Tube Waves Research Articles

Resonant absorption of kink oscillations in coronal flux tubes with continuous magnetic twist

ABSTRACT There are observational evidences for the existence of twisted magnetic field in the solar corona. Here, we have investigated resonant damping of the magnetohydrodynamic (MHD) kink waves in magnetic flux tubes. A realistic model of the tube with continuous magnetic twist and radially inhomogeneous density profile has been considered. We have obtained the dispersion relation of the kink wave using the solution to the linear MHD equations outside the density inhomogeneity and the appropriate connection formula to the solutions across the thin transitional boundary layer. The dependence of the oscillation frequency and damping rate of the waves on the twist parameter and longitudinal wavenumber has been investigated. For the flux tube parameters considered in this paper, we obtain rapid damping of the kink waves comparable to the observations. In order to justify this rapid damping, depending on the sign of the azimuthal kink mode number, $m=+1$ or $-1$, the background magnetic field must have left- or right-handed twisted profile, respectively. For the model considered here, the resonant absorption occurs only when the twist parameter is in a range specified by the density contrast.

Unsteady interaction of nanosecond surface sliding discharge with plane shock wave

In this study, we present experimental results on fast intensified charge-coupled device imaging and current measurements of a nanosecond surface sliding discharge interacting with a plane shock wave in a shock tube. A shock wave with Mach number 1.9–3.5 passed through the discharge area while the electric pulse was switched on. The discharge is initiated by a voltage pulse of 25 kV; the discharge current is ∼1 kA. The discharge current in high-speed airflows including plane shock waves has been measured and the dynamics of the discharge radiation was studied with the nanosecond time resolution. It is shown that the inhomogeneity of the airflow-density field leads to a change in the discharge current and the structure of the discharge radiation strongly depending on the shock wave position in the discharge gap. The dynamics of the energy input is determined by the interaction of the shock wave with the discharge region, as measurements of the discharge current reveal.

The Effect of Weak Magnetic Twist on Resonant Absorption of Slow Sausage Waves in Magnetic Flux Tubes

Abstract Observations show that twisted magnetic flux tubes are present throughout the Sun’s atmosphere. The main aim of this work is to obtain the damping rate of sausage modes in the presence of weak magnetic twist. Using the connection formulae obtained by Sakurai et al., we investigate resonant absorption of the sausage modes in the slow continuum under photospheric conditions. We derive the dispersion relation and solve it numerically, and consequently obtain the frequencies and damping rates of the slow surface sausage modes. We conclude that the magnetic twist can result in strong damping in comparison with the untwisted case.

Synergistic Role of Oxidative Stress and Blood-Brain Barrier Permeability as Injury Mechanisms in the Acute Pathophysiology of Blast-induced Neurotrauma

Blast-induced traumatic brain injury (bTBI) has been recognized as the common mode of neurotrauma amongst military and civilian personnel due to an increased insurgent activity domestically and abroad. Previous studies from our laboratory have identified enhanced blood-brain barrier (BBB) permeability as a significant, sub-acute (four hours post-blast) pathological change in bTBI. We also found that NADPH oxidase (NOX)-mediated oxidative stress occurs at the same time post-blast when the BBB permeability changes. We therefore hypothesized that oxidative stress is a major causative factor in the BBB breakdown in the sub-acute stages. This work therefore examined the role of NOX1 and its downstream effects on BBB permeability in the frontal cortex (a region previously shown to be the most vulnerable) immediately and four hours post-blast exposure. Rats were injured by primary blast waves in a compressed gas-driven shock tube at 180 kPa and the BBB integrity was assessed by extravasation of Evans blue and changes in tight junction proteins (TJPs) as well as translocation of macromolecules from blood to brain and vice versa. NOX1 abundance was also assessed in neurovascular endothelial cells. Blast injury resulted in increased extravasation and reduced levels of TJPs in tissues consistent with our previous observations. NOX1 levels were significantly increased in endothelial cells followed by increased superoxide production within 4 hours of blast. Blast injury also increased the levels/activation of matrix metalloproteinase 3 and 9. To test the role of oxidative stress, rats were administered apocynin, which is known to inhibit the assembly of NOX subunits and arrests its function. We found apocynin completely inhibited dye extravasation as well as restored TJP levels to that of controls and reduced matrix metalloproteinase activation in the sub-acute stages following blast. Together these data strongly suggest that NOX-mediated oxidative stress contributes to enhanced BBB permeability in bTBI through a pathway involving increased matrix metalloproteinase activation.

