Symmetric Force Research Articles

The OWLeS IOP2b Lake-Effect Snowstorm: Dynamics of the Secondary Circulation

Intense lake-effect snowfall results from a long-lake-axis-parallel (LLAP) precipitation band that often forms when the flow is parallel to the long axis of an elongated body of water, such as Lake Ontario. The intensity and persistence of the localized precipitation along the downwind shore and farther inland suggests the presence of a secondary circulation that helps organize such a band, and maintain it for some time as the circulation is advected inland. Unique airborne vertical-plane dual-Doppler radar data are used here to document this secondary circulation in a deep, well-organized LLAP band observed during intensive observing period (IOP) 2b of the Ontario Winter Lake-effect Systems (OWLeS) field campaign. The circulation, centered on a convective updraft, intensified toward the downwind shore and only gradually weakened inland. The question arises as to what sustains such a circulation in the vertical plane across the LLAP band. WRF Model simulations indicate that the primary LLAP band and other convergence zones observed over Lake Ontario during this IOP were initiated by relatively shallow airmass boundaries, resulting from a thermal contrast (i.e., land-breeze front) and differential surface roughness across the southern shoreline. Airborne radar data near the downwind shore of the lake indicate that the secondary circulation was much deeper than these shallow boundaries and was sustained primarily by rather symmetric solenoidal forcing, enhanced by latent heat release within the updraft region.

Influence of Antisymmetrical Actions at Reinforced Concrete Cable-Stayed Bridge

Calculation of the cross-sectional properties of the concrete box-girder with two-cells using Vlasov`s theory and Generalized Beam Theory. Calculation of the internal forces of the St. Venant torsion, the warping torsion, the antisymmetrical and the symmetrical distortion on the basis analogy with beam on the elastic foundation loaded by transverse actions and tension force at the end of the beam. Creating of influence lines of torsion, antisymmetrical distortion and symmetrical distortion internal forces of large concrete cable-stayed bridge with continuous box-girders with two spans. The application for real reinforced concrete Harp bridge over pond Jordán near Tábor in Czech Republic is investigated. Evaluation of internal lines for action according to Eurocode EN 1991-2.

A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment

We present an experimental study of flows in a cylindrical rotating annulus convectively forced by local heating in an annular ring at the bottom near the external wall and via a cooled circular disk near the axis at the top surface of the annulus. This new configuration is distinct from the classical thermally driven annulus analogue of the atmosphere circulation, in which thermal forcing is applied uniformly on the sidewalls, but with a similar aim to investigate the baroclinic instability of a rotating, stratified flow subjected to zonally symmetric forcing. Two vertically and horizontally displaced heat sources/sinks are arranged, so that in the absence of background rotation, statically unstable Rayleigh–Bénard convection would be induced above the source and beneath the sink, thereby relaxing strong constraints placed on background temperature gradients in previous experimental configurations based on the conventional rotating annulus. This better emulates local vigorous convection in the tropics and polar regions of the atmosphere while also allowing stably-stratified baroclinic motion in the central zone of the annulus, as in mid-latitude regions in the Earth’s atmosphere. Regimes of flow are identified, depending mainly upon control parameters that in turn depend on rotation rate and the strength of differential heating. Several regimes exhibit baroclinically unstable flows which are qualitatively similar to those previously observed in the classical thermally driven annulus. However, in contrast to the classical configuration, they typically exhibit more spatio-temporal complexity. Thus, several regimes of flow demonstrate the equilibrated co-existence of, and interaction between, free convection and baroclinic wave modes. These new features were not previously observed in the classical annulus and validate the new setup as a tool for exploring fundamental atmosphere-like dynamics in a more realistic framework. Thermal structure in the fluid is investigated and found to be qualitatively consistent with previous numerical results, with nearly isothermal conditions, respectively, above and below the heat source and sink, and stably-stratified, sloping isotherms in the near-adiabatic interior.

Circular Cylinder Drag Reduction By Three-Electrode Plasma Symmetric Forcing

This study reports an efficient reduction of the drag exerted by a flow on a cylinder when the former is forced with a plasma actuator. A three-electrode plasma device (TED) disposed on the surface of the body is considered, and the effect of the actuation frequency and amplitude is studied. Particle image velocimetry (PIV) measurements provided a detailed information that was processed to obtain the time-averaged drag force and to compare the performances of TED actuator and the canonical dielectric discharge barrier actuator. For the Reynolds number considered (Re = 5500), excitations with the TED actuator were more efficient, achieving drag reductions that attained values close to 40% with high net energy savings. The reduction of coherent structures using the instantaneous vorticity fields and a clustering technique allowed us to gain insight into the physical mechanisms involved in these phenomena. This highlights that the symmetrical forcing of the wake flow at its resonant frequency with the TED promotes symmetrical vorticity patterns which favor drag reductions.

