Academic literature on the topic 'High Frequency Forcing'
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Journal articles on the topic "High Frequency Forcing"
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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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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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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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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.
Dissertations / Theses on the topic "High Frequency Forcing"
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MARTINEZ, MARIANO ALVARO MIGUEL. "Cavity Flows Control by High Frequency Forcing." Doctoral thesis, Politecnico di Torino, 2012. http://hdl.handle.net/11583/2496949.
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In the surfaces of vehicles, such as airplanes, cars and trains, open cavities are an important source of aerodynamically generated noise, drag increase, and dangerous pressure fluctuations. Many different methods have been tested to suppress these undesired effects; some of them reached a partial success. In general, it can be seen that methods which are simple, economic and easy to apply achieve an effect that is restricted to a small operational range, while those which offer a wider operational range are complex and expensive. It has been recently discovered that exciting the flow at high frequencies it is possible to suppress or weaken the undesired effects in a wide range of flow conditions. Additionally, this forcing can be generated by a simple Von Karman wake. In this way both advantages, simplicity and a wide operational range, can be achieved at the same time. The physical mechanisms leading to cavity flow control by high frequency forcing is not well understood yet. The author of this thesis intends to offer a contribution toward a physical explanation for them, based on the analysis of data acquired with the Time Resolved PIV technique.
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Oxlade, Anthony. "High-frequency pulsed jet forcing of an axisymmetric bluff body wake." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/24116.
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The turbulent wake of a bullet-shaped axisymmetric bluff body is forced using a periodic pulsed jet located immediately beneath the point of separation. The Reynolds number based on body diameter is Re_D = 188,000, the ratio of body length to diameter is L/D = 6.48, and the ratio of body diameter to boundary layer momentum thickness at separation is D/θ = 92. A parametric study of the response of the mean base pressure to the governing variables of forcing frequency and forcing amplitude is performed. At high forcing frequencies (several times that of the shear-layer frequency) the area-weighted mean base pressure is increased by as much as 33%. A detailed investigation of the forced and unforced wake is made using random and phase-locked two-component PIV, and modal decomposition of pressure fluctuations on the base of the body. In addition to the well known shear-layer and vortex shedding wake structures, a dominant very-low-frequency mode with azimuthal wavenumber m=±1 and Strouhal number St_D ≈ 0.0015 is identified in the forced and unforced wake. This mode spatially modulates the coherent and incoherent wake oscillations in an orbit around the central axis of the body. Statistical axisymmetry is recovered by the random variations in radius and azimuthal angle of this orbit. This feature of the turbulent wake appears to be an unsteady manifestation of the SS regime of the laminar wake (the latter is a steady state with reflectional symmetry). Although the pulsed jet provides zero-net-mass flux, a non-linear interaction with the wake creates a finite momentum flux. The pressure on the base of the body (and hence drag) can be increased or decreased dependent upon the forcing frequency and amplitude. Increasing the base pressure by forcing at high frequency is a unique form of direct wake control in that it does not target the dominant flow instabilities. Instead, the jet introduces a row of closely spaced single-sign vortices that advect within the separating shear layer, remaining coherent for less than half a body diameter. In the time average these vortices generate a narrow region of large enstrophy bounded on each side by a strong shear layer; the latter being associated with high dissipation and non-local pressure recovery. The magnitude of the base pressure recovery is shown to be proportional to the strength of the jet vortices and is accompanied by broadband suppression of energy across all azimuthal wavenumbers with no preferential mode selection. The pressure recovery is proportional to forcing frequency, reaching saturation at approximately 5 times the natural frequency of the shear layer. This frequency dependence is a direct result of progressively reduced coupling between the jet perturbation and both the shear layer, and the vortex shedding wake instabilities. The latter are responsible for enhancing entrainment and turbulence production, thereby competing directly with the pressure recovery mechanism.
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Szubert, Damien. "Physics and modelling of unsteady turbulent flows around aerodynamic and hydrodynamic structures at high Reynold number by numerical simulation." Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/15129/2/szubert_1.pdf.
