Shock Waves最新文献

The explosive dispersal of particles is an important problem in multiphase physics and is of considerable interest due to its many applications. Simulations that examine particulate dispersal in such flows have employed a variety of methods, including Euler–Lagrange, Euler–Euler, and dusty gas. The appropriate choice of methodology depends on the balance between accuracy and computational cost. In general, if the particles are very small and tracer-like, a cheaper dusty gas approach will be sufficient. In this paper, we present a series of two-dimensional numerical simulations investigating particle and fluid time scales in the context of the explosive dispersal of particles within a spherical shock-tube problem. Using the timescales, the appropriateness of the equilibrium Eulerian approach in calculating the particle velocity is investigated. With increasing particle inertia, the equilibrium Eulerian approximation offers a good compromise between accuracy and computational efficiency, where the particle velocity becomes an algebraic function of the fluid velocity, acceleration, and particle time scale. Different blast parameters, for which the calculation of particle velocity based solely on the flow acceleration and particle time scale is valid, were studied and presented. Initial particle size, volume fraction, blast pressure, and temperature ratio were varied, and the resulting effects on the particle time scale, fluid time scale, and the Stokes number are presented. It was found that the Stokes number is a valid predictor of the viability of the equilibrium Eulerian approximation. For values of the Stokes number below unity, there was good agreement between the equilibrium Eulerian and the Euler–Euler methods. It was observed that the most significant factor impacting the Stokes number, and consequently, the accuracy of the equilibrium Eulerian approximation, is the particle size.

This work presents the experimental investigation of the influence of methane addition to ({hbox {CH}}_{{4}})–(hbox {H}_{{2}})–air mixture ((varphi = 0.8)–1.6) on the critical conditions for transition to detonation in a (90^{circ }) wedge. Similar to (hbox {H}_{{2}})–air mixtures investigated previously, for ({hbox {CH}}_{{4}})–({hbox {H}}_{{2}})–air mixtures results showed three ignition modes: (i) flame ignition with ignition delay time longer than 1 µs, (ii) strong ignition with instantaneous transition to detonation, and (iii) weak ignition with delayed transition to detonation. In a stoichiometric mixture with 5% ({hbox {CH}}_{{4}}) (i.e., 95% ({hbox {H}}_{{2}}) in fuel), the transition to detonation corresponds to a shock velocity of roughly 752 m/s (an increase of 37 m/s from that obtained in (hbox {H}_{{2}})–air) corresponding to (M = 1.89). In general, 5% (hbox {CH}_{{4}}) addition caused an increase of 3.25–5.03% in the critical shock wave velocity necessary for transition to detonation for all lean and rich mixtures considered. Also, similar to that found for ({hbox {H}}_{{2}})–air mixtures, the transition-to-detonation velocity increased for a leaner and richer mixture. Moreover, it was observed that methane addition in general increased the pressure limit at the wedge tip necessary for the transition to detonation.

The influence of viscosity on the formation of the Mach reflection of shock waves in a steady argon flow between two symmetrically arranged wedges is numerically studied at the free-stream Mach number of 2 and the Reynolds number of 1000. A two-shock configuration is shown to form at the wedge angle (theta _text {w}=10.9^circ ) rather than a three-shock configuration predicted by shock polars. The Mach reflection appears as the wedge angle increases, i.e., viscosity leads to a delay of the transition from regular to Mach reflection.

This note presents a promising shock tube system to study the motion of a solid body induced by a propagating shock wave. The system consists of a horizontally-placed shock tube with a spring-loaded knife-edge to rupture the diaphragm separating the driver and driven sections. It also includes a solid-body injecting device installed on the floor of the test section. The injection timing of the solid body and the diaphragm rupture are synchronized using a digital delay circuit. Key features include suspending a solid body in midair without supports and adjusting its posture, allowing precise control of initial conditions and enabling more detailed shock wave interaction studies. A series of experiments with a rectangular solid body in three different initial postures successfully demonstrated the system’s capabilities.

Weighted-type finite-difference schemes are a class of widely used nonlinear schemes that can capture strong discontinuities accurately and efficiently. For the Euler equations without source terms, poor convergence of weighted-type schemes is a widely known difficulty in finding steady-state solutions with strong shock waves. The primary reason for this lies in the fact that classical weighted-type schemes produce spurious oscillations near strong discontinuities. Recently, a novel weighted-type scheme has been developed. The nonlinear weights of the new scheme are fourth-order accurate and do not reduce the accuracy at the high-order critical points, which is beneficial for steady-state convergence. In this paper, we compare the convergence performances of classical and new weighted-type schemes in detail. Several benchmark problems containing shock waves, contact discontinuities, and rarefaction waves were used to compare the convergence performance among different weighted-type schemes. The results show that the new weighted-type scheme basically eliminates slight post-shock oscillations, and the residual settles to machine zero. Compared to classical weighted-type schemes, the steady-state convergence performance of the new weighted-type scheme is significantly improved.