Prosthetic model, but not stiffness or height, affects the metabolic cost of running for athletes with unilateral transtibial amputations.

Running-specific prostheses enable athletes with lower limb amputations to run by emulating the spring-like function of biological legs. Current prosthetic stiffness and height recommendations aim to mitigate kinematic asymmetries for athletes with unilateral transtibial amputations. However, it is unclear how different prosthetic configurations influence the biomechanics and metabolic cost of running. Consequently, we investigated how prosthetic model, stiffness, and height affect the biomechanics and metabolic cost of running. Ten athletes with unilateral transtibial amputations each performed 15 running trials at 2.5 or 3.0 m/s while we measured ground reaction forces and metabolic rates. Athletes ran using three different prosthetic models with five different stiffness category and height combinations per model. Use of an Ottobock 1E90 Sprinter prosthesis reduced metabolic cost by 4.3 and 3.4% compared with use of Freedom Innovations Catapult [fixed effect (β) = -0.177; P < 0.001] and Össur Flex-Run (β = -0.139; P = 0.002) prostheses, respectively. Neither prosthetic stiffness (P ≥ 0.180) nor height (P = 0.062) affected the metabolic cost of running. The metabolic cost of running was related to lower peak (β = 0.649; P = 0.001) and stance average (β = 0.772; P = 0.018) vertical ground reaction forces, prolonged ground contact times (β = -4.349; P = 0.012), and decreased leg stiffness (β = 0.071; P < 0.001) averaged from both legs. Metabolic cost was reduced with more symmetric peak vertical ground reaction forces (β = 0.007; P = 0.003) but was unrelated to stride kinematic symmetry (P ≥ 0.636). Therefore, prosthetic recommendations based on symmetric stride kinematics do not necessarily minimize the metabolic cost of running. Instead, an optimal prosthetic model, which improves overall biomechanics, minimizes the metabolic cost of running for athletes with unilateral transtibial amputations.NEW & NOTEWORTHY The metabolic cost of running for athletes with unilateral transtibial amputations depends on prosthetic model and is associated with lower peak and stance average vertical ground reaction forces, longer contact times, and reduced leg stiffness. Metabolic cost is unrelated to prosthetic stiffness, height, and stride kinematic symmetry. Unlike nonamputees who decrease leg stiffness with increased in-series surface stiffness, biological limb stiffness for athletes with unilateral transtibial amputations is positively correlated with increased in-series (prosthetic) stiffness.

Buckling delamination of the PZT/Metal/PZT sandwich circular plate-disc with penny-shaped interface cracks

The axisymmetric buckling delamination of the Piezoelectric/Metal/Piezoelectric (PZT/Metal/PZT) sandwich circular plate with interface penny-shaped cracks is investigated. The case is considered where open-circuit conditions with respect to the electrical displacement on the upper and lower surfaces, and short-circuit conditions with respect to the electrical potential on the lateral surface of the face layers are satisfied. It is assumed that the edge surfaces of the cracks have an infinitesimal rotationally symmetric initial imperfection and the development of this imperfection with rotationally symmetric compressive forces acting on the lateral surface of the plate is studied by employing the exact geometrically non-linear field equations and relations of electro-elasticity for piezoelectric materials. The sought values are presented in the power series form with respect to the small parameter which characterizes the degree of the initial imperfection. The zeroth and first approximations are used for investigation of stability loss and buckling delamination problems. It is established that the equations and relations related to the first approximation coincide with the corresponding ones of the three-dimensional linearized theory of stability of electro-elasticity for piezoelectric materials. The quantities related to the zeroth approximation are determined analytically, however the quantities related to the first approximation are determined numerically by employing Finite Element Method (FEM). Numerical results on the critical radial stresses acting in the layers of the plate are presented and discussed. In particular, it is established that the piezoelectricity of the face layer material causes an increase (a decrease) in the values of the critical compressive stress acting in the face (core) layer.

