Global Response Characteristics Research Articles

Global design methodology for semi-submersible hulls of floating wind turbines

Designing the support structures for floating wind turbines (FWTs) involves a step-by-step process, typically in three phases, i.e., starting from concept selection and determining the overall dimensions for a given configuration, and finally, determining the detailed specifications. The global design of the hull is used to select the floater concept and determine the global dimensions for the subsequent detailed design. This global design process is a crucial part of the entire design cycle because it has a significant impact on the detailed design, which can be computationally expensive. However, because FWTs are still in the infancy stage of development, there is a lack of effective methods and procedures for determining the global dimensions of floaters. Additionally, the global response characteristics of different hull layouts are not yet well understood. To address these limitations, this paper deals with a global design of semi-submersible floaters to support large-scale wind turbines (using a 10-MW turbine as an example). The paper proposes and describes a methodology and procedures for the global design in detail, specifying criteria and values for a case study. Furthermore, a sensitivity study is conducted to determine the global dimensions of the floaters. Then, intact stability is studied to evaluate their safety in operation under wind loads. Subsequently, global aspects of structural design are analyzed based on wave-induced response amplitude operators (RAOs) of cross-sectional forces and moments, taking into account representative environmental conditions for an offshore site. These analyses provide a foundation for the subsequent detailed design. The results indicate that the spacing of the column centerline and the outer column radius are the most critical design variables for the global response of the semi-submersible hull. Moreover, increasing the outer column radius or freeboard can effectively enhance intact stability. In addition, the study highlights the importance of critical wave periods that cause splitting or prying force on the hull columns in the structural design. Finally, the main findings and conclusions are summarized. This study presents a simplified yet innovative methodology for the global design of mainstream semi-submersible hulls for FWTs. It addresses the lack of effective methods for determining global dimensions and provides valuable insights into the structural design considerations.

Flow-Induced Motion and Energy Conversion of the Cir-T-Att Oscillator in a Flow Field with a High Reynolds Number

The present study aims to systematically investigate the effects of a high Reynolds number on the flow-induced motion and energy conversion of the Cir-T-Att oscillator. Experiments are conducted in six Reynolds number ranges (2.89 × 104~6.51 × 104 < Re < 7.71 × 104~15.21 × 104) in regimes of TrSL2, TrSL3 and TrBL0. The system total damping ratio (ζtotal) is adjusted by varying excitation voltages (VB) with a controllable magnetic damping system. The VB is varied from 0 V to 165 V, corresponding to 0.082 ≤ ζtotal ≤ 1.153. The amplitude, frequency, fluid force, spectral content, active power, and upper limit of power output are analyzed. The results show that the Reynolds number affects both amplitude and global response characteristics. The active power increases with an increasing Reynolds number within the upper branch. The maximum power output of the Cir-T-Att oscillator reaches 10.43 W, appearing at Re = 14.67 × 104 (D = 0.16 m, ζtotal = 0.468, Ur = 6.34), while the maximum upper limit of power output is 17.80 W at Re = 15.21 × 104 (D = 0.16 m, ζtotal = 0.678, Ur = 6.57).

Unveiling the Effects of Viscous Friction on the Full Annular Rubs in a Piecewise Smooth Rotor/Stator Rubbing System

