The Experts below are selected from a list of 30144 Experts worldwide ranked by ideXlab platform
Owen M. Griffin - One of the best experts on this subject based on the ideXlab platform.
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Kinematic and dynamic evolution of deep water breaking waves
Journal of Geophysical Research: Oceans, 1996Co-Authors: Owen M. Griffin, Rodney D. Peltzer, Henry T. Wang, William W. SchultzAbstract:Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave Energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential Energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a modeRate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase-time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise-linear algorithms for two-dimensional potential flow were developed and used by Schultz et al. [1994] to compare the onset of breaking for several methods of Energy Input to the unsteady wave system. The computations show that steep plunging waves occur when Energy Input Rates are large. The various Energy Input methods exhibit similar breaking trends in the limit as the Energy Input Rate becomes small in that incipient spilling breakers form when the potential Energy is approximately 52 to 54% of the Energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest.
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Potential Energy in Steep and Breaking Waves
Journal of Fluid Mechanics, 1994Co-Authors: William W. Schultz, Jin Huh, Owen M. GriffinAbstract:Abstract : We find that potential Energy rather than wave height is a better experimental and analytic criterion for determining when wave breaking will occur. A simple two-dimensional, periodic algorithm is developed and used to compare breaking onset criteria for Energy Input from (1) converging sidewalls, (2) a submerged disturbance and (3) wave focusing. Wave-breaking criteria (potential Energy or the more classical peak-to-peak wave height) are a function of the Rate of Energy Input. Large plunging waves occur for large Energy Input Rates with a smooth transition to smaller spilling waves for lesser Energy Input Rates. The first two kinds of Energy Input show similar trends in the limit as the Energy Input Rate becomes small. The third case, wave focusing, is the subject of an ongoing investigation. The effects of wave modulation and reflection are also discussed.
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Potential Energy in steep and breaking waves By WILLIAM W. SCHULTZl, JIN HUH'T
1994Co-Authors: Owen M. GriffinAbstract:We find that the RMS wave height (square root of the potential Energy) rather than peak-to-peak wave height is a better experimental and analytic criterion for determining when a regular, two-dimensional deep-water wave will break. A spectral algorithm for two-dimensional potential flow is developed and used to compare breaking onset criteria for Energy Input from (i) converging sidewalls, (ii) a submerged disturbance, and (iii) wave focusing. We also find that wave-breaking criteria (potential Energy or the more classical peak-to-peak wave height) are a function of the Rate of Energy Input, Large plunging waves occur when Energy Input Rates are large. As Energy Input Rates become smaller there is a smooth transition to smaller spilling waves. The various Energy Input methods show similar breaking trends in the limit as the Energy Input Rate becomes small - waves break when the potential Energy becomes approximately 52% of the Energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest. The effects of wave modulation and reflection are briefly discussed and shown not to affect the potential Energy breaking criterion significantly. The experimental scatter of the RMS wave height is shown to be half that of wave steepness during incipient breaking in wave packets.
Petros J. Ioannou - One of the best experts on this subject based on the ideXlab platform.
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Is spontaneous generation of coherent baroclinic flows possible
Journal of Fluid Mechanics, 2019Co-Authors: Nikolaos A. Bakas, Petros J. IoannouAbstract:Geophysical turbulence is observed to self-organize into large-scale flows such as zonal jets and coherent vortices. Previous studies of barotropic $\unicode[STIX]{x1D6FD}$-plane turbulence have shown that coherent flows emerge from a background of homogeneous turbulence as a bifurcation when the turbulence intensity increases. The emergence of large-scale flows has been attributed to a new type of collective, symmetry-breaking instability of the statistical state dynamics of the turbulent flow. In this work, we extend the analysis to stratified flows and investigate turbulent self-organization in a two-layer fluid without any imposed mean north–south thermal gradient and with turbulence supported by an external random stirring. We use a second-order closure of the statistical state dynamics, that is termed S3T, with an appropriate averaging ansatz that allows the identification of statistical turbulent equilibria and their structural stability. The bifurcation of the statistically homogeneous equilibrium state to inhomogeneous equilibrium states comprising zonal jets and/or large-scale waves when the Energy Input Rate of the excitation passes a critical threshold is analytically studied. Our theory predicts that there is a large bias towards the emergence of barotropic flows. If the scale of excitation is of the order of (or larger than) the deformation radius, the large-scale structures are barotropic. Mixed barotropic–baroclinic states with jets and/or waves arise when the excitation is at scales shorter than the deformation radius with the baroclinic component of the flow being subdominant for low Energy Input Rates and insignificant for higher Energy Input Rates. The predictions of the S3T theory are compared with nonlinear simulations. The theory is found to accuRately predict both the critical transition parameters and the scales of the emergent structures but underestimates their amplitude.