A novel flux-switching permanent magnet machine with v-shaped magnets

In this paper, firstly a novel 6-stator-coil/17-rotor-pole (6/17) flux-switching permanent magnet (FSPM) machine with V-shaped magnets, deduced from conventional 12/17 FSPM machines is proposed to achieve more symmetrical phase back-electromotive force (back-EMF), and smaller torque ripple by comparing with an existing 6/10 V-shaped FSPM machine. Then, to obtain larger electromagnetic torque, less torque ripple, and easier mechanical processing, two improved variants based on the original 6/17 V-shaped topology are proposed. For the first variant, the separate stator-core segments located on the stator yoke are connected into a united stator yoke, while for the second variant the stator core is a whole entity by adding magnetic bridges at the ends of permanent magnets (PMs). Consequently, the performances of the three 6/17 V-shaped FSPM machines, namely, the original one and the two variants, are conducted by finite element analysis (FEA). The results reveal that the first variant exhibits significantly larger torque and considerably improved torque per magnet volume, i.e., the magnet utilization ratio than the original one, and the second variant exhibits the smallest torque ripple, least total harmonic distribution (THD) of phase back-EMF, and easiest mechanical processing for manufacturing.

Static instability of beam-type NEMS subjected to symmetric electrostatic actuation based on couple stress theory and Timoshenko beam theory

The behavior of nanoelectromechanical systems subjected to symmetrical electrostatic actuation and symmetrical Casimir intermolecular force has been investigated. Two different phenomena (i.e. electromechanical bifurcation and electromechanical buckling) have been considered to explore the static electromechanical instability of such systems. The Timoshenko beam model has been employed to find the effect of shear deformation on these systems. Modified couple stress theory has been used to investigate size-dependency. Besides, the compressive and tensile residual stresses of nanobridges have been measured on the basis of electromechanical buckling. The governing equations and corresponding boundary conditions have been obtained by means of the principle of minimum potential energy. Finally, following validation of results, the effects of material length scale, length, shear deformation, beam geometry, and gap distance on the symmetric electromechanical behavior have been discussed and examined.

Brownian ratchets: How stronger thermal noise can reduce diffusion.

We study diffusion properties of an inertial Brownian motor moving on a ratchet substrate, i.e., a periodic structure with broken reflection symmetry. The motor is driven by an unbiased time-periodic symmetric force that takes the system out of thermal equilibrium. For selected parameter sets, the system is in a non-chaotic regime in which we can identify a non-monotonic dependence of the diffusion coefficient on temperature: for low temperature, it initially increases as the temperature grows, passes through its local maximum, next starts to diminish reaching its local minimum, and finally it monotonically increases in accordance with the Einstein linear relation. Particularly interesting is the temperature interval in which diffusion is suppressed by the thermal noise, and we explain this effect in terms of transition rates of a three-state stochastic model.

Odd Harmonics in Exoplanet Photometry: Weather or Artifact?

Photometry of short-period planetary systems allows astronomers to monitor exoplanets, their host stars, and their mutual interactions. In addition to the transits of a planet in front of its star and the eclipses of the planet by its star, researchers have reported flux variations at the orbital frequency and its harmonics: planetary reflection and/or emission and Doppler beaming of starlight produce one peak per orbit, while ellipsoidal variations of a tidally distorted star and/or planet produce two maxima per orbit. Researchers have also reported significant photometric variability at three times the orbital frequency, sometimes much greater than the predictions of tidal theory. The reflected phase variations of a homogeneous planet contains power at even orbital harmonics-important for studies of ellipsoidal variations-but cannot contain odd orbital harmonics. We show that odd harmonics can, however, be produced by an edge-on planet with a time-variable map, or an inclined planet with a North-South (N-S) asymmetric map. Either of these scenarios entail weather: short-period planets are expected to have zero obliquity and hence N-S symmetric stellar forcing. North-South asymmetry in a giant planet would therefore suggest stochastic localized features, such as weather. However, we find that previous claims of large-amplitude odd modes in Kepler photometry are artifacts of removing planetary transits rather than modeling them. The only reliable claims of odd harmonics remain HAT-P-7b and Kepler-13Ab, for which the third mode amplitude is 6-8% of the planetary flux. Although time-variable albedo maps could in principle explain these odd harmonics, upper-limits on the infrared variability of hot Jupiters make this hypothesis unlikely. We urge theorists to study the effects of close-in planets on stellar atmospheres, as this remains the only plausible hypothesis.

