Transient Kinetic Energy Research Articles

Energy Budget Characterisation of the Optimal Disturbance in Stratified Shear Flow

Stratified Taylor–Couette flow (STCF) undergoes transient growth. Recent studies have shown that there exists transient amplification in the linear regime of counter-rotating STCF. The kinetic budget of the optimal transient perturbation is analysed numerically to simulate the interaction of the shear production (SP), buoyancy flux (BP), and other energy components that contributes to the total optimal transient kinetic energy. These contributions affect the total energy by influencing the perturbation to extract kinetic energy (KE) from the mean flow. The decay of the amplification factor resulted from the positive amplification of both BP and SP, while the growth is attributed to the negative and positive amplification of BP and SP, respectively. The optimal SP is positively amplified, implying that there is the possibility of constant linear growth. These findings agree with the linear growth rate for increasing values of Grashof number.

Decadal co‐variation of the aerosols over East Asia and the East Asian jet streams in the boreal winter

AbstractThe changes in the East Asian polar‐front jet (EAPJ) and the East Asian subtropical jet (EASJ) profoundly impact the weather and climate in East Asia. However, the link between aerosols and the jet streams is still unclear. Here, we investigated the decadal co‐variation of aerosols and the East Asian jet streams in the boreal winter during the period of 1980–2019. In synch with a positive change in aerosol optical depth over East Asia, 300‐hPa winds show an equatorward shift of the land branch of the EASJ, and weakened EAPJ and oceanic branch of the EASJ. This can be linked to the enhanced meridional temperature gradient along 30°–50° N but weakened in northern regions and the decreasing synoptic‐scale transient eddy kinetic energy over subtropical Pacific. Relative importance estimation of aerosols and ocean signals emphasized the contributions of aerosols in jet variations. In turn, meteorological conditions related to jet streams also contribute to variations in aerosols, the decadal co‐variations are a result of their interactions, particularly for the oceanic branch of EASJ. The findings would be helpful in providing potential indicators of climate change.

Frequency Dependence of Ocean Kinetic Energy and Respective Changes Over the Period 1983–2018

AbstractRegional horizontal and vertical patterns of ocean kinetic energy (KE) during 1983–2018 are studied using a numerical simulation. The analysis is focused on time‐mean and transient components of KE. For understanding its contribution to the total as function of timescale, the transient part is further sub‐divided into six frequency bands, with periods >16 months, 16–8 months, 8–4 months, 4–1 months, 1 month–5 days and <5 days. Near the surface, besides intensification due to wind‐forced generation, transient KE is enhanced in boundary currents, in equatorial wave guides and in the Southern Ocean. In the deep, KE is enhanced in re‐circulation regions under boundary currents and at eastern sides of topographic ridges. On scales <5 days it is enhanced over shelf and in quiet ocean interior areas. The largest fraction of transient energy resides in the eddy‐populated band 1 week–4 months. Decadal changes of KE are associated with mean flow variations and changes on inter‐ and intra‐annual periods, especially in the Atlantic and Southern Oceans. Changes in the subpolar Atlantic are correlated with the North Atlantic Oscillation and in the Southern Ocean with the Southern Annular Mode. A decrease/increase in Atlantic/Pacific KE is noticed over the investigated period. Trends are found in the Gulf Stream, Kuroshio Current and along the Antarctic Circumpolar Current, which shows an increase in mean and transient energy in the Indian/west Pacific sectors and a decrease in the Atlantic sector. The strong variations between decades suggest that trends are part of multi‐decadal variability.