Experimental evidences show that friction during rotor/stator rubbing may not always obey the Coulomb friction law, which is predominantly adopted in the studies of these kinds. This work is devoted to unveiling the effects of viscous damping in the friction model on forward and backward full annular rubbing responses in a piecewise smooth rotor/stator rubbing system. The effects induced by the viscous friction, which are exclusively different from those with pure Coulomb friction, may possibly be used as the signatures for identifying the presence/extent of viscous friction in rotor/stator rubbing tests. In this work, the boundaries of both forward and backward full annular rub motions are analytically derived based on the stability and bifurcation theory. Due to the introduction of viscous friction, the rotor/stator system changes from a piecewise smooth continuous system to a Fillipov one. The results show that the viscous friction has little influence on the region of backward full annular rubbing responses but has significant effects on the region of forward full annular rub motion. With the increase of the viscous friction, the parameter region of the forward full annular rub shrinks significantly, and the boundary of Hopf bifurcation changes increasingly from supercritical to subcritical. This leads to more complex behaviors in the global response characteristics in comparison with those of the rotor/stator rubbing system with Coulomb friction, for instance, two new quasiperiodic rubbing responses appear beside the quasiperiodic partial rub bifurcating from the synchronous full annular rub, including a new quasiperiodic full annular rub motion that is different from the traditional full annular rubs mentioned above.

Determining the Global Response Characteristics of General Rotor/Stator Rubbing Systems with Hydrodynamic Forces

Determining the Global Response Characteristics of General Rotor/Stator Rubbing Systems with Hydrodynamic Forces

Dynamics and length scales in vertical convection of liquid metals

Using complementary experiments and direct numerical simulations, we study turbulent thermal convection of a liquid metal (Prandtl number $\textit {Pr}\approx 0.03$ ) in a box-shaped container, where two opposite square sidewalls are heated/cooled. The global response characteristics like the Nusselt number ${\textit {Nu}}$ and the Reynolds number $\textit {Re}$ collapse if the side height $L$ is used as the length scale rather than the distance $H$ between heated and cooled vertical plates. These results are obtained for various Rayleigh numbers $5\times 10^3\leq {\textit {Ra}}_H\leq 10^8$ (based on $H$ ) and the aspect ratios $L/H=1, 2, 3$ and $5$ . Furthermore, we present a novel method to extract the wind-based Reynolds number, which works particularly well with the experimental Doppler-velocimetry measurements along vertical lines, regardless of their horizontal positions. The extraction method is based on the two-dimensional autocorrelation of the time–space data of the vertical velocity.

A dynamic model of deep water riser and its global response characteristics induced by ocean internal wave and floating platform

ABSTRACT Because the flow direction of the water above and below the interface of internal wave is opposite, internal wave has a strong shear effect on ocean structures. A dynamic model of internal wave acting on deep water steel catenary riser (DSRC) is built. The global dynamic responses of gas riser and oil riser under the same action of internal wave are both obtained through numerical calculation, and the dynamic response laws of some specified positions are also studied. Then, the maximum value of dynamic response in a certain period of time and its location on the riser is found. At last, the response characteristic of DSCR induced by internal wave is studied in detail when the heave amplitude of floating platform is constant, but the heave frequency changes continuously as well as that the heave frequency is constant, but the heave amplitude changes continuously by some dynamic model experiments.

Programmable mechanical metastructures from locally bistable domes

Mechanical metamaterials have introduced a paradigm shift in materials development by offering property control through geometrical design at the micro/mesoscale. The ensuing design flexibility has opened up avenues to leverage geometrical nonlinearities in the microstructure design to allow for realizing uncharacteristic global functionalities beyond the scope of traditional materials. In this work, we report on a class of programmable stiffness metastructures designed by patterning locally bistable dome units in structural element geometries. The global response characteristics of our metastructures reversibly switch between a plate-like response and a shell-like response as the bistable domes are switched locally between their two stable states. This allows for up to an order of magnitude variation in the global stiffness. Furthermore, we demonstrate the ability of our metastructures to enable in-situ property programmability in the presence of boundary conditions, thus paving the way for successful integration into larger structures. Uniquely, our architecture allows for multi-directional linear and nonlinear response programmability, while operating under different loading environments. As such, the presented designs can potentially serve as blueprint models to realize a new class of load-carrying, robust, programmable systems for soft robotics and morphing aerospace structures.

