Baroclinic Term Research Articles

A modified filter-based model for simulation of unsteady cavitating flows around a NACA66 hydrofoil

Unsteady cavitating turbulent flow around a NACA66 hydrofoil was simulated using a mass transfer cavitation model and a modified filter-based turbulence model in this paper. The modified filter-based turbulence model can accurately predict the pressure coefficient in midplane and shedding frequency of the unsteady cloud cavitation than standard [Formula: see text]–[Formula: see text] model and filter-based turbulence model. The time evolution of transient cavitation cloud structure predicted by the three-turbulence model was compared. The result which was predicted by the modified filter-based turbulence model is in good agreement with the experimental results. The time evolution of re-entrant jet had been analyzed. The instantaneous wall-pressure evolution on the suction surface (SS) predicted by the modified filter-based turbulence model had been analyzed. The cavitation-vortex interaction had been analyzed in this study. The different effects on the cavitation-vortex interaction of the vortex stretching term, vortex dilatation term and baroclinic torque term in the transport equation of vorticity had been discussed.

LES of tip-leakage cavitating flow with special emphasis on different tip clearance sizes by a new Euler-Lagrangian cavitation model

In the present paper, a new cavitation model proposed by Cheng et al. (2020a) combined with LES is utilized to simulate the tip-leakage cavitating flow around a NACA0009 hydrofoil, which considers the effect of non-condensable gas on cavitation. The predicted tip-leakage vortex (TLV) cavitation agrees well with available experimental observations. Based on the numerical results, the influence of different tip clearance sizes on tip-leakage cavitating flow is discussed in detail. It is shown that with the increase of tip clearance size, the intensity of tip-separation vortex (TSV) cavitation decreases, while the intensity of TLV cavitation increases first and then decreases. Their fusion position moves downstream with the increase of tip clearance size. Moreover, the numerical result implies that the size of tip clearance also has an impact on characteristics of sheet cavitation, such as its length and shape. Finally, the vorticity transport equation is utilized to investigate mechanisms of vortex development around TLV cavitation. As tip clearance size increases, the effect of stretching term decreases, while that of dilatation and baroclinic torque terms increases first and then decreases. However, it is found that the stretching term is dominant in all investigated cases regardless of tip clearance size.

Verification and validation of large eddy simulations of turbulent cavitating flow around two marine propellers with emphasis on the skew angle effects

Large eddy simulation (LES) was used to simulate turbulent cavitating flow around a conventional marine propeller (CP) and a highly skewed marine propeller (HSP) with emphasis on the skew angles effects. The LES verification and validation (V&V) analysis was carried out with cavitation influence on the flow structures. The current numerical results demonstrate that LES can give excellent predictions of the transient complex cavitating flows around a CP and a HSP with the numerical results agreeing well with experimental data. This study applies the LES V&V to the cavitating flow around two propellers with a simplified three-equation method. The results show that the LES errors for HSP are smaller than for CP, which is mainly resulted by more skewed blade of HSP than CP. In addition, the cavitation-vortex interactions around the propellers were studied using the relative vorticity transport equation. The results indicate that both the baroclinic torque term and the Coriolis force term have important influences on the vorticity generation and transport in the cavity closure region. Further analyses indicate that most of the important flow structures including the tip vortex, leading edge vortex, trailing vortex and internal jet are reproduced by the current LES simulations. Due to the different geometry features (less skewed blade of CP than HSP), significantly more intense and violent vortical structures and cavitation phenomena are observed on the CP than on the HSP.

