Multiple Shock Interactions Research Articles

A STUDY OF SHOCKWAVE ATTENUATION IN A CHANNEL WITH RIGID OBSTACLE MATRICES

Development of protective barriers for structures against blast wave requires identification of optimum geometrical arrangement that gives the best attenuation characteristics. This paper highlights the comparative assessment towards shock attenuation capabilities of different arrangements of geometries in terms of pressure load dispersion through numerical methods. The simulations were based on Reynolds average Navier-Stokes equations with SST-kω turbulent model and the important flow features that enhance attenuation of the shock wave were captured. The results shows that the flow domain behind the shock front experienced multiple shock interactions that involved shock diffraction and shock reflection. A complex structure with distinct flow features that evolved within the perturbed region strongly affects the shape of the shock. A comparative analysis of geometries shows similar flow features, a hybrid V-shaped obstacle (H-OG), gives the best attenuation but with large flow blockage. The concave-convex domain with circular thin wall (C-OG) gives good attenuation capability with minimum flow blockage but its major setback is design and fabrication. Incorporating V-shaped obstacles into protective devices will greatly reduce the destructive hazards inherent in blast or shock waves.

Numerical modeling of imposed magnetohydrodynamic effects in hypersonic flows

Weakly ionized plasmas, formed in high enthalpy hypersonic flows, can be actively manipulated via imposed magnetic fields—a concept termed magnetohydrodynamic (MHD) flow control. Imposed MHD effects, within flows that exhibit multiple shock interactions, are consequential for emerging aerospace technologies, including the possibility of replacing mechanical control surfaces with magnetic actuation. However, numerical modeling of this flow type remains challenging due to the sensitivity of feature formation and the real gas modeling of weakly ionized, electrically conductive, air plasma. In this work, numerical simulation capabilities have been developed for the study of MHD affected, hypersonic flows, around two-dimensional axisymmetric non-simple geometries. The validated numerical methodology, combined with an advanced 19 species equation of state for air plasma, permits the realistic and efficient simulation of air plasmas in the equilibrium regime. Quantitative agreement is achieved between simulation and experiment for a Mach 5.6 double cone geometry with applied magnetic field. In the context of the magnetic actuation concept, numerical studies are conducted for varied conical surface angle and magnetic field configuration. For simple geometries with an elemental shock type, the MHD enhancement effect produces a self-similar shock structure. This paper demonstrates how, for hypersonic flows with complex shock interactions, the MHD affected flow is not only augmented in terms of shock position but may exhibit topological adaptations in the fundamental flow structure. A classification system is introduced for the emergent flow topologies identified in this work. Fluid-magnetic interactions are explored and explained in terms of the coupled mechanisms leading to (1) differences in magnitude of MHD enhancement effect and (2) structural adaptations of the flow topology. The applied numerical studies examine why increased conical surface angle does not amplify the MHD enhancement effect as expected from the base flow conditions, and the mechanisms by which the magnetic field configuration influences the MHD augmented shock structure. Most critically, classes of conditions are identified that produce topological equivalence between the magnetic interaction effects and a generalized mechanical control surface.

Shock capturing for discontinuous Galerkin methods with application to predicting heat transfer in hypersonic flows

This study is concerned with predicting surface heat transfer in viscous hypersonic flows using high-order discontinuous Galerkin (DG) methods. Currently, finite-volume (FV) schemes are most commonly employed for computing flows in which surface heat transfer is a target quantity; however, these schemes suffer from large sensitivities to a variety of factors, such as the inviscid flux function and the computational mesh. High-order DG methods offer advantages that can mitigate these sensitivities. As such, a simple and robust shock capturing method is developed for DG schemes. The method combines intraelement variations for shock detection with smooth artificial viscosity (AV) for shock stabilization. A parametric study is performed to evaluate the effects of AV on the solution. The shock capturing method is employed to accurately compute double Mach reflection and viscous hypersonic flows over a circular half-cylinder and a double cone, the latter of which involves a complex flow topology with multiple shock interactions and flow separation. Results show this methodology to be significantly less sensitive than FV schemes to mesh topology and inviscid flux function. Furthermore, quantitative comparisons with state-of-the-art FV calculations from an error vs. cost perspective are provided.