Preserving microgrid sustainability through robust islanding detection scheme ensuring cyber-situational awareness

Along with the numerous environmental and operational benefits of renewable energy sources (RESs) integration into modern distribution networks, maintaining system security in these networks during challenging phenomena, especially unintentional islanding can still be problematic in some cases. This paper puts forward a sensitivity-based islanding detection algorithm that utilizes the transient kinetic energy of the RESs to identify the islanding condition in microgrids and also considers cyber-attacks, which may lead to operators making inappropriate operational choices. To be specific, the proposed algorithm finds a generalized index that considers simultaneously the impact of volatility of parameters during islanding and non-islanding conditions and is also robust against data integrity attacks. The generalized index, the so-called sensitivity islanding detection (SID) index, is designed based on the sensitivity of transient kinetic energy over transient influential parameters. Furthermore, a statistical index for islanding detection is introduced based on the standard variation that effectively monitors the volatilities. An unsupervised anomaly detection method that uses kernel density estimation technology and statistical calculations are proposed as a way to find cyber-attacks. Following that, an outlier detection method is used to identify anomalies. The effectiveness of the proposed algorithm is evaluated under various conditions using simulation environments.

Kinematic and hydrodynamic mechanisms of Misgurnus anguillicaudatus during routine turning

Although traditional underwater thrusters are technologically advanced and widely used, they have limitations in propulsion efficiency, flexibility, and noise. Studying the swimming mechanisms of aquatic organisms can provide new insight into submarine propulsion. The kinematics and hydrodynamic mechanisms of Misgurnus anguillicaudatus in the turning process were explored experimentally through particle image velocimetry. Morphological characteristics of Misgurnus anguillicaudatus locomotion were analyzed using the swimming posture and extracted a body trunk curve. The kinematic characteristics of Misgurnus anguillicaudatus during turning maneuvers were further explored through quantified kinematic parameters. The hydrodynamic mechanism of the turning process was analyzed from the perspective of transient kinetic energy, vortex evolution, and pressure characteristics. The body trunk of Misgurnus anguillicaudatus maintained a fluctuating pattern from the beginning of the movement. Relying on periodic body undulations and periodic tail wagging enables the fish to maintain a continuous maneuvering state. The tail wagging in different directions generates a pair of positive and negative vortices and local high-kinetic-energy regions. The combination of pressure and viscous mechanisms creates vorticity. Jets are generated at the interface between converging vortices and opposite spins. The thrust jets provide thrust, and the side jets provide angular momentum to the fish body and the surrounding additional mass. The pull of the negative pressure area on the body along the trough is the main thrust mechanism that enables Misgurnus anguillicaudatus to swim.

Ocean Model Response to Stochastically Perturbed Momentum Fluxes

Abstract Recent studies of various stochastic forcing and subgrid-scale parameterization schemes applied to climate and atmospheric models have revealed a diversity of model responses. These responses include degeneracy in the response to different forcings and compensating model errors. While stochastic parameterization of the ocean eddies is an active area, this has mainly involved idealized models with fewer studies employing ocean general circulation models. Here we examine the sensitivity of a low-resolution climate model to stochastic forcing of the momentum fluxes restricted to regions of the three-dimensional ocean where an eddy-resolving ocean model configuration has high variability. We consider the changes in the modeled energetics of low-resolution simulations in response to increased stochastic forcing. We find that as forcing amplitudes are increased there is enhanced conversion of transient to seasonal potential energy. Additionally, there is a systematic redistribution from seasonal to small-scale transient kinetic energy. Our approach has zero mean noise such that the total kinetic energy spectra remain largely unchanged even as small-scale eddy kinetic energy is increased in the targeted regions. However, we also show that strong stochastic forcing, particularly when applied in the tropics, can induce substantial changes to the ocean steady state that are undesirable. These changes include overly strong vertical mixing leading to unrealistic increases in ocean heat content and latitudinally dependent changes to sea level. We show that judicious selection of the magnitude and spatial extent of the stochastic forcing is required for desirable results. Our results point to the importance of a comprehensive evaluation of ocean model responses to stochastic parameterizations.