Characteristics and dynamics of compressible cavitating flows with special emphasis on compressibility effects

The objective of the paper is to investigate the physics involved in the compressible cavitating flows, with emphasis on the compressibility effects. 3-D numerical simulations were conducted on the open source software platform OpenFOAM, using both the native incompressible cavitation solver interPhaseChangeFOAM and implemented compressible cavitation solver, where the cavitation model and turbulence model are kept the same and differences of the two approaches mainly root in the density variances of pure liquid and pure vapor. Results are presented for the transient sheet/cloud cavitating flows around a Clark-Y hydrofoil fixed at attack of angle α=8° at inlet velocity U=10m/s and cavitation number σ=0.8, where both ensemble averaged statistics and transient characteristics are analyzed. Good agreement can be obtained using both the incompressible and compressible approaches when compared with the experiment data. While it is found that compared with the incompressible approach, the compressible approach can predict the unsteady cavitation evolution and cavity shedding frequency better. With the compressibility effects considered, the time averaged void fraction distribution decreases, and the cavity size (i.e. cavitation area) becomes smaller. The re-entrant flow thickness normalized by local cavity thickness predicted by the compressible approach is larger than that by the incompressible approach, indicating that the compressible approach can predict the re-entrant jet dynamics well. The velocity divergence analysis show that compared with that in incompressible approach, where velocity divergence mainly comes from the mass transfer between phases, in compressible approach, the velocity divergence originates from both the cavitation two-phase fluid compressibility and mass transfer, and the fluid density variance dominates in compressible results. Following, the budget analysis of vorticity transport equation (VTE) show that the vortex stretching term dominates the cavitation vortex dynamics. Compressibility effects will significantly increase the dilatation term and decrease baroclinic term by decreasing the misalignment between density gradient and pressure gradient. Finally, the temperature and density variance in different cavitation structures are presented.

Numerical investigation on inter-blade cavitation vortex in a Franics turbine

Inter-blade cavitation vortex is substantially considered as a particular cavitation flowing phenomenon associated with liquid transportation in Francis turbine. It causes several adverse effects on the pressure and velocity fields and cannot be eliminated by hydraulic design or optimization. This paper presents the numerical and experimental investigations on cavitation fluid for a reduced scale model of Francis turbine. The inter-blade cavitation vortex structure predicted by numerical simulation yields a very good validation against the experimental visualization. The vapor volume caused by cavitation flowing oscillates periodically and is accompanied by the precessing frequency of inter-blade vortex that is equivalent to the rotational frequency of the runner. Flow separation induced by negative incident angle at the leading edge of runner is identified as the main reason for the incipient and development of the inter-blade cavitation vortex. Cavitation-vortex interaction analysis in terms of the relative vorticity transport equation evidently shows that the vortex stretching term and Coriolis force term always significantly influence the vorticity production near the suction side of the runner blades while the dilatation term and baroclinic torque term play decisive roles on vorticity development adjacent to the vortex center.

Influence of inlet vorticity and aspect ratio on axis-switching and mixing characteristics of heated rectangular jets

To investigate interactive characteristics between the inlet condition and axis-switching phenomenon, heated rectangular jets are numerically investigated for seven aspect ratios (AR = 1.0 − 6.0) with three jet temperatures of 300, 500, and 1000 K. The corresponding Reynolds number is 11,400 – 21,000. To examine the relative importance of ωθ and ωx on axis-switching, seven inlets are obtained by an inflow generator. For the flows of nozzle part, various geometry-driven and turbulence-driven vortices are observed. It is shown that the rotational directions of turbulence-driven vortices have the rotational direction hindering the axis-switching flow. By using the crossover location of the jet half-widths and the mixing length, the axis-switching condition depending on AR and jet temperature is explained. The crossover location is changed to the ωx-variation, which is approximately eight times as sensitive as the ωθ-variation. Based on the results of AR and temperature changes, the strong influence on axis-switching flow and mixing is confirmed for the ωx-change. Also, the heat transfer effect on mixing enhancement becomes strong for the weak axis-switching condition. As AR increases, the mixing performance decreases, but mixing enhancement of the axis-switching flow is confirmed for AR ≤ 4. Finally, to explain the ωx-variation related to the axis-switching mechanism, the ωx transport equations are analyzed by three terms of the Prandtl's first kind and the baroclinic torque. The roles of the vortex stretching and baroclinic torque terms are analyzed for the positive effect of the ωx-variation on the axis-switching phenomenon.