Dynamical behavior of the Richtmyer–Meshkov instability-induced turbulent mixing under multiple shock interactions

The dynamical behavior of Richtmyer–Meshkov instability-induced turbulent mixing under multiple shock interactions is investigated by large-eddy simulation. After the initial shockwave–interface interaction, the transmitted wave reverberates between the accelerated interface and the end-wall of the shock tube to form a process of multiple shock interactions. The turbulent mixing zone grows in a different manner under each of the impingements. After the initial shock, it grows as a power law of time. After the reshock and the impingement of the reflected rarefaction wave, it grows with time as a different negative exponential law. When the impingement of the reflected compression wave completes, it grows approximately in a linear fashion. The statistical quantities in the turbulent mixing zone evolve with time in a similar way under multiple impingements, and after the impingement of the reflected compression wave, they all decay asymptotically. Therefore, the turbulent mixing zone behaves in a statistically self-similar pattern. Even though the impingements of different waves result in different abrupt changes of the characteristic scale parameters of mixing turbulence, as a whole, the characteristic scales present a feature of growth, and the characteristic-scale Reynolds numbers present a feature of decay. The mixing flow is continuously anisotropic, yet the anisotropy weakens gradually. Therefore the development of turbulent mixing presents a trend of isotropy.

Effects of the interaction and evolution of interplanetary shocks on “background” solar wind speeds

Solar wind speed plays an important role in the study of space weather prediction. Some workers have used it for measuring the arrival time of solar disturbances at 1 AU. The purpose of this work is to extend our previous study (Wu et al., 2005c) of some Halloween 2003 events by presenting additional physical effects of multiple shock interactions on the solar wind profile during a complex compound event. In order to achieve this goal, we track a group of specific solar events' plasma and magnetic field output as they propagate into interplanetary space. A one‐dimensional, time‐dependent adaptive grid MHD code is used to study the evolution and interaction of shocks from Sun through the heliosphere. The MHD simulation results demonstrate that the solar wind speed might increase about ≈25% after two shocks collide with each other. This kind of interaction can affect the accuracy of the identification of the solar source that causes the interplanetary event (e.g., magnetic cloud, coronal mass ejection, interplanetary shock, or some other interplanetary discontinuity.) In this study we further simulate part of the famous Halloween 2003 events that contain at least four major solar events (flares) during 28 October to 1 November 2003. These major events, simulated by pressure pulses, generated shocks that matched well with ACE (Advanced Composition Explorer) observations as reported by Wu et al. (2005c) in our previous study. The present work presents new details concerning the interplay (such as sunward and antisunward traveling compression and rarefaction waves) between fast forward and fast reverse interplanetary shocks.

Reignition of detonations by reflected shocks

Numerical simulations are used to study the diffraction, decay, and reignition that occurs when a detonation propagates past an increase in cross-sectional area in a rectangular tube. The computations solve the time-dependent two-dimensional equations describing a reactive flow in an argon-diluted stoichiometric hydrogen-oxygen mixture at atmospheric pressure. Previous studies have shown that soon after transmission to a larger area, the reaction front decouples from the leading shock and forms a decaying blast wave (“bubble”) in the larger tube. Then, depending on the initial conditions, the detonation either continues to decay or is reignited as the bubble reflects off confining surfaces. For a strongly overdriven initiating detonation, reignition occurs through an interaction between the bubble and the original contact surface. For a more weakly driven system, reignition can occur in two ways: either in the slip line and Mach stem of the Mach reflection formed when the bubble reflects off the bottom surface of the tube, or by multiple shock interactions that occur when the reflected bubble overtakes the initial detonation front. The computations show the evolution and development of the cellular structure of the steady detonation front.

The role of magnetic loops in particle acceleration at nearly perpendicular shocks

We consider a system in which a planar shock propagates into a plasma where the magnetic field consists of a uniform background component that is nearly parallel to the shock surface (i.e., the shock is nearly perpendicular) plus a random component that is transverse to the background field. The random component is a superposition of Alfvén waves with a wide range of wavelengths and with amplitudes determined from a power spectral density function that is flat at long wavelengths and a power law at short wavelengths. We investigate the characteristics of superthermal ion (proton) distributions accelerated in this system by integrating along particle orbits. The main difference between results reported herein and those reported previously by us is the major role played by magnetic loops in mediating multiple shock interactions. The transverse magnetic fluctuations are composed of short wavelength components that scatter the ions in pitch angle, and long wavelength components that produce a large‐scale spatial meandering of magnetic field lines. At nearly perpendicular shocks, this meandering results in a distribution of transient magnetic loops of various sizes at the shock. Charged particles are guided by the large‐scale spatial variations and can interact with the shock at many points, becoming temporarily trapped along several such loops, and also being scattered by resonant waves during and between shock interactions. A few particles can acquire large energy gains before the field lines along which they propagate convect through the shock. As a specific application of the model, we investigate the acceleration of a monoenergetic seed proton population for both impulsive and continuous time injection at a shock that is on average perpendicular. In the model the typical energy density of the magnetic fluctuations is 10–20% that of the background field. Characteristic features of the accelerated proton distributions are (1) energy spectra with power law tails extending to at least 1000 times the injection energy, with spectral slopes somewhat steeper than those predicted, for example, by diffusive shock acceleration theory; (2) upstream intensity precursors that grow exponentially towards the shock, with e‐folding scales that increase with energy up to a maximum scale of the order of the spatial scale of the meandering field lines; (3) upstream angular distributions with large field‐aligned bidirectional anisotropies throughout the intensity precursor; (4) downstream angular distributions with weaker anisotropies peaked transverse to the magnetic field within an intensity plateau extending from the shock to the convection boundary, and field‐aligned bidirectional distributions beyond the convection boundary. We develop a simplified model that explains the salient features of the upstream precursors in terms of the statistical properties of the long‐wavelength components that comprise the random field and suggest how electron acceleration can be accommodated in our model.

