Shock Motion Research Articles

Effects of micro-ramps on a shock wave/turbulent boundary layer interaction

Stereoscopic particle image velocimetry is used to investigate the effects of micro-ramp sub-boundary layer vortex generators, on an incident shock wave/boundary layer interaction at Mach 1.84. Single- and double-row arrangements of micro-ramps are considered. The micro-ramps have a height of 20% of the unperturbed boundary layer thickness and the measurement planes are located 0.1 and 0.6 boundary layer thicknesses from the wall. The micro-ramps generate packets of individual vortex pairs downstream of their vertices, which produce counter-rotating longitudinal streamwise vortex pairs in a time-averaged view. These structures induce a pronounced spanwise variation of the flow properties, namely the mixing across the boundary layer interface. The probability of reversed-flow occurrence is decreased by 20 and 30% for the single- and double-row configurations, respectively. Both configurations of micro-ramps stabilize the shock motion by reducing the length of its motion by about 20% in the lower measurement plane. The results are summarized by a conceptual model describing the boundary layer’s and interaction’s flow pattern under the effect of the micro-ramps.

Comparison of three large-eddy simulations of shock-induced turbulent separation bubbles

Three different large-eddy simulation investigations of the interaction between an impinging oblique shock and a supersonic turbulent boundary layer are presented. All simulations made use of the same inflow technique, specifically aimed at avoiding possible low-frequency interferences with the shock/boundary-layer interaction system. All simulations were run on relatively wide computational domains and integrated over times greater than twenty five times the period of the most commonly reported low-frequency shock-oscillation, making comparisons at both time-averaged and low-frequency-dynamic levels possible. The results confirm previous experimental results which suggested a simple linear relation between the interaction length and the oblique-shock strength if scaled using the boundary-layer thickness and wall-shear stress. All the tested cases show evidences of significant low-frequency shock motions. At the wall, energetic low-frequency pressure fluctuations are observed, mainly in the initial part of interaction.

Magnetosheath excursion and the relevant transport process at the magnetopause

Abstract. A large-amplitude excursion of the magnetosheath (MS) in quiet solar wind conditions on 17 March 2004 was recorded simultaneously by the Cluster and TC-1 spacecraft. During this period, the IMF Bz was entirely northward. The coherence between the bow shock motion and magnetopause (MP) motion is revealed and the excursion velocities of the bow shock motion are analyzed. In addition, the relevant plasma transport phenomenon in the form of flux fluctuations below the ion gyrofrequency at the MP is exposed and is interpreted as manifestation of the drift instability. Correlated observations on charge accumulation and electrostatic potential perturbation are recorded by electron measurements in high energy regime, and also the eventual cross-field vortex motion in the nonlinear stage and the consequential mass exchange are exhibited. The present investigation gives some new insight into the MS plasma transport mechanism across the subsolar MP region in quiet solar wind conditions during a period of northward IMF.

Density field of supersonic separated flow past an afterbody nozzle using tomographic reconstruction of BOS data

The background oriented Schlieren (BOS) technique has been applied to determine the density field in an oblique shock-separated turbulent boundary flow. Measurements were made for two cases, namely, with/without jet flow from the afterbody which is a nozzle. In addition, oil flow and Schlieren visualizations were carried out—the results show certain upstream features of interest including shock excursions. The mean density field from BOS is discussed along with results from conventional Schlieren flow visualization. The data extracted from the mean density field obtained through BOS have been compared for the jet-off and jet-on cases. The data obtained also show the mean density in the base region (jet-off case) to be about 50% of the freestream density and match the isentropic values for the underexpanded jet at the exit. The study involving shock–boundary interaction, movement of freestream shock over the afterbody in the presence of a jet plume provides understanding of flow physics in a flow regime where whole field velocity measurements are extremely difficult.

Kinematics of a shock wave of arbitrary strength in a non-ideal gas

Singular surface theory is used to study the evolutionary behaviour of an unsteady three-dimensional motion of a shock wave of arbitrary strength propagating through a non-ideal gas. The dynamical coupling between the shock front and the induced discontinuities behind it is investigated by considering an infinite system of transport equations governing the strength of a shock wave and the induced discontinuities behind it. This infinite system, when subjected to a truncation approximation, efficiently describes the shock motion. Disturbances propagating on the shock and the onset of shock-shocks are briefly discussed. For a two-dimensional shock motion, our transport equations bear a structural resemblance to those of geometrical shock dynamics. Attention is drawn to the connection between the transport equation obtained by using the truncation rule and the one obtained by using the characteristic rule. The effects of van der Waals’ excluded volume and wavefront geometry on the evolutionary behaviour of shocks are discussed.

