Force Spectrum Research Articles

Evaluation of Simplified Analysis Procedures for a High‐Rise Reinforced Concrete Core Wall Structure

Nonlinear response history analysis (NLRHA) procedure is one of the most precise and accurate numerical method to compute the seismic demands of high‐rise structures but is complex, rigorous, and time‐consuming and requires a lot of expertise for nonlinear modelling and results interpretation. Therefore, practicing engineers in developing countries like Pakistan still use the simplified analysis procedures to compute the seismic demands. Among the simplified analysis procedures, equivalent lateral force and response spectrum analysis procedures are widely used for the design purpose. However, other procedures have also been proposed in the recent past to accurately capture the higher mode effects in mid‐to‐high‐rise structures. In the current study, results of a forty‐story core wall building are used to check the relative accuracy and ease of application of different simplified analysis procedures. Furthermore, a modal decomposition technique is used to separate the modal responses from the NLRHA results, and a mode wise comparison of different demand parameters for different simplified procedures is performed. The current study has been used to clearly identify the reasons of inaccuracies in different simplified procedures. Furthermore, a simplified analysis procedure is proposed to accurately estimate the seismic demands of high‐rise buildings and the possible solutions to improve their predictions.

In situ visualization of the quasi-periodic crystal growth interface fluctuation by growth interface electromotive force spectrum in a Czochralski system

Growth interface electromotive force (GEMF) visualizes the real-time states of crystal growth rate, supercooling degree and rotation temperature fluctuations.

Single-Molecule Force Spectroscopy: Experiments, Analysis, and Simulations.

The mechanical properties of cells and of subcellular components are important to obtain a mechanistic molecular understanding of biological processes. The quantification of mechanical resistance of cells and biomolecules using biophysical methods matured thanks to the development of nanotechnologies such as optical and magnetic tweezers, the biomembrane force probe, and atomic force microscopy (AFM). The quantitative nature of force spectroscopy measurements has converted AFM into a valuable tool in biophysics. Force spectroscopy allows the determination of the forces required to unfold protein domains and to disrupt individual receptor/ligand bonds. Molecular simulations as a computational microscope allow investigation of similar biological processes with an atomistic detail. In this chapter, we first provide a step-by-step protocol of force spectroscopyexperiments using AFM, including sample preparation, measurements, and analysis and interpretation of the resulting dynamic force spectrumin terms of available theories. Next, we present the background for molecular dynamics (MD) simulations focusing on steered molecular dynamics (SMD) and the importance of bridging computational tools with experimental techniques.

The Influence of Nonlinear Mode on Generalized Aerodynamic Spectrum in the Downwind

Combined with the coherence function of fluctuating wind load, the average wind speed profile index and the mode index, the method of numerical integration was used to calculate the generalized aerodynamic spectrum of nonlinear modes for high-rise buildings in the downwind. The fitting formula for that value was derived by multivariate linear regression analysis, whose reliability was confirmed by the wind tunnel test. At the same time, the effect of nonlinear vibration modes on the generalized aerodynamic spectrum of high-rise buildings was discussed with the vibration index as the parameters:When the vibration mode index is less than 1, the correction coefficient is more than 1, that is, the calculated generalized aerodynamic force spectrum by the traditional method is too small, which is not safe for the structural design, and on the contrary, it is less than 1. Base on the generalized aerodynamic spectrum of the linear mode, it is recommended to use a correction factor of 1.1 in the basic mode of the load code for the design of building structures.

