Longitudinal Disturbances Research Articles

A robust all-fiber active Q-switched 1-µm Yb3+ fiber laser

An all-fiber active Q-switched Yb3+-doped fiber laser at 1 µm is presented. The laser is composed of a ring resonator with an embedded all-fiber Q-switch element, based on a null coupler with an attached piezoelectric transducer (PZT). The PZT is used as an acoustic actuator, for inducing longitudinal acoustic disturbance along the null coupler and causing light coupling between the null coupler’s ports. A stable operation is achieved with an overall average output power of up to 275 mW at various pulse repetition rates (PRR), ranging from 10 to 35 kHz and typical pulse energy of 15 μJ. In addition, a self-monitoring method is implemented by an embedded microcontroller, in order to maintain stable Q-switch performance, in changing environmental conditions. An average power of 8.5 W and pulse energy of 420 μJ at a PRR of 20 kHz are demonstrated in a master oscillator power amplifier containing the Q-switched laser, followed by a power amplifier.

Are UHR patients who present with hallucinations alone at lower risk of transition to psychosis?

The aim of this study was to investigate whether Ultra High Risk for psychosis (UHR) patients who present with hallucinations alone at identification as UHR are at lower risk of transition to psychosis than UHR patients who present with symptoms other than hallucinations or hallucinations plus other symptoms.Our primary dataset was a retrospective “case-control” study of UHR patients (N=118). The second, independent dataset was a long-term longitudinal follow up study of UHR patients (N=416). We performed a survival analysis using Log-rank test and Cox regression to investigate the relationship between symptom variables and transition to a psychotic disorder.Hallucinations alone at baseline were not significantly associated with a reduced risk of transition to psychosis. In the case control study the presence of hallucinations when found in the absence of any thought disorder and visual hallucinations in the absence of substance misuse was associated with a reduced risk of transition to psychosis. In the longitudinal follow-up dataset perceptual disturbance found in the absence of a disorder of affect or emotion was associated with an increased risk of transition to psychosis.

Left atrial strain in patients with arterial hypertension

<p><span>Background: </span>Arterial hypertension (HTN) causes left ventricular (LV) cavity dysfunction even if ejection function (EF) remains preserved. Recent studies have shown that diastolic dysfunction and left atrial (LA) dilatation are also associated with myocardial dysfunction. The aim of the present study was to explore the nature of LA longitudinal function disturbances in hypertensive patients with normal LV and LA structure and conventional function parameters.</p><p><span>Methods: </span>Peak atrial longitudinal strain (PALS) was evaluated in 78 patients with systemic HTN and preserved EF (≥ 55%) divided in 41 patients with diastolic dysfunction but no hypertrophy (group HTNdd), and 37 patients with no diastolic dysfunction or hypertrophy (group eHTN). Results were compared with those from 38 age and gender-matched healthy controls.</p><p><span>Results: </span>Indexed LA area and indexed LA volume were within the normal range and not different between the two patient groups and controls. eHTN group had reduced global PALS (p < 0.001) and four-and two-chamber average PALS (p < 0.001 for both). Similar abnormalities were seen in HTNdd group but to a worse degree (P < 0.01 for both). LV EF was not different between the eHTN and HTNdd groups compared to controls. LV E/e’ ratio was the strongest predictor of reduced global PALS in both eHTN and HTNdd groups.</p><p><span>Conclusion: </span>Asymptomatic untreated HTN patients with preserved LVEF and normal diastolic function have compromised LA strain despite normal cavity size, consistent with preclinical LA myocardial dysfunction.</p>

Exact propagating nonlinear singular disturbances in strongly coupled dusty plasmas