Spatiotemporal dynamics of a buoyancy-driven turbulent fire.

We numerically study the spatiotemporal dynamics and predictability of a buoyancy-driven turbulent fire. A significant transition from order to disorder structures can be observed from the mean degree in the spatial horizontal visibility graph. The gravitational term (baroclinic torque term) in the vorticity equation has a significant impact on the formation of the order (disorder) structure in the near field (far field). The entropy flow transport from temperature to flow velocity fluctuations is predominant near the interface between hot combustion products and ambient air. The transfer entropy is an important measure for determining the predictability of flow velocity fluctuations in the near field obtained by reservoir computing.

Numerical investigation of unsteady cloud cavitating flow around the Clark-Y hydrofoil with adaptive mesh refinement using OpenFOAM

In this paper, the cavitating flow around a three-dimension Clark-Y hydrofoil is investigated by the large eddy simulation (LES) method coupled with the volume of fluid (VOF) model in OpenFOAM. The simulated results are compared with the experimental data and a reasonable agreement is obtained. The cavitation-vortex interaction is studied with the vorticity transport equation and much stronger baroclinic torque term is observed. The adaptive mesh refinement (AMR) method is adopted for improving the accuracy and efficiency of numerical simulation. Comparing with the results presented by the mesh with fine resolution, even the coarse resolution mesh, with AMR method, can precisely characterize identical complex cavity structures and flow details. The local high pressure induced by the reverse flow and small residual vortex structures are obtained by the coarse resolution mesh with the AMR method. Further analyzes of the variation trend of total grid numbers using the AMR in one typical cycle show that the rates of the cloud cavitation shedding and collapse are faster than that of the attached cavity growth. This research illustrates that the AMR is a potential approach to accurately model multi-scale cavitating flows and to better understand cavitation mechanism on the premise of limited computational costs.

Numerical modeling of unsteady cavitating flow over a hydrofoil with consideration of surface curvature

In the present study, a new partially averaged Navier-Stokes model based on a modified SST k-ω model (MSST PANS) is proposed with particular emphasis on the capability to predict the cavitating flow by considering the surface curvature effect. The unsteady cavitating flow around a NACA0015 hydrofoil is investigated by numerical simulation using the MSST PANS model, the SST k-ω model, and the SST k-ω PANS (SST PANS) model. Results indicate that the MSST PANS model predicts the more accurate cavitation shedding dynamics including the cavitation inception, development, split and collapse than the SST k-ω and the SST PANS models. The analysis using the vorticity transport equation depicts that the vortex dilatation and vortex baroclinic torque terms play the dominant role in producing the cavitating vortex during the inception and development of the cavitation. Further, the cavitating vortex shedding downstream induces severe pressure fluctuations on the hydrofoil surface near the trailing edge of the hydrofoil. The dominant frequency predicted by the MSST PANS model is in accordance with the frequency of the cavitating vortex evolution. The study confirms that the MSST PANS model is a promising method to simulate the cavitating flow over a hydrofoil with consideration of surface curvature, which performs more accurately with little increase of computational cost.

Analysis of shock-wave diffraction over double cylindrical wedges. Part II: Vorticity generation

The unsteady aspect of turbulent flow structures generated by a shock-wave diffraction over double cylindrical wedges, with initial diffracting angle of 75∘, are numerically investigated by means of two-dimensional high-fidelity numerical simulation. Different incident-shock-Mach numbers, ranging from transonic to supersonic regimes, are considered. Unlike previous studies where only the total vorticity production is evaluated, the current paper offers more insights into the spatio-temporal behavior of the circulation by evaluating the evolution of the instantaneous vorticity equation balance. The results show, for the first time, that the diffusion of the vorticity due to the viscous effects is quite important compared to the baroclinic term for low Mach numbers regimes, while this trend is inverted for higher Mach numbers regimes. It is also found that the stretching of the vorticity due to the compressibility effects plays an important role in the vorticity production. In terms of pressure impulses, the effect of the first concave surface on the shock strength has been quantified at both earlier and final stages of the shock diffraction process. Unlike the overpressure, the static and the dynamic pressure impulses are shown to be significantly reduced at the end of the first concave surface.

