Shock Waves最新文献

A comprehensive and systematic, computational investigation is presented on the effect of wall roughness on the supersonic flow over a circular cylinder with a Reynolds number of 500. Flow simulations are conducted using ANSYS Fluent. Wall roughness is modeled by a perturbation of the cylinder geometry with harmonic modes of varying amplitude and frequency. Validated smooth cylinder flow simulations for a range of Mach and Reynolds numbers with slip and no-slip wall serve as a reference. Roughness is shown to increase the effective diameter of the cylinder and the drag by displacing the outer flow along the peaks of the roughness elements. For lower frequencies, this effect is less pronounced than for higher roughness frequencies. While for smooth cylinders the vorticity is mostly generated by viscous shear forces, for rough cylinder the baroclinic vorticity generation is shown to be dominant and shown to determine the topology of the recirculating region.

An experimental comparative study of the detonation re-initiation downstream of an orifice plate and the typical deflagration-to-detonation transition in a smooth tube is carried out. In this study, two tube configurations are employed to study the onset of detonation in stoichiometric methane–oxygen mixtures, i.e., a smooth tube and a tube with a single orifice plate placed in the entrance of the self-sustained detonation transmission. Combustion wave velocity measurement and soot-foil visualization are used to characterize the initiation of detonation. The dimensionless parameters correlated with cell size, tube diameter, and orifice diameter are introduced to analyze the detonation initiation process. The results indicate that the dependence of the detonation initiation distance on the initial pressure as a whole is close to inverse proportionality, and the fitting degree is higher for the detonation re-initiation downstream of the orifice plate. The effect of inherent instability of CH(_{4})–2O(_{2}) on the onset of detonation is significantly enhanced when the cell size is smaller than the characteristic dimension of an unobstructed tube, either for deflagration-to-detonation transition in a smooth tube or for the detonation re-initiation downstream of an orifice plate.

The present study aims to investigate the reflection and refraction of a curved shock front as it slides along an air–water interface, using the time-resolved shadowgraph technique. The curved shock front is generated from a free-piston shock tube. The study successfully captured the propagation of a refracted shock wave in water along with that of the reflected shock wave in the air. The refracted shock moves much faster than the incident shock due to a higher acoustic speed in the water. It is seen that the reflected shock initially exhibits a regular reflection (RR), which then transitions to a Mach reflection (MR) as it propagates along the interface. As the shock wave propagates along the air–water interface, the incident shock wave angle with the interface keeps on increasing, leading to RR–MR transition. Shock polar analysis shows that as the Mach reflection structure propagates further along the interface, it transitions from a standard Mach reflection to a non-standard Mach reflection. It is seen that the distance the shock wave propagates along the interface before it transitions from RR to MR increases with the increase in the interface distance (distance between the water surface and the shock tube axis). It is also found that the reflection surface (water or solid) does not seem to have a significant effect on the shock transition criterion, especially the distance at which the shock wave transitions from RR to MR.

An Excel(copyright ) interface has been developed to provide the physical properties of blast waves produced by surface-burst TNT explosions. The format is identical to that of a previous interface that provides the properties of blast waves from free-field TNT explosions. These interfaces have been developed to replace the information previously provided by a program named (text {AirBlast}^{circledR }), which is no longer compatible with modern operating systems. Excel(copyright ) has been chosen as the platform for the new interfaces because it is widely available, and experience has shown that its files have remained compatible with all operating systems as they have been upgraded. The TNT surface-burst interface has been developed using the same database of experimental measurements as (text {AirBlast}^circledR ), which was gathered from an analysis of the blast waves from over 300 explosions, ranging in size from 4 kg to 500 t. The availability of the data from the free-field and the surface-burst interfaces has permitted a comparison between the two types of blast waves, and in particular the confirmation of previous measures of the reflection factor for surface-burst explosions.