High sensitivity measurements of normal force under large amplitude oscillatory shear

The two aims of this publication are to introduce a new and rheometer-independent rheometric tool for measuring the axial normal force in oscillatory shear rheology and to study the normal forces of polyolefin melts under large amplitude oscillatory shear (LAOS). A new plate geometry with an incorporated highly sensitive piezoelectric normal force sensor was designed for a rotational rheometer. The new geometry was used to investigate normal forces of polyethylene (PE) melts under LAOS. The resulting stress and normal force data was compared with the data from measurements in commercial high performance rotational rheometers. The stress and the normal force response were Fourier-transformed and their resulting spectra were analysed. The non-linear contributions to the FT-magnitude spectra (i.e. the intensities of the higher harmonics) were analysed using the framework of the Q-parameter, $Q=I_{3/1}/{\gamma ^{2}_{0}}$ for both the stress spectrum and the normal force spectrum, resulting in the strain-dependent $Q\left (\gamma _{0}\right )$ and $Q_{NF}\left (\gamma _{0}\right )$ , respectively. The newly designed normal force geometry had a sensitivity in the measurement starting from $5\times 10^{-5}$ N up to 20 N, and respectively a signal-to-noise ratio (SNR) of $1:$ 16.000, which is about a factor of 1.8 times better than the best performing commercial rheometers. The new geometry was used to determine $Q\left (\gamma _{0}\right )$ and $Q_{NF}\left (\gamma _{0}\right )$ , to characterize the shear rheological behaviour of the PE melts. Even rather simple rheometers, those without normal force detection, can be extended utilizing the here presented tools for high sensitive FT-rheology analysing the normal forces.

Buffeting response evaluation of long-span bridges with emphasis on the three-dimensional effects of gusty winds

A generalized gust loading model applicable to the buffeting response predictions of both slender, line-like long-span bridges and bridges with small aspect-ratios (the ratio of the span to the width of a deck) is established based on the three-dimensional (3D) theory. The spectra of the buffeting forces are determined by the equivalent 3D two-wavenumber aerodynamic admittance function (AAF) to deal with the 3D effects of gusty winds. The mode shape influence function is introduced to take into account the three-dimensionality of the modal force. Since the mode shape influence function involves the aspect-ratio term, the three-dimensionality of modal force therefore shows a dependence on the aspect-ratio of the bridge deck. The gust response analysis can easily be conducted in the frequency domain through the use of the generalized gust loading model. Numerical examples of bridges with different aspect-ratios in service states and construction states are performed to evaluate the discrepancies in the buffeting displacements calculated by both the generalized gust loading model and the traditional gust loading model. The results show that the traditional gust loading model can generally be applied to the buffeting analysis of long-span bridges with large aspect-ratios in both the service and construction states. However, for bridges with small aspect-ratios, such as in the construction stages, the traditional gust model may overestimate the buffeting response of a bridge deck to a larger degree (neglecting any end effect), indicating the invalidation of the strip assumption in these cases.

Diurnal and Semidiurnal Cyclicity of Radon (222Rn) in Groundwater, Giardino Spring, Central Apennines, Italy

Understanding natural variations of Rn (222Rn) concentrations is the fundamental prerequisite of using this radioactive gas as a tracer, or even precursor, of natural processes, including earthquakes. In this work, Rn concentrations in groundwater were continuously measured over a seven-month period, during 2017, in the Giardino Spring, Italy, together with groundwater levels in a nearby well installed into a fractured regional aquifer. Data were processed to reduce noise, and then analyzed to produce the Fourier spectra of Rn concentrations and groundwater levels. These spectra were compared with the spectrum of tidal forces. Results showed that diurnal and semidiurnal cycles of Rn concentrations, and filtered oscillations of groundwater levels, in the nearby well, are correlated with solar and luni-solar components of tidal forces, and suggested no correlation with the principal lunar components. Therefore, influencing factors linked to solar cycles, such as daily oscillations of temperature and atmospheric pressure, and related rock deformations, may have played a role in Rn concentrations and groundwater levels. An open question remains regarding the correlation, which is documented elsewhere, of Rn concentrations and groundwater levels with the lunar components of the solid Earth tides.

