Abstract
The hydrodynamic characteristics are crucial for accurately analysing floating offshore wind systems. In this paper, the added mass and damping coefficients of a semisubmersible floater are ...examined around the natural periods of the surge, heave, and pitch motion, using computational fluid dynamics (CFD). The OpenFOAM CFD setup is validated against experimental measurements from the free decay tests, and the same setup is used to determine the hydrodynamic coefficients of the platform subjected to forced motions with different amplitudes and periods. The added mass and quadratic damping coefficients obtained from forced oscillations are consistent with the free decay results. Moreover, the added mass coefficients obtained by CFD is significantly higher than the estimations of the potential flow theory: around 10% larger for surge and 22% larger for heave. The damping is almost independent of the frequency while it varies with the motion amplitude. The deviations in the CFD results from the potential flow theory are due to the viscous effects. Besides, viscous damping is dependent on the drag coefficient specified in the Morison’s equation.
Forced oscillation has become one of the main power grid dynamic stability problems, which can cause high amplitude oscillation of the active power in the tie line. The supplementary damping ...controller (SDC) of the voltage source converter based high voltage direct current (VSCHVDC) has demonstrated its effectiveness in suppressing inter-area modal oscillation, but its role in mitigating forced oscillation has not been studied in the literature. In this paper, an active disturbance rejection control (ADRC) based SDC of the VSCHVDC is designed to mitigate forced oscillations. The mitigation effect of the ADRC based SDC on forced oscillation is compared to the traditional one under different scenarios. Simulation results of four-machine two-area AC-DC hybrid system indicate that the traditional SDC is sensitive to the frequency of the disturbance source, and the reactance between the disturbance source and the VSCHVDC, whereas the ADRC based SDC can effectively mitigate the forced oscillation whatever the frequency of disturbance source is near or far from the inter-area natural frequency, and its mitigating effect is almost not affected by the reactance between the disturbance source and the VSCHVDC.
•In the paper the results of simulated vibrations of cracked beam are given.•Realizations of vibration signals are obtained with using of mathematical model in the form of a system of nonlinear ...differential equations.•In the paper the dynamic of changes of characteristics of vibration’s deterministic and stochastic parts are analyzed separately.•It is shown that characteristics of the stochastic part are more effective for the crack detection then characteristics of the deterministic part.•The mathematical model of the cracked beam forced oscillations is proposed.•The whole complex of statistical characteristics is estimated and analyzed within the theory of periodically correlated random processes.
An analysis of forced oscillations of cracked beam using statistical methods for periodically correlated random processes is presented. The oscillation realizations are obtained on the basis of numerical solutions of differential equations of the second order, for the case when applied force is described by a sum of harmonic and stationary random process. It is established that due to crack appearance forced oscillations acquire properties of second-order periodical non-stationarity. It is shown that in a super-resonance regime covariance and spectral characteristics, which describe non-stationary structure of forced oscillations, are more sensitive to crack growth than the characteristics of the oscillation’s deterministic part. Using diagnostic indicators formed on their basis allows the detection of small cracks.
For the past few years, heave plates have been used in the offshore industry due to their favorable hydrodynamic characteristics in oscillating flows, i.e., increased added mass and damping. The ...hydrodynamic coefficients of heave plates are strongly influenced by the proximity of solid boundaries. Assessing how this variation depends on the proximity to the seabed and on the amplitude of the oscillation is the main aim of this paper. In this process, a new model describing the work done by damping in terms of the flow enstrophy is described herein. This new approach is able to provide a direct correlation between the vortex shedding processes and the linear damping coefficient. Numerical simulations have been performed using the finite volume open source solver OpenFOAM. Simulations have been conducted on a solid circular disc oscillating axially in water at various elevations above an impermeable seabed boundary. Results for hydrodynamic coefficients are validated against previously published experimental data. At low KC numbers, a systematic increase in added mass and damping, corresponding to an increase in the seabed proximity, is observed. As seen in experiments, a critical KC where the monotonic trend of the hydrodynamic coefficients with KC is disrupted and that depends on the seabed distance exists. The physical behavior of the flow around the critical KC is explained through an analysis of the flow enstrophy.
•Numerical simulations on a heave plate oscillating near seabed conducted.•Added mass and damping have been found to increase with the proximity to the seabed.•Depth dependent critical KC found, above which A33 deviates from a linear trend.•Alternative way to obtain the damping looking at the dissipation mechanisms devised.•Damping obtained from enstrophy integral and classical damping formulation discussed.
Abstract
Near-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both ...relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency
f
eff
, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves
ω
min
. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where
f
eff
was less than the Coriolis frequency
f
. If everywhere
f
eff
>
f
then NIWs were generated where
ω
min
<
f
and
f
eff
was smallest. The background current not only affected the location of generation, but also the NIWs’ propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because
ω
min
>
f
on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency
ω
within the anomalously low-frequency band (i.e.,
ω
min
<
ω
<
f
eff
), which only exists if the baroclinicity is nonzero. We observed two periods when
ω
min
<
f
on the shelf, but the relative vorticity was positive (i.e.,
f
eff
>
f
) for one of these periods. The process of NIW propagation remained consistent with the local
ω
min
, and not
f
eff
, emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.