George Housner: A Personal Remembrance (1910–2008)

George W. Housner, C F Braun Professor of Engineering, Emeritus, died after a short illness on November 10, 2008, just a few weeks before his 98th birthday. He was in the retirement home in Pasadena where he had lived for several years. For all of us who knew George, this marked the end of an era. Few people have guided and nurtured a field the way George led earthquake engineering over a period of several decades. He had a profound effect on many people and will long be remembered. His impact was so pervasive that he had earned the title of the ‘Father of Earthquake Engineering’. This article records some of my thoughts about this remarkable man.

Experimental Study of a Mach 3 Compression RampInteraction at Re{theta} = 2400

Experiments were performed to investigate the flow in a Mach 2.9 shock wave turbulent boundary-layer interaction at a Reynolds number based on momentum thickness of 2400. The flow configuration was a nominally two-dimensional 24 deg compression ramp, which exhibited a separation bubble in the corner region. Mean flow quantities, including the velocity profile upstream and downstream of the corner, and the wall pressure through the interaction were measured. Filtered Rayleigh scattering was used to visualize the flow structure in the interaction and provide quantitative measurements of the turbulent structure angle and the intermittency of the boundary-layer edge turbulence. The shock motion was characterized by measuring the fluctuating wall pressure. The results indicate that, compared to previous measurements obtained at higher Reynolds numbers of 60-80,000, the separation bubble is approximately twice as long, the root mean square of the wall-pressure fluctuations has a relatively smaller peak, and the intermittency of the wall-pressure signal in the shock-foot region is attenuated. As in the high-Reynolds-number case, the shock motion has a broadband frequency distribution with a peak slightly below 1 kHz. The mean flow quantities, root mean square wall-pressure-fluctuation profile, wall-pressure signal, and shock-motion frequency agree well with the direct numerical simulation of a previous study at matching conditions.

High-Altitude Limit Cycle Flutter of Transonic Wings

In flutter testing, it is often implicitly assumed that the most critical case at any given Mach number will occur at sea level conditions. This is a reasonable expectation, as the highest dynamic pressure at a fixed Mach number will be encountered at sea level, where the air density is highest. In the present paper, a counterexample involving a generic swept wing representative of transport aircraft is presented in which transonic limit cycle flutter is predicted to occur at altitude rather than at sea level. The calculations are based on a time-accurate Eulerian-Lagrangian finite element scheme that fully accounts for both structural and aerodynamic nonlinearities arising from large deflections and shock motion. Flutter onset is around Mach 0.84 and the limit cycle amplitudes grow until Mach 0.95 is reached, at which point they start decreasing, vanishing abruptly around Mach 0.97. The limit cycle flutter persists down to very low air densities, corresponding to altitudes above 75,000 ft, and the maximum limit-cycle-oscillation flutter amplitude does not occur at sea level, but at altitude. If sufficient structural damping is added, the wing will be flutter-free at lower altitudes but not at higher (cruise) altitudes. At Mach 0.95, the wing is stable below about 15,000 ft, even in the absence of structural damping, but experiences strong limit cycle flutter at higher altitudes.

Numerical study of flow field visualizations over a payload shroud at transonic speeds

The article presents a flow field visualization study on a typical payload shroud at transonic Mach numbers range. The numerical simulation over the heat shield is carried out by solving axisymmetric, turbulent, compressible, Reynolds-averaged Navier—Stokes equations. The closure of these equations is achieved using the Baldwin—Lomax turbulence model. The density contour plots are obtained at Mach number 0.80, 0.90, 0.95, and 1.0. The transonic flow field features over the heat shield are captured and compared with the schlieren pictures, and good matching between them is exhibited. The movement of the terminal shock is found to be a non-linear function of freestream Mach number. The location of the shock on the heat shield is also confirmed with the flight data. The numerical studies have shown that the reliable estimation of the transonic flow field can be made in conjunction with the flow visualization technique.

High-speed digital visualization and high-frequency automated shock tracking in supersonic flows

A low-cost, robust, versatile digital shadowgraph visualization system is presented that provides a fast nonintrusive diagnostic for unsteady high-speed flows. The technique is particularly designed for real-time automated tracking of shock positions, enabling high-speed active shock control. The visualization system is based on a high-intensity white LED light source combined with a CMOS-imaging sensor, providing the system with three modes of operation: (1) high-resolution overall instantaneous visualization; (2) high-resolution visualization showing spatial- temporal variations in the flow field, allowing direct identification of areas where changes occur; (3) adjustable windowed visualization at reduced resolution at high frame rate (currently up to 980 Hz) . Experimental results are presented together with numerical simulations based on the high-accuracy NTS Navier-Stokes solver and Roe's flux difference splitting method. The flow studied is an adjustable underexpanded jet flow coming from a nozzle that is placed in a counterflowing Mach-2 flow. The interaction of the two flows results in a complex shock and expansion pattern, providing a challenging configuration for the numerical flow solver. By modulating the jet, high-frequency changes are induced in the interaction pattern, allowing simulation of shock movement in a supersonic inlet. Good correspondence between measured and numerical shock position and angle isfound. (C) 2008 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.2992621]

An asymmetric solar wind termination shock

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, approximately 10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4-5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.

