Showing papers on "Oblique shock published in 2020"

Modern Compressible Flow: With Historical Perspective

Abstract: 1 Compressible Flow - Some History and Introductory Thoughts 2 Integral Forms of the Conservation Equations for Inviscid Flows 3 One-Dimensional Flow 4 Oblique Shock and Expansion Waves 5 Quasi-One-Dimensional Flow 6 Differential Conservation Equations for Inviscid Flows 7 Unsteady Wave Motion 8 General Conservation Equations Revisited: Velocity Potential Equation 9 Linearized Flow 10 Conical Flow 11 Numerical Techniques for Steady Supersonic Flow 12 The Time-Marching Technique: With Application to Supersonic Blunt Bodies and Nozzles 13 Three-Dimensional Flow 14 Transonic Flow 15 Hypersonic Flow 16 Properties of High-Temperature Gases 17 High-Temperature Flows: Basic Examples Appendix A Appendix B An Illustration and Exercise of Computational Fluid Dynamics

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.

Hypersonic Transitional Shock-Wave-Boundary-Layer Interaction on a Flat Plate

Abstract: This work presents an experimental and numerical study of hypersonic transitional shock-wave–boundary-layer interaction, wherein transition occurs between separation and reattachment in the detache...

Unsteady control of supersonic turbulent cavity flow based on resolvent analysis.

Abstract: We use resolvent analysis to determine an unsteady active control setup to attenuate pressure fluctuations in turbulent supersonic flow over a rectangular cavity with a length-to-depth ratio of 6 at a Mach number of 1.4 and a Reynolds number based on cavity depth of 10,000. Large-eddy simulations (LES) and dynamic modal decomposition (DMD) of the supersonic cavity flow reveal the dominance of two-dimensional Rossiter modes II and IV. These predominantly two-dimensional vortical structures generate high-amplitude unsteadiness over the cavity through trailing-edge impingement and create oblique shock waves by obstructing the freestream. To disrupt the undesired formation of vortical structures, we introduce three-dimensional unsteady forcing along the cavity leading edge to generate streamwise vortical structures. Resolvent analysis with respect to the time-averaged base flow is leveraged to determine the optimal combination of forcing frequency and spanwise wavenumber. Instead of selecting the most amplified resolvent forcing modes, we seek the combination of control parameters that yields sustained amplification of the primary resolvent-based kinetic energy distribution over the entire length of the cavity. The sustained amplification is critical to ensure that the selected forcing input remains effective to prevent the formation of the large spanwise vortices. This resolvent-analysis-based flow control guideline is validated with a number of companion LES of the controlled cavity flows. The optimal control setup is verified to be the most effective in reducing the pressure root-mean-square level up to 52% along the aft and bottom cavity walls compared to the baseline cavity flow. The present flow control guideline derived from resolvent analysis should be applicable to flows that require the actuation input to remain effective over an extended region of interest.

Time-Accurate Experimental Investigation of Hypersonic Inlet Buzz at Mach 5

Abstract: Hypersonic inlet buzz is investigated experimentally in a ramjet intake at Mach 5. A two-dimensional planar inlet featuring a double-compression ramp with 10 and 22° inclination to the freestream d...

Hypersonic flow control of shock wave/turbulent boundary layer interactions using magnetohydrodynamic plasma actuators

Abstract: The effect of magnetohydrodynamic (MHD) plasma actuators on the control of hypersonic shock wave/turbulent boundary layer interactions is investigated here using Reynolds-averaged Navier-Stokes calculations with low magnetic Reynolds number approximation. A Mach 5 oblique shock/turbulent boundary layer interaction was adopted as the basic configuration in this numerical study in order to assess the effects of flow control using different combinations of magnetic field and plasma. Results show that just the thermal effect of plasma under experimental actuator parameters has no significant impact on the flow field and can therefore be neglected. On the basis of the relative position of control area and separation point, MHD control can be divided into four types and so effects and mechanisms might be different. Amongst these, D-type control leads to the largest reduction in separation length using magnetically-accelerated plasma inside an isobaric dead-air region. A novel parameter for predicting the shock wave/turbulent boundary layer interaction control based on Lorentz force acceleration is then proposed and the controllability of MHD plasma actuators under different MHD interaction parameters is studied. The results of this study will be insightful for the further design of MHD control in hypersonic vehicle inlets.

Non-ideal compressible flows in supersonic turbine cascades

Abstract: Flows in the close proximity of the vapour–liquid saturation curve and critical point are examined for supersonic turbine cascades, where an expansion occurs through a converging–diverging blade channel. The present study illustrates potential advantages and drawbacks if turbine blades are designed for operating conditions featuring a non-monotonic variation of the Mach number through the expansion process, and non-ideal oblique shocks and Prandtl–Meyer waves downstream of the trailing edge. In contrast to ideal-gas flows, for a given pressure ratio across the cascade, the flow field and the turbine performance are found to be highly dependent on the thermodynamic state at the turbine inlet, in both design and off-design conditions. A potentially advantageous design, featuring stationary points of the Mach number at the blade trailing edge, is proposed, which induces a nearly uniform outlet Mach number distribution in the stator–rotor gap with a low sensitivity to slight variations in the outlet pressure. These findings are relevant for turbomachines involved in high-temperature organic Rankine cycle power systems, in particular for supercritical applications.

Investigation of Combustion Characteristics in a Kerosene-Fueled Supersonic Combustor with Air Throttling

Abstract: The paper presents the results of an experimental and numerical study on the combustion characteristics of a supersonic combustor operation enhanced by pilot hydrogen and air throttling, including ...

Theoretical and Numerical Investigation on Total Pressure Gain in Rotating Detonation Engine

Abstract: The theoretical analysis and numerical simulations were presented in this study to explore the mechanism of total pressure gain in rotating detonation engines (RDEs). One-dimensional and two-dimens...