Investigations on the Effects of Vortex-Induced Vibration with Different Distributions of Lorentz Forces

The control of vortex-induced vibration (VIV) in shear flow with different distributions of Lorentz force is numerically investigated based on the stream function–vorticity equations in the exponential-polar coordinates exerted on moving cylinder for Re = 150. The cylinder motion equation coupled with the fluid, including the mathematical expressions of the lift force coefficient C l , is derived. The initial and boundary conditions as well as the hydrodynamic forces on the surface of cylinder are also formulated. The Lorentz force applied to suppress the VIV has no relationship with the flow field, and involves two categories, i.e., the field Lorentz force and the wall Lorentz force. With the application of symmetrical Lorentz forces, the symmetric field Lorentz force can amplify the drag, suppress the flow separation, decrease the lift fluctuation, and then suppress the VIV while the wall Lorentz force decreases the drag only. With the application of asymmetrical Lorentz forces, besides the above-mentioned effects, the field Lorentz force can increase additional lift induced by shear flow, whereas the wall Lorentz force can counteract the additional lift, which is dominated on the total effect.

Vibration prediction in fault‐tolerant flux‐switching permanent‐magnet machine under healthy and faulty conditions

This study investigates the vibration behaviour of a fault-tolerant flux-switching permanent-magnet (FT-FSPM) machine due to electromagnetic origins. The FT-FSPM machine adopts fault-tolerant teeth and odd rotor pole to obtain more symmetric back electromotive force and reduced cogging torque. However, this topology suffers from high local magnetic force and unbalanced radial force, thus producing significant vibration and noise. In order to predict the vibration characteristic generated by the structure, electromagnetic and structural models are developed. First, the air-gap field is qualitatively analysed by using the rotor permeance to modulate the magnetomotive force. Second, the radial forces during healthy and faulty operations are compared by using finite element method. Then, the most significant vibration modes are calculated to determine how the machine is excited. Furthermore, the vibration behaviour of the FT-FSPM machine under healthy and faulty conditions is predicted by structural analysis. Finally, the experimental measurements are given for verification.

The influence of asymmetric force requirements on a multi-frequency bimanual coordination task

An experiment was designed to determine the impact of the force requirements on the production of bimanual 1:2 coordination patterns requiring the same (symmetric) or different (asymmetric) forces when Lissajous displays and goal templates are provided. The Lissajous displays have been shown to minimize the influence of attentional and perceptual constraints allowing constraints related to neural crosstalk to be more clearly observed. Participants (N=20) were randomly assigned to a force condition in which the left or right limb was required to produce more force than the contralateral limb. In each condition participants were required to rhythmically coordinate the pattern of isometric forces in a 1:2 coordination pattern. Participant performed 13 practice trials and 1 test trial per force level. The results indicated that participants were able to effectively coordinate the 1:2 multi-frequency goal patterns under both symmetric and asymmetric force requirements. However, consistent distortions in the force and force velocity time series were observed for one limb that appeared to be associated with the production of force in the contralateral limb. Distortions in the force produced by the left limb occurred regardless of the force requirements of the task (symmetric, asymmetric) or whether the left or right limb had to produce more force than the contralateral limb. However, distinct distortions in the right limb occurred only when the left limb was required to produce 5 times more force than the right limb. These results are consistent with the notion that neural crosstalk can influence both limbs, but may manifest differently for each limb depending on the force requirements of the task.

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase (CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional (2D) particle-in-cell (PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.

The effect of a negatively chirped laser pulse on the evolution of bubble structure in nonlinear bubble regime

In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The Carrier-Envelope Phase (CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional particle-in-cell simulations show that the utilization of a negatively chirped laser pulse is more effective in controlling the pulse depletion rate, and consequently, the effect of the CEP in the bubble regime. The results indicate that the pulse depletion rate diminishes during the propagation of the pulse in plasma that leads to postponing the effect of Carrier-Envelope Phase (CEP) in plasma response, and therefore, maintaining the stability of the bubble shape for a longer time than the un-chirped laser pulse. As a result, a localized electron bunch with higher maximum energy is prod...

Can Exchange Rate Dynamics in Krugman's Target-Zone Model Be Directly Tested?

