Low-frequency Structures Research Articles

Coherent Structures in the Breakdown Bubble of a Vortex Flow

Helical structures downstream of the breakdown bubble of the Grabowski vortex profile at Reynolds numbers of have been extensively studied through stability analyses in the literature. However, the low-frequency coherent structures inside the bubble, which have been observed in experiments, were not predicted by the stability approach. In the present work, asymptotic stabilities of the Grabowski vortex flow at are carried out to compare against previous works, and then the linear optimal inflow perturbation and the corresponding outcome are studied to unveil the dynamics inside the bubble. It is found that low-frequency inflow perturbations penetrate the bubble and are amplified to coherent structures, whereas high-frequency perturbations are convected around the border of the bubble. These linearized results agree well with previous experimental observations and are validated by direct numerical simulations.

Tense and plural formation in Welsh-English bilingual children with and without language impairment.

Grammatical morphology has been shown to be problematic for children with specific language impairment (SLI) or developmental language disorder (DLD). Most research on this topic comes from widely spoken languages, such as English. Despite Welsh being the most extensively spoken indigenous in the UK after English, and Wales being the only official bilingual country in the UK, our knowledge about the morphosyntactic areas of Welsh that may pose problems for Welsh-speaking children with SLI is limited. Currently, Welsh-speaking speech and language therapists (SLTs) are heavily reliant on the use of informally translated English assessments. This can inadvertently result in a failure to take aspects of Welsh morphosyntax into account that are critical for the assessment and treatment of Welsh-speaking children. This is the first study to examine how Welsh-English bilingual children of early school age with typical development (bi-TD) and with SLI (bi-SLI) perform on production tasks targeting verbal and nominal morphology in Welsh. We targeted areas of Welsh morphosyntax that could potentially be vulnerable for Welsh-speaking children with or at risk of language impairment, such as tense marking and plural formation, and assessed their diagnostic potential. Twenty-eight Welsh-dominant bilingual children participated in the study: 10 bi-SLI and 18 bi-TD. They were administered three elicitation tasks targeting the production of verbal (compound and synthetic past tense) and nominal (plural) morphology in Welsh. The bi-SLI children performed worse than their bi-TD peers across all three tasks. They produced more uninflected verbs in the elicited-production task and were less likely to be prompted to produce the synthetic past, which is a concatenating, low-frequency form of the past tense. They also over-regularized less in the context of plural nouns, and when they did, they opted for high-frequency suffixes. By focusing on aspects of morphosyntactic development which are unique to Welsh, we have increased existing about how verbal and nominal morphology are acquired in Welsh-speaking bi-SLI and bi-TD children. The present results point towards productivity problems for Welsh-speaking bi-SLI children who are adversely influenced by low-frequency structures and fail to over-regularize in the context of verbal and nominal concatenating morphology. From a clinical perspective, targeting synthetic past-tense forms through a prompting task may be a promising assessment and intervention tool that future studies could explore further.

Identification of coherent structures in the flow past a NACA0012 airfoil via proper orthogonal decomposition

Proper orthogonal decomposition, POD, is employed to identify coherent structures in the turbulent flow past a NACA0012 airfoil. An analysis of different POD techniques is presented including the standard snapshot method, the spectral POD (SPOD) method, and a Fourier-POD implementation combined with the SPOD. The latter technique is viable once the present turbulent flow has a homogeneous direction along the airfoil span. An assessment of different vector norms and filter functions employed in the POD reconstruction is presented. The evaluation of the several POD techniques allows the investigation of the physics of the current turbulent flow including the observation of coherent structures and their relation to the airfoil noise generation mechanisms. The SPOD technique is applied assuming periodic and non-periodic temporal signals in the construction of the covariance matrix. Although it slightly alters the spectrum of the POD singular values, the periodic SPOD considerably reduces the noise observed in the singular vectors. The application of the Gaussian filtering allows an enhanced control in the response of the SPOD when compared with a square-box filter. The present POD reconstructions employ norms based on kinetic energy and pressure. For both norms, two-dimensional coherent structures are observed along the turbulent boundary layer and wake regions for the first pair of modes. These structures have a spectral content at the same frequency of the tonal noise radiation by the airfoil. The POD analysis also allows the identification of further coherent structures in the present flow. The kinetic energy norm is able to reconstruct low-frequency structures in the flow field for higher POD modes, while the pressure norm reconstructs high-frequency structures. This behavior is related to the physics captured by each POD norm in the present turbulent flow.