The dynamical response of the strongly coupled dusty plasma medium has recently been described by utilizing the Generalized Hydrodynamic (GHD) model equations. The GHD equations capture the visco-elastic properties of the medium and have been successful in predicting a host of phenomena (e.g., existence of novel transverse shear waves in the fluid medium, modification of longitudinal wave dispersion by elastic effects, etc.) which have found experimental confirmation. In this paper, the nonlinear longitudinal response of the medium governed by GHD equations in strong coupling limit is discussed analytically. The structure of the equations rules out the balance between dispersion and nonlinearity, thereby, forbidding soliton formation. However, a host of new varieties of nonlinear solutions are found to exist, which have singular spatial profiles and yet have conservative properties. For instance, existence of novel conservative shock structures with zero strength is demonstrated, waves whose breaking produces no dissipation in the medium are observed, propagating solutions which produce cusp like singularities can exist and so on. It is suggested that simulations and experiments should look for these novel nonlinear structures in the large amplitude strong coupling limit of longitudinal disturbances in dusty plasmas.

Lateral Flight Stability of Two Hovering Model Insects

The longitudinal disturbance motion of different insects at hovering flight has the same modal structure. Here, we consider the case of lateral motion. The lateral dynamic flight stability of two model insects, hoverfly and honeybee, at hovering flight is studied. The method of computational fluid dynamics is applied to compute the stability derivatives. The techniques of eigenvalue and eigenvector analysis are used to solve the equations of motion. Results show that the lateral disturbance motion of the hoverfly has three natural modes of motion: an unstable divergence mode, a stable oscillatory mode and a stable subsidence mode, and the flight is unstable; while the honeybee has a different modal structure (a stable slow subsidence mode, a stable fast subsidence mode, and a nearly neutrally stable oscillatory mode) and the flight is nearly neutrally stable. The change in modal structure between the two insects is due to their roll-moment/side-velocity derivative having different sign, and the sign difference is because that the hoverfly has a relatively small, but the honeybee has a relatively large, distance between the wing roots and the center of mass. Thus, unlike the case of longitudinal motion, for lateral motion, some insects have different modal structures and stability properties from others.

CFD analysis of free shear layer approximations for a pulsed air jet

We propose a numerical investigation on the validity of the free shear layer approximations for laminar 2D pulsed jets in both isothermal and heated configurations. More precisely, we determine the range of values for the Strouhal number St and amplitude of the perturbation A for which these approximations remains valid. For this, we have considered a two-dimensional air jet exhausting into a quiescent environment. At the nozzle exit, the flow is characterized by a uniform temperature T0 and submitted to a longitudinal and periodic velocity disturbance: u=u0+a∗sin(ωt). A finite volume method is performed to solve the general Navier–Stokes equations governing this flow type. The present study is evaluated by comparing with previous numerical results. Conclusions are drawn about the ability of a model that does not consider the shear layer approximations to handle properly the behavior of pulsed air jet. The discussion relates to the effect of the most significant parameters, such as the pulsation frequency and amplitude, on the flow characteristic fields. The effect of Reynolds and Grashof numbers was also examined. The results state that the free shear layer approximations are valid in different regions of the isothermal or the heated pulsed jet, and this, only for high Strouhal numbers (St⩾1) and for low pulsation amplitude (A<3%) Otherwise, these approximations remain valid for a free pulsed jet only in the fully developed flow region.

Stabilization of Flapping-Wing Micro-Air Vehicles in Gust Environments

This study presents an approach to develop a controller for stabilization of a flapping-wing micro-air vehicle operating in gusty environments. The rigid-wing micro-air vehicle is modeled as a nonlinear periodic system and the periodic shooting method is used to find a trimmed periodic orbit. A linearized discrete-time representation of the system is created about this trimmed periodic orbit. This linearized representation is used for control synthesis based on linear quadratic regulator theory. The kinematic variables defining the wing motion are used as control inputs. The controller is implemented on the nonlinear system model to stabilize the system in the presence of external disturbances, modeled as discrete gusts in this study. The performance of the controller in terms of the gust speed tolerance of the nonlinear, closed-loop system is compared for different sets of controller parameters. The linear quadratic regulator based controller is capable of stabilizing the system under both longitudinal and lateral gust disturbances; however, the maximum gust speed that can be tolerated by a given controller is influenced by a variety of parameters, as discussed in the paper. Numerical simulations show that tolerance of longitudinal gusts is far higher than lateral gust tolerance. The study also shows that lateral control of the micro-air vehicle can be achieved using only the wing-stroke magnitude and wing-stroke offset as the control inputs for each wing.