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Is spontaneous generation of coherent baroclinic flows possible
arXiv: Atmospheric and Oceanic Physics, 2017Co-Authors: Nikolaos A. Bakas, Petros J. IoannouAbstract:Geophysical turbulence is observed to self-organize into large-scale flows such as zonal jets and coherent vortices. Previous studies on barotropic beta-plane turbulence have shown that coherent flows emerge out of a background of homogeneous turbulence as a bifurcation when the turbulence intensity increases and the emergence of large scale flows has been attributed to a new type of collective, symmetry breaking instability of the statistical state dynamics of the turbulent flow. In this work we extend the analysis to stratified flows and investigate turbulent self-organization in a two-layer fluid with no imposed mean north-south thermal gradient and turbulence supported by an external random stirring. We use a second order closure of the statistical state dynamics (S3T) with an appropriate averaging ansatz that allows the identification of statistical turbulent equilibria and their structural stability. The bifurcation of the statistically homogeneous equilibrium state to inhomogeneous equilibrium states comprising of zonal jets and/or large scale waves when the Energy Input Rate of the excitation passes a critical threshold is analytically studied. The theory predicts that when the flow transitions to a statistical state with large-scale structures, these states are barotropic if the scale of excitation is larger than the deformation radius. Mixed barotropic-baroclinic states with jets and/or waves arise when the excitation is at scales shorter than the deformation radius with the baroclinic component of the flow being subdominant for low Energy Input Rates and non-significant for higher Energy Input Rates. The results of the S3T theory are compared to nonlinear simulations. The theory is found to accuRately predict both the critical transition arameters and the scales of the emergent structures but underestimates their amplitude.
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Emergence of non-zonal coherent structures
arXiv: Atmospheric and Oceanic Physics, 2015Co-Authors: Nikolaos A. Bakas, Petros J. IoannouAbstract:Planetary turbulence is observed to self-organize into large-scale structures such as zonal jets and coherent vortices. One of the simplest models that retains the relevant dynamics of turbulent self-organization is a barotropic flow in a beta-plane channel with turbulence sustained by random stirring. Non-linear integrations of this model show that as the Energy Input Rate of the forcing is increased, the homogeneity of the flow is first broken by the emergence of non-zonal, coherent, westward propagating structures and at larger Energy Input Rates by the emergence of zonal jets. The emergence of both non-zonal coherent structures and zonal jets is studied using a statistical theory, Stochastic Structural Stability Theory (S3T). S3T directly models a second-order approximation to the statistical mean turbulent state and allows the identification of statistical turbulent equilibria and study of their stability. Using S3T, the bifurcation properties of the homogeneous state in barotropic beta-plane turbulence are determined. Analytic expressions for the zonal and non-zonal large-scale coherent flows that emerge as a result of structural instability are obtained and the equilibration of the incipient instabilities is studied through numerical integrations of the S3T dynamical system. The dynamics underlying the formation of zonal jets are also investigated. It is shown that zonal jets form from the upgradient momentum fluxes that result from the shearing of the eddies by the emerging infinitesimal large-scale flow. Finally, numerical simulations of the nonlinear equations confirm the characteristics (scale, amplitude and phase speed) of the structures predicted by S3T, even in highly non-linear parameter regimes such as the regime of zonostrophic turbulence.
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A theory for the emergence of coherent structures in beta-plane turbulence
Journal of Fluid Mechanics, 2014Co-Authors: Nikolaos A. Bakas, Petros J. IoannouAbstract:Planetary turbulent flows are observed to self-organize into large-scale structures such as zonal jets and coherent vortices. One of the simplest models of planetary turbulence is obtained by considering a barotropic flow on a beta-plane channel with turbulence sustained by random stirring. Nonlinear integrations of this model show that as the Energy Input Rate of the forcing is increased, the homogeneity of the flow is broken with the emergence of non-zonal, coherent, westward propagating structures and at larger Energy Input Rates by the emergence of zonal jets. We study the emergence of non-zonal coherent structures using a non-equilibrium statistical theory, stochastic structural stability theory (S3T, previously referred to as SSST). S3T directly models a second-order approximation to the statistical mean turbulent state and allows the identification of statistical turbulent equilibria and study of their stability. Using S3T, the bifurcation properties of the homogeneous state in barotropic beta-plane turbulence are determined. Analytic expressions for the zonal and non-zonal large-scale coherent flows that emerge as a result of structural instability are obtained. Through numerical integrations of the S3T dynamical system, it is found that the unstable structures equilibRate at finite amplitude. Numerical simulations of the nonlinear equations confirm the characteristics (scale, amplitude and phase speed) of the structures predicted by S3T.