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This thesis aims at analysing the predictive capabilities of statistical URANS and hybrid RANS-LES methods to model complex flows at high Reynolds numbers and carrying out a physical analysis of the near-region turbulence and coherent structures. This study handles configurations included in the European research programmes ATAAC (Advanced Turbulent Simulation for Aerodynamics Application Challenges) and TFAST (Transition Location Effect on Shock Wave Boundary Layer Interaction). First, the detached flow in a configuration of a tandem of cylinders, positionned behind one another, is investigated at Reynolds number 166000. A static case, corresponding to the layout of the support of a landing gear, is initially considered. The fluid-structure interaction is then studied in a dynamic case where the downstream cylinder, situated in the wake of the upstream one, is given one degree of freedom in translation in the crosswise direction. A parametric study of the structural parameters is carried out to identify the various regimes of interaction. Secondly, the physics of the transonic buffet is studied by means of time-frequency analysis and proper orthogonal decomposition (POD), in the Mach number range 0.70–0.75. The interactions between the main shock wave, the alternately detached boundary layer and the vortices developing in the wake are analysed. A stochastic forcing, based on reinjection of synthetic turbulence in the transport equations of kinetic energy and dissipation rate by using POD reconstruction, has been introduced in the so-called organised-eddy simulation (OES) approach. This method introduces an upscale turbulence modelling, acting as an eddy-blocking mechanism able to capture thin shear-layer and turbulent/non-turbulent interfaces around the body. This method highly improves the aerodynamic forces prediction and opens new ensemble-averaged approaches able to model the coherent and random processes at high Reynolds number. Finally, the shock-wave/boundary-layer interaction (SWBLI) is investigated in the case of an oblique shock wave at Mach number 1.7 in order to contribute to the so-called "laminar wing design" studies at European level. The performance of statistical URANS and hybrid RANS-LES models is analysed with comparison, with experimental results, of integral boundary-layer values (displacement and momentum thicknesses) and wall quantities (friction coefficient). The influence of a transitional boundary layer on the SWBLI is featured.
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Jalón, Rojas Isabel. "Évaluation des changements hydro-sédimentaires de l'estuaire de la Gironde en lien avec les pressions sur le milieu." Thesis, Bordeaux, 2016. http://www.theses.fr/2016BORD0165/document.
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La dynamique sédimentaire estuarienne joue un rôle très important pour la qualité de l'eau, les écosystèmes et la navigation. Les estuaires macrotidaux comme la Gironde se caractérisent par la formation de régions très chargées en matière en suspension (MES), appelées zones de turbidité maximale (ZTM), qui influencent le transport et le dépôt des sédiments fins, l'envasement des chenaux, la consommation d'oxygène dissous et le devenir des polluants. L'objectif de ce travail est de comprendre la dynamique hydro-sédimentaire, particulièrement de la ZTM, dans la section fluviale, encore peu étudié, de l'estuaire de la Gironde en lien avec les facteurs de forçage environnementaux et les perturbations du système (changements hydrologiques et morphologiques naturels et anthropiques). La méthodologie de ce travail est basée sur l'analyse de 10 années de données continues de turbidité enregistrées par le réseau de surveillance MAGEST. L'exploitation de telles séries de données, assez novatrice dans les estuaires, a notamment impliqué le développement d'une méthode d'analyse basée sur la combinaison de plusieurs méthodes spectrales. Cette approche est complétée par l'analyse des profils de turbidité et de vitesse de courant lors de cycles de marée, l'analyse de séries temporelles historiques de marée et l'exploitation d'un modèle semi-analytique 2DV. La dynamique sédimentaire de l'estuaire fluvial est d'abord détaillée à toutes les échelles de temps représentatives. A l'échelle de temps intratidale, la distribution verticale des MES et des courants, en deux points d'une même section transversale, a permis de détailler les mécanismes de transport sédimentaire. Les flux particulaires résiduels (totaux, advection, pompage tidal) ainsi estimés pour plusieurs conditions hydrologiques, démontrent le contrôle du pompage tidal sur les flux de MES lors de l'étiage. A l'échelle de temps subtidale, la réponse de la ZTM aux fluctuations hydrologiques (crues, périodes d'augmentation ou diminution continue du débit, variabilité inter-anuelle) est analysée. Ceci a permis de définir plusieurs indicateurs hydrologiques des caractéristiques de la ZTM, qui suggèrent l'intensification de la ZTM au cours des dernières décennies en lien avec la diminution des débit. La contribution relative des facteurs de forçage à la variabilité de la turbidité a été quantifiée pour différentes régions estuariennes et échelles de temps (saisonnière et plurianuelle). L'application de la méthodologique développée à l'estuaire de la Loire, qui dispose de séries de données similaires (réseau SYVEL), a permis de généraliser ces résultats. Enfin, l'effet des changements pluri-décennaux hydrologiques et morphologiques sur la propagation de la marée et la dynamique sédimentaire est détaillé dans la Garonne tidale. Il ressort une amplification du marnage et de la asymétrie de la marée au cours des six dernières décennies, principalement liée aux changements morphologiques naturels dans la Gironde en aval, les extractions de granulat et le changement de régime hydrologique. L'implémentation d'un modèle semi-analytique a permis de vérifier ces résultats et d'analyser leurs implications sur les concentrations de MES et la limite amont de la ZTMEstuarine suspended sediment dynamics play an important role in water quality, ecosystems and navigation. The formation of regions of high suspended sediments (SS) concentrations, called turbidity maximum zones (TMZ), is a characteristic feature of macrotidal estuaries, such as the Gironde. The TMZ influences the