Oblique shock

About: Oblique shock is a research topic. Over the lifetime, 6551 publications have been published within this topic receiving 119823 citations.

Oblique detonation wave engine performance prediction

Abstract: The performance of an oblique detonation wave engine (ODWE) is evaluated and compared to a diffusive scramjet engine. Diffusive scramjet performance is predicted for both constant area and constant pressure combustion. Detonation wave calculations that utilize a perfect gas approximation vs a real gas model indicate the inadequacy of a perfect gas approach. Hence, a real gas model is utilized hi which the flow is in chemical equilibrium after combustion. Results are provided for both frozen and equilibrium flow through the expansion waves and nozzle. Performance parameters are calculated for H2 and CHi, a range of freestream Mach numbers and fuel flow rates, various inlet configurations, and altitude. Results are for specific thrust and a normalized transverse engine size. The ODWE analysis shows the importance of including the Taylor wave, done here for the first time. An ODWE gives comparable performance to diffusive engines and offers several advantages. The smaller longitudinal size of the ODWE produces less drag and less engine heat transfer. The ODWE offers design flexibility, since the wedge angle that controls the detonation wave can be varied to provide optimum performance over the hypersonic flight envelope. Wedge angle variation also allows for an easy start and restart capability. Nomenclature A = area a = speed of sound 7sp = specific impulse / = length M = Mach number m - mass flow rate p - pressure q = normalized heat addition #00 = dynamic pressure R = specific gas constant T = temperature T = thrust V = flow speed j8 = shock angle j8wlgn = minimum shock angle for ignition y = specific heat ratio 0 = flow turn angle p = density = equivalence ratio

Impact of the rippling of a perpendicular shock front on ion dynamics

Abstract: [1] Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi-perpendicular shock can be rippled. Recent two-dimensional simulations have focused on two different types of shock front rippling: (1) one characterized by a small spatial scale along the front is supported by lower hybrid wave activity, (2) the other characterized by a large spatial scale along the front is supported by the emission of large amplitude nonlinear whistler waves. These two rippled shock fronts are self-consistently observed when the static magnetic field is perpendicular to (so called “B0-OUT” case) or within (so called “B0-IN” case) the simulation plane, respectively. On the other hand, several studies have been made on the reflection and energization of incoming ions with a shock but most have been restricted to a one dimensional shock profile only (no rippling effects). Herein, two-dimensional test particle simulations based on strictly perpendicular shock profiles chosen at a fixed time in two-dimensional Particle-in-cell (PIC) simulations, are performed in order to investigate the impact of the shock front ripples on incident ion (H+) dynamics. The acceleration mechanisms and energy spectra of the test-ions (described by shell distributions with different initial kinetic energy) interacting with a rippled shock front are analyzed in detail. Both “B0-OUT” and “B0-IN” cases are considered separately; in each case, y-averaged (front rippling excluded) and non-averaged (front rippling included) profiles will be analyzed. Present results show that: (1) the incident ions suffer both shock drift acceleration (SDA) and shock surfing acceleration (SSA) mechanisms. Moreover, a striking feature is that SSA ions not only are identified at the ramp but also within the foot which confirms previous 1-D simulation results; (2) the percentage of SSA ions increases with initial kinetic energy, a feature which persists well with a rippled shock front; (3) furthermore, the ripples increase the porosity of the shock front, and more directly transmitted (DT) ions are produced; these strongly affect the relative percentage of the different identified classes of ions (SSA, SDA and DT ions), their average kinetic energy and their relative contribution to the resulting downstream energy spectra; (4) one key impact of the ripples is a strong diffusion of ions (in particular through the frontiers of their injection angle domains and in phase space which are blurred out) which leads to a mixing of the different ion classes. This diffusion increases with the size of the spatial scale of the front ripples; (5) through this diffusion, an ion belonging to a given category (SSA, SDA, or DT) in y-averaged case changes class in non-averaged case without one-to-one correspondence.

The structure of normal shock waves in a binary gas mixture

Abstract: A previous study of normal shock waves through numerical experiments with a simulated gas on a digital computer is extended to binary gas mixtures. The mixture is simulated by two sets of rigid elastic sphere molecules with the appropriate mass and diameter ratios. Velocity profile results for medium strength waves in a mixture of equal parts argon and helium are in qualitative agreement with the continuum calculations of Sherman (1960), but there is no initial acceleration of the argon in mixtures containing a very small initial mole fraction of this gas. The temperature profiles are similar to those for the velocity in that the argon profile lags behind the helium profile. However, when there is a small proportion of heavy gas, the profiles cross-over and the temperature of the heavy gas overshoots the Rankine-Hugoniot downstream value. For very strong shock waves, the overall shock thickness expressed in upstream mean free paths becomes larger, but the profiles are generally similar to those for the medium strength waves.

Electron Acceleration in Non-relativistic Quasi-perpendicular Collisionless Shocks

Abstract: The origin of nonthermal emission observed from a variety of astrophysical objects is still a major unresolved issue in plasma astrophysics. Shocks at SNRs, with the help of a universal acceleration mechanism (i.e., diffusive shock acceleration; DSA), are widely believed to be the most probable acceleration sites of galactic cosmic rays (CRs). The underlying assumption of DSA is that only particles with Larmor radius much larger than the shock width can cross the shock and enter the acceleration process. This is especially challenging for thermal electrons due to their small Larmor radii. In non-relativistic quasi-perpendicular shocks without significant proton acceleration, whether electrons can be injected into DSA by self-driven upstream turbulence is not well-addressed in the literature. In this thesis, I try to answer this question by performing large scale particle-in-cell (PIC) simulations and magnetohydrodynamic-particle-in-cell (MHD-PIC) simulations. 1D PIC simulations show that electrons are injected into DSA through repeated cycles of shock drift acceleration (SDA) and the scattering of self-driven upstream waves. Multi-dimensional PIC simulations show different electron acceleration efficiencies with different background magnetic field geometries. 2D out-of-plane shocks are much more efficient in electron acceleration compared to in-plane shocks and the acceleration efficiency in 3D shocks lies in between 2D in-plane and out-of-plane shocks. I demonstrate that both the pre-acceleration at the shock leading edge and the corrugations at the shock ramp affect the electron acceleration efficiency. For the second half of my thesis, I apply MHD-PIC method to study electron acceleration in oblique shocks for larger transverse size and longer time scale. I develop a simple but more realistic electron injection prescription motivated by PIC simulations. The MHD-PIC simulations reproduce the most essential features of the shock structure and electron acceleration process. Quasi-perpendicular shocks can self-regulate how many particles they can take in response to different injection fractions by creating shock corrugations, making MHD-PIC model more robust for studying long term particle acceleration process without a detailed understanding of microphysics. By combing the results from both PIC simulations and MHD-PIC simulations, we can gain more insights into the physics of electron acceleration at different scales in astrophysical systems.

High time resolution plasma wave and magnetic field observations of the Jovian bow shock

Abstract: High time resolution (60 ms) Voyager magnetometer and plasma wave measurements of a strong (fast Mach number 16), quasi-perpendicular Jovian bow shock reveal an abrupt change in the plasma wave spectrum at the leading edge of the shock foot. Upstream electron plasma waves terminate at the leading edge, and are replaced by a lower-frequency broadband spectrum of ion-acoustic-like waves, which terminates at the main shock ramp. The clear association with the foot region of the lower frequency component suggests that it is generated by reflected ions. If the upstream plasma waves are generated by an escaping electron heat flux, their termination at the leading edge suggests that electrons are heated by the low-frequency waves in the shock foot.