Diffusion and Persistence of Rotor/Stator Synchronous Full Annular Rub Response Under Weak Random Perturbations

In this paper, the diffusion and persistence of synchronous full annular rub in a rotor/stator system under weak random disturbances are studied. First, the model of the rotor/stator rubbing system and the global response characteristics of the corresponding deterministic system are introduced. Then the diffusion features of the synchronous full annular rub response under small random perturbation are quantified and visualized using the stochastic sensitivity function for the discrete-time systems. The shape of the density distribution of the rotor orbit under different noise intensities and rotation speeds are compared and discussed. Finally, using the confidence ellipsoid of the response, the mechanism of noise-induced escape from the synchronous full annular rub to a large amplitude response of dry friction backward whirl is revealed, and the persistence of full annular rub response responses under different conditions is discussed and analyzed in detail. The results may be helpful in studying complex phenomena in random-disturbed rotor/stator rubbing systems.

Seismic Performance of Steel Industrial Storage Racks Subjected to Korea Earthquakes

This study experimentally and analytically examines the seismic vulnerability of steel rack storage frames subjected to Korea earthquakes (2016 Gyeongju earthquake and 2017 Pohang earthquake). To achieve this aim, this study selects a three-story, one-bay steel rack frame with a typical configuration of rack frame in Korea. Firstly, the local behavior for frame components is examined by performing monotonic and/or cyclic load tests and the global response and dynamic characteristics of the subject rack frame are investigated by conducting a shaking table test. The analytical model of the rack frame is then created based on the experimental results and is used to perform nonlinear time history analyses with recorded Korea earthquakes. The seismic demand of the rack frame is considerably affected by the spectral acceleration response, instead of peak ground accelerations (peak floor accelerations). Moreover, the collapse fragility curve of the rack frame is developed using incremental dynamic analyses for the Gyeongju and Pohang earthquakes. Fragility results indicate that the ground motion characteristics of these earthquakes do not significantly affect the frame vulnerability at the collapse state.

Shake table test of a full-scale four-story reinforced concrete structure and numerical representation of overall response with modified IMK model

This paper mainly addresses the validity and verification of our current code-based knowledge through the utilization of test data of benchmark structure in Japan. A full-scale, four-story reinforced concrete building within the 2010 E-Defense test program is revisited to assess the capability of the current state of practice for the prediction of the actual seismic response of reinforced concrete structures by taking advantage of invaluable data and high-quality measurement techniques applied during the tests. The detailing of the structural elements was in compliant with the Japanese seismic design code whereas minor modifications were applied to the final design of the structure to bring it closer to the selectively well-known US design practice. A series of full-scale shake table tests were performed by gradually increasing the two ground motions of the 1995 Hyogo-Ken Nanbu earthquake for reaching to the near collapse limit state. Global dynamic response characteristics of the structure due to damage accumulation were derived from the test data to trace the strength and stiffness deterioration in the force–deformation hysteresis. Even though the story drift angles were exceeded the critical 0.04 level beyond the limitations of code-provisions, the structure has preserved dynamic stability and its intact form. Based on the surveys and measurements performed at the end of each test, shear walls and beam–column joints were sustained the severe damage while the beams and columns were maintained their structural integrity during the entire test program. Parameters of advanced hysteretic models in the moment-resisting and shear wall frames were calibrated using test data for developing analytical models that can represent all modes of the cyclic deterioration under pinching action as an alternative to the customarily used Takeda model in Japan. For the transformation of MDOF system to equivalent SDOF system, a straightforward procedure is applied to adopt the global response characteristics to the versatile hysteretic model. Moreover, the capability of capturing the seismic response of RC system through advanced hysteretic models is justified. The test results and numerical analyses implied the necessity of further improvements on MLIT and US design provisions for the reduction of observed damages on structural elements particularly in shear walls and beam–column joints to achieve reparability goals.

