Helical Oscillation Research Articles

A QPO in Mkn 421 from Archival RXTE Data

We report a 325(−7, +8) day quasi-periodic oscillation (QPO) in the X-ray emission of the blazar Mkn 421 based on data obtained with the Rossi X-ray Timing Explorer satellite. The QPO is seen prominently in the ASM data (at least 15 cycles) due to the fact that it has had near-continuous sampling for more than a decade. The PCA data, where the sampling is not uniform and shows many large gaps, provide supporting evidence at lower significance. The QPOs are an important observable in accretion disks, can be modulated by various orbital timescales, and may be generated by a number of mechanisms. They have been studied extensively in X-ray binaries and should be present in active galactic nuclei (AGN) if they are governed by a common set of physical principles. In jetted sources, QPOs can probe jet–disk interactions or helical oscillations. This QPO has previously been claimed intermittently in X-ray, radio, and gamma-ray data, but the continuous, 15 yr extent (1996–2011) of the ASM observations (in which Mkn 421 is the brightest AGN observed) provides a unique window. The QPO appears to be present for nearly the entire extent of the ASM observations. We explore various physical origins and modulating mechanisms, particularly interpretations of the QPO as a result of disk–jet interactions, due to either an accretion disk limit cycle, jet instabilities, or helical motions. Limit cycle oscillations would not interact with either Keplerian or Lense–Thirring modulated oscillations; however, those associated with jet instabilities or helical motions in the jet would likely be modulated by Lense–Thirring precession.

Global oscillations in low-density round jets with parabolic velocity profiles

Experiments and spatiotemporal stability analysis are carried out to study the global oscillations in laminar low-density round jets with parabolic velocity profiles. The experimental results of laminar low-density jets with parabolic velocity profiles exhibit global axisymmetric oscillations. The spatiotemporal stability results based on base profiles from numerical simulations are consistent with the present experimental results. These results differ from the prediction of stability study by Coenen et al. (Phys. Fluids, vol. 20, 2008, p. 074104). They reported that the low-density jets with near parabolic velocity profiles show global helical oscillations. It is observed that the method used by Coenen et al. is not able to predict the nature of global oscillations observed in experiments for low-density jets with near parabolic profiles. The present spatiotemporal stability results demonstrate that the base flows from Navier–Stokes equations are required to predict the critical conditions observed in experiments for low-density jets with near parabolic velocity profiles. The breakdown distance of globally unstable low-density jets scales with $(Re-Re_c)^{-1/2}$ ( $Re_c$ is the critical Reynolds number), consistent with the scaling law obtained from the nonlinear global mode of the Ginzburg–Landau equation.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Studying Helical Rod Oscillations Exposed to Vortex Flow

Studying Helical Rod Oscillations Exposed to Vortex Flow

A weakly nonlinear analysis of the precessing vortex core oscillation in a variable swirl turbulent round jet

We study the emergence of precessing vortex core (PVC) oscillations in a swirling jet experiment. We vary the swirl intensity while keeping the net mass flow rate fixed using a radial-entry swirler with movable blades upstream of the jet exit. The swirl intensity is quantified in terms of a swirl number . Time-resolved velocity measurements in a radial–axial plane anchored at the jet exit for various values are obtained using stereoscopic particle image velocimetry. Spectral proper orthogonal decomposition and spatial cross-spectral analysis reveal the simultaneous emergence of a bubble-type vortex breakdown and a strong helical limit-cycle oscillation in the flow for where . The oscillation frequency, , and the square of the flow oscillation amplitudes vary linearly with . A solution for the coherent unsteady field accurate up to ( ) is determined from the nonlinear Navier–Stokes equations, using the method of multiple scales. We show that onset of bubble type vortex breakdown at , results in a marginally stable, helical linear global hydrodynamic mode. This results in the stable limit-cycle precession of the breakdown bubble. The variation of with is determined from the Stuart–Landau equation associated with the PVC. Reasonable agreement with the corresponding experimental result is observed, despite the highly turbulent nature of the flow in the present experiment. Further, amplitude saturation results from the time-averaged distortion imposed on the flow by the PVC, suggesting that linear stability analysis may predict PVC characteristics for .