Passive control of vortex-induced vibration by spanwise grooves

The objective of this numerical study is to investigate the effect of spanwise grooves on the suppression of vortex-induced vibration (VIV) and the reduction of drag force. For this purpose, we consider a standard configuration of the elastically mounted circular cylinder, which is free to vibrate in both streamwise and transverse directions with identical natural frequency. We introduce a novel staggered groove configuration whose geometry is especially designed by offsetting the cross-sectional portion of the cylinder continuously along the spanwise direction. We assess the characteristic VIV responses of the proposed staggered groove configuration against the helical surface grooves for the identical dimensions and physical conditions. The staggered and helical groove configurations differ only in their arrangement of cross-section geometry along the spanwise direction. Three-dimensional coupled fluid–structure simulations are conducted at low mass and damping values with moderate Reynolds number of Re=4800. The effective width and the depth of surface grooves are determined to characterize the size effects for the assessment of staggered and helical groove configurations. Results show that the staggered groove configuration is effective in suppressing VIV, wherein the net reductions of 37% in the peak transverse amplitude and about 25% in the mean drag coefficient are observed in comparison to the plain cylinder counterpart. Staggered groove configuration produces three dominant effects by introducing a continuous jump in the cross-sectional geometry along the spanwise direction: (i) reduction of the spanwise correlation, (ii) enhancement of the three-dimensional effects in the near-wake flow, and (iii) broadening of the frequency spectra of fluid forces. As a result of these physical effects, the transfer of energy from the surrounding fluid flow to the vibrating grooved-cylinder system is reduced as compared to its plain cylinder counterpart. Owing to the simplicity of mechanical design and the ease of installation, the proposed passive control concept has a potential application to deepwater marine risers and tall structures in a wind environment.

Design of Rotor Excited Axial Flux-Switching Permanent Magnet Machine

In this paper, a new structure of permanent magnet axial flux-switching machine with segmented rotor has been introduced. The 12-pole stator has been installed between two 8-pole segmented rotors, and the excitation system has been connected to the rotor. This topology enhances the design of flux-switching permanent magnet machines. With this new topology, an increase in the torque density, the reduction the total harmonic distortion, simple construction, and the reduction of cogging torque have been achieved. The harmonic spectrum of the back electromagnetic force has also been improved. This topology provides an optimum power for the electric vehicles because of the short axial length and the high power density along with the low vibration and noise due to lower torque ripple and cogging torque. The three-dimensional time-stepping finite-element analysis is used to determine the flux linkage, back electromagnetic force, the output torque, and the cogging torque. A comparative analysis is made of the proposed topology and related topology in the literature.

Application of atomic force microscopy-based single molecule force spectroscopy in G-quadruplex studies

Telomere plays a crucial role in the physiological and pathological processes of cells. At the end of the telomere, the single-stranded DNA repeat sequence rich in guanine (G) folds in the presence of monovalent metal ions such as Na+ or K+ to form a G-quadruplex structure. This structure can not be extended by telomerase and inhibits the activity of telomerase, thus becoming a potential anticancer target. Stabilizing the formation of DNA G-quadruplex structures by small molecule ligands has become a new strategy for designing many anticancer drugs, and studying the interaction strength of these small molecule ligands with G-quadruplex is thus of particular importance for screening highly effective anticancer drugs. Single molecule force spectroscopy enables direct measurement of the interaction between small molecule ligands and G-quadruplexes. This review highlights the advances of single-molecule force spectroscopy based on atomic force microscopy in the study of the G quadruplex structure and its interaction with small molecule ligands, and summarizes the application and development trend of single molecule force spectrum technology in G quadruplex.

Photoinduced Tip-Sample Forces for Chemical Nanoimaging and Spectroscopy.

Control of photoinduced forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Scanning probe microscopy enables the local detection of photoinduced effects with nano-optical imaging and spectroscopy modalities being used for chemical analysis and the study of physical effects. Recently, the development of a novel scanning probe technique has been reported with local chemical sensitivity attributed to the localization and detection of the optical gradient force between a probe tip and sample surface via infrared vibrationally resonant coupling. However, the magnitude and spectral line shape of the observed signals disagree with theoretical predictions of optical gradient forces. Here, we clarify this controversy by resolving and analyzing the interplay of several photoinduced effects between scanning probe tips and infrared resonant materials through spectral and spatial force measurements. Force spectra obtained on IR-active vibrational modes of polymer thin films are symmetric and match the material absorption spectra in contrast to the dispersive spectral line shape expected for the optical gradient force response. Sample thickness dependence shows continuous increase in force signal beyond the thickness where the optical dipole force would saturate. Our results illustrate that photoinduced force interactions between scanning probe tips and infrared-resonant materials are dominated by short-range thermal expansion and possibly long-range thermally induced photoacoustic effects. At the same time, we provide a guideline to detect and discriminate optical gradient forces from other photoinduced effects, which opens a new perspective for the development of new scanning probe modalities exploiting ultrastrong opto-mechanical coupling effects in tip-sample cavities.

