Journal articles: 'High Frequency Forcing'

1

Yao, Cheng-Gui, Zhi-Wei He, and Meng Zhan. "High frequency forcing on nonlinear systems." Chinese Physics B 22, no. 3 (March 2013): 030503. http://dx.doi.org/10.1088/1674-1056/22/3/030503.

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Sipp, Denis. "Open-loop control of cavity oscillations with harmonic forcings." Journal of Fluid Mechanics 708 (September 12, 2012): 439–68. http://dx.doi.org/10.1017/jfm.2012.329.

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AbstractThis article deals with open-loop control of open-cavity flows with harmonic forcings. Two-dimensional laminar open-cavity flows usually undergo a supercritical Hopf bifurcation at some critical Reynolds number: a global mode becomes unstable and its amplitude converges towards a limit cycle. Such behaviour may be accurately captured by a Stuart–Landau equation, which governs the amplitude of the global mode. In the present article, we study the effect on such a flow of a forcing characterized by its frequency ${\omega }_{f} $, its amplitude ${E}^{\ensuremath{\prime} } $ and its spatial structure ${\mathbi{f}}_{E} $. The system reacts like a forced Van der Pol oscillator. In the general case, such a forcing modifies the linear dynamics of the global mode. It is then possible to predict preferred forcing frequencies ${\omega }_{f} $, at which the global mode may be stabilized with the smallest possible forcing amplitude ${E}^{\ensuremath{\prime} } $. In the case of a forcing frequency close to the frequency of the global mode, a locking phenomenon may be observed if the forcing amplitude ${E}^{\ensuremath{\prime} } $ is sufficiently high: the frequency of the flow on the limit cycle may be modified with a very small forcing amplitude ${E}^{\ensuremath{\prime} } $. In each case, we compute all harmonics of the flow field and all coefficients that enter the amplitude equations. In particular, it is possible to find preferred forcing structures ${\mathbi{f}}_{E} $ that achieve strongest impact on the flow field. In the general case, these are the optimal forcings, which are defined as the forcings that trigger the strongest energy response. In the case of a forcing frequency close to the frequency of the global mode, a forcing structure equal to the adjoint global mode ensures the lowest forcing amplitude ${E}^{\ensuremath{\prime} } $. All predictions given by the amplitude equations are checked against direct numerical simulations conducted at a supercritical Reynolds number. We show that a global mode may effectively be stabilized by a high-frequency harmonic forcing, which achieves suppression of the perturbation frequencies that are lower than the forcing frequency, and that a near-resonant forcing achieves locking of the flow onto the forcing frequency, as predicted by the amplitude equations.

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Ghazanshahi, S. D., S. M. Yamashiro, and V. Z. Marmarelis. "Use of a random forcing for high-frequency ventilation." Journal of Applied Physiology 62, no. 3 (March 1, 1987): 1201–5. http://dx.doi.org/10.1152/jappl.1987.62.3.1201.

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Previous applications of high-frequency oscillatory ventilation (HFOV) have used cyclic forcings with the frequency of oscillation considered to be a fundamental parameter. A question that is addressed in the present study is whether or not periodicity is an essential requirement for this mode of ventilation to occur. It was found possible to adequately ventilate anesthetized and paralyzed cats with volume excursions below the dead-space level using a random band-limited forcing. Experimental conditions were close to a constant flow variance (VARF) state, and arterial CO2 tension varied linearly as a function of the ratio of noise bandwidth and VARF. Periodicity per se did not appear to be a requirement for HFOV to occur, a result consistent with predictions of Taylor dispersion theory.

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Oxlade, Anthony R., Jonathan F. Morrison, Ala Qubain, and Georgios Rigas. "High-frequency forcing of a turbulent axisymmetric wake." Journal of Fluid Mechanics 770 (March 31, 2015): 305–18. http://dx.doi.org/10.1017/jfm.2015.153.

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A high-frequency periodic jet, issuing immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body, its axis aligned with the free stream. It is shown that the base pressure may be varied more or less at will: at forcing frequencies several times that of the shear layer frequency, the time-averaged area-weighted base pressure increases by as much as 35 %. An investigation of the effects of forcing is made using random and phase-locked two-component particle image velocimetry (PIV), and modal decomposition of pressure fluctuations on the base of the model. The forcing does not target specific local or global wake instabilities: rather, the high-frequency jet creates a row of closely spaced vortex rings, immediately adjacent to which are regions of large shear on each side. These shear layers are associated with large dissipation and inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer mode approaches unity gain, is roughly five times the shear layer frequency.

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Lin, Li-Ching, and Mao-Chang Liang. "Meteotsunamis produced by high frequency atmospheric pressure forcing." Terrestrial, Atmospheric and Oceanic Sciences 28, no. 6 (2017): 1033–40. http://dx.doi.org/10.3319/tao.2017.03.20.01.

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Athanasiadis, Panos J., and Maarten H. P. Ambaum. "Do High-Frequency Eddies Contribute to Low-Frequency Teleconnection Tendencies?*." Journal of the Atmospheric Sciences 67, no. 2 (February 1, 2010): 419–33. http://dx.doi.org/10.1175/2009jas3153.1.

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Abstract An isentropic potential vorticity (PV) budget analysis is employed to examine the role of synoptic transients, advection, and nonconservative processes as forcings for the evolution of the low-frequency PV anomalies locally and those associated with the North Atlantic Oscillation (NAO) and the Pacific–North American (PNA) pattern. Specifically, the rate of change of the low-frequency PV is expressed as a sum of tendencies due to divergence of eddy transport, advection by the low-frequency flow (hereafter referred to as advection), and the residual nonconservative processes. The balance between the variances and covariances of these terms is illustrated using a novel vector representation. It is shown that for most locations, as well as for the PNA pattern, the PV variability is dominantly driven by advection. The eddy forcing explains a small amount of the tendency variance. For the NAO, the role of synoptic eddy fluxes is found to be stronger, explaining on average 15% of the NAO tendency variance. Previous studies have not assessed quantitively how the various forcings balance the tendency. Thus, such studies may have overestimated the role of eddy fluxes for the evolution of teleconnections by examining, for example, composites and regressions that indicate maintenance, rather than evolution driven by the eddies. The authors confirm this contrasting view by showing that during persistent blocking (negative NAO) episodes the eddy driving is relatively stronger.