Despite Krugman's (1991) model being a benchmark for modelling target zones, empirical support has been sparse due to the subtle non-linear relationship between the observable exchange rate and underlying unobservable fundamental. This paper provides an alternative approach to derive explicit exchange rate dynamics by approximating a quadratic relationship between the exchange rate and fundamental through a power-series method. The exchange rate dynamics with a parametric class of drift terms of the stochastic fundamental including constant-trend, symmetric and asymmetric mean-reverting forces regarding how central banks intervene are ready for direct empirical tests. The empirical results demonstrate that the derived dynamics following a mean-reverting square-root or double square-root processes adequately fits the exchange rate data of various target-zone systems including the Exchange Rate Mechanism. The model parameters of the exchange rate dynamics under the asymmetric mean-reverting fundamental are shown to be associated with realignment of the currencies' target zones.

Elastic ball under non‐stationary axially symmetrical volume forces

This paper studies deformation of homogeneous elastic isotropic ball under the action of non‐stationary axially symmetrical volume forces. Displacements at the boundary of the ball equal to zero. We used expansion in series of Legendre polynomials and their derivatives, as well as the Laplace transform by time. The solution is represented in integral form with kernels in form of Green functions. We have established the structure of such kernels and found their original functions. The present study presents examples of calculations of kernels and deformation of a ball under various external forces.

Rotational invariance conditions in elasticity, gradient elasticity and its connection to isotropy

For homogeneous higher-gradient elasticity models we discuss frame-indifference and isotropy requirements. To this end, we introduce the notions of local versus global SO(3)-invariance and identify frame-indifference (traditionally) with global left SO(3)-invariance and isotropy with global right SO(3)-invariance. For specific restricted representations, the energy may also be local left SO(3)-invariant as well as local right SO(3)-invariant. Then we turn to linear models and consider a consequence of frame-indifference together with isotropy in nonlinear elasticity and apply this joint invariance condition to some specific linear models. The interesting point is the appearance of finite rotations in transformations of a geometrically linear model. It is shown that when starting with a linear model defined already in the infinitesimal symmetric strain , the new invariance condition is equivalent to the isotropy of the linear formulation. Therefore, it may also be used in higher-gradient elasticity models for a simple check of isotropy and for extensions to anisotropy. In this respect we consider in more detail variational formulations of the linear indeterminate couple-stress model, a new variant of it with symmetric force stresses and general linear gradient elasticity.

Comparative study of biased flux permanent magnet machines with doubly salient permanent magnet machines considering with influence of flux focusing

Different from the doubly salient permanent magnet (PM) machines (DSPMMs), the stator/rotor pole number combination (Ns/Nr) in biased flux PM machines (BFPMMs) is flexible and Nr can be any integer except the phase number and its multiples. When Ns/Nr differ by one, 6-stator pole BFPMM exhibits bipolar phase flux-linkage and symmetrical phase back-electromotive force waveforms. Based on the optimal 6/7 Ns/Nr and the rated copper loss, the torque density of BFPMMs is enhanced by 19%, 18.8% and 40.3% respectively when employing the flux-focusing structures with inner-type, outer-type and combined-type. Meanwhile, the PM utilization efficiency is also enhanced. Moreover, based on the inner-type flux-focusing structure, the optimized 6/7 Ns/Nr BFPMM exhibits about 18% higher average torque and 80% lower torque ripple than the optimized 6/4 Ns/Nr DSPMM. Further, the unbalance between phases which is observed in DSPMMs is overcome in the BFPMMs. The analyses are validated experimentally on a prototype machine.

Input-dependent performance study of a nonlinear piezoelectric energy harvester

This work reports an input-dependent performance study of a nonlinear piezoelectric energy harvester with introduced magnetic interaction. The performances of the novel harvester with two external magnet arrays (I and II) are compared. Array II that has symmetric magnetic force yields better voltage output under frequency sweep test. As such, the energy harvesting capacity with Array II is performed under two vibration inputs (I and II). Under excitation Input I with periodic varying frequency, experimental results show that the nonlinear piezoelectric harvester outperforms its linear counterpart (no magnetic interaction) at alternating input bandwidths. A 104.5% improvement of root mean square voltage output (318.2% of power output) is obtained under excitation of 0.334 g (root mean square) and bandwidth of 7 Hz. No advantage is observed under Input II consisting of one principal and finite non-principal components. However, detailed study indicates that the amplitude of the principal component and the amplitude ratio of the non-principal components to the principal component in Input II are essential to maintain large-amplitude periodic motion. Our work provides useful insights into the design, characterization, and application of nonlinear energy harvesters with external magnetic forces based on a priori knowledge of input.