometry, and kinematic characteristics of compressible vortex rings (CVRs) were systematically analyzed. The inertial effect causes the initial wavefront to gradually transition from a Gaussian-shape waveform to a triangular waveform during its propagation, eventually coalescing into a WSW. The overpressure, density jump, and shock Mach number at the WSW location all increase with the increasing train speed, while the WSW thickness decreases accordingly. The formation distance of the WSW is inversely proportional to the amplitude of the initial wavefront gradient, and the WSW directly causes the occurrence of the exit sonic boom. The MPW amplitude has significant directionality with a largest value in the axial direction. Within the speed range of 450–700 km/h, the sound pressure level of the MPW exceeds the hearing threshold and even reaches the feeling threshold. The evolution of CVRs includes primary CVR, secondary CVR, and Kelvin–Helmholtz vortices. Primary CVR has the greatest impact on the axial MPW among them. The occurrence of CVRs will cause a second small noise level other than the sonic boom. [ABSTRACT FROM AUTHOR]

The evolution of a double-layer gas cylinder under various Mach numbers (M = 1.27, 1.5, 1.7, and 2.1) and Atwood numbers (A = 0.504, 0.392, 0.194, and −0.202 for the first layer) are studied numerically. At M ≥ 1.5 and A = 0.504, a bulge is generated near the upstream pole of the outer cylinder due to the impingement of a reflected shock wave, which promotes the formation of an upstream jet. At a higher Mach number, the evolution of the jet is suppressed under the influence of a higher pressure upstream of the jet head. The compressibility effects are quantified by the widths and heights of the gas cylinders. The Atwood number is associated with nonlinear acoustic effects, and the sign of A results in a significant variation in the wave patterns. The development of vortex pairs slows down with the decreasing Atwood numbers in the scenario of A > 0 for the first layer gas cylinder, while vortex pairs emerge and propagate in both upstream and downstream directions from the outer interface in the scenario of A < 0 for the first layer. As the Mach numbers and the magnitude of the Atwood numbers are increased, the mixing of various gases is promoted by detecting the circulation and mixed mass. The net circulation can be predicted by the linear summation of the Picone and Boris model and the Samtaney and Zabusky model under various Mach and Atwood numbers. [ABSTRACT FROM AUTHOR]

Shock-tube experiments are performed on the convergent Richtmyer–Meshkov (RM) instability of a multimode interface. The temporal growth of each Fourier mode perturbation is measured. The hydrodynamic instabilities, including the RM instability and the additional Rayleigh–Taylor (RT) effect, imposed by the convergent shock wave on the dual-mode interface, are investigated. The mode-coupling effect on the convergent RM instability coupled with the RT effect is quantified. It is evident that the amplitude growths of all first-order modes and second-order harmonics and their couplings depend on the variance of the interface radius, and are influenced by the mode-coupling from the very beginning. It is confirmed that the mode-coupling mechanism is closely related to the initial spectrum, including azimuthal wavenumbers, relative phases and initial amplitudes of the constituent modes. Different from the conclusion in previous studies on the convergent single-mode RM instability that the additional RT effect always suppresses the perturbation growth, the mode-coupling might result in the additional RT effect promoting the instability of the constituent Fourier mode. By considering the geometry convergence, the mode-coupling effect and other physical mechanisms, second-order nonlinear solutions are established to predict the RM instability and the additional RT effect in the cylindrical geometry, reasonably quantifying the amplitude growths of each mode, harmonic and coupling. The nonlinear solutions are further validated by simulations considering various initial spectra. Last, the temporal evolutions of the mixed mass and normalized mixed mass of a shocked multimode interface are calculated numerically to quantify the mixing of two fluids in the cylindrical geometry. [ABSTRACT FROM AUTHOR]

In this work, the stability of hypersonic flow over a curved compression ramp is studied using several stability analysis tools and direct numerical simulations (DNS). The free-stream Mach number and the unit Reynolds number are 7.7 and $4.2 \times 10^6$ m $^{-1}$ , respectively. Corner rounding is considered to alter the separation bubble flow so as to suppress the intrinsic instability of the compression-ramp flow. The variation of intrinsic instability is confirmed by global stability analysis. Subsequently, resolvent analysis is employed to examine the response of intrinsically stable flows to external disturbances. It is shown that the considered flows strongly amplify low-frequency streamwise streaks with a preferential spanwise wavelength. This result is verified using DNS by introducing a random forcing upstream of the separation point. Furthermore, both resolvent analysis and DNS demonstrate that the separation bubble contributes little to the selection of the spanwise wavelength of streamwise streaks. The combined effects of convective and intrinsic instabilities are also explored using DNS. A better agreement with experimental data is achieved after introducing upstream disturbances in an inherently unstable flow. [ABSTRACT FROM AUTHOR]