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7

Capotondi, Antonietta, Prashant D. Sardeshmukh, and Lucrezia Ricciardulli. "The Nature of the Stochastic Wind Forcing of ENSO." Journal of Climate 31, no. 19 (October 2018): 8081–99. http://dx.doi.org/10.1175/jcli-d-17-0842.1.

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El Niño–Southern Oscillation (ENSO) is commonly viewed as a low-frequency tropical mode of coupled atmosphere–ocean variability energized by stochastic wind forcing. Despite many studies, however, the nature of this broadband stochastic forcing and the relative roles of its high- and low-frequency components in ENSO development remain unclear. In one view, the high-frequency forcing associated with the subseasonal Madden–Julian oscillation (MJO) and westerly wind events (WWEs) excites oceanic Kelvin waves leading to ENSO. An alternative view emphasizes the role of the low-frequency stochastic wind components in directly forcing the low-frequency ENSO modes. These apparently distinct roles of the wind forcing are clarified here using a recently released high-resolution wind dataset for 1990–2015. A spectral analysis shows that although the high-frequency winds do excite high-frequency Kelvin waves, they are much weaker than their interannual counterparts and are a minor contributor to ENSO development. The analysis also suggests that WWEs should be viewed more as short-correlation events with a flat spectrum at low frequencies that can efficiently excite ENSO modes than as strictly high-frequency events that would be highly inefficient in this regard. Interestingly, the low-frequency power of the rapid wind forcing is found to be higher during El Niño than La Niña events, suggesting a role also for state-dependent (i.e., multiplicative) noise forcing in ENSO dynamics.

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8

Condon, M., A. Deaño, and A. Iserles. "Simulation of memristors in presence of high-frequency forcing function." Electronics Letters 48, no. 12 (2012): 684. http://dx.doi.org/10.1049/el.2012.1051.

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Waseda, Takuji, Humio Mitsudera, Bunmei Taguchi, and Kunio Kutsuwada. "Significance of High-Frequency Wind Forcing in Modelling the Kuroshio." Journal of Oceanography 61, no. 3 (June 2005): 539–48. http://dx.doi.org/10.1007/s10872-005-0061-z.

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10

Shiogama, Hideo, Toru Terao, Hideji Kida, and Tatsuya Iwashima. "Roles of Low- and High-Frequency Eddies in the Transitional Process of the Southern Hemisphere Annular Mode." Journal of Climate 18, no. 6 (March 15, 2005): 782–94. http://dx.doi.org/10.1175/jcli-3303.1.

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Abstract The effects of low- and high-frequency eddies (time scales longer and shorter than 10 days, respectively) on the transitional processes of the Southern Hemisphere “Annular Mode” are investigated, based on NCEP–NCAR daily reanalysis data for the period 1979–2001. Special attention is focused on the zonal symmetry/asymmetry and the temporal evolution of the eddy forcing. For the poleward transitional process, the effects of low-frequency eddies precede those of high-frequency eddies in driving the jet transition. Quasi-stationary Rossby waves propagating along the polar jet with wavelengths of 7000 km play an important role. The waves, originally come from the Indian Ocean through the waveguide associated with the polar jet, dissipate equatorward over the eastern Pacific Ocean. This anomalous equatorward dissipation of wave activity induces an anomalous poleward momentum flux, which is responsible for changes in the polar jet over the Pacific Ocean during the beginning stage. Following the low-frequency eddy forcing, momentum forcing anomalies due to the high-frequency eddies rapidly appear. This forcing continues to drive the polar jet poleward over the whole of longitude, while the low-frequency eddies have completed their role of inducing the anomalous poleward momentum flux during the earlier stage. For the equatorward transitional events, the roles of the low-frequency eddy forcing differ from that in the poleward ones. Anomalous equatorward momentum fluxes due to low-frequency eddies appear simultaneously with that due to high-frequency eddies. Quasi-stationary Rossby waves with wavelengths of 7000 km propagate southeastward through the waveguide over the Pacific Ocean. The convergence of their wave activity results in the deceleration of the westerlies over the higher latitudes of the Pacific Ocean. On the other hand, the high-frequency eddy forcing contributes to the equatorward jet drift longitudinally over the whole of the hemisphere.

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11

KIYA, M., M. SHIMIZU, and O. MOCHIZUKI. "Sinusoidal forcing of a turbulent separation bubble." Journal of Fluid Mechanics 342 (July 10, 1997): 119–39. http://dx.doi.org/10.1017/s0022112097005521.

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A turbulent separation bubble is forced by single- and double-frequency sinusoidal disturbances, with the emphasis placed on the reattachment length as a function of the forcing amplitude and frequency. The separation bubble is that formed along the side of a blunt circular cylinder with a square leading edge. In single-frequency forcing, the reattachment length attains a minimum at a particular forcing frequency, F, which scales with the frequency of shedding of vortices from the reattachment region of the separated shear layer. A flow model is presented to interpret the frequency F. Forcing of sufficiently high amplitude eliminates the recirculating region in a range of the forcing frequency. Flow visualization and a survey of the mean flow and turbulence properties demonstrate how the flow in the separated shear layer is modified by the forcing. In double-frequency forcing, the superposition of the F-component on its higher or subharmonic components is considered. A non-resonant combination of the two frequencies is also considered.

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12

Ukita, Jinro, and Douglas G. Martinson. "An efficient adjustable-layering thermodynamic sea-ice model formulation for high-frequency forcing." Annals of Glaciology 33 (2001): 253–60. http://dx.doi.org/10.3189/172756401781818194.

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AbstractRecent observations suggest that high-frequency forcing events have profound influence on the air-sea-ice interactions in the polar region. Studying these events with sea-ice models requires close examination of the model sensitivity that may arise from the high-frequency variability of the forcing. We show that the maximum layer thickness is dictated by the time-scale of the forcing variability and that the computation of the surface temperature develops enhanced sensitivity at high-frequency forcing. We resolve these constraints by developing an "adjustable-layering" thermodynamic formulation for ice and snow that re-computes the number of layers required each time-step to satisfy this maximum thickness, which preserves the total enthalpy and general internal thermal gradients. The conservation equations form a tri-diagonal system ideal for a fast and efficient implicit solution. Furthermore, we resolve the issue of the high sensitivity of the surface flux balance by solving the linearized version of the flux boundary condition simultaneously with the overall conservation system. In this paper we develop the analyses specifying the model requirements, describe the model system and test its algorithmic implementation.