The conservative version of a method of the characteristics with a floating grid in problems of gas dynamics

Test calculations of the motion and reflection of individual waves in a long tube have been performed applying newly developed and known schemes with a floating grid. Waves in the corresponding experiment were formed using a specially designed single-cycle generator. The version presented is shown to have a significant advantage in calculation time over the discontinuity decay method, in which the computational grid density must be increased significantly to obtain the desired correspondence with the real wave pattern.

The Effect of Degenerate Plasma on the Frequency Spectra of Slow Waves in Helix Traveling-Wave Tube

The dispersion relationship of electromagnetic waves in a helix traveling-wave tube consisting of a dielectric rod and a degenerate plasma layer was investigated and the presence of terahertz (THz) waves in this helix waveguide was shown. Furthermore, the permissible frequency region for the presence of slow waves in this helix waveguide was obtained. In addition to the effect of the dielectric radius, the plasma radius and the dielectric permittivity on the dispersion relationship were investigated. In the following, by injecting the electron beam in this helix waveguide, the electromagnetic waves were excited and the effect of geometric parameters on the growth rate of these THz waves was investigated.

Combustion characteristics of hydrogen-air mixture in pulse detonation engines

The literature review reveals that the supersonic combustion wave remains a helpful method for improving the efficiency of pulse detonation engines (PDEs). A number of experimental studies have been conducted on detonation waves, which are formed by deflagration-to-detonation (DDT) transition waves. In this work, a straight PDE tube with a length of 1200 mm and a circular cross section measuring 60 mm in diameter is considered for combustion analysis. The combustion mechanism of stoichiometric hydrogen-air mixture is modeled by a three-dimensional Navier-Stokes turbulence model with a one-step reduced chemical kinetic reaction model using ANSYS Fluent software. The detonation tube contains obstacles with various blockage ratios of 0.5, 0.6 and 0.7 with 60 mm spacing (S) between them. Initial boundary conditions of 0.1 MPa pressure and 293 K temperature are applied to the hydrogen-air mixture to initiate combustion. The objective of the present work is to analyze the combustion flame generation and development of a stable detonation wave in the PDE tube. The flame rapidly develops and accelerates due to the burning of unburnt fuel particles in the leading zone and reduces the DDT run-up length.

Experimental testing of semirigid corrugated baffles for the suppression of tube waves in vertical seismic profile data

Multichannel borehole hydrophone strings are a low-cost, low-risk, alternative to borehole clamping geophones. Vertical seismic profile (VSP) data collected with hydrophones, however, suffer from high-amplitude coherent tube-wave noise. This reduces the usable data to the first arrivals and traveltimes for check-shot surveys. To significantly reduce tube-wave noise from VSP data acquired with hydrophones, we have designed and tested a novel tube-wave attenuation baffle. The effectiveness of the baffle was first verified in a laboratory-scale experiment and then in a borehole drilled into a hardrock environment. The laboratory experiments tested the performance of four different baffle topologies, whereby the best performing topology was the semirigid corrugated pipe baffle. This design reduced the amplitude of the tube wave with more than 40 dB and was logistically easy to deploy. The field experiment investigated the effectiveness of three different semirigid corrugated pipe baffle topologies in a PQ (123 mm) diamond drillhole in Western Australia. Here, we found that the semirigid corrugated pipe baffle was effective in disrupting tube-wave propagation. The 100 mm diameter baffle achieved an impressive 60 dB of tube-wave attenuation, whereas the 50 mm baffle had a modest attenuation of 10–15 dB. This suggests that the performance of this new type of baffle is best when the diameter of the baffle is closely matched to the diameter of the borehole. The results of these experiments have significant implications because hydrophone arrays with a large number of receivers are comparatively inexpensive and simpler to deploy than borehole geophone counterparts. The development of hydrophone arrays that are free of interfering borehole modes could allow VSPs to be acquired in situations in which seismic-polarity information is not required and could help VSP gain traction in cases in which the cost of acquisition has precluded its use until now.

