Shock Waves最新文献

An important phenomenon in the gas–liquid two-phase mixtures is a sudden change in the flow that may lead to transition of the flow regime from non-homogeneous slip flow to homogenous flow. This phenomenon is called mixing shock and has been investigated by several researchers. In the present paper, a more comprehensive model is proposed by including the entrainment ratio in the governing equations. Moreover, parametric studies are performed to assess the importance of this parameter in different conditions. The results of the present study indicate that at Euler numbers less than four the effect of gas mass flow is insignificant. However, at higher Euler numbers the differences of the present and previous models are noticeable. Since two possible solutions for the aftershock state exist, four criteria—entropy change across the shock, possibility of an expansion shock, positivity of the Euler number, and choking flow condition—are considered to identify the correct solution. The results indicate that the flow after the shock could only be of a subsonic type, and the mixing shock is compressive. A comparison of the possible realizable zones for the developed model and the previous models indicates that the developed model predicts a larger area for the realizable solution of the mixing shock. A comparison between analytical results and experimental data shows that the developed model predicts reasonable results.

Multiple elastic shock waves carry the information on elastic properties under dynamic extreme conditions, but may complicate the interpretation of wave structure including the elastic–plastic transition. On the basis of the acoustic wave-equation analysis, we predict the absence or presence of multiple elastic shock waves in a single crystal subjected to shock loading along a specific crystallographic orientation. Typical FCC and BCC single crystals are taken as validation and application cases. Large-scale molecular dynamics simulations are performed for Cu and Ta; double-wave or triple-wave structures of elastic shock waves (quasilongitudinal and quasitransverse) are observed in the simulations, and the multi-wave structures are in excellent agreement with the wave-equation analysis. Also, the acoustic wave-equation analysis is used to analyze MD calculations, as well as the complex structure of the shock wave during plastic deformation. Free-surface velocity history, transverse velocity history of free surface, and ultrafast X-ray diffraction are explored as experimental means to resolve multiple elastic shock waves.

This study aims to validate the new developments in our in-house spectroscopic code (KAT-LIF) to perform NO-LIF simulations for detonation conditions, as well as evaluating the capabilities of the NO-LIF diagnostic for characterizing H(_2)-air detonations. This objective was achieved in several steps. First, our in-house spectroscopic tool, KAT-LIF, was updated to perform NO-LIF simulations by notably developing a database of NO(A-X) transitions, currently unavailable in conventional spectroscopic databases, as well as collecting and implementing species-specific line broadening, line shifting, and quenching parameters for NO-LIF. Second, the validation of KAT-LIF was performed by comparing the simulation results with pre-existing simulation tools (LIFSim and LIFBASE) and experimental NO-LIF measurements in a laminar CH(_4)-air flame and H(_2)-air detonation. The validation results present satisfactory agreement of KAT-LIF and other simulation tools (LIFBASE, LIFSim) with experimental results for several conditions. For example, less than 20% discrepancy between the simulated and experimental NO-LIF profiles is observed for stoichiometric H(_2)-air detonation, initially at 20 kPa and 293 K. Third, qualitative and quantitative capabilities of the NO-LIF technique for detonation characterization are discussed, which include: shock detection, induction zone length measurements, and quantitative number density measurements.

The aim of the present paper was to report on an experimental study of the characterization of a blast wave initiated by a solid explosive and its interaction with a rigid obstacle in the form of a hemicylinder. Pressure transducers located along the path of the blast wave and high-speed imaging allow (1) the measurement of the overpressure at different locations and (2) the characterization of the blast wave inception, propagation, and reflection off the hemicylinder. The scaling effect has been investigated by performing experiments in two different facilities, where one is at twice the scale of the other.