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13

Verbitsky, Mikhail Y., Michel Crucifix, and Dmitry M. Volobuev. "ESD Ideas: Propagation of high-frequency forcing to ice age dynamics." Earth System Dynamics 10, no. 2 (April 24, 2019): 257–60. http://dx.doi.org/10.5194/esd-10-257-2019.

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Abstract. Palaeoclimate records display a continuous background of variability connecting centennial to 100 kyr periods. Hence, the dynamics at the centennial, millennial, and astronomical timescales should not be treated separately. Here, we show that the nonlinear character of ice sheet dynamics, which was derived naturally from the ice-flow conservation laws, provides the scaling constraints to explain the structure of the observed spectrum of variability.

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14

Bartuccelli, M., G. Gentile, and J. A. Wright. "Stable dynamics in forced systems with sufficiently high/low forcing frequency." Chaos: An Interdisciplinary Journal of Nonlinear Science 26, no. 8 (August 2016): 083108. http://dx.doi.org/10.1063/1.4960614.

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15

Rocha, Alfredo, Paulo Melo-Gonçalves, Carlos Marques, Juan Ferreira, and José M. Castanheira. "High-frequency precipitation changes in southeastern Africa due to anthropogenic forcing." International Journal of Climatology 28, no. 9 (September 5, 2007): 1239–53. http://dx.doi.org/10.1002/joc.1596.

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16

Son, Donggun, and Jungil Lee. "A Periodically Rotating Distributed Forcing of Flow over a Sphere for Drag Reduction." Mathematics 11, no. 3 (January 30, 2023): 706. http://dx.doi.org/10.3390/math11030706.

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In the present study, we propose a periodically rotating distributed forcing for turbulent flow over a sphere for its drag reduction. The blowing/suction forcing is applied on a finite slot of the sphere surface near the flow separation, and unsteady sinusoidal forcing velocities are azimuthally distributed on the sphere surface. This forcing profile periodically rotates in the azimuthal direction over time with a forcing frequency, satisfying the instantaneous zero net mass flux. The Reynolds number considered is Re=104 and large eddy simulations are conducted to assess the control performance. It is shown that the drag reduction performance varies with the forcing frequency, and the control results are classified into low-frequency ineffective, effective drag reduction, and high-frequency saturation regimes. With forcing frequencies in the effective drag reduction regime, a helical vortex is generated from the forcing on the sphere and evolves in the shear layer, and this vortex is responsible for the separation delay and flow reattachment resulting in the base pressure recovery and drag reduction. The maximum drag reduction is about 44% with the forcing frequency in the effective drag reduction regime, while controls in other regimes do not produce a drag reduction.

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Ham, Yoo-Geun, Jong-Seong Kug, and Mi-Jung Lim. "Rectification Feedback of High-Frequency Atmospheric Variability into Low-Frequency Zonal Flows in the Tropical Pacific." Journal of Climate 25, no. 14 (July 15, 2012): 5088–101. http://dx.doi.org/10.1175/jcli-d-11-00303.1.

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Abstract In this study, the rectification process of high-frequency (HF) zonal-wind variability on the low-frequency (LF) zonal wind is investigated through an idealized experiment using an atmospheric general circulation model (AGCM). Through an idealized AGCM experiment with a fixed SST boundary forcing, it is shown that there is positive (negative) correlation between HF (2–90-day period) zonal-wind variance and LF (3-month average) zonal wind where the HF zonal-wind variance is positively (negatively) skewed because the stronger HF westerly (easterly) wind events than HF easterly (westerly) wind events induce a residual westerly (easterly), and it results in an additional rectified LF westerly (easterly) anomaly. This means that, over regions with positively skewed HF zonal winds, LF westerly anomalies are generated due to the residuals of the HF zonal winds. It implies that the LF zonal wind can be generated through internal processes of the atmosphere without external forcing and the interaction between LF and HF is not a one-way process from LF to HF but, rather, a two-way interaction process.

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Humphreys, L. D., P. J. McKenna, and K. M. O’Neill. "High frequency shaking induced by low frequency forcing: Periodic oscillations in a spring–cable system." Nonlinear Analysis: Real World Applications 11, no. 5 (October 2010): 4312–25. http://dx.doi.org/10.1016/j.nonrwa.2010.04.003.

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Wang, Linhui, Huiwang Gao, Jie Shi, and Lian Xie. "A Numerical Study on the Impact of High-Frequency Winds on the Peru Upwelling System during 2014–2016." Journal of Marine Science and Engineering 7, no. 5 (May 25, 2019): 161. http://dx.doi.org/10.3390/jmse7050161.

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The contribution of high-frequency wind to the Peruvian upwelling system during 2014–2016 was studied using the Regional Ocean Modeling System (ROMS), forced by four different temporal resolution (six-hourly, daily, weekly, and monthly) wind forcing. A major effect of the high-frequency wind is its warming of the water at all depths along the Peruvian coast. The mechanism for the temperature changes induced by high-frequency wind forcing was analyzed through heat budget analysis, which indicated a three-layer structure. Vertical advection plays a leading role in the warming of the mixed layer (0–25 m), and enhanced vertical mixing balances the warming effect. Analysis suggests that around the depths of 25–60 m, vertical mixing warms the water by bringing heat from the surface to deeper depths. In waters deeper than 60 m, the effect of vertical mixing is negligible. The differences among the oceanic responses in the sensitivity experiments suggest that wind forcing containing variabilities at higher than synoptic frequencies must be included in the atmospheric forcing in order to properly simulate the Peru upwelling system.

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Lovejoy, Shaun, and Costas Varotsos. "Scaling regimes and linear/nonlinear responses of last millennium climate to volcanic and solar forcings." Earth System Dynamics 7, no. 1 (February 23, 2016): 133–50. http://dx.doi.org/10.5194/esd-7-133-2016.

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Abstract. At scales much longer than the deterministic predictability limits (about 10 days), the statistics of the atmosphere undergoes a drastic transition, the high-frequency weather acts as a random forcing on the lower-frequency macroweather. In addition, up to decadal and centennial scales the equivalent radiative forcings of solar, volcanic and anthropogenic perturbations are small compared to the mean incoming solar flux. This justifies the common practice of reducing forcings to radiative equivalents (which are assumed to combine linearly), as well as the development of linear stochastic models, including for forecasting at monthly to decadal scales. In order to clarify the validity of the linearity assumption and determine its scale range, we use last millennium simulations, with both the simplified Zebiak–Cane (ZC) model and the NASA GISS E2-R fully coupled GCM. We systematically compare the statistical properties of solar-only, volcanic-only and combined solar and volcanic forcings over the range of timescales from 1 to 1000 years. We also compare the statistics to multiproxy temperature reconstructions. The main findings are (a) that the variability in the ZC and GCM models is too weak at centennial and longer scales; (b) for longer than ≈ 50 years, the solar and volcanic forcings combine subadditively (nonlinearly) compounding the weakness of the response; and (c) the models display another nonlinear effect at shorter timescales: their sensitivities are much higher for weak forcing than for strong forcing (their intermittencies are different) and we quantify this with statistical scaling exponents.