Coupling Modes of an Underexpanded Twin Axisymmetric Jet

An experimental investigation into the coupling behavior of screeching axisymmetric twin supersonic jets is presented. Acoustic measurements and schlieren photography are used to identify four distinct coupling modes over a wide range of pressure ratios and nozzle spacings. Nozzle spacing is found to have a significant effect on the selection of these modes as well as the transition between them. A discrete jump in the peak screech frequency is seen to accompany most but not all mode switches. Statistical analysis of schlieren photography provides evidence that the modes consist of two flapping (symmetric and antisymmetric) and two different helical oscillations. These coupling modes are shown to have a limited effect on the convection velocity of the large-scale structures within the jet, and as such, the characteristic length scales are investigated as a source of mode switching.

Numerical study of the flow and the near acoustic fields of an underexpanded round free jet generating two screech tones

The flow and near acoustic fields of a supersonic round free jet are explored using a compressible large eddy simulation. At the exit of a straight pipe nozzle, the jet is underexpanded, and is characterized by a Nozzle Pressure Ratio of 4.03 and a Temperature Ratio of 1. It has a fully expanded Mach number of 1.56, an exit Mach number of 1, and a Reynolds number of 6×104. Flow snapshots, mean flow fields and convection velocity in the jet shear layers are consistent with experimental data and theoretical results. Furthermore, two screech tones are found to emerge in the pressure spectrum calculated close to the nozzle. Using a Fourier decomposition of the pressure fields, the two screech tones are found to be associated with anticlockwise helical oscillation modes. Besides, the frequencies of the screech tones and the associated oscillation modes both agree with theoretical predictions and measurements. Moreover, pressure fields filtered at the screech frequencies reveal the presence of hydrodynamic-acoustic standing waves. In those waves, the regions of highest amplitude in the jet are located in the fifth and the sixth cells of the shock cell structure. The two screech tones therefore seem to be linked to two different loops established between the nozzle and the fifth and sixth shock cells, respectively. In the pressure fields, three other acoustic components, namely the low-frequency mixing noise, the high-frequency mixing noise and the broadband shock-associated noise, are noted. The directivity and frequency of the mixing noise are in line with numerical and experimental studies. A production mechanism of the mixing noise consisting of sudden intrusions of turbulent structures into the potential core is discussed. Then, the broadband shock-associated noise is studied. This noise component is due to the interactions between the turbulent structure in the shear layers and the shocks in the jet. By analyzing the near pressure fields, this noise component is found to be produced mainly in the sixth shock cell. Finally, using the size of this shock cell in the classical theoretical model of this noise component, a good agreement is found with the simulation results.

Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets

The aeroacoustic feedback loop establishing in a supersonic round jet impinging on a flat plate normally has been investigated by combining compressible large-eddy simulations and modelling of that loop. At the exit of a straight pipe nozzle of radius $r_{0}$, the jet is ideally expanded, and has a Mach number of 1.5 and a Reynolds number of$6\times 10^{4}$. Four distances between the nozzle exit and the flat plate, equal to$6r_{0}$,$8r_{0}$,$10r_{0}$and$12r_{0}$, have been considered. In this way, the variations of the convection velocity of the shear-layer turbulent structures according to the nozzle-to-plate distance are shown. In the spectra obtained inside and outside of the flow near the nozzle, several tones emerge at Strouhal numbers in agreement with measurements in the literature. At these frequencies, by applying Fourier decomposition to the pressure fields, hydrodynamic-acoustic standing waves containing a whole number of cells between the nozzle and the plate and axisymmetric or helical jet oscillations are found. The tone frequencies and the mode numbers inferred from the standing-wave patterns are in line with the classical feedback-loop model, in which the loop is closed by acoustic waves outside the jet. The axisymmetric or helical nature of the jet oscillations at the tone frequencies is also consistent with a wave analysis using a jet vortex-sheet model, providing the allowable frequency ranges for the upstream-propagating acoustic wave modes of the jet. In particular, the tones are located on the part of the dispersion relations of the modes where these waves have phase and group velocities close to the ambient speed of sound. Based on the observation of the pressure fields and on frequency–wavenumber spectra on the jet axis and in the shear layers, such waves are identified inside the present jets, for the first time to the best of our knowledge, for a supersonic jet flow. This study thus suggests that the feedback loop in ideally expanded impinging jets is completed by these waves.