transport and deposition of fine sediments, channel siltation, oxygen conditions and the particulate transport of pollutants. This work aims to understand the hydro-sedimentary dynamics of the fluvial Gironde estuary, still poorly studied, in relation with environmental forcings and system perturbations (natural and anthropic hydrological and morphological changes). The methodology of this work is based on the analysis of the 10-years continuous time series of turbidity recorded by the MAGEST monitoring network. The exploitation of such time series, quite innovative in estuaries, required the development of an analysis method based on the combination of spectral techniques. This approach is completed by the analysis of turbidity and current velocity profiles over tidal cycles, the analysis of historical tide time series, and the exploitation of a 2DV semi-analytical model. First, SS dynamics of the fluvial Gironde is detailed at all representative time scales. At the intratidal time scale, the mechanisms of SS transport were described from the vertical depth of SSC and current velocities at two points of the same section. Residual fluxes (total, advective and tidal pumping), estimated for different hydrological conditions, demonstrated the control of tidal pumping on SS fluxes during periods of low river flow. At the subtidal time scale, the TMZ response to hydrological fluctuations (floods, periods of continuous river flow increase and decrease, interannual changes) was analyzed. Hydrological indicators of the TMZ features were thus defined, which suggest the TMZ intensification over the last decades in relation to the river flow decrease. The relative contributions of environmental forcings to the turbidity variability were quantified for different estuarine regions and time scales (seasonal and multiannual). The application of the same methodology to the Loire estuary, which counts on equivalent time series (SYVEL network), allowed the generalization of these results. Finally, the impact of pluri-decades hydrological and morphological changes on tidal propagation and suspended sediments dynamics is detailed in the tidal Garonne. Both tidal range and asymmetry appear to be amplified over the last six decades, mainly due to natural changes of the down Gironde, gravel extraction in the tidal Garonne and hydrological regime shifts. The implementation of an idealized model allowed verifying such results and analyzing their implications for SS concentrations and the upper TMZ limit
Books on the topic "High Frequency Forcing"
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Cornejo-Rodriguez, M. Pilar. Propagation and forcing of high frequency sea level variability in the eastern equatorial Pacific. 1987.
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Gao, Yanhong, and Deliang Chen. Modeling of Regional Climate over the Tibetan Plateau. Oxford University Press, 2017. http://dx.doi.org/10.1093/acrefore/9780190228620.013.591.
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The modeling of climate over the Tibetan Plateau (TP) started with the introduction of Global Climate Models (GCMs) in the 1950s. Since then, GCMs have been developed to simulate atmospheric dynamics and eventually the climate system. As the highest and widest international plateau, the strong orographic forcing caused by the TP and its impact on general circulation rather than regional climate was initially the focus. Later, with growing awareness of the incapability of GCMs to depict regional or local-scale atmospheric processes over the heterogeneous ground, coupled with the importance of this information for local decision-making, regional climate models (RCMs) were established in the 1970s. Dynamic and thermodynamic influences of the TP on the East and South Asia summer monsoon have since been widely investigated by model. Besides the heterogeneity in topography, impacts of land cover heterogeneity and change on regional climate were widely modeled through sensitivity experiments.In recent decades, the TP has experienced a greater warming than the global average and those for similar latitudes. GCMs project a global pattern where the wet gets wetter and the dry gets drier. The climate regime over the TP covers the extreme arid regions from the northwest to the semi-humid region in the southeast. The increased warming over the TP compared to the global average raises a number of questions. What are the regional dryness/wetness changes over the TP? What is the mechanism of the responses of regional changes to global warming? To answer these questions, several dynamical downscaling models (DDMs) using RCMs focusing on the TP have recently been conducted and high-resolution data sets generated. All DDM studies demonstrated that this process-based approach, despite its limitations, can improve understandings of the processes that lead to precipitation on the TP. Observation and global land data assimilation systems both present more wetting in the northwestern arid/semi-arid regions than the southeastern humid/semi-humid regions. The DDM was found to better capture the observed elevation dependent warming over the TP. In addition, the long-term high-resolution climate simulation was found to better capture the spatial pattern of precipitation and P-E (precipitation minus evapotranspiration) changes than the best available global reanalysis. This facilitates new and substantial findings regarding the role of dynamical, thermodynamics, and transient eddies in P-E changes reflected in observed changes in major river basins fed by runoff from the TP. The DDM was found to add value regarding snowfall retrieval, precipitation frequency, and orographic precipitation.Although these advantages in the DDM over the TP are evidenced, there are unavoidable facts to be aware of. Firstly, there are still many discrepancies that exist in the up-to-date models. Any uncertainty in the model’s physics or in the land information from remote sensing and the forcing could result in uncertainties in simulation results. Secondly, the question remains of what is the appropriate resolution for resolving the TP’s heterogeneity. Thirdly, it is a challenge to include human activities in the climate models, although this is deemed necessary for future earth science. All-embracing further efforts are expected to improve regional climate models over the TP.