Generalized stability criterion for controlling solidification segregation upon twin-roll casting

• Systematical framework integrating dendritic growth model, overall solidification kinetic model and generalized stability was constructed. • Processing parameters of the casting speed and the flow speed of cooling water in twin-roll casting are coupled. • Sub-rapid solidification kinetics, concentration and segregation by different processing parameters couple were established. • New strategy was proposed: high initial driving force suppresses centerline segregation, and high final generalized stability suppresses edge segregation. Macro- and micro-segregation formed upon twin-roll casting (TRC) can be inherited from sub-rapid solidification to solid-state transformation, even to plastic deformation, thus deteriorating drastically mechanical properties of as-produced thin sheets. Although many works focusing mainly on controlling fields of thermal, concentration and convection have been reported, how to control artificially and quantitatively the segregation using a theoretical connection between processing parameters and solidification models, has not been realized, yet. Regarding it, a systematical framework integrating non-equilibrium dendritic growth and overall solidification kinetics with the TRC parameters, was constructed applying a generalized stability (GS) conception deduced from transient thermodynamic driving force Δ G t and transient kinetic energy barrier Q eff t evolving upon solidification. Departing from this framework considering synergy of thermodynamics and kinetics (i.e., thermo-kinetic synergy), a criterion of high Δ G t -high GS guaranteed that the macro (i.e., the centerline) and the micro (i.e., the edge) segregation can be suppressed by increasing Δ G t and GS at the beginning and the ending stage of sub-rapid solidification, respectively. This typical thermo-kinetic combination producing the microstructure can be inherited into the plastic deformation, as reflected by corresponding strength-ductility combinations. This work realized quantitative controlling of TRC by a theoretical connection between processing parameters and solidification models, where, an optimization for sub-rapid solidification segregation using the GS conception including Δ G t and Q eff t has been performed.

Experimental evaluation of the transient propagation fracture properties of rocks under dynamic mode I loading: An insight from digital image correlation

Accurate measurement of dynamic fracture properties is crucial for the safety and stability of the engineering applications. In this study, the dynamic mode I fracture tests are conducted on the notched semi-circular bend (NSCB) specimens with the split Hopkinson pressure bar (SHPB) loading system. By virtue of digital image correlation (DIC) technique combined with high-speed photography, the velocity fields of specimen are revealed, and thus an improved method is proposed to accurately calculate the transient kinetic energy and transient propagation fracture toughness of NSCB specimens under dynamic mode I loading. Our method has obvious merits in its specimen preparation and calculation accuracy since no specific assumptions are required. In addition, the crack propagation gauge (CPG) system is also applied into the tests to capture the transient crack propagation velocity and verify the testing results obtained by DIC technique. The results show that the transient propagation velocity of rocks continues to decrease with a three-stage evolution during the process of rock fracture, while the transient fracture energy of rock gradually increases. Moreover, the transient propagation fracture toughness is inversely proportional to the transient crack propagation velocity. Furthermore, the initiation fracrture toughness, propagation fracture toughness, propagation velocity and fracture energy all feature significant loading rate dependence.

The Future Intensification of the North Atlantic Winter Storm Track: The Key Role of Dynamic Ocean Coupling

AbstractClimate models project an intensification of the wintertime North Atlantic Ocean storm track, over its downstream region, by the end of this century. Previous studies have suggested that ocean–atmosphere coupling plays a key role in this intensification, but the precise role of the different components of the coupling has not been explored and quantified. In this paper, using a hierarchy of ocean coupling experiments, we isolate and quantify the respective roles of thermodynamic (changes in surface heat fluxes) and dynamic (changes in ocean heat flux convergence) ocean coupling in the projected intensification of North Atlantic transient eddy kinetic energy (TEKE). We show that dynamic coupling accounts for nearly all of the future TEKE strengthening as it overcomes the much smaller effect of surface heat flux changes to weaken the TEKE. We further show that by reducing the Arctic amplification in the North Atlantic, ocean heat flux convergence increases the meridional temperature gradient aloft, causing a larger eddy growth rate and resulting in the strengthening of North Atlantic TEKE. Our results stress the importance of better monitoring and investigating the changes in ocean heat transport, for improving climate change adaptation strategies.Significance StatementBy the end of this century, the North Atlantic Ocean storm track is projected to intensify on its eastward flank. Such intensification will have large societal impacts, mostly over western Europe. Thus, it is critical to better understand the mechanism underlying the intensification of the storm track. Here we investigate the role of ocean coupling in the future intensification of the North Atlantic storm track and find that ocean heat transport processes are responsible for the strengthening of the storm track. Our results suggest that better monitoring the changes in ocean heat transport will hopefully improve climate change adaption strategies.