First comparison of wave observations from CoMP and AIA/SDO

Waves have long been thought to contribute to the heating of the solar corona and the generation of the solar wind. Recent observations have demonstrated evidence of quasi-periodic longitudinal disturbances and ubiquitous transverse wave propagation in many different coronal environments. This paper investigates signatures of different types of oscillatory behaviour, both above the solar limb and on-disk, by comparing findings from the Coronal Multi-channel Polarimeter (CoMP) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) for the same active region. We study both transverse and longitudinal motion by comparing and contrasting time-distance images of parallel and perpendicular cuts along/across active region fan loops. Comparisons between parallel space-time features in CoMP Doppler velocity and transverse oscillations in AIA images are made, together with space-time analysis of propagating quasi-periodic intensity features seen near the base of loops in AIA. Signatures of transverse motions are observed along the same magnetic structure using CoMP Doppler velocity (Vphase=600-750km/s, P=3-6mins) and in AIA/SDO above the limb (P=3-8mins). Quasi-periodic intensity features (Vphase=100-200km/s, P=6-11mins) also travel along the base of the same structure. On the disk, signatures of both transverse and longitudinal intensity features were observed by AIA; both show similar properties to signatures found along structures anchored in the same active region three days earlier above the limb. Correlated features are recovered by space-time analysis of neighbouring tracks over perpendicular distances of <2.6Mm.

Nonlinear Alfvén wave dynamics at a 2D magnetic null point: ponderomotive force

Context : In the linear, {\beta}=0 MHD regime, the transient properties of MHD waves in the vicinity of 2D null points are well known. The waves are decoupled and accumulate at predictable parts of the magnetic topology: fast waves accumulate at the null point; whereas Alfv\'en waves cannot cross the separatricies. However, in nonlinear MHD mode conversion can occur at regions of inhomogeneous Alfv\'en speed, suggesting that the decoupled nature of waves may not extend to the nonlinear regime. Aims: We investigate the behaviour of low-amplitude Alfv\'en waves about a 2D magnetic null point in nonlinear, {\beta}= 0 MHD. Methods: We numerically simulate the introduction of low-amplitude Alfv\'en waves into the vicinity of a magnetic null point using the nonlinear LARE2D code. Results: Unlike in the linear regime, we find that the Alfv\'en wave sustains cospatial daughter disturbances, manifest in the transverse and longitudinal fluid velocity, owing to the action of nonlinear magnetic pressure gradients (viz. the ponderomotive force). These disturbances are dependent on the Alfv\'en wave and do not interact with the medium to excite magnetoacoustic waves, although the transverse daughter becomes focused at the null point. Additionally, an independently propagating fast magnetoacoustic wave is generated during the early stages, which transports some of the initial Alfv\'en wave energy towards the null point. Subsequently, despite undergoing dispersion and phase-mixing due to gradients in the Alfv\'en-speed profile (Grad c_A \neq 0) there is no further nonlinear generation of fast waves. Conclusions: We find that Alfv\'en waves at 2D cold null points behave largely as in the linear regime, however they sustain transverse and longitudinal disturbances - effects absent in the linear regime - due to nonlinear magnetic pressure gradients.