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Holdsworth, Amber M., and Paul G. Myers. "The Influence of High-Frequency Atmospheric Forcing on the Circulation and Deep Convection of the Labrador Sea." Journal of Climate 28, no. 12 (June 11, 2015): 4980–96. http://dx.doi.org/10.1175/jcli-d-14-00564.1.

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Abstract The influence of high-frequency atmospheric forcing on the circulation of the North Atlantic Ocean with emphasis on the deep convection of the Labrador Sea was investigated by comparing simulations of a coupled ocean–ice model with hourly atmospheric data to simulations in which the high-frequency phenomena were filtered from the air temperature and wind fields. In the absence of high-frequency atmospheric forcing, the strength of the Atlantic meridional overturning circulation and subpolar gyres was found to decrease by 25%. In the Labrador Sea, the eddy kinetic energy decreased by 75% and the average maximum mixed layer depth decreased by between 20% and 110% depending on the climatology. In particular, high-frequency forcing was found to have a greater impact on mixed layer deepening in moderate to warm years whereas in relatively cold years the temperatures alone were enough to facilitate deep convection. Additional simulations in which either the wind or temperature was filtered revealed that the wind, through its impact on the bulk formulas for latent and sensible heat, had a greater impact on deep convection than the temperature.

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22

Isakson, Marcia J., Autumn N. Kidwell, and Mark Story. "Effects of tidal forcing in an estuarine environment on high-frequency acoustics." Journal of the Acoustical Society of America 143, no. 3 (March 2018): 1729. http://dx.doi.org/10.1121/1.5035637.

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23

Lovejoy, S., and C. A. Varotsos. "Scaling regimes and linear and nonlinear responses of last millennium climate models to volcanic and solar forcings." Earth System Dynamics Discussions 6, no. 2 (September 25, 2015): 1815–62. http://dx.doi.org/10.5194/esdd-6-1815-2015.

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Abstract. At scales much longer than the deterministic predictability limits (about 10 days), the statistics of the atmosphere undergo a drastic transition, the high frequency weather acts as a random forcing on the lower frequency macroweather. In addition, up to decadal and centennial scales the equivalent radiative forcings of solar, volcanic and anthropogenic perturbations are small compared to the mean incoming solar flux. This justifies the common practice of reducing forcings to radiative equivalents (which are assumed to combine linearly), as well as the development of linear stochastic models, including for forecasting at monthly to decadal scales. In order to clarify the validity of the linearity assumption and determine its range of validity, we use last Millennium simulations, both with the simplified Zebiac–Cane (ZC) model and the NASA GISS E2-R fully coupled GCM. We systematically compare the statistical properties of solar only, volcanic only and combined solar and volcanic forcings over the range of time scales from one to 1000 years. We also compare the statistics to multiproxy temperature reconstructions. The main findings are: (a) that the variability of the ZC and GCM models are too weak at centennial and longer scales, (b) for longer than &approx; 50 years, the solar and volcanic forcings combine subadditively (nonlinearly) compounding the weakness of the response, (c) the models display another nonlinear effect at shorter scales: their sensitivities are much higher for weak forcing than for strong forcing (their intermittencies are different) and we quantify this with statistical scaling exponents.

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Zaman, Sakhi, Latif Ullah Khan, Irshad Hussain, and Lucian Mihet-Popa. "Fast Computation of Highly Oscillatory ODE Problems: Applications in High-Frequency Communication Circuits." Symmetry 14, no. 1 (January 9, 2022): 115. http://dx.doi.org/10.3390/sym14010115.

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The paper demonstrates symmetric integral operator and interpolation based numerical approximations for linear and nonlinear ordinary differential equations (ODEs) with oscillatory factor x′=ψ(x)+χω(t), where the function χω(t) is an oscillatory forcing term. These equations appear in a variety of computational problems occurring in Fourier analysis, computational harmonic analysis, fluid dynamics, electromagnetics, and quantum mechanics. Classical methods such as Runge–Kutta methods etc. fail to approximate the oscillatory ODEs due the existence of oscillatory term χω(t). Two types of methods are presented to approximate highly oscillatory ODEs. The first method uses radial basis function interpolation, and then quadrature rules are used to evaluate the integral part of the solution equation. The second approach is more generic and can approximate highly oscillatory and nonoscillatory initial value problems. Accordingly, the first-order initial value problem with oscillatory forcing term is transformed into highly oscillatory integral equation. The transformed symmetric oscillatory integral equation is then evaluated numerically by the Levin collocation method. Finally, the nonlinear form of the initial value problems with an oscillatory forcing term is converted into a linear form using Bernoulli’s transformation. The resulting linear oscillatory problem is then computed by the Levin method. Results of the proposed methods are more reliable and accurate than some state-of-the-art methods such as asymptotic method, etc. The improved results are shown in the numerical section.

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Williams, C. Rosie, Richard C. A. Hindmarsh, and Robert J. Arthern. "Frequency response of ice streams." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 468, no. 2147 (June 27, 2012): 3285–310. http://dx.doi.org/10.1098/rspa.2012.0180.

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Changes at the grounding line of ice streams have consequences for inland ice dynamics and hence sea level. Despite substantial evidence documenting upstream propagation of frontal change, the mechanisms by which these changes are transmitted inland are not well understood. In this vein, the frequency response of an idealized ice stream to periodic forcing in the downstream strain rate is examined for basally and laterally resisted ice streams using a one-dimensional, linearized membrane stress approximation. This reveals two distinct behavioural branches, which we find to correspond to different mechanisms of upstream velocity and thickness propagation, depending on the forcing frequency. At low frequencies (centennial to millennial periods), slope and thickness covary hundreds of kilometres inland, and the shallow-ice approximation is sufficient to explain upstream propagation, which occurs through changes in grounding-line flow and geometry. At high frequencies (decadal to sub-decadal periods), penetration distances are tens of kilometres; while velocity adjusts rapidly to such forcing, thickness varies little and upstream propagation occurs through the direct transmission of membrane stresses. Propagation properties vary significantly between 29 Antarctic ice streams considered. A square-wave function in frontal stress is explored by summing frequency solutions, simulating some aspects of the dynamical response to sudden ice-shelf change.