Bath Energy for Correlated Initial States vs. Information Flow in Local Dephasing Channels

Variations of the bath energy are compared with the information flow in local dephasing channels. Special correlated initial conditions are prepared from the thermal equilibrium of the whole system, by performing a selective measurement on the qubit. The spectral densities under study are ohmic-like at low frequencies and include logarithmic perturbations of the power-law profiles. The bath and the correlation energy alternately increase or decrease, monotonically, over long times, according to the value of the ohmicity parameter, following logarithmic and power laws. Consider initial conditions such that the environment is in a thermal state, factorized from the state of the qubit. In the superohmic regime the long-time features of the information flow are transferred to the bath and correlation energy, by changing the initial condition from the factorized to the specially correlated, even with different temperatures. In fact, the low-frequency structures of the spectral density that provide information backflow with the factorized initial condition, induce increasing (decreasing) bath (correlation) energy with the specially correlated initial configuration. By performing the same change of initial conditions, the spectral properties providing information loss, produce decrease (increase) of the bath (correlation) energy.

Causal Realization of Rate-Independent Linear Damping for the Protection of Low-Frequency Structures

AbstractThe devastating low-frequency ground motions of the 2011 Great East Japan Earthquake produced large displacements in low-frequency structures previously thought to be safe, including base-i...

Analysis of Airfoil Stall Control Using Dynamic Mode Decomposition

Airfoil stall is a major inhibitor of aircraft performance. Many methods have been successfully shown to inhibit or eliminate stall. However, the underlying dynamics of control remains relatively obscure because of the highly nonlinear nature of turbulence. A major challenge is the a priori estimation of actuation parameters without resorting to trial and error. Such estimation of parameters requires a physics-based understanding of the flowfield and its response to actuation. To study this, high-fidelity large-eddy simulations are employed to analyze active control by exploring the key dynamic modes, without and with control, through dynamic mode decomposition. A NACA 0015 stalled airfoil is considered at a Reynolds number of 100,000 and a 15 deg angle of attack. The results suggest that the dominant mode representing stall has an effective Strouhal number of two. Simulations are then performed by modeling control at using a nanosecond-pulsed dielectric barrier discharge near the leading edge. Introduction of control at this frequency is shown to successfully mitigate stall. Further analyses show that control tends to excite and destabilize predominantly higher frequencies, but it specifically stabilizes the low-frequency large-scale structures associated with stall. The results establish promise of the methodology in a priori estimation of control frequency.

Low-frequency photonic band structures in graphene-like triangular metallic lattice

We study the low frequency photonic band structures in triangular metallic lattice, displaying Dirac points in the frequency spectrum, and constructed upon the lowest order regular polygonal tiles. We show that, in spite of the unfavourable geometrical conditions intrinsic to the structure symmetry, the lowest frequency photonic bands are formed by resonance modes sustained by local structure patterns, with the corresponding electric fields following a triangular distribution at low structure filling rate and a honeycomb distribution at high filling rate. For both cases, the lowest photonic bands, and thus the plasma gap, can be described in the framework of a tight binding model, and analysed in terms of local resonance modes and their mutual correlations. At high filling rate, the Dirac points and their movement following the structure deformation are described in the same framework, in relation with local structure patterns and their variations, as well as the particularity of the metallic lattice that enhances the topological anisotropy.