Laser-Induced Electronic and Vibronic Dynamics in the Pyrene Molecule and Its Cation.

Among polycyclic aromatic hydrocarbons, pyrene is widely used as an optical probe thanks to its peculiar ultraviolet absorption and infrared emission features. Interestingly, this molecule is also an abundant component of the interstellar medium, where it is detected via its unique spectral fingerprints. In this work, we present a comprehensive first-principles study on the electronic and vibrational response of pyrene and its cation to ultrafast, coherent pulses in resonance with their optically active excitations in the ultraviolet region. The analysis of molecular symmetries, electronic structure, and linear optical spectra is used to interpret transient absorption spectra and kinetic energy spectral densities computed for the systems excited by ultrashort laser fields. By disentangling the effects of the electronic and vibrational dynamics via ad hoc simulations with stationary and moving ions, and, in specific cases, with the aid of auxiliary model systems, we rationalize that the nuclear motion is mainly harmonic in the neutral species, while strong anharmonic oscillations emerge in the cation, driven by electronic coherence. Our results provide additional insights into the ultrafast vibronic dynamics of pyrene and related compounds and set the stage for future investigations on more complex carbon-conjugated molecules.

Spreading and retraction kinetics for impact of nanodroplets on hydrophobic surfaces

Impact dynamics of nanodroplets has recently gained extensive attention because of its potential applications in nanoscale inkjet printing, nanodroplet spray cooling, and nanocoating. In this study, a nanodroplet impacting unheated, flat, smooth, and hydrophobic surfaces is investigated via molecular dynamics simulations. The emphasis is placed on spreading and retraction kinetics, i.e., time-dependent wetting radius or r–τ relation, where r and τ are the normalized wetting radius and time. On the basis of an energy conservation approach, an analytical model of r–τ kinetics is developed for impacting nanodroplets. Hypotheses of cylinder droplet and extensional flow are employed to calculate the transient kinetic energy and viscous dissipation rate, which are found to be the most appropriate for impacting nanodroplets. The model is tested in a range of Weber numbers from We = 15 to 60, Reynolds numbers from Re = 11.07 to 22.19, and surface wettability θ0 = 105° and 125°. The tests show that the mean relative deviation ranges from 2.22% to 5.47%, and hence, the developed model captures the spreading and retraction kinetics of a nanodroplet impacting hydrophobic surfaces with satisfactory accuracy. Furthermore, it is found that the model can also be extended to predict the retraction kinetics of nanodroplets on hydrophilic surfaces for high Weber numbers.

Identification of transient stability boundaries for power systems with multidimensional uncertainties using index-specific parametric space

This paper introduces a method for the identification of transient stability boundaries in power systems with multidimensional uncertainties. This method uses a parametric space rather than the state-space to identify the critical operating states. First, different transient stability indices are used to map the state-space of system into the index-specific parametric spaces. Then, these index-specific parametric spaces are characterized to identify their practicality and effectiveness to reflect the system’s state-space information accurately and to quantify uncertainties. Finally, the transient stability boundaries are identified using the index-specific parametric spaces considering multidimensional uncertainties. The indices studied in this paper are on basis of rotor-angle difference, transient kinetic energy, and transient potential energy. This method is verified using a 16-machine, 68-bus computer model of the New England Test System and the New York Power System in MATLAB and DigSILENT environments. The uncertainty in this paper is represented in up to 4-dimensions and is modeled in form of loading conditions and generation dispatch uncertainties. This method is further applicable to higher dimensions and other forms of uncertainties and has the utility for probabilistic studies and predictive analytics.