Temperature and precipitation projection at 1.5 and 2°C increase in global mean temperature

Based on climate model outputs from Coupled Model Intercomparison Project Phase 5 (CMIP5), we investigated the global temperature and precipitation changes when global mean temperature rises by 1.5 and 2°C relative to the pre- industrial period (1861–1900). Multi-model ensemble mean (MME) shows that for the scenarios RCP2.6, RCP4.5, RCP6.0 and RCP8.5, global mean near-surface temperature (GMST) may reach 1.5°C warming relative to the pre- industrial level (1861–1900) around the year 2036, 2028, 2033 and 2025, respectively; and reach 2°C warming in 2049 (RCP4.5), 2056 (RCP6.0) and 2039 (RCP8.5). For RCP2.6 scenario, the 2°C warming could not be seen in the multi-model ensemble mean before 2100, but can be reached in different years in single model simulations including GFDL-CM3, IPSL-CM5A-LR, IPSL-CM5A-MR and MIROC-ESM. The timing when the GMST reaches a specific warming threshold is primarily related to radiative forcing and CO2-equivalent concentrations, which show similar value when the GMST rises by the same value. Projections under all RCPs from 25 CMIP5 models show that only 6 models under RCP2.6 scenario can project a warming lower than 1.5°C relative to the pre-industrial era before 2100, while the temperature increase under other three scenarios will be more than 1.5°C. It is therefore necessary to develop a set of lower emission scenarios to limit the global warming below 1.5°C. The investigations on inter-model differences show that the model with large transient climate response (TCR), also known as warm models, may reach the 1.5 and 2°C temperature increase earlier than those with low TCR (cold models), though other factors can also affect the model to reach 1.5 or 2°C temperature increase. For the future changes of temperature and precipitation at global scale, the MME results show little distinction among different scenarios when GMST rises by the same value, indicating that the global and regional response characteristics of temperature and precipitation are independent of the RCP scenarios definition (defined by the radiative forcing by 2100). Therefore, it is possible to investigate the changes in temperature increase 0.5 under RCP8.5 scenarios. The results show that regional responses are almost the same when GMST rises by each additional 0.5°C, indicating that the temperature and precipitation will essentially change linearly. These changes are characterized by more temperature increase in higher latitudes than in low latitudes, more temperature rise in land than in ocean, and increased precipitation in wet areas and decreased precipitation in dry areas, as was commonly detected in multiple studies. This suggests that the global warming impacts could be evaluated based on the multi-model ensemble projections under any RCP scenario with as much as a collection of models. This study also shows that in China, the regional mean temperature and precipitation changes are larger than the global mean when the GMST rises by 1.5 and 2°C. The temperature may increase across all China, with the warming increases from southeast to northwest. The precipitation will increase in most areas but it may decrease in the eastern part south of 30°N, based on the MME results.

Acoustic response of vortex breakdown modes in a coaxial isothermal unconfined swirling jet