Band structure calculation of SH waves in nanoscale multilayered piezoelectric phononic crystals using radial basis function method with consideration of nonlocal interface effects

In this paper, the radial basis function (RBF) collocation method based on the nonlocal Eringen piezoelectricity theory is developed to compute the band structures of nanoscale multilayered piezoelectric phononic crystals taking account of nonlocal interface effects. Detailed calculations are performed for anti-plane transverse waves propagating obliquely or vertically in the system. The correctness of the present method is verified by comparing the numerical results with those obtained by applying the transfer matrix method in the case of nonlocal perfect interfaces. The effects of nonlocal interface imperfections are considered by comparing with the nonlocal perfect interfaces. In addition, the influences of the piezoelectric constant, the nanoscale size, the impedance ratio and the incidence angle on the cut-off frequency and band structures are investigated and discussed in detail. Numerical results show that the nonlocal interface discontinuity has more obvious effect on the low-frequency band structures at the microscopic scale than at the macroscopic scale. Furthermore, at the macroscopic scale, the nonlocal interface imperfection has an obvious effect on the high frequency waves, but the effect on the low frequency waves is not obvious, and the nonlocal interface imperfection has no effect on the cut-off frequency at the microscopic scale.

Concept study of a novel energy harvesting-enabled tuned mass-damper-inerter (EH-TMDI) device for vibration control of harmonically-excited structures

A novel dynamic vibration absorber (DVA) configuration is introduced for simultaneous vibration suppression and energy harvesting from oscillations typically exhibited by large-scale low-frequency engineering structures and structural components. The proposed configuration, termed energy harvesting-enabled tuned mass-damper-inerter (EH-TMDI) comprises a mass grounded via an in-series electromagnetic motor (energy harvester)-inerter layout, and attached to the primary structure through linear spring and damper in parallel connection. The governing equations of motion are derived and solved in the frequency domain, for the case of harmonically-excited primary structures, here modelled as damped single-degree- of-freedom (SDOF) systems. Comprehensive parametric analyses proved that by varying the mass amplification property of the grounded inerter, and by adjusting the stiffness and the damping coefficients using simple optimum tuning formulae, enhanced vibration suppression (in terms of primary structure peak displacement) and energy harvesting (in terms of relative velocity at the terminals of the energy harvester) may be achieved concurrently and at nearresonance frequencies, for a fixed attached mass. Hence, the proposed EH-TMDI allows for relaxing the trade-off between vibration control and energy harvesting purposes, and renders a dual-objective optimisation a practically-feasible, reliable task.

Swirl-switching phenomenon in turbulent flow through toroidal pipes

Direct numerical simulations (DNSs) are performed to investigate turbulent flows in toroidal pipes with two different curvatures. By means of proper orthogonal decomposition (POD), dominant coherent structures in the flow are identified. The most energetic structures in the strongly curved pipes (with curvature κ=0.1 and κ=0.3 and Reb=11,700 based on bulk velocity and diameter) are very similar in structure for both configurations studied. The dominant modes (shape and frequency) appear to match the coherent structures responsible for the swirl switching phenomenon found earlier in experimental and numerical studies of turbulent flow in spatially developing 90° bends, with which the current results are compared. The fact that turbulence in the toroidal pipe features low-frequency coherent structures very similar to swirl switching is relevant as it may challenge the current hypothesis regarding the origin of swirl switching, which is usually connected to the conditions in the upstream straight section. However, the toroidal pipe is homogeneous in the streamwise direction and does as such not feature a straight part.

Design of radial phononic crystal using annular soft material with low-frequency resonant elastic structures

Using FEM, we theoretically study the vibration properties of radial phononic crystal (RPC) with annular soft material. The band structures, transmission spectra, and displacement fields of eigenmode are given to estimate the starting and cut-off frequency of band gaps. Numerical calculation results show that RPC with annular soft material can yield low-frequency band gaps below 350 Hz. Annular soft material decreases equivalent stiffness of the whole structure effectively, and makes corresponding band gaps move to the lower frequency range. Physical mechanism behind band gaps is the coupling effect between long or traveling wave in plate matrix and the vibrations of corrugations. By changing geometrical dimensions of plate thickness e, the length of silicone rubber h2, and the corrugation width b, we can control the location and width of the first band gap. These research conclusions of RPC structure with annular soft material can potentially be applied to optimize band gaps, generate filters, and design acoustic devices.