The influence of South American regional climate on the simulation of the Southern Hemisphere extratropical circulation

This paper presents new modeling evidence showing the added value of high-resolution information from South America (SA) in the simulation of the Southern Hemisphere (SH) extratropical circulation. LMDZ, a coarse-resolution atmospheric global general circulation model constitutes the main tool for this investigation. Parallel to the control simulation, a two-way nesting (TWN) simulation of LMDZ is performed with an interactive coupling to the same model, but with a higher-resolution zoom over SA. The third simulation is a perfect boundary simulation for which re-analysis information from ERA-Interim is used to nudge LMDZ, but only over SA. Results indicate that enhanced resolution over SA improves the representation of the most important processes that influence extratropical eddy activity. The local improvement is followed by a better representation of the global extratropical circulation, especially in austral summer. The regional climate enhancement over SA has positive effects on simulation of the midlatitude jet position during the austral summer by significantly reducing the bias of the mean zonal kinetic energy outside the nudged zone. On the other hand, the wintertime general circulation outside the nudged-zone shows a limited bias-reduction for the regional-driven simulations, especially in the case of the TWN system. However, improvements of the TWN system compared to the control experiment are noticed in early stages of cyclone lifecycle, as it is identified in a better simulation of transient meridional heat transport and transient kinetic energy intensity. The findings of the present study suggest, thus, that improvements in resolution over SA effectively excite the simulation of the mean atmospheric circulation in the SH.

Causes and underlying dynamic processes of the mid-winter suppression in the North Pacific storm track

Baroclinic wave activity in the North Pacific exhibit peaks in late fall and early spring, and a local minimum in midwinter, when by linear baroclinic instability theory it should attain its maximum. This counterintuitive phenomenon, or “midwinter suppression” (MWM) as called, is investigated with a functional analysis apparatus, multiscale window transform (MWT), and the MWT-based theory of canonical transfer and localized multi-scale energetics analysis, together with a feature tracking technique, using the data from the European Centre for Medium-Range Weather Forecasts ReAnalysis (ERA-40). It is found that the MWM results from a variety of different physical processes, including baroclinic canonical transfer, diabatic effect, energy flux divergence, and frictional dissipation. On one hand, baroclinic canonical transfer and diabatic effect achieve their respective maxima in late fall. More transient available potential energy is produced and then converted to transient kinetic energy, resulting in a stronger storm track in late fall than in midwinter. On the other hand, in early spring, although baroclinic instability and buoyancy conversion are weak, energy flux convergences are substantially strengthened, leading to a net energy inflow into the storm track. Meanwhile, frictional dissipation is greatly reduced in spring; as a result, less transient energy is dissipated in early spring than in midwinter. It is further found that the weakening of baroclinic canonical transfer in midwinter (compared to late fall) is due to the far distance between the storm and the jet stream (located at its southernmost point), which suppresses the interaction between them. Regarding the increase in energy flux convergence in early spring, it appears to originate from the increase (enhancement) in the number (strength) of storms from the upstream into the Pacific.

Laser induced dynamic fracture of fused silica: Experiments and simulations

Fused silica samples were subjected to laser induced shock loading. Laser flux was varied in order to obtain different amounts and characteristics of damage in the samples. Three dimensional damage and fracture maps of two identical samples impacted by high and low laser flux values were obtained using both optical microscopy and X-ray computed micro-tomography. Three prevalent fracture and damage patterns were identified. Peridynamic approach was used to simulate the laser impact conditions on the samples in order to explain the causes of the observed fracture and damage morphologies. A proprietary shock physics code, ESTHER, was used to calculate the transient kinetic energy imparted to the samples based on the experimental laser flux values. The kinetic energy values were then integrated over time and provided target values to match for the peridynamic impact conditions. The main fracture patterns were captured by peridynamic simulations with reasonable quantitative accuracy. Explanations for initiation and propagation of each of the fracture patterns were presented based on the peridynamic dynamic fracture simulations. Limitations of the computational approach and recommendations for future work is provided.