An Optimal Estimation of Aerodynamic Parameters for GPS Guided Missile

The disturbance factors in the missile flight are obvious,so the analytical equations of missile longitudinal disturbance of movement is derived . The missile body longitudinal aerodynamic parameters are fitting out with wind tunnel experimental data. The equation of system state takes the aerodynamic coefficient errors interference source and GPS error sources as the state variables, and the system measurement equation adopts GPS pseudo-range measurements. In passive segment, the aerodynamic coefficients disturbing errors sources of th missile longitudinal plane is estimated optimally with extended Kalman filter. The results show that the method accelerate the convergence speed of the disturbance source errors, and improve the estimation accuracy of the perturbation errors.

On Ponderomotive Effects Induced by Alfvén Waves in Inhomogeneous 2.5D MHD Plasmas

Where spatial gradients in the amplitude of an Alfvén wave are non-zero, a nonlinear magnetic-pressure gradient acts upon the medium (commonly referred to as the ponderomotive force). We investigate the nature of such a force in inhomogeneous 2.5D MHD plasmas by analysing source terms in the nonlinear wave equations for the general case of inhomogeneous B and ρ, and consider supporting nonlinear numerical simulations. Our equations indicate that there are two distinct classes of ponderomotive effect induced by Alfvén waves in general 2.5D MHD, each with both a longitudinal and transverse manifestation. i) Geometric effects: Gradients in the pulse geometry relative to the background magnetic field cause the wave to sustain cospatial disturbances, the longitudinal and transverse daughter disturbances – where we report on the transverse disturbance for the first time. ii) ∇(c A) effects: Where a pulse propagates through an inhomogeneous region (where the gradients in the Alfvén-speed profile c A are non-zero), the nonlinear magnetic-pressure gradient acts to accelerate the plasma. Transverse gradients (phase mixing regions) excite independently propagating fast magnetoacoustic waves (generalising the result of Nakariakov, Roberts, and Murawski (Solar Phys. 175, 93, 1997)) and longitudinal gradients (longitudinally dispersive regions) perturb along the field (thus creating static disturbances in β=0, and slow waves in β≠0). We additionally demonstrate that mode conversion due the nonlinear Lorentz force is a one-way process, and does not act as a mechanism to nonlinearly generate Alfvén waves due to propagating magnetoacoustic waves. We conclude that these ponderomotive effects are induced by an Alfvén wave propagating in any MHD medium, and have the potential to have significant consequences on the dynamics of energy transport and aspects of dissipation provided the system is sufficiently nonlinear and inhomogeneous.

Tibial bowing in children

Abstract There is a broad collection of different conditions that can lead to tibial bowing. In this article we review the causes of tibial deformity in children and give an overview of the management of this group. Readers should gain an awareness of the varying presentations of these different conditions and be able to differentiate between the relatively benign and the more challenging pathologies that present in childhood. An appreciation of the natural history and classification of these disorders and an understanding of the various management options will allow the appropriate counselling of affected families. In this article we will review normal childhood development and cover the causes of genu varum deformity, such as Blount's disease, metabolic disorders and skeletal dysplasias. We will also consider the challenging condition of congenital pseudarthrosis of the tibia and review some of the congenital longitudinal growth disturbances that can affect the tibia.

Longitudinal and transverse disturbances in unbounded, gas-fluidized beds

A temporal linear stability analysis is performed on the continuum-averaged equations of motion for the fluid and dispersed particle phases which describe a uniform, boundless fluidized bed. The present analysis considers disturbances ranging from purely transverse to purely longitudinal, with respect to the flow direction of the base-state fluid velocity. It is found that disturbances over the entire horizontal-vertical wavenumber spectrum may be unstable, although the dominant disturbance is either the classically recognized longitudinal disturbance travelling in the vertical direction or a non-oscillating, purely transverse disturbance. The nature of the preferred mode depends upon the dimensionless parameters of the system. For all parameter cases the growth constants of the two modes remain within an order of magnitude of each other, thereby highlighting the importance of both modes in stability considerations of this system. Dispersion relations for each of the separated horizontal and vertical modes are derived and long-wavelength analyses are performed on the corresponding relations. It is shown that the unstable transverse mode shares similar mechanisms with the longitudinal mode, and that the dominant stabilizing mechanism for each mode is the same for closely-packed beds.