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Tibaldi, Stefano, and Franco Molteni. Atmospheric Blocking in Observation and Models. Oxford University Press, 2018. http://dx.doi.org/10.1093/acrefore/9780190228620.013.611.
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The atmospheric circulation in the mid-latitudes of both hemispheres is usually dominated by westerly winds and by planetary-scale and shorter-scale synoptic waves, moving mostly from west to east. A remarkable and frequent exception to this “usual” behavior is atmospheric blocking. Blocking occurs when the usual zonal flow is hindered by the establishment of a large-amplitude, quasi-stationary, high-pressure meridional circulation structure which “blocks” the flow of the westerlies and the progression of the atmospheric waves and disturbances embedded in them. Such blocking structures can have lifetimes varying from a few days to several weeks in the most extreme cases. Their presence can strongly affect the weather of large portions of the mid-latitudes, leading to the establishment of anomalous meteorological conditions. These can take the form of strong precipitation episodes or persistent anticyclonic regimes, leading in turn to floods, extreme cold spells, heat waves, or short-lived droughts. Even air quality can be strongly influenced by the establishment of atmospheric blocking, with episodes of high concentrations of low-level ozone in summer and of particulate matter and other air pollutants in winter, particularly in highly populated urban areas.Atmospheric blocking has the tendency to occur more often in winter and in certain longitudinal quadrants, notably the Euro-Atlantic and the Pacific sectors of the Northern Hemisphere. In the Southern Hemisphere, blocking episodes are generally less frequent, and the longitudinal localization is less pronounced than in the Northern Hemisphere.Blocking has aroused the interest of atmospheric scientists since the middle of the last century, with the pioneering observational works of Berggren, Bolin, Rossby, and Rex, and has become the subject of innumerable observational and theoretical studies. The purpose of such studies was originally to find a commonly accepted structural and phenomenological definition of atmospheric blocking. The investigations went on to study blocking climatology in terms of the geographical distribution of its frequency of occurrence and the associated seasonal and inter-annual variability. Well into the second half of the 20th century, a large number of theoretical dynamic works on blocking formation and maintenance started appearing in the literature. Such theoretical studies explored a wide range of possible dynamic mechanisms, including large-amplitude planetary-scale wave dynamics, including Rossby wave breaking, multiple equilibria circulation regimes, large-scale forcing of anticyclones by synoptic-scale eddies, finite-amplitude non-linear instability theory, and influence of sea surface temperature anomalies, to name but a few. However, to date no unique theoretical model of atmospheric blocking has been formulated that can account for all of its observational characteristics.When numerical, global short- and medium-range weather predictions started being produced operationally, and with the establishment, in the late 1970s and early 1980s, of the European Centre for Medium-Range Weather Forecasts, it quickly became of relevance to assess the capability of numerical models to predict blocking with the correct space-time characteristics (e.g., location, time of onset, life span, and decay). Early studies showed that models had difficulties in correctly representing blocking as well as in connection with their large systematic (mean) errors.Despite enormous improvements in the ability of numerical models to represent atmospheric dynamics, blocking remains a challenge for global weather prediction and climate simulation models. Such modeling deficiencies have negative consequences not only for our ability to represent the observed climate but also for the possibility of producing high-quality seasonal-to-decadal predictions. For such predictions, representing the correct space-time statistics of blocking occurrence is, especially for certain geographical areas, extremely important.
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Goswami, B. N., and Soumi Chakravorty. Dynamics of the Indian Summer Monsoon Climate. Oxford University Press, 2017. http://dx.doi.org/10.1093/acrefore/9780190228620.013.613.