Drag reduction using aerospikes has been explored extensively due to the consequences associated with the range, manoeuvrability, and structural limitations of supersonic vehicles. The objective of the present work is to introduce steps as a novel aft-geometry configuration for a sharp-tipped aerospike to enhance drag reduction. The conventional and stepped spikes are of aspect ratio 1.5. Axisymmetric viscous flow simulations and wind-tunnel tests are conducted at a Mach number of 2.43 to analyze the drag reduction phenomena. The viscous simulations provide insight into the shock structure and the recirculation zones. Schlieren images obtained from the experiments in the wind tunnel reveal that the shock angles and locations are in reasonable agreement with the viscous flow simulations. The stepped geometry introduces multiple shocks which eventually reduce the strength of the reattachment shock. A detailed comparison of the location of the steps reveals the effect of the recirculation zones and the interaction of oblique shocks and expansion fans on the extent of drag reduction. The simulations indicate that an enhanced reduction in the wave drag ranging from 9.3 to 21.1% may be achieved over a conventional aerospike as the step locations are varied. The maximum drag reduction potential offered by the steps may be realized in practice using an actively adapting telescopic aerospike.

Modeling of the chemistry and thermodynamics is crucial in numerical simulations that attempt to accurately simulate reactive flows such as flame acceleration and detonation phenomena. The current study explores how a four-species, four-step combustion mechanism performs to predict ignition processes in various premixed hydrocarbon fuel mixtures when compared to detailed chemical kinetic mechanisms. A key objective of this research is to determine how well this model, which has been modified to include only three species transport equations, performs at predicting fundamental combustion properties that are important for flame acceleration and detonation applications. On comparison with full chemistry mechanisms, the four-step model demonstrates an ability to predict the ignition time, reaction stiffness, thermodynamic state, and detonation stability-parameter to a high level of accuracy, for ignition processes over a wide range of initial temperatures and densities. With the ignition structures and key detonation stability parameters correctly predicted, we conclude that the four-step model is an effective and economic tool for studying complex explosion phenomena in situations where pre-combustion temperature and density are constantly changing, such as deflagration-to-detonation transition by flame acceleration or shock–flame interaction.

The safe operation and performance of a mixed compression air intake critically depend on the nature of shock wave/boundary layer interactions (SBLIs). The interaction between the ramp boundary layer and the cowl shock at the ramp–isolator junction plays a key role. In this experimental study, a modified backward-facing step called “notch” is used at the ramp–isolator junction to control the SBLI in a rectangular intake at Mach 3. The unstart and performance characteristics are evaluated and compared with the baseline, “faceted” configuration. The intake was unstarted by varying the back-pressure using a choke flap located at the exit in a quasi-steady manner. The surface and rake pressure measurements in addition to the shadowgraph and oil flow visualizations were taken to characterize the effect of flow control. The results showed that the notch anchors the separation bubble at the ramp–isolator junction and helps mitigate the strength of SBLI. The notched intake static pressures are relatively lower as compared to the baseline configuration suggesting reduced severity of structural loads. The floor boundary layer is energized by the notch leading to better efficiency and flow uniformity. There is an increase in the margin of unstart due to the presence of the notch by 7–10%.

The violent eruption of the volcano at Hunga Tonga–Hunga Ha‘apai island on January 15, 2022, generated an intense pressure wave registered by instruments all over the world. Using public reports posted on social media, we have used the arrival time of the first passage of the wave to measure its velocity, found to be a constant (1114pm 2) km/h ((309pm 1) m/s). An empirical pressure–distance relation that utilizes measurements from a large range of sources is used to estimate an energetic output. We find that this Hunga Tonga–Hunga Ha‘apai volcanic eruption released approximately the equivalent of 61 Mt, which is considerably larger than the 1980 eruption of Mount St. Helens and slightly higher than the yield of Tsar Bomba, the largest human-made explosion in history.

Measurements of the time of arrival of shock waves from explosions can serve as powerful markers of the evolution of the shock front for determining crucial parameters driving the blast. Using standard theoretical tools and a simple ansatz for solving the hydrodynamics equations, a general expression for the Mach number of the shock front is derived. Dimensionless coordinates are introduced allowing a straightforward visualization and direct comparison of blast waves produced by a variety of explosions, including chemical, nuclear, and laser-induced plasmas. The results are validated by determining the yield of a wide range of explosions, using data from gram-size charges to thermonuclear tests.