Active Control of the Longitudinal-Lateral Vibration of a Shaft-Plate Coupled System

The coupled longitudinal-lateral vibration of a shaft-plate system and its suppression by means of a feedback control scheme are discussed. A simplified model of the system is established through synthesis of frequency response functions (FRFs) and verified with the finite element method (FEM). This analytical model describes the coupled longitudinal-lateral vibration of the system induced by longitudinal periodic excitation at the free end of the shaft. Based on this model, vibration control via longitudinal actuation on the shaft and active vibration cancellation are studied. The active control scheme is based on an adaptive feedback scenario and a novel mechanism of adaptation of the controller’s gain, which is proposed for time-varying dynamics induced by the variation of the axial spring stiffness. Simulation results have demonstrated that the control scheme is effective in attenuating vibration of the system. Furthermore, axial actuation on the shaft is able to cancel the effect of the longitudinal disturbance acting at the free end of the shaft and consequently reduces the internal forces as well as the vibration in the plate. However, deviation of the actuation force from the shaft axis will deteriorate control of the lateral vibration and sufficiently small deviation needs to be guaranteed.

A new guidance law for a tactical surface-to-surface missile

Modern tactical surface-to-surface missiles, equipped with strapdown inertial navigation systems, achieve very good accuracy compared with free-flight rockets. The probable range dispersion mainly depends on instruments errors and longitudinal disturbances such as rocket motor total-impulse deviation as well as differences between the estimated and actual values of the axial force and head wind. This paper gives an extension of the correlated velocity concept for surface-to-surface missiles without a thrust-terminating mechanism. The calculated parameters of the correlated velocity are stored into the memory of an onboard guidance computer. On the basis of the correlated velocity concept, the modified proportional navigation with the adjustment of the time-to-go of the missile to the target was proposed. It is shown that the new guidance law can compensate for the longitudinal disturbances of different levels successfully. The effectiveness of the proposed guidance method was confirmed by means of the calculated probable range and lateral dispersion for the anticipated disturbances in the guidance system.

The homogenized equations of motion for an activated elastic lamination in plane strain

AbstractThe equations of motion for an elastic laminar spatial‐temporal composite are investigated. It is assumed that the composite is binary, that is, it is assembled of two original constituents capable of changing (in space‐time) their material density, as well as their material stiffness. The condition of plane strain was then imposed on the composite. The paper begins by attempting to evaluate the materials' average Lagrangian (action density). In doing so, it immediately becomes apparent that expressions are needed for average momentum and stress. Both quantities are found to depend linearly on average strain and average velocity. After calculating the general Euler equations of motion, isotropy was assumed, and two additional forces (one being of a Coriolis nature) were found in the averaged equations of elastodynamics due to the presence of simultaneous change in both inertial and elastic properties of the original material constituents. The appearance of these two forces is a consequence of both dynamics and plane strain; the Coriolis type force disappears in the case of one dimensional strain that arises when longitudinal dynamic disturbances propagate along an elastic bar.

Thermoconvective instabilities in an inclined porous channel heated from below

The thermoconvective instability in an inclined rectangular channel filled with a fluid saturated porous medium and heated from below with a uniform flux is investigated. A stationary parallel buoyant flow with a linear temperature change in the transverse direction is considered. The linear stability to transverse and longitudinal roll disturbances of this basic state is examined. The thermoconvective instability onset of transverse rolls occurs when the Darcy–Rayleigh number exceeds a critical value, that increases with the inclination angle. The critical Darcy–Rayleigh number is discontinuous at the inclination angle 23.4749° above the horizontal. It is shown that, when the inclination angle exceeds 31.3618°, either the transverse rolls are stable or a second discontinuous transition to a higher branch of instability occurs. The longitudinal rolls may be unstable for every inclination except for the vertical. A stability diagram is sketched displaying the geometrical conditions (inclination angle, aspect ratio) such that either the transverse rolls or the longitudinal rolls is the preferred mode of instability.