The present experimental work is concerned with the study of amplitude dependent acoustic response of an isothermal coaxial swirling jet. The excitation amplitude is increased in five distinct steps at the burner’s Helmholtz resonator mode (i.e., 100 Hz). Two flow states are compared, namely, sub-critical and super-critical vortex breakdown (VB) that occur before and after the critical conical sheet breakdown, respectively. The geometric swirl number is varied in the range 2.14-4.03. Under the influence of external pulsing, global response characteristics are studied based on the topological changes observed in time-averaged 2D flow field. These are obtained from high resolution 2D PIV (particle image velocimetry) in the longitudinal-mid plane. PIV results also illustrate the changes in the normalized vortex core coordinates (rvcc/(rvcc)0 Hz, yvcc/(yvcc)0 Hz) of internal recirculation zone (IRZ). A strong forced response is observed at 100 Hz (excitation frequency) in the convectively unstable region which get amplified based on the magnitude of external forcing. The radial extent of this forced response region at a given excitation amplitude is represented by the acoustic response region (b). The topological placement of the responsive convectively unstable region is a function of both the intensity of imparted swirl (characterized by swirl number) and forcing amplitude. It is observed that for sub-critical VB mode, an increase in the excitation amplitude till a critical value shifts the vortex core centre (particularly, the vortex core moves downstream and radially outwards) leading to drastic fanning-out/widening of the IRZ. This is accompanied by ∼30% reduction in the recirculation velocity of the IRZ. It is also observed that b < R (R: radial distance from central axis to outer shear layer-OSL). At super-critical amplitudes, the sub-critical IRZ topology transits back (the vortex core retracts upstream and radially inwards) and finally undergoes a transverse shrinkage (rvcc/rvcc0Hz decreases by ∼20%) when b ≥ R. In contrast, the vortex core of super-critical breakdown mode consistently spreads radially outwards and is displaced further downstream. Finally, the IRZ fans-out at the threshold excitation amplitude. However, the acoustic response region b is still less than R. This is explained based on the characteristic geometric swirl number (SG) of the flow regimes. The super-critical flow mode with higher SG (hence, higher radial pressure drop due to rotational effect which scales as ΔP ∼ ρuθ2 and acts inwards towards the center line) compared to sub-critical state imposes a greater resistance to the radial outward spread of b. As a result, the acoustic energy supplied to the super-critical flow mode increases the degree of acoustic response at the pulsing frequency and energizes its harmonics (evident from power spectra). As a disturbance amplifier, the stronger convective instability mode within the flow structure of super-critical VB causes the topology to widen/fan-out severely at threshold excitation amplitude.

Adaptive structural control using global vibration sensing and model updating based on local infrared imaging

SUMMARY This paper presents a hybrid structural health monitoring system that was connected to an adaptive structural control algorithm to improve the control performance. The proposed vibration-based global damage detection method is combined with a local damage identification method using sonic infrared imaging. The numerical model and the monitor are updated for continuous structural monitoring and control during future earthquakes. The previously developed damage diagnosis technique is enhanced by including an equivalent simplified lumped-mass model deduced from a complex frame structure. Changes in global dynamic response characteristics due to damaged members or joints are first observed and the damage can be detected and located during the earthquake event. After the earthquake event, local damage is inspected in detail using sonic infrared imaging. The example of a three-story steel frame structure with a damaged column and a damaged joint is presented to demonstrate and evaluate the usefulness and effectiveness of the proposed concept. Results from numerical simulations indicate that the adaptive control strategy based on model updating improves the control performance. Copyright © 2011 John Wiley & Sons, Ltd.

The global responses characteristics of a rotor/stator rubbing system with dry friction effects

Rotor/stator rubbing systems may undertake a number of quite different responses. Recent experiments on rotor/stator rubbing have revealed that two or three different responses may coexist. In this paper the global response characteristics of a general rotor/stator rubbing system, which takes into account the dominant factors in the process of rotor/stator rubbing, especially, the dry friction effect that is mostly neglected in the previous works and is the main factor for the self-excited dry friction backward whirl, are studied. The different solutions of the piecewise nonlinear system are derived and their stability are analyzed to get the existence boundaries of the different responses. An overall picture of the global response characteristics of this model is then obtained by drawing the existence boundaries in a same parameter space. The present results provide good understanding on the coexistence of different rubbing responses observed in tests. Moreover, deeper insight into the types of coexistence of different rubbing responses and their relationship with the system parameters is gained.

Determination of the global responses characteristics of a piecewise smooth dynamical system with contact

In this paper the global response characteristics of a piecewise smooth dynamical system with contact, which is specifically used to describe the rotor/stator rubbing systems, is studied analytically. A method to derive the global response characteristics of the model is proposed by studying each piece of the equations corresponding to different phases of the rotor motion, i.e., the phase without rubbing, the phase with rubbing and the phase of self-excited backward whirl. After solving the typical responses in each phase and deriving the corresponding existence boundaries in the parameter space, an overall picture of the global response characteristics of the model is obtained. As is shown, five types of the coexistences of the different rotor responses and deep insights into the interactive effect of parameters on the dynamic behavior of the model are gained.