Unsteady shock wave dynamics

An experimental study of an oscillating normal shock wave subject to unsteady periodic forcing in a parallel-walled duct has been conducted. Measurements of the pressure rise across the shock have been taken and the dynamics of unsteady shock motion have been analysed from high-speed schlieren video (available with the online version of the paper). A simple analytical and computational study has also been completed. It was found that the shock motion caused by variations in back pressure can be predicted with a simple theoretical model. A non-dimensional relationship between the amplitude and frequency of shock motion in a diverging duct is outlined, based on the concept of a critical frequency relating the relative importance of geometry and disturbance frequency for shock dynamics. The effects of viscosity on the dynamics of unsteady shock motion were found to be small in the present study, but it is anticipated that the model will be less applicable in geometries where boundary layer separation is more severe. A movie is available with the online version of the paper.

Wavelet‐based simulation of spectrum‐compatible aftershock accelerograms

AbstractIn damage‐based seismic design it is desirable to account for the ability of aftershocks to cause further damage to an already damaged structure due to the main shock. Availability of recorded or simulated aftershock accelerograms is a critical component in the non‐linear time‐history analyses required for this purpose, and simulation of realistic accelerograms is therefore going to be the need of the profession for a long time to come. This paper attempts wavelet‐based simulation of aftershock accelerograms for two scenarios. In the first scenario, recorded main shock and aftershock accelerograms are available along with the pseudo‐spectral acceleration (PSA) spectrum of the anticipated main shock motion, and an accelerogram has been simulated for the anticipated aftershock motion such that it incorporates temporal features of the recorded aftershock accelerogram. In the second scenario, a recorded main shock accelerogram is available along with the PSA spectrum of the anticipated main shock motion and PSA spectrum and strong motion duration of the anticipated aftershock motion. Here, the accelerogram for the anticipated aftershock motion has been simulated assuming that temporal features of the main shock accelerogram are replicated in the aftershock accelerograms at the same site. The proposed algorithms have been illustrated with the help of the main shock and aftershock accelerograms recorded for the 1999 Chi–Chi earthquake. It has been shown that the proposed algorithm for the second scenario leads to useful results even when the main shock and aftershock accelerograms do not share the same temporal features, as long as strong motion duration of the anticipated aftershock motion is properly estimated. Copyright © 2008 John Wiley & Sons, Ltd.

Analysis of shock motion in shockwave and turbulent boundary layer interaction using direct numerical simulation data

Direct numerical simulation data of a Mach 2.9, 24○ compression ramp configuration are used to analyse the shock motion. The motion can be observed from the animated DNS data available with the online version of the paper and from wall-pressure and mass-flux signals measured in the free stream. The characteristic low frequency is in the range of (0.007–0.013) U∞/δ, as found previously. The shock motion also exhibits high-frequency, of O(U∞/δ), small-amplitude spanwise wrinkling, which is mainly caused by the spanwise non-uniformity of turbulent structures in the incoming boundary layer. In studying the low-frequency streamwise oscillation, conditional statistics show that there is no significant difference in the properties of the incoming boundary layer when the shock location is upstream or downstream. The spanwise-mean separation point also undergoes a low-frequency motion and is found to be highly correlated with the shock motion. A small correlation is found between the low-momentum structures in the incoming boundary layer and the separation point. Correlations among the spanwise-mean separation point, reattachment point and the shock location indicate that the low-frequency shock unsteadiness is influenced by the downstream flow. Movies are available with the online version of the paper.

Gain Scheduling Control of Nonlinear Shock Motion Based on Equilibrium Manifold Linearization Model

The equilibrium manifold linearization model of nonlinear shock motion is of higher accuracy and lower complexity over other models such as the small perturbation model and the piecewise-linear model. This paper analyzes the physical significance of the equilibrium manifold linearization model, and the self-feedback mechanism of shock motion is revealed. This helps to describe the stability and dynamics of shock motion. Based on the model, the paper puts forwards a gain scheduling control method for nonlinear shock motion. Simulation has shown the validity of the control scheme.

Intranuclear Distribution and Local Dynamics of RNA Polymerase II during Transcription Activation

Transcription activation causes dramatic changes in a gene's compaction and macromolecular associations and, in some cases, triggers the translocation of the gene to a nuclear substructure. Here, we evaluate the location, movement, and transcriptional dynamics of Drosophila heat shock (HS) genes both by two-photon microscopy in live polytene nuclei and by FISH in diploid nuclei. The different HS loci occupy separate nuclear positions. Although these loci decondense upon HS, they do not undergo a detectable net translocation nor are they preferentially localized to the nuclear periphery or interior. Additionally, fluorescence recovery after photobleaching reveals that, shortly after HS, newly recruited RNA polymerase II (Pol II) enters elongation via an "efficient entry" mode, which is followed by the progressive establishment of transcription "compartments" at Hsp70 loci where concentrated Pol II is used in a "local recycling" mode. Pol II at highly transcribed developmental loci exhibits dynamics resembling combinations of these Hsp70 transcription modes.