Sulfur hexafluoride bubble evolution in shock accelerated flow with a transverse density gradient

Sulfur hexafluoride (SF6) is a colorless, odorless, non-toxic, non-flammable stable gas, which has been widely adopted as the heavy gas in the Richtmyer–Meshkov instability study. In this paper, a computational analysis of SF6 bubble evolution in shock-accelerated flow with a transverse density gradient is presented. The influences of different incident shock Mach numbers on various interactions were clarified using high-resolution computation schemes. The results showed that the incident shock wave becomes curved during propagation because of the transverse density gradient. Based on this, two separate shock-focusing processes were identified when Ma = 1.21 and three separate shock-focusing processes were identified when Ma = 2.0. However, the shock-focusing intensity was weaker than previously observed in a flow field with a uniform density distribution. High- and relevant-pressure impingement played vital roles in the formation of three jets near the downstream pole of the SF6 bubble in both cases. In addition, impingement by incident and reflected shocks could induce additional vorticities in the bubble region and promote increased bubble volumes, but these increased bubble volumes could weaken the average vorticity. Upon increasing the incident shock Mach number, the effective bubble volume decreased with the enhanced shock intensity, but the vorticities were strengthened. Furthermore, analyzing the factors that affected vorticity evolution allowed us to find that the compression term had a stronger influence on vorticity evolution than the baroclinic term or the viscosity term. All of these studies complement the Richtmyer–Meshkov instability study.

Dynamical connection between the stratospheric Arctic vortex and sea surface temperatures in the North Atlantic

We used two long-term reanalysis datasets and time-slice simulations to examine the decadal relationship between the stratospheric Arctic vortex (SAV) and sea surface temperature anomalies (SSTAs) in the North Atlantic and the dynamic mechanisms involved in the linkage between the two. Our results show that there is a significant decadal linkage between SSTAs over the North Atlantic and the SAV, where warmed (cooled) SSTAs over the North Atlantic in association with its principal mode correspond to a weakened (strengthened) SAV. The warmed North Atlantic SSTAs tend to result in a weakened SAV via two dynamic processes: (1) constructive interference at high latitudes with a ridge in the Atlantic sector and a trough in the Pacific accompanied by a negative North Atlantic Oscillation-like pattern over the North Atlantic and a weakened Aleutian low over the North Pacific; and (2) more wavenumber-1 waves propagated into the Arctic stratosphere by modifying the baroclinic term of the zonal mean background state and altering the propagating conditions around the tropopause over the Arctic. Results from reanalysis and model simulations both suggest that a strengthening wave intensity in the high-latitude troposphere and more upward propagation of the planetary wavenumber-1 wave in response to the warmed North Atlantic SSTAs conjunctly contribute to the increased planetary wave flux in the Arctic stratosphere, facilitating a weakened SAV. These results provide a new understanding of what dynamic processes control the SAV, and will help to predict the stratosphere on decadal timescales.

Turbulent structures of shock-wave diffraction over 90° convex corner

The turbulent structures and long-time flow dynamics of shock diffraction over 90° convex corner associated with an incident shock Mach number Ms = 1.5 are investigated by large eddy simulation (LES). The average evolution of the core of the primary vortex is in agreement with the previous two dimensional studies. The Type-N wall shock structure is found to be in excellent agreement with the previous experimental data. The turbulent structures are well resolved and resemble those observed in the experimental findings. Subgrid scale dissipation and subgrid scale activity parameter are quantified to demonstrate the effectiveness of the LES. An analysis based on turbulent-nonturbulent interface reveals that locally incompressible regions exhibit the universal teardrop shape of the joint probability density function of the second and third invariants of the velocity gradient tensor. Stable focus stretching (SFS) structures dominate throughout the evolution in these regions. Stable node/saddle/saddle structures are found to be predominant at the early stage in locally compressed regions, and the flow structures evolve to more SFS structures at later stages. On the other hand, the locally expanded regions show a mostly unstable nature. From the turbulent kinetic energy, we found that the pressure dilatation remains important at the early stage, while turbulent diffusion becomes important at the later stage. Furthermore, the analysis of the resolved vorticity transport equation reveals that the stretching of vorticity due to compressibility and stretching of vorticity due to velocity gradients plays an important role compared to diffusion of vorticity due to viscosity as well as the baroclinic term.