Antecedents of farmers’ willingness to participate in short food supply chains

PurposeDespite the increasing consumers’ intent to support short food networks, the expansion of short food supply chains (SFSCs) remains limited. The purpose of this paper is to identify potential antecedents of farmers’ willingness to participate in SFSCs.Design/methodology/approachUsing data from a sample of Greek farmers the authors examined whether farmers’ citizenship behaviour, the levels of their perceived competencies and the degree to which they feel accepted by their communities affect their willingness to engage in SFSCs.FindingsResults indicate that producers’ citizenship behaviour does indeed have a positive impact on willingness to participate in SFSCs, whereas their perceptions of the acceptance they enjoy within their communities also significantly predict this willingness. On the contrary, self-perceived lack of communication and collaboration competencies diminishes this willingness.Originality/valueTo the best of our knowledge, this study is one of the first attempts to explore the role of farmers’ competencies in their willingness to participate in SFSCs. In addition, by integrating concepts derived from multiple disciplines, our work adds new factors in the wide spectrum of forces that impel or suppress farmers’ willingness to take part in alternative food distribution networks.

Direct numerical simulation of a turbulent Couette–Poiseuille flow: Turbulent statistics

The direct numerical simulation of a fully developed turbulent Couette–Poiseuille flow is performed to investigate the modification of turbulent statistics on the bottom and top walls compared to those in a pure Poiseuille flow. The streamwise mean velocity profile shows that an extended logarithmic layer for a Couette–Poiseuille flow is developed from each wall to the centerline. In addition, the turbulent intensities and Reynolds shear stress on the bottom wall are found to be larger than those in the Poiseuille flow, whereas it is reversed on the top wall due to reduction of the velocity shear. The quadrant analysis of the Reynolds shear stress reveals that large Q2 and Q4 event motions are continuously created throughout the entire flow near the centerline, leading to active momentum transport between the bottom and top walls for the C-type. Inspection of the pre-multiplied streamwise and spanwise energy spectra shows that distinct secondary outer peaks are created for all velocity components and a plateau, called the kx−1 region, is presented in the logarithmic region. Based on an analysis of the net force spectra, three spectral ranges in the wavelength space, corresponding to small- (λx/δ ≤ 1), large- (1 ≤ λx/δ ≤ 10) and very-large-scale (λx/δ ≥ 10) motions for a Couette–Poiseuille flow, are proposed, and very-large-scale structures are highly energetic and contribute more than half of the streamwise turbulent kinetic energy and Reynolds shear stress, where λx is the streamwise wavelength and δ is the channel half height.

Flow-induced loading on and unsteady flow structure in the wake of bluff perforated plates at zero incidence

Flow over rectangular plates positioned at zero incidence can result in high-amplitude forces on the plates. In this study, we apply particle image velocimetry (PIV) and direct force measurements to investigate the effect of transverse perforations on the flow-induced loading on and the flow structure in the near-wake of the plates. We compare plates with different characteristic diameter of the perforations, as well as a reference configuration without perforations, in terms of the spectra of the flow-induced forces, frequencies of the trailing edge vortex shedding and boundary layer profiles at the trailing edge at different planes across the perforation patterns for a range of inflow velocities. The three-dimensionality of the near-wake of the perforated plate is related to the proximity of the individual perforations to the trailing edge of the plate. In the vicinity of the perforations, transverse oscillations of the flow velocity in the wake become suppressed as the diameter of the perforations increases.