Numerical investigation of the influence of blade helicity on the performance characteristics of vertical axis tidal turbines

Previous research has shown that helical vertical axis turbines exhibit lower torque fluctuation levels than straight-bladed turbines; however little is known of the impact of blade helicity on turbine performance characteristics. To investigate these relationships the hydrodynamic characteristics of straight and helical-bladed vertical axis turbines were investigated using Three-Dimensional (3D) Computational Fluid Dynamics (CFD) models using a commercial Unsteady Reynolds Averaged Navier-Stokes (URANS) solver. Simulations of power output, torque oscillations, and mounting forces were performed for turbines with overlap angles from 0° to 120° and section inclination angles from −15° to 45°. Results indicated that straight-bladed turbines with 0° blade overlap generated the highest power output. Helical turbines were found to generate decreasing power outputs as blade overlap angle increased due to the resultant blade inclination to the inflow. Blade section inclination to the inflow was also found to influence power output. Some benefits of helical-bladed turbines over their straight-bladed counterparts were established; helical turbine torque oscillation levels and mounting forces were reduced when compared to straight-bladed turbines. For both straight and helical-bladed turbines maximum mounting force levels were found to exceed the average force levels by more than 40%, with large cyclical loading forces identified.

SNS junctions in nanowires with spin-orbit coupling: Role of confinement and helicity on the subgap spectrum

We study normal transport and the subgap spectrum of superconductor-normal-superconductor (SNS) junctions made of semiconducting nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on the role of confinement effects in long ballistic junctions. In the normal regime, scattering at the two contacts gives rise to two distinct features in conductance: Fabry-Perot resonances and Fano dips. The latter arise in the presence of a strong Zeeman field $B$ that removes a spin sector in the leads (helical leads), but not in the central region. Conversely, a helical central region between nonhelical leads exhibits helical gaps of half-quantum conductance, with superimposed helical Fabry-Perot oscillations. These normal features translate into distinct subgap states when the leads become superconducting. In particular, Fabry-Perot resonances within the helical gap become parity-protected zero-energy states (parity crossings), well below the critical field ${B}_{c}$ at which the superconducting leads become topological. As a function of Zeeman field or Fermi energy, these zero modes oscillate around zero energy, forming characteristic loops, which evolve continuously into Majorana bound states as $B$ exceeds ${B}_{c}$. The relation with the physics of parity crossings of Yu-Shiba-Rusinov bound states is discussed.

Investigation of the Global Mode in Swirling Combustor Flows: Experimental Observations and Local and Global Stability Analysis

Self-excited helical flow oscillations are frequently observed in gas turbine combustors. In the present work a new approach is presented tackling this phenomenon with stability concepts. Three reacting swirling flows are investigated. All of them undergo vortex breakdown, but only two of them show self-excited global flow oscillations at well-defined frequencies. The oscillations feature a precession of the vortex core and synchronized Kelvin-Helmholtz instabilities in the shear layers. Based on the mean flow fields, local and global linear hydrodynamic stability analyses are carried out. The dampening effect of the Reynolds stresses is accounted for by an eddy viscosity estimated from the experimental results. Both the local and the global analysis successfully identify linear global modes as being responsible for the large-scale flow oscillations and successfully predict their frequency. However, only the global analysis accurately predicts a globally stable flow field for the case without the oscillation, while the local analysis overpredicts the global growth rate. The predicted spatial distribution of the amplitude functions agree very well to the experimentally identified global mode. This successful application of global and local stability concepts to a complex and practically relevant flow configuration paves the way for the application of theoretically-founded passive and active control strategies.