Abstract Background Epidemiological evidence suggests the existence of a direct link between allergic rhinitis (AR) and asthma. Several studies also support the presence of small-airway dysfunction ...(SAD) in non-asthmatic children with AR. However, it remains unknown whether SAD can predict the progression of AR to asthma. Our objective was to explore the existence of SAD in non-asthmatic children with AR and to assessed its ability to predict the development of asthma. Methods Seventy-three 6-year-old children with intermittent moderate-severe AR but without asthma symptoms/medication within the last two years, underwent spirometry and measurement of respiratory resistance (Rrs) and reactance (Xrs) before and after bronchodilation (BD) (300 mcg salbutamol). Lung function measurements were performed in the absence of nasal symptoms and repeated at AR exacerbation. SAD was defined as >30% decrease in Rrs or >50% increase in Xrs at 6 or 8 Hz post-BD. Participants were followed for five years. Results Twenty-three children (31.5%) developed asthma; this group presented significant post-BD changes in Rrs and Xrs, but only at AR exacerbation. The ability of these changes to predict the development of asthma was exceptional and superior to that of the spirometric parameters. SAD (22 children, 30.1%), emerged as the single most efficient predictor of asthma, independently of other risk factors such as parental asthma, personal history of eczema and type of allergic sensitisation. Conclusion SAD precedes the development of asthma in children with AR. Changes in respiratory impedance at AR exacerbation may assist in identifying those at risk to progress to asthma.
The article presents a dynamic model of interaction of rollers on the example of printing cylinders of an offset machine. Based on the solution of the contact problem, the interaction forces of the ...cylinders are determined, direct modeling of dynamic loads and vibrations is carried out, the amplitudes of vibrations and pressure changes are determined. Options for reducing loads are considered. The disadvantages of existing models of printing machines are analyzed and taken into account.
The self similar branching arrangement of the airways makes the respiratory system an ideal candidate for the application of fractional calculus theory. The fractal geometry is typically ...characterized by a recurrent structure. This study investigates the identification of a model for the respiratory tree by means of its electrical equivalent based on intrinsic morphology. Measurements were obtained from seven volunteers, in terms of their respiratory impedance by means of its complex representation for frequencies below 5 Hz. A parametric modeling is then applied to the complex valued data points. Since at low-frequency range the inertance is negligible, each airway branch is modeled by using gamma cell resistance and capacitance, the latter having a fractional-order constant phase element (CPE), which is identified from measurements. In addition, the complex impedance is also approximated by means of a model consisting of a lumped series resistance and a lumped fractional-order capacitance. The results reveal that both models characterize the data well, whereas the averaged CPE values are supraunitary and subunitary for the ladder network and the lumped model, respectively.
The forced oscillations of hair bundle of inner hair cells of auditory nervous system evoked by external force from steady state are related to the fast adaption of hair cells, which are very ...important for auditory amplification. In the present paper, comprehensive and deep understandings to nonlinear dynamics of forced oscillations are acquired in four aspects. Firstly, the complex dynamics underlying the twitch (fast recoil of displacement
X
which is fast variable) induced from Case-1 and Case-2 steady states by external pulse force are obtained. With help of vector fields and nullclines, the phase trajectory of forced oscillations is identified to be an evolution process between two equilibrium points corresponding to zero force and pulse force, respectively, and then the twitch is obtained as the behavior running along the nonlinear part of
X
-nullcline. Especially, twitch observed in experiment are classified into 6 types, which are induced by negative change of force, negative and positive changes of force, and positive change of force, respectively, and further build relationships to three subcases of Case-2 steady state with N-shaped
X
-nullcline (equilibrium point locates on the left, middle, and right branches of
X
-nullcline, respectively). Secondly, the experimental observation of fatigue of twitch induced by continual two pulse forces, i.e. the reduced amplitude of the latter twitch when interval between two forces is short, is also explained as a nonlinear behavior beginning from an initial value different from that of the former one. Thirdly, the experimental observation of transition between sustained oscillations and steady state induced by pulse force can be simulated for Case-1 steady state with Z-shaped
X
-nullcline instead of Case-2, due to that there exists bifurcations with respect to external force for Case-1 while no bifurcations for Case-2. Last, the threshold phenomenon induced by simple pulse stimulation exists for Case-1 steady state rather than Case-2, due to that the upper and lower branches of Z-shaped
X
-nullcline close to the middle branch exhibit coexisting behaviors of variable
X
while N-shaped
X
-nullcline does not. The nonlinear dynamics of forced oscillations are helpful for explanations to the complex experimental observations, which presents potential measures to modulate the functions of twitch such as the fast adaption.
Asymptotic approximations of “phase functions” for linear second-order differential equations, whose solutions are highly oscillatory, can be obtained using Borůvka's theory of linear differential ...transformations coupled to Liouville–Green (WKB) asymptotics. A numerical method, very effective in case of asymptotically polynomial coefficients, is extended to other cases of rapidly growing coefficients. Zeros of solutions can be computed without prior evaluation of the solutions themselves, but the method can also be applied to Initial- and Boundary-Value problems, as well as to the case of forced oscillations. Numerical examples are given to illustrate the performance of the algorithm. In all cases, the error turns out to be of the order of that made approximating the phase functions.
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