Hypersonic flows over a compression ramp with different leading-edge radii are numerically investigated. Flow separation occurs on the cold-wall compression ramp with a free-stream Mach number of 7.7 and a unit Reynolds number of $4.2 \times 10^6$ m $^{-1}$. By performing direct numerical simulations (DNS), it is shown that the separation bubble enlarges when the leading-edge radius is increased from zero up to a critical value. Beyond the critical radius, the separation bubble conversely shrinks as the radius is further increased. Global stability analysis (GSA) is employed to investigate the three-dimensional instability of the two-dimensional base flows. It is found that the inherent instability in the flow field also exhibits a reversal trend, that is, the flow system firstly becomes more unstable and then tends to be more stable with increasing leading-edge radius. The growth rate and spanwise wavelength of the unstable modes identified by GSA are verified by DNS. Accompanying the occurrence of three-dimensionality, streamwise heat-flux streaks are formed on the ramp surface downstream of reattachment. The present study demonstrates that a proper blunting of the leading edge can suppress flow separation, reduce aerodynamic heating and stabilise the flow system for a hypersonic compression-ramp flow. [ABSTRACT FROM AUTHOR]

A hypersonic shock wave/laminar boundary-layer interaction over a canonical 25−55° double-cone configuration is numerically investigated. A moderate-enthalpy flow of 5 MJ kg−1 with a Mach number of 9.87 and a unit Reynolds number of 1.5 × 105 m−1 is considered. Special emphasis is given to the influence of leading-edge bluntness. The results indicate that the double-cone flow is insensitive to small bluntness in terms of shock structures, separation region sizes and surface pressure and heat flux distributions. A critical nose radius is observed, beyond which the separation bubble grows dramatically. The numerical data are analysed and interpreted based on a triple-deck formulation. It is shown that the sudden change in flow features is mainly caused by pressure overexpansion on the first cone due to leading-edge bluntness, such that the skin friction upstream of the separation is significantly reduced and the upstream pressure can no longer resist the large adverse pressure gradient induced by shock impingement. An estimation of the critical radius is established based on the pressure correlations of Blick & Francis (AIAA J., vol. 4 (3), 1966, pp. 547–549) for spherically blunted cones. Simulations at a higher enthalpy with the presence of both vibrational relaxation and air chemistry show a similar trend with increasing nose radius. The proposed criterion agrees well with the experimental observations. [ABSTRACT FROM AUTHOR]

Numerical simulations were performed to investigate the re-initiation mechanism of a diffracted detonation wave near the critical channel width for a weakly unstable gas. Two scenarios were examined: diffraction of a planar detonation wave and of a cellular detonation wave inside the inlet channel. The results revealed that the critical channel width predicted using a cellular detonation wave is smaller than that predicted using a planar detonation wave. The re-initiation mechanisms are described in detail by tracing massless particles along both the plane of symmetry and the re-initiation path. For planar detonation diffractions, a compression wave is formed in the far field behind the diffracted shock. Re-initiation is closely related to the amplification of this compression wave and its coalescence with the diffracted shock. Depending on the inlet channel width, the strength of the reflected rarefaction wave is responsible for weakening the strength of the compression wave and its coalescence with the diffracted shock, consequently hindering the reaction of particles behind the diffracted shock wave. In cellular cases, the continuous collisions of transverse waves, which generate local explosion sites, sustain detonation wave propagation. [ABSTRACT FROM AUTHOR]

The jet formation of gas inhomogeneities under the impact of a planar shock wave is an interesting phenomenon that is closely related to shock convergence. In this study, a series of heavy gas inhomogeneities with very simple geometries (square, rectangle, circle, and triangle) are numerically reproduced to trace the source of the jet formation. Special attention is given to the wave patterns that lead to the formation of specific jet forms. The shock-accelerated multicomponent flow is simulated by solving inviscid compressible Euler equations. An up-wind characteristic space-time conservation element and solution element scheme is adopted, and a five-equation model is used to treat the gas interface. The jet types that emerge in the experimental images are explained based on the numerical results, and a typical shock pattern that ensures the jet formation is uncovered. It is found that, physically, the jet is initiated by the impact of the internal Mach stem, and the jet growth is nourished by the high speed gas flow induced by this Mach stem. The width of the jet is determined by the height of the internal Mach stem. Geometrically, a focal wedge enveloped by slip lines emerges in the gas inhomogeneity, in which the gas is accelerated. It is found that the existence of the focal wedge and the coordinates of the wedge tip can be used as qualitative criteria to illustrate the mechanism of the jet formation. These criteria provide a more intuitive basis for understanding the direction, scale, and process of jet formation. [ABSTRACT FROM AUTHOR]

The characteristic space–time conservation element and solution element (CE/SE) schemes proposed by Shen et al. (2015) [15] are straightforward extended to multidimensional schemes on 2D rectangular meshes which strictly follow the space–time conservation law. The schemes for solving both scalar conservation laws and compressible Euler equations with shock waves are developed. They are accurate and robust with CFL widely ranging from 0 to nearly 1. In Euler solvers, the frequently-used Harten, Lax and van Leer (HLL) Riemann solver, contact discontinuity restoring HLLC Riemann solver and Roe Riemann solver are employed to calculate the upwind fluxes as examples. When standard grid-aligned Riemann solvers are employed, the carbuncle phenomena are significantly suppressed when comparing with conventional upwind schemes. If rotated Riemann solvers are employed, nearly carbuncle-free results are obtained. Several well understood numerical examples are carried out to demonstrate that the 2D characteristic CE/SE schemes can simultaneously capture shocks and details of complex flow structures very well. [ABSTRACT FROM AUTHOR]