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Yang, Mingchuan, Xinye Shao, Guanchang Xue, Botao Liu, and Yanyong Su. "Regularized Zero-Forcing Dirty Paper Precoding in a High-Throughput Satellite Communication System." Electronics 11, no. 19 (September 28, 2022): 3106. http://dx.doi.org/10.3390/electronics11193106.

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In order to maximize the available data rate and spectrum utilization efficiency, a high-throughput satellite communication system adopts the full spectrum reuse scheme, which will cause serious co-frequency interference. In this paper, a forward link model, considering the effects of free space loss, rainfall attenuation, and beam gain, is established, and the classical low-complexity of the zero-forcing precoding algorithm is improved in order to solve the serious co-frequency interference. Moreover, the regularized zero-forcing precoding algorithm considering the influence of system noise is studied, and a low complexity regularized zero-forcing dirty paper precoding algorithm is proposed, whose basic principle is to sort users based on the principle of channel maximum norm selection and practical application scenarios. Simulation results show that it can encode users sequentially, according to the channel conditions, to maximize the SINR (signal-to-interference-plus-noise ratio) and increase the throughput of the system.

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Haine, T. W. N., S. Zhang, G. W. K. Moore, and I. A. Renfrew. "On the impact of high-resolution, high-frequency meteorological forcing on Denmark Strait ocean circulation." Quarterly Journal of the Royal Meteorological Society 135, no. 645 (October 2009): 2067–85. http://dx.doi.org/10.1002/qj.505.

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28

SAMIMY, M., J. H. KIM, J. KASTNER, I. ADAMOVICH, and Y. UTKIN. "Active control of high-speed and high-Reynolds-number jets using plasma actuators." Journal of Fluid Mechanics 578 (April 26, 2007): 305–30. http://dx.doi.org/10.1017/s0022112007004867.

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Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

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Bodega, P. S., P. Kaira, C. Beta, D. Krefting, D. Bauer, B. Mirwald-Schulz, C. Punckt, and H. H. Rotermund. "High frequency periodic forcing of the oscillatory catalytic CO oxidation on Pt (110)." New Journal of Physics 9, no. 3 (March 19, 2007): 61. http://dx.doi.org/10.1088/1367-2630/9/3/061.

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Jayne, Steven R., and Robin Tokmakian. "Forcing and Sampling of Ocean General Circulation Models: Impact of High-Frequency Motions*." Journal of Physical Oceanography 27, no. 6 (June 1997): 1173–79. http://dx.doi.org/10.1175/1520-0485(1997)027<1173:fasoog>2.0.co;2.

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Peng, K., F. Avallone, and M. Kotsonis. "Unsteady disturbances in a swept wing boundary layer due to plasma forcing." Physics of Fluids 34, no. 11 (November 2022): 114115. http://dx.doi.org/10.1063/5.0124818.

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This work investigates the response of a transitional boundary layer to spanwise-invariant dielectric barrier discharge plasma actuator (PA) forcing on a [Formula: see text] swept wing at a chord Reynolds number of [Formula: see text]. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar–turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling low-frequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling high-frequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge.

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DANDOIS, JULIEN, ERIC GARNIER, and PIERRE SAGAUT. "Numerical simulation of active separation control by a synthetic jet." Journal of Fluid Mechanics 574 (February 15, 2007): 25–58. http://dx.doi.org/10.1017/s0022112006003995.

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Direct numerical simulation (DNS) and large-eddy simulation (LES) are carried out to investigate the frequency effect of zero-net-mass-flux forcing (synthetic jet) on a generic separated flow. The selected test case is a rounded ramp at a Reynolds number based on the step height of 28 275. The incoming boundary layer is fully turbulent withRθ=1410. The whole flow in the synthetic jet cavity is computed to ensure an accurate description of the actuator effect on the flow field. In a first step, DNS is used to validate LES of this particular flow. In a second step, the effect of a synthetic jet at two reduced frequencies of 0.5 and 4 (based on the separation length of the uncontrolled case and the free-stream velocity) is investigated using LES. It is demonstrated that, with a proper choice of the oscillating frequency, separation can be drastically reduced for a velocity ratio between the jet and the flow lower than one. The low frequency is close to the natural vortex shedding frequency. Two different modes of the synthetic jet have been identified. Avorticity-dominated modeis observed in the low-frequency forcing case for which the separation length is reduced by 54%, while anacoustic-dominated modeis identified in the high-frequency forcing case for which the separation length is increased by 43%. The decrease of the separation length in the low-frequency forcing case is correlated with an increase of the turbulent kinetic energy level and consequently with an increase of the entrainment in the separated zone. A linear inviscid stability analysis shows that the increase of the separation length in the high-frequency forcing case is due to a modification of the mean velocity profile suggested by Stanek and coworkers. The result is a lower amplification of the perturbations and consequently, a lower entrainment into the mixing layer. To our knowledge, it is the first time that Stanek's hypothesis has been assessed, thanks to numerical simulations of fully turbulent flow.

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Alexeev, V. V., E. N. Odarenko, and A. A. Shmat'ko. "Theory of the O-Type Resonant Oscillator Subject to Powerful External High-Frequency and Parametric Low-Frequency Forcing." Telecommunications and Radio Engineering 53, no. 2 (1999): 65–67. http://dx.doi.org/10.1615/telecomradeng.v53.i2.120.

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Mondal, Sirshendu, Samadhan A. Pawar, and R. I. Sujith. "Forced synchronization and asynchronous quenching of periodic oscillations in a thermoacoustic system." Journal of Fluid Mechanics 864 (February 1, 2019): 73–96. http://dx.doi.org/10.1017/jfm.2018.1011.