Numerical investigation on cavitating jet inside a poppet valve with special emphasis on cavitation-vortex interaction

A thorough understanding of the internal flow dynamics inside a poppet valve makes sense for an improved control performance. Hence, a numerical simulation for the cavitating jet is performed with main concern on the cavitation-vortex interaction. The periodic cavitation structure, along with corresponding variation in vorticity distribution and large scale eddy, is presented for the quasi-steady flow from numerical results. As expected, cavitation inception exhibits great dependence on coherent structure, which is organized as paired vortex rings. Besides, cavitation is persistently located at core of vortex, indicating strong correlation between cavitation and vortex dynamics. The cavitation-vortex correlation is further discussed with analysis of vorticity transport equation, involving vortex stretching, dilatation and baroclinic torque terms. At incepting stage, vortex stretching is the dominant factor, responsible for vortex growth and the elliptical shape of cavitation ring. In comparison, the dilatation term could produce either enhancement or suppression in local vorticity, depending on the volumetric variation induced by cavitation. During the collapse stage, the bubble collapse creates baroclinic vorticity, and contributes to 3-dimensional vorticity. The most significant finding of the present study is the suppression of vorticity during the growing process of cavitation, contributing to an enhanced understanding of the cavitation-vortex interaction.

Development of high resolution methods for analyzing multi-fluid problems

High resolution multi-fluid solvers are developed, using a fifth order weighted essentially non-oscillatory approach for spatial reconstruction and a third order Runge-Kutta scheme for temporal integration. Several flux evaluation schemes, comprising of exact and approximate Riemann solvers available in the literature are incorporated. Multi-fluid simulation capability is achieved using the ghost fluid method. Two-dimensional geometry is considered, and the Euler equations of gas dynamics are used as the basic flow model in view of the fact that the viscosity effects are negligible for most of the time of interest. Relative performance of these solvers is evaluated for several one- and two-dimensional test cases, before simulating the shock-bubble interaction. Three Mach numbers and air-Helium gas combination is studied and the results are presented in this study. Numerical schlieren images are rendered for qualitative study while time evolution of several integral features such as the axial and lateral deformations, the translational velocity of the bubble, the circulation in the flow-field, and the stretching rates of the bubbles are analyzed for quantitative studies. The results are consistent with the strength of baroclinic source term.

Relations of the Low-Level Extratropical Cyclones in the Southeast Pacific and South Atlantic to the Atlantic Multidecadal Oscillation

AbstractThe relations of the low-level extratropical cyclones in the southeastern Pacific and South Atlantic with the sea surface temperature (SST) anomalies associated with the Atlantic multidecadal oscillation (AMO) during the summer and winter of the 1979–93 cold AMO (CAMO) and 2003–17 warm AMO (WAMO) are analyzed. During both seasons and in both AMO phases, the cyclone trajectories defined by cyclone local counts exceeding 10 events per grid box occur approximately in the areas with the AMO-related positive SST anomalies. The cyclone densities in most latitudes during both seasons are higher in the CAMO than in the WAMO. Thus, the cyclone density in the study domain presents a reduction trend during the 1979–2017 period. The large-scale northward SST anomalous gradients between the bands north and south of 40°S increase the long-wave baroclinicity in the midlatitudes in the WAMO, and the southward SST anomalous gradients decrease it in the CAMO. Consequently, the short-wave baroclinicity is higher in the WAMO than in the CAMO in the southeastern Pacific midlatitudes. Thus, the cyclones are more energetic in the WAMO than in the CAMO. In the South Atlantic region off the Argentinean coast, both the barotropic and baroclinic conversion terms are positive, indicating an increase of the kinetic energy of the short waves. The low-level cyclones in the southeastern Pacific and South Atlantic are modulated by the AMO. As far as we know, the relation of the SH low-level extratropical cyclones to the AMO documented here was not studied before.