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Lifeline for about one-sixth of the world’s population in the subcontinent, the Indian summer monsoon (ISM) is an integral part of the annual cycle of the winds (reversal of winds with seasons), coupled with a strong annual cycle of precipitation (wet summer and dry winter). For over a century, high socioeconomic impacts of ISM rainfall (ISMR) in the region have driven scientists to attempt to predict the year-to-year variations of ISM rainfall. A remarkably stable phenomenon, making its appearance every year without fail, the ISM climate exhibits a rather small year-to-year variation (the standard deviation of the seasonal mean being 10% of the long-term mean), but it has proven to be an extremely challenging system to predict. Even the most skillful, sophisticated models are barely useful with skill significantly below the potential limit on predictability. Understanding what drives the mean ISM climate and its variability on different timescales is, therefore, critical to advancing skills in predicting the monsoon. A conceptual ISM model helps explain what maintains not only the mean ISM but also its variability on interannual and longer timescales.The annual ISM precipitation cycle can be described as a manifestation of the seasonal migration of the intertropical convergence zone (ITCZ) or the zonally oriented cloud (rain) band characterized by a sudden “onset.” The other important feature of ISM is the deep overturning meridional (regional Hadley circulation) that is associated with it, driven primarily by the latent heat release associated with the ISM (ITCZ) precipitation. The dynamics of the monsoon climate, therefore, is an extension of the dynamics of the ITCZ. The classical land–sea surface temperature gradient model of ISM may explain the seasonal reversal of the surface winds, but it fails to explain the onset and the deep vertical structure of the ISM circulation. While the surface temperature over land cools after the onset, reversing the north–south surface temperature gradient and making it inadequate to sustain the monsoon after onset, it is the tropospheric temperature gradient that becomes positive at the time of onset and remains strongly positive thereafter, maintaining the monsoon. The change in sign of the tropospheric temperature (TT) gradient is dynamically responsible for a symmetric instability, leading to the onset and subsequent northward progression of the ITCZ. The unified ISM model in terms of the TT gradient provides a platform to understand the drivers of ISM variability by identifying processes that affect TT in the north and the south and influence the gradient.The predictability of the seasonal mean ISM is limited by interactions of the annual cycle and higher frequency monsoon variability within the season. The monsoon intraseasonal oscillation (MISO) has a seminal role in influencing the seasonal mean and its interannual variability. While ISM climate on long timescales (e.g., multimillennium) largely follows the solar forcing, on shorter timescales the ISM variability is governed by the internal dynamics arising from ocean–atmosphere–land interactions, regional as well as remote, together with teleconnections with other climate modes. Also important is the role of anthropogenic forcing, such as the greenhouse gases and aerosols versus the natural multidecadal variability in the context of the recent six-decade long decreasing trend of ISM rainfall.
Book chapters on the topic "High Frequency Forcing"
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Read, W. A. "High-Frequency, Glacial-Eustatic Sequences in Early Namurian Coal-Bearing Fluviodeltaic Deposits, Central Scotland." In Orbital Forcing and Cyclic Sequences, 413–28. Oxford, UK: Blackwell Publishing Ltd., 2009. http://dx.doi.org/10.1002/9781444304039.ch25.
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Goldhammer, R. K., E. J. Oswald, and P. A. Dunn. "High-Frequency, Glacio-Eustatic Cyclicity in the Middle Pennsylvanian of the Paradox Basin: An Evaluation of Milankovitch Forcing." In Orbital Forcing and Cyclic Sequences, 243–83. Oxford, UK: Blackwell Publishing Ltd., 2009. http://dx.doi.org/10.1002/9781444304039.ch18.
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Longo, G., B. d'Argenio, V. Ferreri, and M. Iorio. "Fourier Evidence for High-Frequency Astronomical Cycles Recorded in Early Cretaceous Carbonate Platform Strata, Monte Maggiore, Southern Apennines, Italy." In Orbital Forcing and Cyclic Sequences, 77–85. Oxford, UK: Blackwell Publishing Ltd., 2009. http://dx.doi.org/10.1002/9781444304039.ch7.
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Epanchintsev, Timofei, Sergei Pravdin, and Alexander Panfilov. "Spiral Wave Drift Induced by High-Frequency Forcing. Parallel Simulation in the Luo–Rudy Anisotropic Model of Cardiac Tissue." In Lecture Notes in Computer Science, 378–91. Cham: Springer International Publishing, 2018. http://dx.doi.org/10.1007/978-3-319-93698-7_29.
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Forkner, Rob M., Linda A. Hinnov, Robert K. Goldhammer, and Laurie A. Hardie. "The Allocyclic Interpretation of the ‘Latemar Cycles’ (Middle Triassic, the Dolomites, Italy) and Implications for High-Frequency Cyclostratigraphic Forcing." In Perspectives in Carbonate Geology, 215–38. Chichester, West Sussex, UK: John Wiley & Sons, Ltd, 2012. http://dx.doi.org/10.1002/9781444312065.ch14.