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We perform an experimental and theoretical study to investigate the interaction between an external harmonic excitation and a self-excited oscillatory mode ($f_{n0}$) of a prototypical thermoacoustic system, a horizontal Rijke tube. Such an interaction can lead to forced synchronization through the routes of phase locking or suppression. We characterize the transition in the synchronization behaviour of the forcing and the response signals of the acoustic pressure while the forcing parameters, i.e. amplitude ($A_{f}$) and frequency ($f_{f}$) of forcing are independently varied. Further, suppression is categorized into synchronous quenching and asynchronous quenching depending upon the value of frequency detuning ($|\,f_{n0}-f_{f}|$). When the applied forcing frequency is close to the natural frequency of the system, the suppression in the amplitude of the self-excited oscillation is known as synchronous quenching. However, this suppression is associated with resonant amplification of the forcing signal, leading to an overall increase in the response amplitude of oscillations. On the other hand, an almost 80 % reduction in the root mean square value of the response oscillation is observed when the system is forced for a sufficiently large value of the frequency detuning (only for $f_{f}<f_{n0}$). Such a reduction in amplitude occurs due to asynchronous quenching where resonant amplification of the forcing signal does not occur, as the frequency detuning is significantly high. Further, the results from a reduced-order model developed for a horizontal Rijke tube show a qualitative agreement with the dynamics observed in experiments. The relative phase between the acoustic pressure ($p^{\prime }$) and the heat release rate ($\dot{q}^{\prime }$) oscillations in the model explains the occurrence of maximum reduction in the pressure amplitude due to asynchronous quenching. Such a reduction occurs when the positive coupling between $p^{\prime }$ and $\dot{q}^{\prime }$ is disrupted and their interaction results in overall acoustic damping, although both of them oscillate at the forcing frequency. Our study on the phenomenon of asynchronous quenching thus presents new possibilities to suppress self-sustained oscillations in fluid systems in general.

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35

Shabtai, Y., and N. Gavriely. "Frequency and amplitude effects during high-frequency vibration ventilation in dogs." Journal of Applied Physiology 66, no. 3 (March 1, 1989): 1127–35. http://dx.doi.org/10.1152/jappl.1989.66.3.1127.

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High-frequency external body vibration, combined with constant gas flow at the tracheal carina, was previously shown to be an effective method of ventilation in normal dogs. The effects of frequency (f) and amplitude of the vibration were investigated in the present study. Eleven anesthetized and paralyzed dogs were placed on a vibrating table (4–32 Hz). O2 was delivered near the tracheal carina at 0.51.kg-1.min-1, while mean airway pressure was kept at 2.4 +/- 0.9 cmH2O. Table vertical displacement (D) and acceleration (a), esophageal (Pes), and tracheal (Ptr) peak-to-peak pressures, and tidal volume (VT) were measured as estimates of the input amplitude applied to the animal. Steady-state arterial PCO2 (PaCO2) and arterial PO2 (PaO2) values were used to monitor overall gas exchange. Typically, eucapnia was achieved with f greater than 16 Hz, D = 1 mm, a = 1 G, Pes = Ptr = 4 +/- 2 cmH2O, and VT less than 2 ml. Inverse exponential relationships were found between PaCO2 and f, a, Pes, and Ptr (exponents: -0.69, -0.38, -0.48, and -0.54, respectively); PaCO2 decreased linearly with increased displacement or VT at a fixed frequency (17 +/- 1 Hz). PaO2 was independent of both f and D (393 +/- 78 Torr, mean +/- SD). These data demonstrate the very small VT, Ptr, and Pes associated with vibration ventilation. It is clear, however, that mechanisms other then those described for conventional ventilation and high-frequency ventilation must be evoked to explain our data. One such possible mechanism is forcing of flow oscillation between lung regions (i.e., forced pendelluft).

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Coccolo, Mattia, Grzegorz Litak, Jesús M. Seoane, and Miguel A. F. Sanjuán. "Optimizing the Electrical Power in an Energy Harvesting System." International Journal of Bifurcation and Chaos 25, no. 12 (November 2015): 1550171. http://dx.doi.org/10.1142/s0218127415501710.

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In this paper, we study the vibrational resonance (VR) phenomenon as a useful mechanism for energy harvesting purposes. A system, driven by a low frequency and a high frequency forcing, can give birth to the vibrational resonance phenomenon, when the two forcing amplitudes resonate and a maximum in amplitude is reached. We apply this idea to a bistable oscillator that can convert environmental kinetic energy into electrical energy, that is, an energy harvester. Normally, the VR phenomenon is studied in terms of the forcing amplitudes or of the frequencies, that are not always easy to adjust and change. Here, we study the VR generated by tuning another parameter that is possible to manipulate when the forcing values depend on the environmental conditions. We have investigated the dependence of the maximum response due to the VR for small and large variations in the forcing amplitudes and frequencies. Besides, we have plotted color coded figures in the space of the two forcing amplitudes, in which it is possible to appreciate different patterns in the electrical power generated by the system. These patterns provide useful information on the forcing amplitudes in order to produce the optimal electrical power.

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Manzi, V., R. Gennari, S. Lugli, M. ROVERI, N. Scafetta, and B. C. Schreiber. "High-Frequency Cyclicity In the Mediterranean Messinian Evaporites: Evidence For Solar-Lunar Climate Forcing." Journal of Sedimentary Research 82, no. 12 (December 20, 2012): 991–1005. http://dx.doi.org/10.2110/jsr.2012.81.

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38

Nesje, Atle, and Truls Johannessen. "What were the Primary Forcing Mechanisms of High-Frequency Holocene Climate and Glacier Variations?" Holocene 2, no. 1 (March 1992): 79–84. http://dx.doi.org/10.1177/095968369200200110.

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39

Wu, Yang, Zhaomin Wang, Chengyan Liu, and Xia Lin. "Impacts of High-Frequency Atmospheric Forcing on Southern Ocean Circulation and Antarctic Sea Ice." Advances in Atmospheric Sciences 37, no. 5 (April 18, 2020): 515–31. http://dx.doi.org/10.1007/s00376-020-9203-x.

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Weijer, Wilbert. "High-frequency wind forcing of a channel model of the ACC: normal mode excitation." Ocean Modelling 9, no. 1 (January 2005): 31–50. http://dx.doi.org/10.1016/j.ocemod.2004.04.001.

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41

Plazotta, Simon, and Jonathan Zinsl. "High-frequency limit of non-autonomous gradient flows of functionals with time-periodic forcing." Journal of Differential Equations 261, no. 12 (December 2016): 6806–55. http://dx.doi.org/10.1016/j.jde.2016.09.003.

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42

Luo, D. H., X. J. Sun, D. G. Huang, and G. Q. Wu. "Flow control effectiveness of synthetic jet on a stalled airfoil." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 225, no. 9 (July 6, 2011): 2106–14. http://dx.doi.org/10.1177/0954406211407255.