EOG Responses in Anesthetized Freely Breathing Rats

In mammals, access of odor molecules to the olfactory receptor neurons is controlled by respiratory activity. Thus, anesthetized, freely breathing rats were used to record from the olfactory mucosa in the intact nasal cavity (electroolfactogram or EOG) so as to study global response characteristics to odor stimuli. During alternation of the inspiratory phases of odor sampling and expiratory phases, the response was a succession of individual EOG events synchronized with respiration. These were characterized by a steep decrease that started approximately 100-150 ms after the beginning of inhalation, reached its maximum at the transition between inspiration and expiration and was followed by a slower rise until the next inhalation. They were greater during the first respiratory cycles following odor stimulation onset. Thereafter their amplitudes decreased throughout odor delivery, but a significant EOG signal was still present at the end of short (10 s) and long (60 s) odor presentations. Amplitude increased with odor concentration, but much less than expected from concentration changes. Lastly, for some odors EOG responses persisted well beyond the end of stimulation. These results are in agreement with the respiratory synchronization of mitral cell activities observed during short odor presentations and long duration odor exposures. They underline again the importance of taking into account the respiratory activity in studies on the functioning of the olfactory system.

An evaluation of higher-order effects on the eigenfrequencies of composite structures

The evaluation of the performances of higher-order models for the simulation of the response of laminated structures, namely in the prediction of global response characteristics such as the eigenfrequencies, is considered. An FEM analysis based on single-layer higher-order models has been carried out and the results for various cases of stacking sequences and aspect ratios have been compared. The same results have also been compared with the ones obtained with the classical laminate theory (CLT) in order to establish the predicting capability of the models and their utility in real practice.

Assessment of computational models for sandwich panels and shells

A study is made of the effects of variation in the material and geometric parameters of curved sandwich panels on the accuracy of the static response predicted by nine different modeling appraoches based on two-dimensional shell theories. The standard of comparison is taken to be the exact three-dimensional thermoelasticity solutions, and the quantities compared include gross response characteristics (e.g. strain energy components, average through-the-thickness displacements and rotations); detailed, through-the-thickness distributions of displacements and stresses; and sensitivity coefficients of the response quantities (derivatives of response quantities with respect to material and geometric parameters of the sandwich structure). Extensive numerical studies are conducted to assess the accuracy of both the global and detailed response characteristics and their sensitivity coefficients obtained by nine two-dimensional modeling approaches. For accurate determination of detailed through-the-thickness distributions such as transverse stresses, either high-order discrete three-layer models or predictor-corrector approaches are required. The potential of predictor-corrector approaches for predicting the thermomechanical response of sandwich panels and shells with complicated geometry is also discussed.

The Global Response Characteristics of the BVP Neuronal Model to Periodic Pulse Trains

The Global Response Characteristics of the BVP Neuronal Model to Periodic Pulse Trains

Stress and free vibration analyses of multilayered composite plates

A two-phase computational procedure is presented for the accurate prediction of the vibration frequencies, stresses and deformations in multilayered composite plates. In the first phase a two-dimensional first-order shear deformation theory is used to predict the global response characteristics (vibration frequencies, ‘average’ through-the-thickness displacements and rotations) as well as the in-plane stress and strain components in the different layers. In the second phase, equilibrium equations and constitutive relations of the three-dimensional theory of elasticity are used to: (1) calculate the transverse stresses and strains as well as the transverse strain energy densities in the different layers; (2) provide better estimates for the composite shear correction factors; and (3) calculate corrected values for the vibration frequencies, displacements, and in-plane strains and stresses. For simply supported plates the predictions of the proposed procedure are shown to be in close agreement with exact three-dimensional elasticity solutions for a wide range of lamination and geometric parameters. Also, the potential of the proposed procedure for use in conjunction with large-scale finite element models of composite structures is discussed.