Investigation on Particle Dispersion in Pulsing Jet Using Vortex Method

The evolution of particle dispersion in pulsing jet is simulated by the vortex method. The particles are traced using the Lagrangian approach. The objective of this study is to examine the effects of excitations on particle dispersion in the jet flow. In the present study, the longitudinal periodic disturbance is activated at the jet exit to form a pulsing jet. The results display spatially symmetric vortex structures, that can be generated periodically, and the leap-frogging process is also achieved successfully. In addition, the development of the vortex structures strongly influences on the particle dispersion. Results show that the symmetric particle distributions are obtained in pulsing jet attributed to the symmetric vortex structures. Furthermore, for the largest Stokes number (St=23.6 in this study), the particle dispersion process can be modified by this forcing type, and achieved a better dispersion variance using low-frequency disturbance.

A kinetic model for chemical reactions without barriers: transport coefficients andeigenmodes

The kinetic model of the Boltzmann equation proposed in the work of Kremer and Soares2009 for a binary mixture undergoing chemical reactions of symmetric type which occurwithout activation energy is revisited here, with the aim of investigating in detail thetransport properties of the reactive mixture and the influence of the reaction process on thetransport coefficients. Accordingly, the non-equilibrium solutions of the Boltzmannequations are determined through an expansion in Sonine polynomials up to the first order,using the Chapman–Enskog method, in a chemical regime for which the reactionprocess is close to its final equilibrium state. The non-equilibrium deviations areexplicitly calculated for what concerns the thermal–diffusion ratio and coefficients ofshear viscosity, diffusion and thermal conductivity. The theoretical and formalanalysis developed in the present paper is complemented with some numericalsimulations performed for different concentrations of reactants and products ofthe reaction as well as for both exothermic and endothermic chemical processes.The results reveal that chemical reactions without energy barrier can induce anappreciable influence on the transport properties of the mixture. Oppositely tothe case of reactions with activation energy, the coefficients of shear viscosityand thermal conductivity become larger than those of an inert mixture whenthe reactions are exothermic. An application of the non-barrier model and itsdetailed transport picture are included in this paper, in order to investigate thedynamics of the local perturbations on the constituent number densities, andvelocity and temperature of the whole mixture, induced by spontaneous internalfluctuations. It is shown that for the longitudinal disturbances there exist twohydrodynamic sound modes, one purely diffusive hydrodynamic mode and one kineticmode.

Disturbance Field Characteristics of a Transversely Excited Burner

Transverse acoustic instabilities in premixed, swirl-stabilized flames are an important problem in low NOx combustors. Transverse excitation of swirling flames involves complex interactions between acoustic waves and fluid mechanic instabilities. This paper presents high-speed PIV characterization of the flow field characteristics of a swirling, annular jet under reacting and nonreacting conditions. These data show that the flame response to transverse acoustic excitation is a superposition of acoustic and vortical disturbances that fluctuate in both the longitudinal and transverse direction. In the nozzle near-field region, the disturbance field is a complex superposition of short wavelength and convecting vortical disturbances, as well as longer wavelength transverse and longitudinal acoustic disturbances. Very near the nozzle, distinct vortical structures are evident that are associated with the separating inner and outer annulus shear layers. Their relative phasing on the left and right side of the burner annulus changes by 180° under conditions where the burner centerline is nominally at a transverse acoustic velocity node and antinode. These suggest that the dominant excited instability mode of the annular jet changes from axisymmetric to helical as the structure of the acoustic mode shape changes. Farther downstream, these structures disappear rapidly and the disturbance field is dominated by the longer wavelength, transverse acoustic field.