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Essefi, Elhoucine. "High Resolution Cyclostratigraphy During the Last Two Millennia Based on the Clayey Fraction Within the Mhabeul Wetland (Southeastern Tunisia)." In Climatic and Environmental Significance of Wetlands, 139–56. IGI Global, 2022. http://dx.doi.org/10.4018/978-1-7998-9289-2.ch008.
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High-resolution proxy-based paleoenvironmental records derived from wetlands provide important insights into climate changes over centennial to millennial timescales. This work aims to study the climatic cyclicity of the clayey fraction along a 78 cm core from the saline system of Mhabeul, located in Southeastern Tunisia. Based on the age model of Essefi et al., the core covers the last two millennia (»2100 yr). A high-resolution sampling of 2mm was carried out to obtain 380 samples evenly distributed along the core. In terms of analyses, the cumulative curve of the grain size distribution carried out by Fritsch laser apparatus percentages of the Clayey fraction underwent the spectral analysis. Based on the spectral analysis, millennial cycles of » 2500 yr, and 1000 yr are related to solar forcing. Solar irradiation is most likely responsible for the cyclic characteristics at 500 yr (600 yr) frequency.
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Gray, Ian, Andrea Acquaviva, and Neil Audsley. "Designing Resource-Constrained Embedded Heterogeneous Systems to Cope with Variability." In Advances in Systems Analysis, Software Engineering, and High Performance Computing, 75–101. IGI Global, 2014. http://dx.doi.org/10.4018/978-1-4666-6194-3.ch004.
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As modern embedded systems become increasingly complex, they also become susceptible to manufacturing variability. Variability causes otherwise identical hardware elements to exhibit large differences in dynamic and static power usage, maximum clock frequency, thermal resilience, and lifespan. There are currently no standard ways of handling this variability from the software developer's point of view, forcing the hardware vendor to discard devices that fall below a certain threshold. This chapter first presents a review of existing state-of-the-art techniques for mitigating the effects of variability. It then presents the toolflow developed as part of the ToucHMore project, which aims to build variability-awareness into the entire design process. In this approach, the platform is modelled in SysML, along with the expected variability and the monitoring and mitigation capabilities that the hardware presents. This information is used to automatically generate a customised variability-aware runtime, which is used by the programmer to perform operations such as offloading computation to another processing element, parallelising operations, and altering the energy use of operations (using voltage scaling, power gating, etc.). The variability-aware runtime affects its behaviour according to modelled static manufacturing variability and measured dynamic variability (such as battery power, temperature, and hardware degradation). This is done by moving computation to different parts of the system, spreading computation load more efficiency, and by making use of the modelled capabilities of the system.
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Finn, Ed. "Counting Bitcoin." In What Algorithms Want. The MIT Press, 2017. http://dx.doi.org/10.7551/mitpress/9780262035927.003.0006.
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This chapter uses the growing dominance of algorithmic high frequency trading in finance to frame a reading of Bitcoin and related cryptocurrencies. By defining the unit of exchange through computational cycles, Bitcoin fundamentally shifts the faith-based community of currency from a materialist to an algorithmic value system. Algorithmic arbitrage is forcing similar transitions in the attribution of value and meaning in many spaces of cultural exchange, from Facebook and Google’s Page Rank algorithm to journalism. The fundamental shift from valuing the cultural object itself to valuing the networks of relations that the object establishes or supports leads to new practices and aesthetics of production, where form and genre give way to memes and nebulous collaborative works. Using Bitcoin and its underlying blockchain technology as an example of this new value model, the chapter considers the consequences of programmable value for the notion of a public sphere in the twenty-first century, an era when arbitrage trumps content.
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Oliveira Guimarães, Sullyandro. "Climate Models Accumulated Cyclone Energy Analysis." In Current Topics in Tropical Cyclone Research. IntechOpen, 2020. http://dx.doi.org/10.5772/intechopen.91268.
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Looking at the connection between tropical cyclones and climate changes due to anthropogenic and natural effects, this work aims for information on understanding and how physical aspects of tropical cyclones may change, with a focus on accumulated cyclone energy (ACE), in a global warming scenario. In the present climate evaluation, reasonable results were obtained for the ACE index; the Coupled Model Intercomparison Project Phase 6 (CMIP6) models with lower horizontal and vertical resolution showed more difficulties in representing the index, while Max Planck Institute model demonstrated ability to simulate the climate with more accurate, presenting values of both ACE and maximum temperature close to NCEP Reanalysis 2. The MPI-ESM1-2-HR projections suggest that the seasons and their interannual variations in cyclonic activity will be affected by the forcing on the climate system, in this case, under the scenario of high GHG emissions and high challenges to mitigation SSP585. The results indicate to a future with more chances of facing more tropical cyclone activity, plus the mean increase of 3.1°C in maximum daily temperatures, and more heavy cyclones and stronger storms with more frequency over the North Atlantic Ocean may be experimented, as indicated by other studies.