Measurement of the intrinsic variability within protein crystals: implications for sample-evaluation and data-collection strategies

The advent of micro-focused X-ray beams has led to the development of a number of advanced methods of sample evaluation and data collection. In particular, multiple-position data-collection and helical oscillation strategies are now becoming commonplace in order to alleviate the problems associated with radiation damage. However, intra-crystal and inter-crystal variation means that it is not always obvious on which crystals or on which region or regions of a crystal these protocols should be performed. For the automation of this process for large-scale screening, and to provide an indication of the best strategy for data collection, a metric of crystal variability could be useful. Here, measures of the intrinsic variability within protein crystals are presented and their implications for optimal data-collection strategies are discussed.

Tomographic reconstruction of heat release rate perturbations induced by helical modes in turbulent swirl flames

Swirl flows with vortex breakdown are widely used in industrial combustion systems for flame stabilization. This type of flow is known to sustain a hydrodynamic instability with a rotating helical structure, one common manifestation of it being the precessing vortex core. The role of this unsteady flow mode in combustion is not well understood, and its interaction with combustion instabilities and flame stabilization remains unclear. It is therefore important to assess the structure of the perturbation in the flame that is induced by this helical mode. Based on principles of tomographic reconstruction, a method is presented to determine the 3-D distribution of the heat release rate perturbation associated with the helical mode. Since this flow instability is rotating, a phase-resolved sequence of projection images of light emitted from the flame is identical to the Radon transform of the light intensity distribution in the combustor volume and thus can be used for tomographic reconstruction. This is achieved with one stationary camera only, a vast reduction in experimental and hardware requirements compared to a multi-camera setup or camera repositioning, which is typically required for tomographic reconstruction. Different approaches to extract the coherent part of the oscillation from the images are discussed. Two novel tomographic reconstruction algorithms specifically tailored to the structure of the heat release rate perturbations related to the helical mode are derived. The reconstruction techniques are first applied to an artificial field to illustrate the accuracy. High-speed imaging data acquired in a turbulent swirl-stabilized combustor setup with strong helical mode oscillations are then used to reconstruct the 3-D structure of the associated perturbation in the flame.

1121 SWIRL FLAME INTERACTION WITH TRANSVERSE AND AXIAL ACOUSTIC FORCING

The present study is an experimental investigation on the effect of axial and transverse acoustic excitation on a swirl-stabilized flame. A single burner test-rig with transverse extensions to facilitate transverse acoustic excitation is employed in this experimental study. The swirl flow is established using a generic radial swirl generator. Results presented here include phase-averaged crosswise flow field oscillations at isothermal flow conditions and partial orthogonal decomposition (POD) analysis of flow field acquired in the streamwise plane at isothermal and reacting conditions. Through analysis of radial velocity profile oscillations obtained from crosswise measurements and modal decomposition of the velocity field in the streamwise direction, conclusions have been derived for flow and flame response to transverse forcing. It is found that while out-of-phase transverse forcing can interact with natural helical oscillations in the swirl flow, the effect of in-phase forcing is dominantly observed in the form of flow response to induced axial oscillations. The study complements previous investigations on swirl flow and flame interaction with transverse acoustics, and provides information for future efforts on the issue of thermoacoustic instability in annular combustors.

An approach to the experimental determination of the bending stiffness of nanosize shells

The problem of the experimental determination of elastic moduli of nanoscale objects is of present interest. The determination of the elastic moduli of thin macroscopic shells is usually based on experiments with plates. It is known that, when grown using certain techniques, nanoobjects are obtained only in the form of shells. Therefore, it is necessary to develop a method for determining the elastic moduli of nanoobjects on the basis of experiments with shells. Experimental determination of the bending stiffness of nanosize shells presents a serious problem, because for such widespread nanoobjects as nanotubes and fullerenes under arbitrary deformation, the material is subjected to both bending and tension. Therefore, all parameters (e.g., natural frequencies) that can be measured directly are complicated functions of both bending and tension stiffness. In recent years, together with nanotubes and fullerenes, nanoobjects of a more intricate configuration have been obtained [1‐4]. Nanosize cylindrical helices [1, 3] are of particular interest in connection with the possible experimental determination of bending stiffness. This is due to the fact that (1) in helical shells under arbitrary deformation, the material is mainly bent, so that the material tension effect can be neglected when interpreting experimental data; and (2) the natural oscillation shapes of helical shells are much more easily observed than those of cylindrical shells associated with pure bending of the material. The latter statement is illustrated in Fig. 1, which presents the first four helical shell oscillation shapes. The analysis of helical shell dynamics [5] presented below may be a theoretical foundation for experimental testing of the applicability of the continuum theory to (a) the calculation of mechanical characteristics of nanoobjects and (b) the experimental determination of the bending stiffness of nanoshells.