Experimental survey of linear and nonlinear inertial waves and wave instabilities in a spherical shell

We experimentally study linear and nonlinear inertial waves in a spherical shell with a radius ratio of ${\it\eta}=1/3$. The shell rotates with a mean angular velocity ${\it\Omega}_{0}$ around its vertical axis. This rotation is overlaid by a time-periodic libration of the inner sphere in the range $0<{\it\omega}_{lib}<2{\it\Omega}_{0}$ to excite inertial waves with a defined frequency. In the first part, we investigate linear inertial waves. The influence of the libration amplitude and the libration frequency on the waves and further the efficiency of the forcing to excite linear inertial waves will be discussed. For this, qualitative data from Kalliroscope visualisation in a meridional laser plane, as well as quantitative particle image velocimetry (PIV) data in a horizontal plane, have been analysed. A simple two-dimensional ray-tracing model is applied for the meridional plane to interpret the visualisations with respect to energy focusing and wave attractors. For sufficiently high/low libration amplitudes/frequencies, the Stewartson layer, a vertical shear layer tangential to the inner sphere’s equator, becomes unstable. This so-called ‘supercritical’ regime, where centrifugal and shear instabilities occur, allows for nonlinear wave coupling. PIV analyses in the horizontal laser plane in the corotating frame show low-frequency structures that correspond to Rossby-wave instabilities of the Stewartson layer. Some of these are travelling retrograde and are trapped near the Stewartson layer, others are travelling prograde filling the whole gap outside the Stewartson layer. Since libration can be viewed as a time-periodic variation of differential rotation, we assume that these two different structures are related to either the retrograde $(Ro_{d}<0)$ or the prograde $(Ro_{d}>0)$ phase of the libration cycle. The experimental results confirm theoretical, numerical as well as other experimental studies on Stewartson-layer instabilities.

Formation of low-frequency periodic structures in a pulsed magnetron discharge

Periodic plasma structures are observed in non-sputtering magnetron discharge (NSMD) that is the transient quasi-stationary low-voltage regime between the high-current magnetron discharge (HCIMD) and an arc. The fast camera imaging synchronized with the magnetic probe diagnostics reveals the correlation between the observed rotation of the plasma inhomogeneities and the magnetic field perturbation behaviour. The frequencies of the periodic processes fall into kHz-range. A simple analytical model of the ionization instability in crossed electric and magnetic fields is suggested for the low-pressure discharge case. Using the model, the possible ranges of wavelengths and frequencies for the plasma inhomogeneities are evaluated. The results show good agreement between the experimental data and theory.

Experimental characterisation of walking locomotion on rigid level surfaces using motion capture system

Low-frequency structures, such as footbridges and long-span floors, are often sensitive to variations in dynamic loading induced by pedestrians. As a result, the design of these structures using traditional deterministic approaches is being replaced by stochastic load models that can accommodate different styles of walking. To inform development and facilitate wider implementation of the new stochastic approaches, a database of experimental data characterising both inter- and intra-subject variability of gait parameters is required. This study aims to contribute to the development of such a database by providing a set of data for walking over rigid level surfaces.The motion capture system Vicon was used for simultaneous monitoring of the kinematic and kinetic gait parameters. Ten test subjects walking at 13 different speeds participated in the experimental programme. Novel experimental data on pacing rate, step length, step width, angular positions of the legs and the trunk, and the force amplitude were collected and statistically characterised. The acquired data are suitable for calibration of the bipedal pedestrian models intended for civil engineering applications.

Mesoscale Eddies Affect Near-Surface Turbulent Exchange: Evidence from Lidar and Tower Measurements

AbstractThe eddy-covariance technique tends to underestimate turbulent heat fluxes, which results in nonclosure of the surface energy balance. This study shows experimental evidence that mesoscale turbulent organized structures, which are inherently not captured by the standard eddy-covariance technique, can affect near-surface turbulent exchange. By using a combined setup of three Doppler wind lidars above a cropland-dominated area in Germany, low-frequency turbulent structures were detected in the surface layer down to a few meters above ground. In addition, data from two micrometeorological stations in the study area were analyzed with respect to energy balance closure. In accordance with several previous studies, the data confirm a strong friction velocity dependence of the energy balance residual. At both stations, the energy balance residual was found to be positively correlated with the vertical moisture gradient in the lower atmospheric boundary layer, but at only one station was it correlated with the temperature gradient. This result indicates that mesoscale transport probably contributes more to the latent heat flux than to the sensible heat flux, but this conclusion depends largely on the measurement site. Moreover, flow distortion due to tower mountings and measurement devices affects the energy balance closure considerably for certain wind directions.