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"Mitigating Impacts of Natural Hazards on Fishery Ecosystems." In Mitigating Impacts of Natural Hazards on Fishery Ecosystems, edited by Hans W. Paerl and Benjamin L. Peierls. American Fisheries Society, 2008. http://dx.doi.org/10.47886/9781934874011.ch9.
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<em>Abstract</em>.—Since the mid-1990s, the U.S. mid-Atlantic region has witnessed a sudden rise in hurricane and tropical storm landfalls. In particular, eastern North Carolina has been impacted by eight hurricanes and six tropical storms in the past decade, and this relatively high frequency is forecast to continue for the next several decades. Each of the past storms exhibited unique hydrologic and nutrient loading scenarios for the Pamlico Sound, the United States’ second largest estuarine system and its largest subestuary, the Neuse River estuary. This variability represents a challenge to nutrient management aimed at protecting water quality and ensuring optimal fisheries habitat conditions. Different rainfall amounts among hurricanes led to variable freshwater and nutrient discharge and hence variable nutrient, organic matter, and sediment enrichment. These enrichments differentially affected physical and chemical properties (salinity, water residence time, transparency, stratification, dissolved oxygen), phytoplankton primary production, and phytoplankton community composition. The contrasting effects were accompanied by biogeochemical perturbations (hypoxia, enhanced nutrient cycling), benthic and planktonic habitat alterations, and possibly food web disturbances. Floodwaters from the two largest hurricanes, Fran (1996) and Floyd (1999), exerted multimonth to multiannual effects on hydrology, nutrient loads, productivity, biotic composition, and habitat condition. In contrast, relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical mixing and storm surges but exhibited relatively minor hydrologic, nutrient, and biotic impacts. Both hydrologic and wind forcing are important drivers and must be integrated with nutrient loading in assessing short- and long-term ecological impacts of these storms. These climatic forcings cannot be managed but must be considered when developing water quality management strategies for these and other large estuarine ecosystems faced with increasing frequencies and intensities of hurricane activity.
Conference papers on the topic "High Frequency Forcing"
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Stanek, Michael, Ganesh Raman, Valdis Kibens, John Ross, Jessaji Odedra, and James Peto. "Control of cavity resonance through very high frequency forcing." In 6th Aeroacoustics Conference and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 2000. http://dx.doi.org/10.2514/6.2000-1905.
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Roa, Mario, Stephen A. Schumaker, and Doug G. Talley. "High Frequency Transverse Acoustic Forcing of Cryogenic Impinging Jets at High Pressure." In 52nd AIAA/SAE/ASEE Joint Propulsion Conference. Reston, Virginia: American Institute of Aeronautics and Astronautics, 2016. http://dx.doi.org/10.2514/6.2016-5086.
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Benton, Stuart I., and Miguel R. Visbal. "High-Frequency Forcing to Delay Dynamic Stall at Relevant Reynolds Number." In 47th AIAA Fluid Dynamics Conference. Reston, Virginia: American Institute of Aeronautics and Astronautics, 2017. http://dx.doi.org/10.2514/6.2017-4119.
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Mangold, Tobias O., Alessandro Orchini, Christian Oliver Paschereit, Jonas P. Moeck, and Myles D. Bohon. "Flame Response of a Lean Premixed Swirl Flame to High Frequency Azimuthal Forcing." In ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/gt2022-84211.
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Abstract This experimental study explores fundamental qualitative differences of the response of a lean-premixed turbulent swirl flame to azimuthal acoustic excitation in the low- (LF), medium- (MF), and high-frequency (HF) regime. An experimental test rig with a cylindrical combustion chamber is equipped with an array of azimuthally distributed acoustic driver outlets in the burner front plate, allowing for the excitation of the flame with a longitudinal wave, and different azimuthal wave types. Phase-averaged chemiluminescence images show that in the LF regime the flame responds to both longitudinal and azimuthal forcing schemes with hydrodynamically induced intensity fluctuation patterns at relatively small wavelength. Azimuthal forcing generates helical intensity patterns, at the same wavelengths but with lower intensities compared to longitudinal forcing. A fundamentally different flame response is observed in the HF regime: Longitudinal forcing generates only a very weak flame response, and the amplitude of the flame response to azimuthal forcing type depends on the forced azimuthal mode. No hydrodynamic structures are evident anymore, but the flame response now uniformly covers each of the flame sides, indicating a purely acoustic response.