High-temperature laminar flame speed measurements in a shock tube

High-temperature methane and propane laminar flame speed measurements were conducted behind reflected shock waves in a shock tube. A high-power Nd:YAG laser was used to spark-ignite the shock-heated gas mixtures and initiate laminar flame propagation. High-speed, OH* endwall imaging was used to record the propagation of the spherically expanding flames in time, and a non-linear stretch correlation was applied and used to determine the unburned, unstretched laminar flame speed. “Low-temperature” (<600 K) flame speed results are presented for stoichiometric methane/air and propane/air mixtures at initial unburned gas conditions of 489–573 K and 391–556 K, respectively, and 1 atm. The low-temperature measurements show close agreement with available literature data and kinetic modeling results, thereby validating the shock-tube laminar flame speed measurement approach. “High-temperature” (>750 K) flame speed results are presented for a propane/21% O2-47% N2-32% He mixture (ϕ = 0.8) at initial unburned gas conditions of 764–832 K, 1 atm. The high-temperature measurements fall between kinetic model predictions, but the kinetic model results show significant disagreement, highlighting the need for high-temperature flame speed validation data of this kind. We believe that these results represent the first laminar flame speed measurements conducted in a shock tube, and that the high-temperature results are the highest-temperature, 1-atm flame speed measurements available in the literature.

Propagating modes of the travelling wave in a microwave plasma torch with metallic enclosure

For the discharges sustained by the travelling electromagnetic waves, the wave propagation characteristics are of great importance to discharge maintenance and stability. This study aims to investigate the propagating electromagnetic modes of the travelling wave in a cylindrical discharge tube bound by a metallic enclosure based on a microwave plasma torch. In certain particular circumstances, it is found that the cylindrical discharge tube with metallic enclosure is able to change from a one-conductor circular waveguide to a two-conductor-like coaxial waveguide. Such a change allows the travelling wave to propagate in the transverse electromagnetic mode, which has rarely been noticed before. Regarding this transition, a new criterion is proposed to determine the formation of the two-conductor-like waveguide structure. Existence conditions of different propagating modes of the travelling wave are further discussed with respect to different glass tube wall thicknesses and dielectric properties. The results indicate that it is possible to control the propagating modes of the travelling wave for different purposes by choosing a proper glass tube.

The Effect of a Residual Stress on Wave Propagation in a Fluid-Filled Thick Elastic Tube

The propagation of harmonic waves in an elastic tubes filled fluid is presented in this study. The tube material is considered to be incompressible, homogeneous, isotropic, initially axially stretched, inflated and thick elastic like human arteries. The viscous fluid is assumed to be incompressible and Newtonian. The differential equations of both materials are obtained in cylindrical coordinates. The analytical solutions of the equations of motion for the fluid, numerical solutions of the equations of motion for the tube have been found. The residual circumferential strain in the unloaded state of artery causes opening angle. The dispersion relation is presented as a function of the axial stretch, opening angle, internal pressure and material parameters. The effects of these parameters are shown and discussed on graphics.

Ignition and detonation onset behind incident shock wave in the shock tube

The paper analyses in details and describes the process of ignition kernel formation and subsequent detonation onset behind the shock wave propagating in the shock tube. To get the overall pattern of the process a series of one- and two-dimensional calculations are carried out with the use of a dissipation-free numerical technique. It is shown that one of the leading roles in the process of ignition kernel formation belongs to the non-steady flow dynamics establishing behind the shock wave. The development of the boundary layer determines both the temperature re-distribution in the near-wall region and the conditions for gas-dynamic acceleration of the flow. With an account of thermal runaway, the most intensively heated region corresponds to the area between the inner margin of the boundary layer and the contact surface separating driver gas and the test mixture. After localized ignition takes place the forming reaction wave propagates out from the ignition epicenter. Reaction wave propagates behind the outrunning compression wave through the already reacting mixture. Shock-induced compression of the test mixture provides conditions for the self-sustained acceleration of the combustion wave, and finally, the detonation onset takes place.