William W. Schultz - One of the best experts on this subject based on the ideXlab platform.
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Kinematic and dynamic evolution of deep water breaking waves
Journal of Geophysical Research: Oceans, 1996Co-Authors: Owen M. Griffin, Rodney D. Peltzer, Henry T. Wang, William W. SchultzAbstract:Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave Energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential Energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a modeRate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase-time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise-linear algorithms for two-dimensional potential flow were developed and used by Schultz et al. [1994] to compare the onset of breaking for several methods of Energy Input to the unsteady wave system. The computations show that steep plunging waves occur when Energy Input Rates are large. The various Energy Input methods exhibit similar breaking trends in the limit as the Energy Input Rate becomes small in that incipient spilling breakers form when the potential Energy is approximately 52 to 54% of the Energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest.
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Potential Energy in Steep and Breaking Waves
Journal of Fluid Mechanics, 1994Co-Authors: William W. Schultz, Jin Huh, Owen M. GriffinAbstract:Abstract : We find that potential Energy rather than wave height is a better experimental and analytic criterion for determining when wave breaking will occur. A simple two-dimensional, periodic algorithm is developed and used to compare breaking onset criteria for Energy Input from (1) converging sidewalls, (2) a submerged disturbance and (3) wave focusing. Wave-breaking criteria (potential Energy or the more classical peak-to-peak wave height) are a function of the Rate of Energy Input. Large plunging waves occur for large Energy Input Rates with a smooth transition to smaller spilling waves for lesser Energy Input Rates. The first two kinds of Energy Input show similar trends in the limit as the Energy Input Rate becomes small. The third case, wave focusing, is the subject of an ongoing investigation. The effects of wave modulation and reflection are also discussed.
Izuru Takewaki - One of the best experts on this subject based on the ideXlab platform.
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Building earthquake resilience in sustainable cities in terms of Input Energy
Sustainable Cities and Society, 2014Co-Authors: K. Kojima, Kohei Fujita, Izuru TakewakiAbstract:Abstract Input Energy to building structures during earthquakes is an important index to measure the influence of earthquake ground motions on building structures. Such Input Energy can be defined after the structural system is specified and the Input mechanism is described clearly. The Energy Input to structures consists mainly of the Energy dissipated by hysteretic deformation and that by viscous damping. The excessive dependence on the former mechanism leads to unrepairable and unpreferable states of structures after earthquakes which should be avoided from the viewpoint of sustainability of building structures and cities. In this sense, the measure of Energy is appropriate from the viewpoint of total management of buildings in a sustainable city. Then the upper bound of earthquake Input Energy is derived and discussed under uncertain conditions on Input ground motions. It is shown that the earthquake Energy Input Rate is another key parameter for measuring the instantaneous effect of earthquake ground motions on structural responses. A historical review is also made on the development of treatment of earthquake Input Energy into buildings and on its role into greater building earthquake resilience in sustainable cities.
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Critical Excitation for Earthquake Energy Input Rate
Critical Excitation Methods in Earthquake Engineering, 2013Co-Authors: Izuru TakewakiAbstract:This chapter discusses a critical excitation method for earthquake Energy Input Rate. It explores a new probabilistic critical excitation method for identifying the critical frequency content of ground motions maximizing the mean earthquake Energy Input Rate to structures. The critical excitation problem includes a double maximization procedure with respect to time and to the power spectral density (PSD) function. The key to finding the critical frequency content is the order exchange in the double maximization procedure. No mathematical programming technique is required in the proposed method. It is shown that the proposed technique is systematic and the critical excitation can be found extremely efficiently within a reasonable accuracy. Extension of the proposed method is discussed for a more general ground motion model. The chapter discusses the process to derive a new expression on the probabilistic earthquake Input Energy and its Rate in terms of uniformly modulated and nonuniformly modulated ground motion models. The process to formulate a new critical excitation problem with the probabilistic earthquake Energy Input Rate as the criticality measure is mentioned. A deterministic expression of earthquake Energy Input Rate to a base-isolated building model is also presented in order to capture the properties of earthquake Energy Input Rate in more detail.