An inviscid analysis of swept oblique shock reflections

Abstract: An inviscid flow model is presented to gain a basic understanding of the reflection of a swept oblique shock from a planar wall. The analytical model is constructed to describe the fundamental influence of sweep on this shock configuration, which has been commonly studied as an unswept non-dimensional shock boundary layer interaction (SBLI). Transformation of model parameters into a plane perpendicular to the sweep angle reduces the resultant flow to a two-parameter system. An equivalency between this configuration and others commonly assessed is presented with advisory notes on the definition of effective coordinate systems. Inviscid shock detachment has been associated with the onset of quasi-conical SBLI spanwise development; see (Settles & Teng, AIAA J., vol. 22 (2), 1984, pp. 194–200). Its occurrence for this SBLI configuration is determined for a range of conditions and compared to experimental observations of swept SBLIs claiming cylindrical/conical similarity scalings. Finally, influence of a zero-mass flux plane associated with typical experimental and numerical analyses is presented with an accompanying model for the shock structure. While this paper serves as a useful resource when designing swept impinging oblique SBLI studies, it also provides a vital benchmark for this complex configuration and helps to unify various SBLI configurations that are often analysed in isolation.

Two-stage growth mode for lift-off mechanism in oblique shock-wave/jet interaction

Abstract: The lift-off characteristics of supersonic streamwise vortex in oblique shock-wave/jet interaction (OS/JI for short), extracted from a wall-mount ramp injector in scramjet, is studied through Large-eddy simulation method. Shocked helium jet is deformed into a pair of streamwise vortex with a co-rotating companion vortex showing the lift-off characteristic immediately after shock. Based on the objective coordinate system in frame of oblique shock structure, it is found that the nature of three-dimensional lift-off structure of a shockinduced streamwise vortex is inherently and precisely controlled by a two-stage growth mode of structure kinetics of a shock bubble interaction (SBI for short). The striking similar structures between OS/JI and SBI support the proposition that the lift-off of streamwise vortex is the result of a underlying two-dimensional vortical motion. By considering the first stage impulsive linear growth rate, an improved vortex propagation model suitable for SBI is proposed and validated. The lift-off phenomena of both numerical OS/JI case in this paper and wall-mounted ramp injector cases in literature are well explained under the two-stage structure kinetics model of SBI. This model further predicts that for higher free stream Mach number (M > 5), increasing ramp compression shows little effect on elevating streamwise vortex while evident lift-off may occur for lower Mach number (M < 3.5), which offers the new way for preliminary design of streamwise vortex-based ramp injector in scramjet.

The effect of flow confinement on laminar shock-wave/boundary-layer interactions

Abstract: Numerical work on shock-wave/boundary-layer interactions (SBLI) to date has largely focused on span-periodic quasi-two-dimensional (quasi-2-D) configurations that neglect the influence lateral confinement has on the core flow. The present study is concerned with the effect of flow confinement on Mach 2 laminar SBLI in rectangular ducts. An oblique shock generated by a wedge forms a conical swept SBLI with sidewall boundary layers before reflecting from the bottom wall of the domain. Multiple large regions of flow-reversal are observed on the sidewalls, bottom wall and at the corner intersection. The main interaction is found to be strongly three-dimensional and highly dependent on the geometry of the duct. Comparison to quasi-2-D span-periodic simulations showed sidewalls strengthen the interaction by 31 % for the baseline configuration with an aspect ratio of one. The length of the shock generator and subsequent trailing edge expansion fan position was shown to be a critical parameter in determining the central separation length. By shortening the length of the shock generator, modification of the interaction and suppression of the central interaction is demonstrated. Parametric studies of shock strength and duct aspect ratio were performed to find limiting behaviours. For the largest aspect ratio of four, three-dimensionality was visible across 30 % of the span width away from the wall. The topology of the three-dimensional separation is shown to be similar to ‘owl-like’ separations of the first kind. Reflection of the initial conical swept SBLI is found to be the most significant factor determining the flow structures downstream of the main interaction.

Unusual Location of the Geotail Magnetopause Near Lunar Orbit: A Case Study

Abstract: The Earth's magnetopause is highly variable in location and shape and is modulated by solar wind conditions. On 8 March 2012, the ARTEMIS probes were located near the tail current sheet when an interplanetary shock arrived under northward interplanetary magnetic field conditions and recorded an abrupt tail compression at ∼(‐60, 0, ‐5) RE in Geocentric Solar Ecliptic coordinate in the deep magnetotail. Approximately 10 minutes later, the probes crossed the magnetopause many times within an hour after the oblique interplanetary shock passed by. The solar wind velocity vector downstream from the shock was not directed along the Sun‐Earth line but had a significant Y component. We propose that the compressed tail was pushed aside by the appreciable solar wind flow in the Y direction. Using a virtual spacecraft in a global magnetohydrodynamic (MHD) simulation, we reproduce the sequence of magnetopause crossings in the X‐Y plane observed by ARTEMIS under oblique shock conditions, demonstrating that the compressed magnetopause is sharply deflected at lunar distances in response to the shock and solar wind VY effects. The results from two different global MHD simulations show that the shocked magnetotail at lunar distances is mainly controlled by the solar wind direction with a timescale of about a quarter hour, which appears to be consistent with the windsock effect. The results also provide some references for investigating interactions between the solar wind/magnetosheath and lunar nearside surface during full moon time intervals, which should not happen in general.

Using High-Frequency Pulsed Supersonic Microjets to Control Resonant High-Speed Cavity Flows

Abstract: A novel actuator concept is evaluated in a series of active flow control experiments on a resonant high-speed cavity flow. The actuator generates pulsed supersonic microjets by using the resonance ...