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Synthetic jet has attracted much attention for its use as an effective active flow control technique. With the aim of achieving a deeper understanding of the mechanisms and the critical fluid flow phenomena associated with synthetic jet, 2D simulations are conducted, in which a synthetic jet is placed on a NACA 0012 airfoil’s upper surface near the leading edge simulating the periodic blowing and suction control at Re = 5 × 105 and an angle of attack of 18°. Important control parameters, such as jet amplitude and frequency are investigated over a wide range and the jet width is also considered. Numerical results show that the recirculation region on the airfoil is decreased with a maximum increase in the lift by about 40 per cent and a reduced drag. The synthetic jet is found to be particularly effective at high forcing levels when the forcing frequency fe is close to the natural shedding frequency of the airfoil, and however become ineffective at low forcing levels. Use of higher forcing frequencies further decreases the drag of the airfoil but also reduces the lift.

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43

Chagnon, Jeffrey M. "Gravity Waves, Dynamical Resistance, and Forcing Efficiency." Journal of the Atmospheric Sciences 67, no. 6 (June 1, 2010): 2039–51. http://dx.doi.org/10.1175/2009jas3244.1.

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Abstract The effect of the dynamical response associated with high-frequency gravity waves on the total energy generated by imposed heating is examined in a 2D linear compressible model. The work performed by waves against a sustained forcing is defined as the dynamical resistance. The dynamical resistance is minimized and forcing efficiency maximized for basic-state and forcing configurations that yield a wave response whose phase varies minimally relative to the forcing. When generated against a forcing-relative background flow, waves that have a deep vertical scale relative to the forcing depth impose less resistance than waves of a shallow vertical scale. The efficiency of an ensemble of forcing elements is shown to differ significantly from that corresponding to an isolated forcing. If the forcing elements are all of the same sign (e.g., are all warmings), then the efficiency increases with decreasing separation between elements.

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44

Emerson, Benjamin, and Tim Lieuwen. "Dynamics of harmonically excited, reacting bluff body wakes near the global hydrodynamic stability boundary." Journal of Fluid Mechanics 779 (August 21, 2015): 716–50. http://dx.doi.org/10.1017/jfm.2015.450.

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This paper describes linear and nonlinear interactions between forced axial acoustic oscillations and the global mode of the reacting wake. This work is motivated by the problem of combustion instabilities, where acoustic oscillations associated with natural combustor modes excite hydrodynamic instabilities of the flow that, in turn, induce heat release oscillations. Wake flows with density stratification can be globally stable or unstable at high Reynolds numbers, and so the density change across the flame has significant influence on the natural flame and flow dynamics. Measurements were obtained in a facility in which flame density ratio, lip velocity and forcing frequency are independently varied using 5 kHz particle image velocimetry and Mie scattering measurements. By varying the density ratio, the hydrodynamic global mode growth rate can be systematically varied. In addition, measurements and analyses were performed where the forcing frequency is varied relative to the global mode frequency. While axial forcing excites a varicose response of the shear layers, the sinuous mode is the most rapidly growing. As expected, forcing at a frequency near the wake’s global mode frequency leads to rapid growth in vortical disturbance amplitude, and the symmetric vortices quickly stagger as they convect downstream leading to a large scale, sinuous flapping of the wake and flame. A linear, local stability analysis, together with a nonlinear analysis, help elucidate the physics that govern the vortex staggering. The study concludes with an analysis of the heat release dynamics. Significantly, the study shows that the heat release exhibits quite different sensitivities than the fluid dynamics; e.g. axial forcing of the flow near its global mode frequency leads to a reduction in heat release oscillations. This is true even though this forcing frequency maximizes the local degree of vortically induced flame flapping. Thus, the results of this study show some phenomena that contradict conventional notions, namely that conditions which align the frequency of a hydrodynamic global mode with that of an acoustic mode may lead to diminished forced heat release oscillations in bluff body combustors.

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45

Waugh, William, B. J. Gallacher, and J. S. Burdess. "A High-Sensitivity Resonant Sensor Realised Through the Exploitation of Nonlinear Dynamic Behaviour." Additional Conferences (Device Packaging, HiTEC, HiTEN, and CICMT) 2010, DPC (January 1, 2010): 001379–400. http://dx.doi.org/10.4071/2010dpc-wa24.

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Existing MEMS viscometers and density meters typically measure vibrational characteristics such as resonant frequency, bandwidth and quality factor. In order to significantly improve sensitivity to changes in viscosity and/or density the proposed sensor will exploit nonlinear dynamic behaviour and measure the frequency separation between singular jump points in the frequency response function. By using a one-mode approximation when excited near resonance, the dynamics of a clamped-clamped slender beam in fluid is that of a standard Duffing oscillator. With harmonic forcing of sufficient magnitude, a bistable region, bounded by amplitude jump points, is seen to occur. The width of this bistable region is dependent on the damping ratio of the system, which is shown to be a function of the dynamic viscosity and density. Experiments with clamped-clamped silicon beams in a range of gases demonstrate that the sensitivity of the proposed nonlinear sensor at low viscosity can be at least an order of magnitude better than that of conventional devices. Forcing magnitude and control is identified as being critical to the measured width of the bistable region. Beam dimensions can be chosen to optimise measurements for the desired application.

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Dobricic, S., C. Dufau, P. Oddo, N. Pinardi, I. Pujol, and M. H. Rio. "Assimilation of SLA along track observations in the Mediterranean with an oceanographic model forced by atmospheric pressure." Ocean Science Discussions 9, no. 2 (April 10, 2012): 1577–98. http://dx.doi.org/10.5194/osd-9-1577-2012.

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Abstract. A large number of SLA observations at a high along track horizontal resolution are an important ingredient of the data assimilation in the Mediterranean Forecasting System (MFS). Recently new higher frequency SLA products have become available, and the atmospheric pressure forcing has been implemented in the numerical model used in the MFS data assimilation system. In a set of numerical experiments we show that in order to obtain the most accurate analyses the ocean model should include the atmospheric pressure forcing and the observations should contain the atmospheric pressure signal. When the model is not forced by the atmospheric pressure the high frequency filtering of SLA observations, however, improves the quality of the analyses. It is further shown that MFS analyses, produced by an assimilation system given by the numerical model and the high frequency SLA observations, have a correct power spectrum at high wave numbers and they filter efficiently the SLA assimilated observations which, on the other hand, are contaminated by high wavenumber noise.