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Instantaneous earthquake Input Energy and sensitivity in base-isolated building
The Structural Design of Tall and Special Buildings, 2009Co-Authors: Kaoru Yamamoto, Kohei Fujita, Izuru TakewakiAbstract:The Input Energy and Energy Input Rate to a base-isolated (BI) building during an earthquake are considered and formulated in the frequency domain. The frequency-domain approach for computation of Input Energy and Energy Input Rate has different remarkable advantages compared with the conventional time-domain approach. It is demonstRated that the Input Energy can be of a compact form via the frequency integration of the product between the Input component (squared Fourier amplitude spectrum of acceleration) and the structural model component (so-called Energy transfer function). Furthermore, the Energy Input Rate can also be of a similar form via the frequency integration of the product between the instantaneous power spectrum and the Energy transfer function. With the help of this compact form, it is shown that the formulation in the frequency domain is essential for deriving arbitrary-order closed-form sensitivities of the Input Energy and Energy Input Rate with respect to uncertain stiffness and damping coefficients in the BI storey. The closed-form sensitivity expressions provide us with information on the most unfavourable variation of the uncertain parameters that leads to the maximum Input Energy and Input Rate. Copyright © 2009 John Wiley & Sons, Ltd.
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Chapter 12 – Critical Excitation for Earthquake Energy Input Rate
Critical Excitation Methods in Earthquake Engineering, 2007Co-Authors: Izuru TakewakiAbstract:Publisher Summary This chapter discusses a critical excitation method for earthquake Energy Input Rate. It explores a new probabilistic critical excitation method for identifying the critical frequency content of ground motions maximizing the mean earthquake Energy Input Rate to structures. The critical excitation problem includes a double maximization procedure with respect to time and to the power spectral density (PSD) function. The key for finding the critical frequency content is the order interchange in the double maximization procedure. No mathematical programming technique is required in the proposed method. It is shown that the proposed technique is systematic and the critical excitation can be found extremely efficiently within a reasonable accuracy. Extension of the proposed method is discussed to a more general ground motion model. The chapter discusses the process to derive a new expression on the probabilistic earthquake Input Energy and its Rate in terms of uniformly modulated and non-uniformly modulated ground motion models. The process to formulate a new critical excitation problem with the probabilistic earthquake Energy Input Rate as the criticality measure is mentioned.
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chapter 12 critical excitation for earthquake Energy Input Rate
Critical Excitation Methods in Earthquake Engineering, 2007Co-Authors: Izuru TakewakiAbstract:Publisher Summary This chapter discusses a critical excitation method for earthquake Energy Input Rate. It explores a new probabilistic critical excitation method for identifying the critical frequency content of ground motions maximizing the mean earthquake Energy Input Rate to structures. The critical excitation problem includes a double maximization procedure with respect to time and to the power spectral density (PSD) function. The key for finding the critical frequency content is the order interchange in the double maximization procedure. No mathematical programming technique is required in the proposed method. It is shown that the proposed technique is systematic and the critical excitation can be found extremely efficiently within a reasonable accuracy. Extension of the proposed method is discussed to a more general ground motion model. The chapter discusses the process to derive a new expression on the probabilistic earthquake Input Energy and its Rate in terms of uniformly modulated and non-uniformly modulated ground motion models. The process to formulate a new critical excitation problem with the probabilistic earthquake Energy Input Rate as the criticality measure is mentioned.
T. V. Laitinen - One of the best experts on this subject based on the ideXlab platform.
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New interpretation of magnetospheric Energy circulation
Geophysical Research Letters, 2006Co-Authors: Tuija Pulkkinen, Minna Palmroth, Eija Tanskanen, Pekka Janhunen, Hej Koskinen, T. V. LaitinenAbstract:[1] New results from the global MHD simulation GUMICS-4 show that the Energy transfer Rate from the solar wind into the magnetosphere is a function of also the past values of the IMF. Furthermore, comparison of the Energy Input Rate with the conversion and dissipation Rates in the magnetosphere and ionosphere shows that the Energy entering the magnetosphere is processed by the system without delay. This and earlier observations lead us to suggest that the role of Energy Input during the substorm growth phase is to facilitate the configuration change rather than to store Energy for later dissipation. While the change of magnetotail stability requires configuration change, the substorm energetics after the onset is quite directly driven by the solar wind Energy Input.