Multiple transonic shock-wave/turbulent boundary-layer interaction in a circular duct

An experimental study is described in which detailed pitot, static, and wall pressure measurements have been obtained for multiple transonic shock-wave/turbulent boundary-layer interactions in a circular duct at a freestream Mach number of 1.49, a unit Reynolds number of 4.90xl0/m, and a blockage of 5.15%. The details of the flowfield show the formation of a series of normal shock waves with successively decreasing strength and decreasing distance between the successive shock waves up to the point where a terminal shock occurs. A one-dimensional flow model based on the boundary-layer displacement thickness is postulated to explain the formation of the series of normal shock waves. A comparison with the results from our previous study involving a single shock interaction suggests that the effect of increased blockage is to promote multiple shock interactions and produce a lower pressure recovery, a less retarded boundary-layer flow, and an increase in the overall length of the interaction.

Integrated Scramjet Nozzle/Afterbody Performance Analysis

A two-dimensiona l flowfield computer program developed for preliminary performance analysis of integrated scramjets is presented. Scramjet flowfields are computed by means of the shock-capture technique, with real gas thermodynamic properties for frozen and equilibrium compositions. The program computes internal and external flowfields with multiple shock interactions. Interaction of the exhaust (underexpanded or overexpanded) with the external stream and vehicle afterbody also is considered. Forces and moments are computed due to stream thrust and external surface pressure distributions. Typical program results for parametric nozzle/afterbody design and off-nominal performance analysis are presented. N recent years, increased attention has been given to the application of supersonic combustion ramjets (scramjets) to airbreathing launch vehicles, advanced civil aviation, and hypersonic research aircraft. One of the scramjet technology areas requiring particular attention is the interaction of the airbreathing propulsion system with the vehicle aerodynamics and the resulting effects on the overall vehicle performance. Particularly significant and as yet not well defined is the interaction of the vehicle afterbody and the exhaust of the integrated scramjet.M The integrated scramjet propulsion system utilizes the vehicle afterbody as an extension of the nozzle, producing increased effective exhaust velocities and thrust at the expense of incurred trim penalties. Therefore, a requirement exists for the means to define accurately scramjet nozzle/afterbody exhaust flowfields. A critical review of this problem indicated that detailed analysis should include consideration of real gas three-dimensional flowfields with chemical-kinetic effects, multiple shocks, heat transfer, boundary layer growth, and interaction with the inviscid flow. In view of the complexity of the problem, simplified practical methods of analysis must be developed which retain the most essential features of the flow phenomena, fulfill preliminary design requirements, and produce realistic results. At the present time, the computer programs of Refs. 5 and 6 appear to be the only published programs specifically developed to meet this requirement. These programs consider inviscid and adiabatic quasi-three-dimensional flowfields with real gas equilibrium hydrogen-air chemistry. These programs are based on the reference plane technique, with a table lookup thermodynamic properties routine. This paper represents a simpler scramjet flowfield analysis computer program suitable for preliminary design analyses. Since the majority of the current scramjet configurations are of the modular quasi-two-dimensional type, it is felt that for preliminary design .purposes the flowfield can be described sufficiently well by means of a two-dimensional program. The two-dimensional scramjet flowfield program7 considers inviscid and adiabatic supersonic flow in ducts and coflowing dissimilar streams with real or ideal gas properties. The methodology used in this program is the shock capture technique (SCT) described in Refs. 8 and 9. Thermodynamic properties are evaluated by means of a special-purpose sub

Experimental Investigation of Hypersonic Buzz on a Delta Configuration

N experimental study is described which confirms the existence of hypersonic buzz in the separated region of a downward deflected control surface. Wind-tunnel tests in helium at M = 17.5 clearly demonstrated the occurrence of large-scale pressure oscillations throughout a substantial range of angles of attack and control surface deflections on a blunt slab, delta configuration. Measurements of surface pressure fluctuations and observations of high-speed schlieren photographs served to delineate the boundaries of the phenomenon and to identify the physical parameters necessary for the onset of a buzz condition. Prominent spectral energy peaks in surface pressures were observed and correlated with the instability. These conditions were found to be strongly dependent on the presence of multiple shock interactions relative to the location of a reattaching shear layer.