Numerical study of chemically reacting flow in a shock tube using a high-order point-implicit scheme

The shock wave/boundary-layer interaction of chemically reacting flow in a shock tube is studied using a high-order point-implicit solver. The solver employs a high-resolution weighted essentially non-oscillatory (WENO) scheme to capture the complex shock structures, together with a point-implicit method to overcome the stiffness of the chemical production term in the multicomponent Navier–Stokes equations. The numerical code is carefully validated with three benchmark tests, which demonstrates the robustness and good performance of the combined numerical methods. The unsteady interaction process between the shock wave and boundary layer in a two-dimensional shock tube is clearly captured with detailed flow patterns in the simulation. Simulation results show that regular vortex arrangements appear in the flow field for the case of Mach number 2.37, while for the case of Mach number 3.15, the vortex structures break up and chemical nonequilibrium effects become apparent. The influence of real gas effects on shock wave/boundary-layer interaction is further identified on the temperature field and triple point trajectory.

A Conservative Version of the Method of Characteristics with a Floating Grid in Problems of Gas Dynamics

Test calculations of the motion and reflection of individual waves in a long tube have been performed applying newly developed and known schemes with a floating grid. Waves in the corresponding experiment were formed using a specially designed single-cycle generator. The version presented is shown to have a significant advantage in calculation time over the discontinuity decay method, in which the computational grid density must be increased significantly to obtain the desired correspondence with the real wave pattern.

Transverse waves in coronal flux tubes with thick boundaries: The effect of longitudinal flows

Observations show that transverse magnetohydrodynamic (MHD) waves and flows are often simultaneously present in magnetic loops of the solar corona. The waves are resonantly damped in the Alfvén continuum because of plasma and/or magnetic field nonuniformity across the loop. The resonant damping is relevant in the context of coronal heating, since it provides a mechanism to cascade energy down to the dissipative scales. It has been theoretically shown that the presence of flow affects the waves propagation and damping, but most of the studies rely on the unjustified assumption that the transverse nonuniformity is confined to a boundary layer much thinner than the radius of the loop. Here we present a semi-analytic technique to explore the effect of flow on resonant MHD waves in coronal flux tubes with thick nonuniform boundaries. We extend a published method, which was originally developed for a static plasma, in order to incorporate the effect of flow. We allowed the flow velocity to continuously vary within the nonuniform boundary from the internal velocity to the external velocity. The analytic part of the method is based on expressing the wave perturbations in the thick nonuniform boundary of the loop as a Frobenius series that contains a singular term accounting for the Alfvén resonance, while the numerical part of the method consists of solving iteratively the transcendental dispersion relation together with the equation for the Alfvén resonance position. As an application of this method, we investigated the impact of flow on the phase velocity and resonant damping length of MHD kink waves. With the present method, we consistently recover results in the thin boundary approximation obtained in previous studies. We have extended those results to the case of thick boundaries. We also explored the error associated with the use of the thin boundary approximation beyond its regime of applicability.

Ignition delay time and H2O measurements during methanol oxidation behind reflected shock waves

To improve detailed chemical kinetics models, the oxidation of methanol was investigated behind reflected shock waves in shock tubes. Ignition delay times of methanol–air mixtures, with Ar as diluent, were studied between 940 and 1540 K in a heated shock tube, for pressures up to 14.9 atm and for equivalence ratios of 0.5, 1.0, and 2.0. Water profiles were measured by utilizing a laser absorption technique in the 1350-to-1600-K temperature range, at an average pressure of 1.3 atm and for similar equivalence ratios. The present study shows the ignition delay times of methanol to be in very good agreement with results from the literature (Fieweger et al., 1997), whereas the other conditions have never been investigated before. The ignition delay time data are also in good agreement with modern detailed kinetics mechanisms such as the AramcoMech 3.0 model. The water time-history profiles were modeled using well-known literature mechanisms. Discrepancies were observed between these kinetics mechanisms, and poor predictions were observed for the lower temperatures investigated. Sensitivity and rate-of-production analyses were performed using 3 literature mechanisms (namely, AramcoMech 3.0, Princeton, and JetSurfII). Discrepancies were found among the models when predicting important reactions dominating the oxidation of methanol as well as the rate-of-production of H2O.