A Forecast of the Heliospheric Termination-Shock Position by Three-dimensional MHD Simulations

The effects of heliospheric disturbances on the position of the termination shock (TS) are examined using a three-dimensional MHD model. Variations in the solar wind ram pressure due to the interplanetary shock waves drive the TS away from its steady state equilibrium position and emit shocks and waves downstream. Transmitted/emitted disturbances propagating from the TS to the heliopause (HP) are partially reflected at the HP, and the reflected waves return and collide with the TS. Thus, besides upstream solar wind disturbances, the TS location RTS changes in response to incident downstream disturbances associated with waves reflected from the HP produced by earlier supersonic solar wind disturbances. To determine the time-varying RTS, we incorporate Voyager 2 (V2) plasma data as a boundary condition into our 3D MHD simulations, which allows us to forecast the termination shock movement for nearly a year after the present V2 data. Our simulations indicate that the TS was at ~90 AU along the Sun-V1 line on 2007 August 14, the last tentative available date of the V2 data. After this, our simulation forecasts that RTS will decrease to a minimum distance in late 2007 or early 2008. This decrease will be mainly caused by the heliosheath returned pulse driven by the 2006 March event. Whether V2 will cross the TS or not in this period depends on the future solar wind ram pressure and also on the degree of the north-south asymmetry of the heliospheric structure. Some quantitative discussions are given.

Simulations of transient shock motion within a biological contoured-shock-tube system

This study is motivated by the author’s interest in developing needle-free powdered vaccine/drug delivery systems. One system configuration is called the Contoured Shock Tube (CST). Of great importance is the behaviour of a transonic gas flow with a strongly nonlinear starting process, which accelerates powdered vaccines in micro-form to a sufficient momentum to penetrate the outer layer of human skin or mucosal tissue. In this paper, an established Modified Implicit Flux Vector Splitting (MIFVS) solver for the Navier–Stokes equations is extended to numerically study these transient transonic gas flows. A low Reynolds number k – ϵ turbulence model, with the compressibility effect considered, is integrated into the MIFVS solver to predict the turbulent structures and interactions with inherent shock systems. The MIFVS is first calibrated for NASA validation case, NPARC, and the resulting flow characteristic are compared with experimental date and simulations published. The MIFVS calculation with the modified k – ϵ model shows the best agreement. Subsequently, the MIFVS is applied to model the transient gas flow within a biolistic CST prototype. Comparison with experimental pressure traces shows the MIFVS captures gas flow mechanics with more accuracy than calculations with a commercial code (Fluent). This illustrates that the MIFVS is well-suited to model the strongly nonlinear fluid dynamics associated with the CST biolistic particle delivery system.

Boundary driven phase transitions of the first order for systems of conservationlaws

We argue that a driven system with two particle species and hardcore interactionsgenerically undergoes at least two different types of boundary-driven first-order phasetransitions. One observes them in succession, bringing the system from a statewith a small density of right-movers to a fully jammed state, by keeping oneboundary fixed and changing gradually conditions at the other boundary. As inone-component systems, the phase transitions are caused by shock motion. MonteCarlo simulations and PDEs numerical integration of a specific model serve as anexample.

Effect of a Laser Pulse on a Normal Shock

A numerical study was conducted to understand the effect of a single laser pulse on a normal shock and shock boundary layer interaction. The goal is to examine the capability of a pulsed laser energy deposition to momentarily move a normal (terminal) shock upstream in a mixed-compression inlet so as to counteract the effect of a disturbance that would move the normal shock downstream. Two numerical models were used. The perfect gas model for energy pulse was developed at Rutgers University, and the commercial software GASPex was used as the flow solver. The real gas model was developed at the University of Minnesota. The research was conducted in two phases. First, the 3-D interaction of a laser pulse with an isolated normal shock at Mach 2 was examined using the perfect gas and real gas models. A detailed comparison of the computed flowfields indicates that the principal details of the interaction are accurately predicted by the perfect gas model. Second, the perfect gas model was used to simulate the 2-D interaction of a laser pulse with a normal shock including the effects of the interaction of the shock wave with a turbulent boundary layer. Three different dimensionless energy levels (∈ = 1, 10, and 100) were considered. The interaction at ∈ = 100 demonstrated a prominent upstream movement of the normal shock and a significant though temporary increase in the length of the separation region due to interaction of the compression wave (induced by the energy spot) with the separated boundary layer.