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Emmerson, Paul R., Mike J. Lewis, Neil A. Barton, Steinar Orre, and Knud Lunde. "Flow Induced Vibration Analysis of Topside Piping at High Pressure." In ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/omae2020-18760.
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Abstract Flow induced vibration (FIV) from high velocity multiphase flow is a common source of vibration concern in process piping, potentially leading to fatigue failures and hydrocarbon leaks. FIV screening methods tend to be conservative for multiphase flows and are typically only validated for simple single bends at low pressure. FE can predict the response of a system if a sensible forcing function is provided. CFD can be used to predict realistic forcing functions in complex combinations of bends and tees, typically seen in process piping systems. FIV studies were performed on a topside production system operated by Equinor, carrying multiphase flow at high pressure (∼69 bara) conditions, where significant vibration was measured. The study assessed different vibration simulation methodologies, combining FE analysis with forcing functions based on both correlations and CFD simulations. The aim was to gain a better understanding of the accuracy and limitations of calculation methods typically used to assess fatigue. CFD simulations predicted similar force magnitudes but higher frequency forcing at 69 bara compared to equivalent simulations at atmospheric pressure (at the same liquid and gas superficial velocities). The forcing function correlations used do not predict higher frequency forcing at high pressure, which has a significant impact on the predicted vibration. Care is required when undertaking this type of analysis. It is important to have an accurate FE model of the as-built pipework and supports as well as a forcing function which accurately represents the fluid forces on the bends. For the case simulated here the magnitude and peak frequency of the forcing function had a significant influence on the response of the structure. Forcing functions based on correlated data from tests at low pressure should be used carefully for high pressure systems. In addition, the inclusion of phasing of the forces at each bend can influence the structural response, and simulations performed in the frequency domain do not consider this. A combination of CFD and FE modelling offers a potentially powerful tool for assessing and diagnosing multiphase FIV problems in hydrocarbon production piping systems.
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Samimy, Mo, and Jeff Kastner. "Effects of Forcing Frequency on the Control of an Impinging High Speed Jet." In 41st Aerospace Sciences Meeting and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 2003. http://dx.doi.org/10.2514/6.2003-6.
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Stanek, Michael, Ganesh Raman, Valdis Kibens, John Ross, Jessaji Odedra, and James Peto. "Suppression of cavity resonance using high frequency forcing - The characteristic signature of effective devices." In 7th AIAA/CEAS Aeroacoustics Conference and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 2001. http://dx.doi.org/10.2514/6.2001-2128.
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Lilley, Alexander J., Subrata Roy, and Miguel R. Visbal. "On the effect of high-frequency plasma actuator forcing for prevention of dynamic stall." In AIAA SCITECH 2023 Forum. Reston, Virginia: American Institute of Aeronautics and Astronautics, 2023. http://dx.doi.org/10.2514/6.2023-0353.
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Cruikshank, Ross, and Philippe Lavoie. "Modulated High-Frequency Distributed Forcing of the Wake of a Blunt Trailing Edge Profiled Body." In 2018 AIAA Aerospace Sciences Meeting. Reston, Virginia: American Institute of Aeronautics and Astronautics, 2018. http://dx.doi.org/10.2514/6.2018-0794.
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Denisov, Alexey, and Abhishek Ravi. "Premixed Swirling Flame Response to Acoustic Forcing Studied With High-Speed PIV and OH* Chemiluminescence." In ASME Turbo Expo 2015: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/gt2015-43419.
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Studies of swirling flames have been boosted by technological advances in high-speed lasers and cameras. Temporal and spatial evolution of swirling flows has been revealed by high-speed particle image velocimetry (PIV). We have studied the response of a perfectly premixed swirling flame to weak acoustic perturbations induced by a pair of loudspeakers upstream of the burner. Phase-resolved response of the flame was observed with PIV and OH* chemiluminescence measurements running at 12 times the forcing frequency. The flow dynamics was not affected by the flame compared to non-reacting conditions and the flame responded to flow variations by changing its angle. Proper orthogonal decomposition analysis revealed that the strongest coherent structure in the flow was precessing vortex core that caused transversal variation of the heat release without producing acoustic oscillations. Axisymmetric vortices were not observed at this level of acoustic forcing, but precession modes were modulated at acoustic frequency as additional frequency peaks appeared at the sum and the difference of precession and forcing frequencies. Average time of vortex convection from the burner to the flame is close to the delay of the flame response to acoustic forcing, measured by microphones. This supports the importance of vortex propagation to acoustic modulation of flame heat release.