The propagation mechanism of detonation wave in a round tube filled with larger blockage ratio orifice plates

In this study, the detonation propagation mechanisms for the stoichiometric hydrogen-oxygen mixture are explored systematically in a circular tube with 6-m in length and an inner diameter of 90-mm. The continuous orifice plates with BR = 0.93 are adopted to investigate the characteristics of detonation diffraction, failure and initiation. High-speed piezoelectric pressure transducers are used to obtain the average velocity, and the smoked foil technique is adopted to record the detonation cellular patterns. The results indicate that three various propagation regimes can be observed, i.e., steady detonation, quasi-detonation and fast flame. In the smooth tube, only the steady detonation and fast flame modes are seen. When the initial pressure is greater than the critical value, the detonation can propagate at about the theoretical CJ velocity. Near the critical pressure, a sudden velocity drop is observed. Of note is that the single-headed spin and double-headed detonation cannot occur because of the limitation of the aspect ratio. In the tube filled with obstacles, the averaged wave velocity is decayed severely. Only the mechanisms of the quasi-detonation and fast flame can be seen. In the quasi-detonation mode, the critical value of d/λ is greater than 7.36, which is far larger than 1. Two different detonation ignition regimes produced by the shock reflection from the wall are observed, i.e., the initiation positions occur in the vicinity of the tube wall and the surface of the orifice plate.

Assessment of the Fluidic Control Boundary Nozzle within a Pulse Detonation Rocked Engine by Unstructured Meshes CE/SE Method

The simulation model of pneumatic Fluidic Control Boundary Nozzle(Fluidic nozzle) is built to study its operation and propulsive augmentation performance in the unsteady PDRE system. The numerical simulation employed an adaptable unstructured-mesh CE/SE method, which is found capable to solve flows with strong gradients in detonation tube and complicated wave interaction patterns in fluidic nozzle. It is found that during the detonation and earlier exhaust stage, the whole section of fluidic nozzle is used to expand and increase the thrust without any over-expansion appearing in the nozzle. Conversely, during the later exhaust stage, purge and refilling stage, the annular control surface by subsonic jet of fluidic nozzle is successfully formed on the outside of main stream to avoid harmful shock waves of over-expansion. Thus, the real expansion ratio and thrust efficient are optimized. It is also found that easier for the bell fluidic nozzle to control the outer interface of main supersonic flow, while it is easier for the conical fluidic nozzle to perfect the real expansion ratio of main flow. The research results enrich the research system of PDRE nozzle.

Mitigation of H2/air gaseous detonation via utilization of PAN-based carbon fibre felt

This paper first studied the mitigation of the hydrogen/air mixture detonation wave by tuning the tube inner wall with the absorbing material of polyacrylonitrile (PAN)-based carbon fibre felt. The experimental tests were performed in a single-trial circular cross-section tube filled with premixed hydrogen and air detonative mixture. The pressure values and flame front propagation were measured by means of pressure transducers and photodiodes respectively. The attenuation regimes of detonation wave in walled tubes with different thicknesses and layouts of absorption material were compared. The PAN-based carbon fibre felt makes a significant attenuation on the detonation propagation. The decoupling of leading shock wave and flame front can be observed under the effect of this absorbing layer. The ultimate strength close to the tube end and propagation velocity of the combustion wave decrease with the increase of felt thickness. When the interval layout felt is adopted, the spacing distance has almost no impact on the attenuation effectiveness. When the sectional layout is adopted, the effectiveness of detonation mitigation is however improved for a higher proportion of the absorbing material.

Visualizing the superposition principle of sound waves in both-closed-end resonance tube

In this study, we visualized the superposition principle of sound waves in a both-closed-end (BCE) resonance tube. The resonance tube is constructed using an acrylic tube with 2.64 cm internal diameter and 150 cm length, polystyrene beads to illustrate the positions of displacement nodes and antinodes and two identical detachable speakers as source of sound waves at both ends of the tube. The superposition principle of sound waves in a BCE tube is performed for two conditions: speakers at both ends are in-phase and 180° out-of-phase with each other. The findings indicate that the superposition principle of sound waves in a BCE tube changes the modes of harmonic series in the tube depending on the phase of speakers. The harmonic series in BCE tubes occur at the even mode (n = 2, 4, 6…) for in-phase speakers and the odd mode (n = 1, 3, 5…) for out-of-phase speakers. The experimental values of harmonic frequencies differ by less than 6.09% compared to the theoretical values, confirming that the experiment can be used to visualize the superposition principle of sound waves in a resonance tube.