In the present work, operation characteristics of a disk-type rotating detonation engine (DRDE) with a constant chamber area were experimentally studied for various total mass flow rates and a wide variety of equivalence ratios of hydrogen–air mixtures. From the direct visualizations, the rotating detonation wave was found to propagate near the outer wall of the combustion chamber, regardless of the wave mode. For the present test conditions, single- and double-wave modes are observed, depending on the equivalence ratio of the mixture. The pressure gain was evaluated based on a one-dimensional flow model together with the chamber static pressure measured with the capillary tube average pressure technique. Although the present DRDE configuration provided a negative pressure gain for all the test conditions, it was found that the single-wave mode was superior to the double-wave mode in terms of the pressure gain.

Pre-ignition is an undesired combustion event known to restrict kinetic modeling validation. Previous methane oxidation studies reported premature ignition as part of ignition delay time measurements in shock tubes. In this context, the effect on the pre-ignition propensity and auto-ignition behavior of stoichiometric methane mixtures at different dilution levels of (hbox {N}_2), Ar, He, and (hbox {CO}_2) was studied at 10 bar and 25 bar and temperatures between 1080 K and 1350 K. In addition to conventional sidewall pressure and endwall light emission measurements, a high-speed imaging setup was utilized to visualize the ignition process. Relevant physicochemical parameters to describe and predict the pre-ignition phenomenon were used. The results suggest that dilution levels up to (80%) of bath gas are not successful in mitigating early ignition occurrence and its effects at moderate pressures. Replacing (hbox {N}_2) by He was found to suppress early ignition at 10 bar, attributed to an enhanced dissipation of temperature inhomogeneities in the test gas section. The present findings demonstrate that (hbox {CO}_2) has potential for pre-ignition heat release mitigation, while Ar was confirmed to promote premature ignition. To the best of our knowledge, we present the first detailed study on pre-ignition mitigation for methane mixtures in shock tubes, where further insights into its ignition non-idealities are given.

The deformation and breakup characteristics of liquid rocket propellant 2 (RP-2) droplets are experimentally investigated in a shock tube. The RP-2 droplets are subjected to a weak shock wave, a strong shock, and a detonation wave to deduce the impacts of high-speed and supersonic reacting flows on droplet deformation and breakup. High-speed shadowgraph and schlieren imaging techniques are employed to characterize droplet morphologies, deformation rates, and displacement of the droplet centroid. The results reveal that the transition from a shock wave to a detonation suppresses the deformation of the droplet and augments small-scale breakup. A shift in dominant breakup mechanisms is linked to a significant increase in the Weber number due to an increase in flow velocities and temperatures when transitioning to the detonation case. The experimental data are combined with a droplet stability analysis to predict the “child” (or fragments of the initial “parent” droplet) droplet sizes of each test condition. The child droplet size is shown to decrease as the flow regime transitions toward a detonation. An analytical mass stripping model was also used to determine that the total mass stripped from the parent droplet increased when approaching supersonic reacting conditions. The child droplet sizes and mass stripping rate will ultimately influence evaporation timescales and ignition in supersonic reacting flows, which is important for the development of detonation-based propulsion and power systems.

A new reflected shock tunnel capable of generating hypersonic environments at realistic flight enthalpies has been commissioned at Sandia. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight-enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25-cm core flow with 4.6-MJ/kg total enthalpy is achieved over an approximately 1-ms test time. The condition was refined using analysis and a heavier piston, leading to an increase in test time. A novel high-speed molecular tagging velocimetry method is applied using in situ nitric oxide to measure the freestream velocity of approximately 3016 m/s. Companion simulation data show good agreement in exit velocity, pitot pressure, and core flow size.

When departure from the ideal gas equation of state is considered, the Noble–Abel stiffened gas model is an appealing and versatile candidate due to its simple form. The linear interaction approximation formalism is extended to consider non-ideal gas effects introduced by this equation of state. Kovásznay decomposition and adequate definition of the energy of disturbances are provided in the context of this equation of state. Changes with respect to ideal gas are investigated on transfer functions, critical angle, and compression factor. Those differences yield concrete effects on the damping and transfer of fluctuations across shock waves. Those changes are further illustrated by considering the interaction of an entropy spot with a Mach 3 stationary shock wave.