Common envelope jets supernova (CEJSN) impostors resulting from a neutron star companion

We propose a new type of repeating transient outburst initiated by a neutron star (NS) entering the envelope of an evolved massive star, accreting envelope material and subsequently launching jets which interact with their surroundings. This interaction is the result of either a rapid expansion of the primary star due to an instability in its core near the end of its nuclear evolution, or due to a dynamical process which rapidly brings the NS into the primary star. The ejecta can reach velocities of |${\approx } 10^4 \, \rm {km\, s^{-1}}$| despite not being a supernova, and might explain such velocities in the 2011 outburst of the luminous blue variable progenitor of SN 2009ip. The typical transient duration and kinetic energy are weeks to months, and up to |${\approx } 10^{51} \, \rm {erg}$|⁠, respectively. The interaction of an NS with a giant envelope might be a phase in the evolution of the progenitors of most NS–NS binary systems that later undergo a merger event. If the NS spirals in all the way to the core of the primary star and brings about its complete disruption we term this a ‘common envelope jets supernova’ (CEJSN), which is a possible explanation for the peculiar supernova iPTF14hls. For a limited interaction of the NS with the envelope we get a less luminous transient, which we term a CEJSN impostor.

Markovian inhomogeneous closures for Rossby waves and turbulence over topography

Manifestly Markovian closures for the interaction of two-dimensional inhomogeneous turbulent flows with Rossby waves and topography are formulated and compared with large ensembles of direct numerical simulations (DNS) on a generalized$\unicode[STIX]{x1D6FD}$-plane. Three versions of the Markovian inhomogeneous closure (MIC) are established from the quasi-diagonal direct interaction approximation (QDIA) theory by modifying the response function to a Markovian form and employing respectively the current-time (quasi-stationary) fluctuation dissipation theorem (FDT), the prior-time (non-stationary) FDT and the correlation FDT. Markov equations for the triad relaxation functions are derived that carry similar information to the time-history integrals of the non-Markovian QDIA closure but become relatively more efficient for long integrations. Far from equilibrium processes are studied, where the impact of a westerly mean flow on a conical mountain generates large-amplitude Rossby waves in a turbulent environment, over a period of 10 days. Excellent agreement between the evolved mean streamfunction and mean and transient kinetic energy spectra are found for the three versions of the MIC and two variants of the non-Markovian QDIA compared with an ensemble of 1800 DNS. In all cases mean Rossby wavetrain pattern correlations between the closures and the DNS ensemble are greater than 0.9998.

Visualization of the formation of cyclopentylcarbene using time-resolved photoelectron imaging spectroscopy

Ultrafast electronic relaxation dynamics in cyclohexene are studied using femtosecond photoelectron imaging spectroscopy. Transient photoelectron kinetic energy distributions and photoelectron angular distributions are obtained and analyzed. Photoelectron bands are discussed and assigned to the ionization of the related electronic states. The formation process of cyclopentylcarbene, together with the ultrafast relaxation of the involved electronic states, is evidenced by the transient photoelectron images. The lifetime for the observed carbene is very short and determined to be 395 (±67) fs. The ionization dynamics of the produced cyclopentylcarbene are also discussed via an analysis of the corresponding photoelectron band.

Blunt-body paradox and transient growth on a hypersonic spherical forebody

The laminar-turbulent transition in hypersonic flow on a blunt hemisphere is studied with Navier-Stokes and parabolized stability equations. Maximum transient kinetic energy growth is found close to the sonic point, which could explain the transition observed in wind tunnel and flight experiments.

Online Calculation for the Optimal Reclosing Time of Transmission Lines

Circuit breakers should be reclosed at the optimal reclosing time in order to enhance the transient stability of a power system after disturbances. This article proposes an online optimal reclosing time calculation based on the extended equal area criterion of transmission lines. A multi-machine system is first clustered into an equivalent two-machine system with a fuzzy c-means algorithm. Then a transient kinetic energy function of system oscillation magnitudes is used to determine the optimal reclosing time in an online manner. The performance of the proposed online calculation method for optimal reclosing time is evaluated on multi-machine power system benchmarks with comparison to the conventional optimal reclosing method. The results have shown that the stability of the system is enhanced by the proposed optimal reclosing time in different case studies. Moreover, the value of optimal reclosing time is smaller with the proposed method compared to the conventional optimal reclosing method. An accurate predictive reclosing time can be obtained by the proposed method.