Experimental and numerical investigation of wave induced forces and motions of partially submerged bodies near a fixed structure in irregular waves

This paper is the third of a series covering investigations of the wave induced force and motion behavior of ice masses near a fixed structure. The objective of this phase of the study was to determine whether the effects observed in regular waves (Sayeed et al. (2017b, 2018)) still held in irregular seas. In the present case, wave characteristics measured in front of the fixed structure showed that the significant wave heights follow a standing wave pattern generated by the superposition of the incident and reflected peak frequency wave. Thus it may be inferred that the significant surge and heave forces and motions will be increased or decreased at the node and antinode locations in front of the structure as previously observed for regular waves. Force spectra generated from regular wave force data (Sayeed et al. 2017b) and wave measurements from the present study showed that beyond the nodal location at Dλp=0.25, the significant surge force goes down and significant heave force goes up as the body gets close to the structure. Direct measurements of irregular wave forces on a fixed berg model at different proximity to the fixed structure demonstrate the change in significant wave forces based on the ratio of separation distance to peak wavelength. When the berg body is positioned closer to the fixed structure, the normalized significant forces in the horizontal direction decrease, and the normalized significant forces in the vertical direction increase. Significant differences were found between the directly measured force spectra and those produced by linear superposition of regular wave force transfer functions. This is attributed to non-linear effects arising from the iceberg profile and low freeboard.Free floating motion experiments showed smaller differences in the surge and heave velocity spectra due to the presence of the structure. This is because there is insufficient motion data at any specific location to capture the motion responses for all the frequencies in the spectrum. The proximity effect on wave loads and wave induced motions in the vertical direction is found to be more pronounced than the same in the horizontal direction.Numerical simulations using RANS based commercial CFD code Flow3D© are compared to the experimental results. Selected test cases from both force and motion experiments are simulated in irregular waves generated from selected experimental wave spectra. Numerical results for irregular waves demonstrate the ability to simulate random waves and resulting forces and motions with trends comparable to the experimental results.

LES simulation and experimental validation of the unsteady aerodynamics of blunt wind turbine airfoils

In order to investigate the unsteady performance of blunt wind turbine airfoils caused by boundary layer separation and wake eddies, this paper studies the aerodynamic performance by large eddy simulation (LES) and wind tunnel experiment at a Reynolds number of 2.62 × 10ˆ5. The blunt airfoils are obtained by enlarging the trailing edge of the DU 91-W2-250 airfoil to 6% and 10% chords symmetrically on both pressure and suction sides of the airfoil. The simulation was carried out with the incompressible finite-volume Navier-Stokes code EllipSys3D; and, the experiment was done in a wind tunnel with a cross-section of 0.5 m × 0.5 m by measuring the surface pressure and wake velocities using ESP-64HD pressure scanner and TSI hot-wire anemometer. The unsteady wake was captured by hot-wire in the wind tunnel, and LES with EllipSys3D. Both experiment and LES show that the spectrum of aerodynamic forces has a broadband nature which is in coincidence with the wake eddies, implying that the unsteady Kármán vortex sheet is the driving mechanism of the force fluctuation. Moreover, the trailing edge size affects the separation bubbles and transition process in the boundary layer. It shows that the boundary layer near the leading edge is unstable in the spanwise direction, which is characterized by low frequency waves.

Compilation of NC lathe dynamic cutting force spectrum based on two-dimensional mixture models

The load spectrum of numerical control machine tool (NCMT) is the basis data of reliability design, life prediction, and reliability bench test with simulated actual load conditions. Static cutting force spectrum, which can be compiled by empirical formula, cannot accurately reflect the real load conditions. Hence, a method for compiling the spectrum of dynamic turning force is proposed by using two-dimensional mixture models. Dynamic turning force is measured in the laboratory based on a large amount of cutting process data collected in the user field. Mean–frequency and amplitude–frequency matrices of dynamic turning force are obtained by the rainflow counting method. The probability distributions of mean and amplitude are established by using Weibull mixture models (WMM) according to the characteristics of the dynamic turning force, and the two main problems of this method are the determination of the number of mixture components and the estimation of parameters. Gray relation analysis is used to quantify the multi-objective evaluation matrix, and thus the number of mixture components is determined. Parameters are estimated using the improved expectation maximization (EM) algorithm, where the initial values of parameters are obtained by the K-means++ and Bayesian random classification. Finally, the joint probability distribution function between the mean and amplitude is established by the Copula function. Thus, the dynamic cutting force spectrum is compiled. Case analysis results show that the mean and amplitude distribution models with high fitting precision can be obtained using the proposed method. Furthermore, the precision of the load spectrum of NCMT is improved.