Low-frequency photonic bands in metallic lattices: a tight-binding description

Low-frequency photonic band structures in two-dimensional metallic lattices are investigated through both numerical and tight-binding approaches. The metallic structures, displaying respectively four and six fold rotational symmetries, are constructed upon different sets of adjustable structure units, allowing probing the contribution of different structure configurations to the band formation. We show that the low-frequency band structures can be described in the tight-binding framework, and analyzed in terms of local resonance modes and their mutual correlations.

Modulating state transition and mechanical properties of viscoelastic resins from maize zein through interactions with plasticizers and co-proteins

Viscoelastic properties have been observed in maize zein above its glass transition temperature; however, current understanding of how these viscoelastic polymers can be further manipulated for optimal performance is limited. Using resins formed via precipitation from aqueous ethanolic environments, this study presents a framework for understanding how state transition and viscoelastic properties of zein can be controlled through interactions with plasticizers and co-proteins. Resins plasticized with oleic acid exhibited reduced water absorption and glass transition temperatures and formed low elasticity/high extensibility resins. Incorporation of casein increased water absorption and glass transition temperatures and imparted a four-fold increase in material strength/elasticity, as compared to zein alone. Plasticizers and co-proteins influenced zein secondary structure in the resin systems by decreasing and increasing low-frequency β-sheet structures (1640-1615 cm-1), respectively. This work demonstrates that specific protein/plasticizer and protein/co-protein interactions are capable of promoting fundamental differences to zein's behavior in viscoelastic systems and could serve as a basis for improving the functional properties of this underutilized material in various food and biomaterial applications.

Study on flow behind backward-facing step in a narrow channel

Both statistics and dynamics of the flow-field behind a backward-facing step in a narrow channel is studied experimentally using time-resolved PIV technique. The channel width is 4 step heights, Reynolds number based on the step height was about 35 thousands. Secondary flow effect is detected clearly in the region behind the step resulting in strong 3D flow structure just behind the step. The low-frequency quasi-periodical structures appearing in the region behind the step and in the region of the flow reattachment close to the channel bottom are studied using the Oscillation Pattern Decomposition (OPD) technique. Strouhal numbers of those phenomena are very low, order of 0.01. Typical time-mean structures as well as dynamical structures are to be presented.

Vibration Modeling of Arc-Based Cantilevers for Energy Harvesting Applications

Abstract Cantilever beams are widely used for designing transducers for low-frequency vibration energy harvesting. However, in order to keep the dimensions within reasonable constraints, a large tip mass is generally required for reducing the resonance frequency below 100 Hz which has adverse effect on the reliability. This study provides a breakthrough toward realizing low-frequency micro-scale transduction structures. An analytical out-of-plane vibration model for standalone arc-based cantilever beams was developed that includes provisions for shear and rotary inertia, multidirectional arcs, and multiple layers. The model was applied to a multilayered cantilever beam (10-mm wide and 0.1-mm thick) composed of three arcs, and the results indicate that the fundamental bending mode of the beam was 38 Hz for a silicon substrate thickness of 100 μm. The model was validated with modal experimental results from an arc-based cantilever made out of aluminum.

Dynamics of secondary flow behind backward-facing step in a narrow channel

Dynamics of the flow-field behind a backward-facing step in a narrow channel is studied experimentally using time resolved PIV technique. Secondary flow represented by vortical structures is studied using Oscillation Pattern decomposition technique. The low-frequency quasi-periodical structures appearing in the region just behind the step close to the channel bottom are studied. Typical dynamics of vortex structures involving contra-rotating vortex pair train, vortices coalescence and splitting have been observed.