Sound sources of the screech tone radiated from circular supersonic jet oscillating in the helical mode

The generation mechanism of the screech tone in the helical oscillation mode is investigated using a series of instantaneous photographs. From the analysis of the photographs, six evanescent sound sources are observed as prominent points along the jet axis. These sound sources move along circular orbits in the planes perpendicular to the jet axis and slightly downstream of the rear edges of each shock cell. The speed of the moving sound sources is supersonic and moving Mach cones generated behind the moving sound sources form the helical-shaped wave fronts of the screech tone. The existence of the moving Mach cones about the jet axis was confirmed by comparing Schlieren photographs and drawings of the envelopes of the moving Mach cones. [Work supported by The Mitsubishi Foundation.]

Gyroscopic motion of superfluid-trapped atomic condensates

The gyroscopic motion of a trapped Bose gas containing a vortex is studied. Wemodel the system as a classical top, as a superposition of coherent hydrodynamicstates, by solution of the Bogoliubov equations, and by integration of thetime-dependent Gross–Pitaevskii equation. The frequency spectrum of Bogoliubovexcitations, including quantum frequency shifts, is calculated and the quantalprecession frequency is found to be consistent with experimental results, althougha small discrepancy does exist. The superfluid precession is found to be welldescribed by the classical and hydrodynamic models. However, the frequencyshifts and helical oscillations associated with vortex bending and twisting requirea quantal treatment. In gyroscopic precession, the Kelvin modes m = ±1are the dominant features giving a vortex kink or bend, while them = +2mode is found to be the dominant Kelvin wave associated with vortex twisting.

2428 超音速円形ジェットから放射されるスクリーチ・トーンの音源に付随するマッハ・コーン

The generation mechanism of the screech tone in the helical oscillation mode is investigated using a series of instantaneous schlieren photographs. From the photographs, six evanescent sound sources are observed as prominent points along the jet axis. These sound sources move along circular orbits in the planes perpendicular to the jet axis just downstream of the rear edges of each shock cell. The speed of moving sound sources is supersonic and the moving Mach cones generated behind the moving sound sources form the helical-shaped wave front of the screech tone. Existence of the moving Mach cones about the jet axis was confirmed by comparing schlieren photographs and drawings of the envelopes of the moving Mach cones. The helical vortical structure appears to overlap on the envelopes of the moving Mach cones.

Sound Sources of Screech Tone Radiated from Circular Supersonic Jet Oscillating in the Helical Mode

The generation mechanism of the screech tone in the helical oscillation mode is investigated using a series of instantaneous schlieren photographs. From the photographs, six evanescent sound sources are observed as prominent points along the jet axis. These sound sources move along circular orbits in the planes perpendicular to the jet axis and just downstream of the rear edges of each shock cell. The speed of moving sound sources is supersonic and the moving Mach cones generated behind the moving sound sources form the helical-shaped wave front of the screech tone. Existence of the moving Mach cones about the jet axis was confirmed by comparing schlieren photographs and drawings of the envelopes of the moving Mach cones. The helical vortical structure appears to overlap on the envelopes of the moving Mach cones.

2008 ヘリカル・モードで振動する不足膨張円形ジェットの周りで周回するマッハ・コーン

Existence of Mach cone and helical vortical structure in the helical oscillation mode of underexpanded circular jet was confirmed by using schlieren instantaneous photographs and drawings of envelopes of the Mach cones by the superposition of spherical sound waves radiated from two moving sound sources about the jet axis at a supersonic speed. The enveloes of a Mach cone are observed as a V-shaped pattern composed of a pair of clear fine lines starting from a prominent point, which rotates about the jet axis. The helical vortical structure is observed as bright pattern of the gathering of the tiny specks around the jet. It always apperars to overlap on the envelopes of the moving Mach cones.