Force Spectrum Microscopy Using Mitochondrial Fluctuations of Control and ATP-Depleted Cells

A single-cell assay of active and passive intracellular mechanical properties of mammalian cells could give significant insight into cellular processes. Force spectrum microscopy (FSM) is one such technique, which combines the spontaneous motion of probe particles and the mechanical properties of the cytoskeleton measured by active microrheology using optical tweezers to determine the force spectrum of the cytoskeleton. A simpler and noninvasive method to perform FSM would be very useful, enabling its widespread adoption. Here, we develop an alternative method of FSM using measurement of the fluctuating motion of mitochondria. Mitochondria of the C3H-10T1/2 cell line were labeled and tracked using confocal microscopy. Mitochondrial probes were selected based on morphological characteristics, and their mean-square displacement, creep compliance, and distributions of directional change were measured. We found that the creep compliance of mitochondria resembles that of particles in viscoelastic media. However, comparisons of creep compliance between controls and cells treated with pharmacological agents showed that perturbations to the actomysoin network had surprisingly small effects on mitochondrial fluctuations, whereas microtubule disruption and ATP depletion led to a significantly decreased creep compliance. We used properties of the distribution of directional change to identify a regime of thermally dominated fluctuations in ATP-depleted cells, allowing us to estimate the viscoelastic parameters for a range of timescales. We then determined the force spectrum by combining these viscoelastic properties with measurements of spontaneous fluctuations tracked in control cells. Comparisons with previous measurements made using FSM revealed an excellent match.

Measured aerodynamic forces on a full scale 2MW turbine in comparison with EllipSys3D and HAWC2 simulations

Design loads on turbines are normally simulated with an aeroelastic model using an engineering BEM type model with the turbulent inflow generated with a turbulence model like the Mann model. There are several fundamental uncertainties in this approach, e.g. how well the unsteady induction in response to the turbulent flow is computed. However, within the last few years full 3D CFD rotor computations with turbulent inflow have been performed which can provide detailed insight into this complex load response. In the present work we present computations with the EllipSys3D solver on the 80m diameter NM80 turbine used in the DANAERO project where surface pressure measurements at four radial positions were conducted. The aerodynamic loads integrated from the pressure distributions have been derived and compared with computations by the aeroelastic code HAWC2. Overall a very good correlation is found by comparing PSD spectra of the measured sectional blade forces with HAWC2 simulations using specific flow input from the meteorology mast at six heights. In another comparison using purely turbulent inflow for the simulations on the NM80 rotor some deviations between the force spectra are found between EllipSys3D results and HAWC2 simulations at the inboard part of the blade and at high frequencies.

Atomic-scale forces induced by a hydrogen molecule trapped in a tunneling junction

Hydrogen molecules can be trapped in the nanocavity formed by the tip of a scanning tunneling microscope and a metal or molecular surface, and produce sharp inelastic non-linearities in the tunneling spectra. Here, we study the interaction effects of hydrogen in a tunneling junction created over Manganese phthalocyanines molecules in two oxidation states. The effect of hydrogen in the tunneling spectra varies strongly depending on the molecular species, but its force spectrum is fairly independent on the molecular state. We find that in mild tunneling conditions hydrogen-induced forces are weakly attractive during a small range of tip sample distance. The van der Waals interaction shows a maximum value of 140 pN, which faintly depends on electrostatic variations along the surface. These results show that AFM can be employed to resolve the complex interaction landscape of a trapped hydrogen molecule and deduce fainter effects such as molecular deformations or dipolar fields.