Reduced-scale experiments offer a controlled and safe environment for studying the effects of blasts on structures. Traditionally, these experiments rely on the detonation of solid or gaseous explosive mixtures, with only limited understanding of alternative explosive sources. This paper presents a detailed investigation of the blasts produced by exploding aluminum wires for generating shock waves of controlled energy levels. We meticulously design our experiments to ensure a precise quantification of the underlying uncertainties and conduct comprehensive parametric studies. We draw practical relationships of the blast intensity with respect to the stand-off distance and the stored energy levels. The analysis demonstrates self-similarity of blasts with respect to the conventional concept of the scaled distance, a desirable degree of sphericity of the generated shock waves, and high repeatability. Finally, we quantify the equivalence of the reduced-scale blasts from exploding wires with high explosives, including TNT. This experimental setup and the present study demonstrate the high degree of robustness and effectiveness of exploding aluminum wires as a tool for controlled blast generation and reduced-scale structural testing.

Velocity field measurements by means of PIV are used in this work to characterize the flow in a shock wave–boundary layer interaction. For a free-stream Mach number of (M_infty =2.56), the flow over a flat plate model is deflected by a (16^circ ) wedge. For these flow conditions, an unsteady dual-state solution is observed where the shock switches between a regular reflection and a Mach reflection. This non-periodic mode switching is atypical for a shock wave–boundary layer interaction and causes significant changes in the flow field. The PIV measurements enable the Mach number and the flow direction to be determined from the measured velocity. In this way, both the position of the shocks and the flow deflection across the shocks can be reliably identified. Our analysis shows that regular reflection rarely occurs and that Mach reflection with varying Mach stem height is present for about (85%) of the measurement time. We provide evidence here that the transition to regular reflection is related to a temporarily thickening of the boundary layer ahead of the shock interaction, which is caused by the breathing of the separation bubble below the shock interaction. This phenomenon results in compression waves that alter the Mach number and flow direction in the region upstream of the shock system, enabling a momentary transition to a regular reflection.

Detonation propagation dynamics in circularly curved channels are investigated using both experimental and geometric modeling approaches. Quasi-two-dimensional curved channels with a range of channel widths and curve radii were tested. Experimentally, three propagation modes were observed: a stable propagation mode featuring a flat detonation front and steady near-CJ propagation, an unstable mode with varying frontal structures and velocity oscillations, and failure to propagate. Experimental data from the current study and those in the literature show that for a given ratio between channel width and detonation cell width, there exists a critical inner-to-outer radius ratio that sets apart the stable and unstable propagation modes. A regime map is proposed in the present work to describe the observed propagation modes. The regime map highlights the competition between the focusing effect of the outer concave boundary (with respect to the transverse waves) and the diverging effect of the inner convex boundary in addition to the effect from the channel-to-cell width ratio. With a reduced channel-to-cell width ratio, the inner-to-outer radius ratio critical to sustained detonation propagation must increase. Geometric modeling results are found to be in agreement with experimental observations. In addition, geometric modeling was used to test channel geometries beyond what has been experimentally tested and to provide a rational explanation for the regime map.

As the detonation product cloud from a high explosive detonation expands, an arresting flow is generated at the interface between these products and the surrounding air. Eventually this flow forms an inward-travelling shock wave which coalesces at the origin and reflects outwards as a secondary shock. Whilst this feature is well known and often reported, there remains no established method for predicting the form and magnitude of the secondary shock. This paper details an empirical superposition method for modelling the secondary shock, based on the physical analogy of the secondary loading pulse resembling the blast load from a smaller explosive relative to the original. This so-called dummy charge mass is determined from 58 experimental tests using PE4, PE8, and PE10, utilising Monte Carlo sampling to account for experimental uncertainty, and is found to range between 3.2–4.9% of the original charge mass. A further 18 “unseen” datapoints are used to rigorously assess the performance of the new model, and it is found that reductions in mean absolute error of up to 40%, and typically 20%, are achieved compared to the standard model which neglects the secondary shock. Accuracy of the model is demonstrated across a comprehensive range of far-field scaled distances, giving a high degree of confidence in the new empirical method for modelling the secondary shock from high explosives.

The induction time in shock tubes with different surface roughnesses and different mixture densities was measured, local features of self-ignition were described, and the results obtained were compared with the results for tubes with other diameters in order to determine the effect of gasdynamic parameters on the formation of ignition kernels and ignition in general. It was discovered that ignition at temperature range of 904–1200 K for (rho _{textrm{5}} = 2.80,{hbox {kg/m}}^{textrm{3}}) and 1020–1120 K for (rho _{textrm{5}} = 1.53,{hbox {kg/m}}^{textrm{3}}) is determined by the ignition kernel that forms near the tube axis and is a consequence of the gasdynamic effect at the tube axis (axial effect), but is not explained by the adiabatic compression of the mixture due to the expansion of gas from the reflected shock wave bifurcation stagnation region. An increase in the size of the bifurcation structure due to an increase in tube surface roughness does not affect ignition at these temperatures, but expands the ignition range at lower temperatures, in which multi-kernels or volumetric ignition is observed.