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Kotz, Maximilian, Leonie Wenz, and Anders Levermann. "Footprint of greenhouse forcing in daily temperature variability." Proceedings of the National Academy of Sciences 118, no. 32 (August 2, 2021): e2103294118. http://dx.doi.org/10.1073/pnas.2103294118.

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Changes in mean climatic conditions will affect natural and societal systems profoundly under continued anthropogenic global warming. Changes in the high-frequency variability of temperature exert additional pressures, yet the effect of greenhouse forcing thereon has not been fully assessed or identified in observational data. Here, we show that the intramonthly variability of daily surface temperature changes with distinct global patterns as greenhouse gas concentrations rise. In both reanalyses of historical observations and state-of-the-art projections, variability increases at low to mid latitudes and decreases at northern mid to high latitudes with enhanced greenhouse forcing. These latitudinally polarized daily variability changes are identified from internal climate variability using a recently developed signal-to-noise-maximizing pattern-filtering technique. Analysis of a multimodel ensemble from the Coupled Model Intercomparison Project Phase 6 shows that these changes are attributable to enhanced greenhouse forcing. By the end of the century under a business-as-usual emissions scenario, daily temperature variability would continue to increase by up to a further 100% at low latitudes and decrease by 40% at northern high latitudes. Alternative scenarios demonstrate that these changes would be limited by mitigation of greenhouse gases. Moreover, global changes in daily variability exhibit strong covariation with warming across climate models, suggesting that the equilibrium climate sensitivity will also play a role in determining the extent of future variability changes. This global response of the high-frequency climate system to enhanced greenhouse forcing is likely to have strong and unequal effects on societies, economies, and ecosystems if mitigation and protection measures are not taken.

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Rigas, Georgios, Aimee S. Morgans, and Jonathan F. Morrison. "Weakly nonlinear modelling of a forced turbulent axisymmetric wake." Journal of Fluid Mechanics 814 (February 9, 2017): 570–91. http://dx.doi.org/10.1017/jfm.2017.32.

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A theory is presented where the weakly nonlinear analysis of laminar globally unstable flows in the presence of external forcing is extended to the turbulent regime. The analysis is demonstrated and validated using experimental results of an axisymmetric bluff-body wake at high Reynolds numbers, $Re_{D}\sim 1.88\times 10^{5}$, where forcing is applied using a zero-net-mass-flux actuator located at the base of the blunt body. In this study we focus on the response of antisymmetric coherent structures with azimuthal wavenumbers $m=\pm 1$ at a frequency $St_{D}=0.2$, responsible for global vortex shedding. We found experimentally that axisymmetric forcing ($m=0$) couples nonlinearly with the global shedding mode when the flow is forced at twice the shedding frequency, resulting in parametric subharmonic resonance through a triadic interaction between forcing and shedding. We derive simple weakly nonlinear models from the phase-averaged Navier–Stokes equations and show that they capture accurately the observed behaviour for this type of forcing. The unknown model coefficients are obtained experimentally by producing harmonic transients. This approach should be applicable in a variety of turbulent flows to describe the response of global modes to forcing.

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Levine, Aaron F. Z., and Fei-Fei Jin. "Noise-Induced Instability in the ENSO Recharge Oscillator." Journal of the Atmospheric Sciences 67, no. 2 (February 1, 2010): 529–42. http://dx.doi.org/10.1175/2009jas3213.1.

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Abstract The conceptual El Niño–Southern Oscillation (ENSO) recharge oscillator model is used to study the linear stability of ENSO under state-dependent noise forcing. The analytical framework developed by Jin et al. is extended to more fully study noise-induced instability of ENSO. It is shown that in addition to the noise-induced positive contribution to the growth rate of the ensemble mean (first moment) evolution of the ENSO cycle, there is also a noise-induced instability for the ensemble spread (second moment). These growth rates continue to increase as the strength of the multiplicative noise increases. In both the analytical solution and the numerical model, the criticality threshold for instability of the second moment occurs at a lower value of the parameter that measures multiplicative forcing than the threshold for the first moment. The noise-induced instability not only enhances ENSO activity but also results in a large ensemble spread and thus may reduce the effectiveness of ENSO prediction. As in the additive noise forcing case, the low-frequency variability in the forcing is the important part for forcing El Niño events and the high-frequency forcing alone cannot effectively excite ENSO.

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Kim, Jinwon, Yu Gu, and K. N. Liou. "The Impact of Direct Aerosol Radiative Forcing on Surface Insolation and Spring Snowmelt in the Southern Sierra Nevada." Journal of Hydrometeorology 7, no. 5 (October 1, 2006): 976–83. http://dx.doi.org/10.1175/jhm541.1.

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Abstract To understand the regional impact of the atmospheric aerosols on the surface energy and water cycle in the southern Sierra Nevada characterized by extreme variations in terrain elevation, the authors examine the aerosol radiative forcing on surface insolation and snowmelt for the spring of 1998 in a regional climate model experiment. With a prescribed aerosol optical thickness of 0.2, it is found that direct aerosol radiative forcing influences spring snowmelt primarily by reducing surface insolation and that these forcings on surface insolation and snowmelt vary strongly following terrain elevation. The direct aerosol radiative forcing on surface insolation is negative in all elevations. It is nearly uniform in the regions below 2000 m and decreases with increasing elevation in the region above 2000 m. This elevation dependency in the direct aerosol radiative forcing on surface insolation is related to the fact that the amount of cloud water and the frequency of cloud formation are nearly uniform in the lower elevation region, but increase with increasing elevation in the higher elevation region. This also suggests that clouds can effectively mask the direct aerosol radiative forcing on surface insolation. The direct aerosol radiative forcing on snowmelt is notable only in the regions above 2000 m and is primarily via the reduction in the surface insolation by aerosols. The effect of this forcing on low-level air temperature is as large as −0.3°C, but its impact on snowmelt is small because the sensible heat flux change is much smaller than the insolation change. The direct aerosol radiative forcing on snowmelt is significant only when low-level temperature is near the freezing point, between −3° and 5°C. When low-level temperature is outside this range, the direct aerosol radiative forcing on surface insolation has only a weak influence on snowmelt. The elevation dependency of the direct aerosol radiative forcing on snowmelt is related with this low-level temperature effect as the occurrence of the favored temperature range is most frequent in high elevation regions.

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Journal articles on the topic 'High Frequency Forcing'

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