A Comparative Study on the Effect of Upstream Trough on Intensity Changes of Two Types of Tropical Cyclones during Extratropical Transition

Based on the China Meteorological Administration (CMA) tropical cyclone (TC) database and the reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), the post-transition intensifying tropical cyclones (ITC) and weakening tropical cyclones (WTC) which landed in China and underwent extratropical transition (ET) are discussed in this paper. The TCs with upper-level trough are selected in order to identify different effects of the trough on the ITC and the WTC. The dynamic composite analysis is applied to explore their structure characteristics, environment fields and dynamic diagnosis. Results show that affected by the South Asia high and the subtropical high, the trough of ITC (WTC) extends from northwest to southeast (northeast to southwest) in the ET process. Thus, the zonal wind shear of ITC drops off after ET due to the approach of trough and its northwest-southeast direction, while the zonal and total wind shears of WTC continue to increase because of the steering westerly flow at the upper level. In terms of the ITC, the cold air carried by the upper-level trough intrudes into TC inner area and mostly encircles the TC center, making the ITC characterized by a warm seclusion. While for the WTC, the cold air only wanders on the northwest side of TC without further intrusion. The upper-level divergence is also in favor of the ITC by the pumping influence. According to the diagnostic analysis of moist potential vorticity, the moist baroclinity can lead to changes in vertical vorticity to some extent. The vertical vorticity budget analysis further indicates that there is stronger and wider positive vorticity advection in the upper troposphere near the TC center for ITC. The contribution of the baroclinic term to the growth of vertical vorticity is more significant in ITC than WTC but it is also deeply influenced by the strength of upper-level trough.

A direct numerical simulation study of the influence of flame-generated vorticity on reaction-zone-surface area in weakly turbulent premixed combustion

Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., σ = 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.

Pressure fluctuation–vortex interaction in an ultra-low specific-speed centrifugal pump

To provide a comprehensive understanding of the pressure fluctuation–vortex interaction in non-cavitation and cavitation flow, in this article, the unsteady flow in an ultra-low specific-speed centrifugal pump was investigated by numerical simulation. The uncertainty of the numerical framework with three sets of successively refined mesh was verified and validated by a level of 1% of the experimental results. Then, the unsteady results indicate that the features of the internal flow and the pressure fluctuation were accurately captured in accordance with the closed-loop experimental results. The detailed pressure fluctuation at 16 monitoring points and the monitoring of the vorticity suggest that some inconsistent transient phenomena in frequency spectrums show strong correlation with the evolution of vortex, such as abnormal increasing amplitudes at the monitoring points near to the leading edge on the suction surface and the trailing edge on the pressure surface in the case of lower pressurization capacity of impeller after cavitation. Further analysis applies the relative vortex transport equation to intuitionally illustrate the pressure fluctuation–vortex interaction by the contribution of baroclinic torque, viscous diffusion and vortex convection terms. It reveals that the effect of viscous diffusion is weak when the Reynolds number is much greater than 1. Pressure fluctuation amplitude enlarges on the suction side of blade near to the leading edge due to the baroclinic torque in cavitation regions, whereas the abnormal increase of pressure fluctuation after cavitation on the pressure surface of blade approaching the trailing edge results from the vortex convection during vortices moving downstream with the decrease of available net positive suction head at the same instance.

The energy cascade associated with daily variability of the North Atlantic Oscillation

An analysis of anomalies of the energy cascade associated with daily fluctuations of the North Atlantic Oscillation (NAO) is presented here. The analysis consists of a detailed version of the Lorenz energy cycle, which decomposes the energy flows into baroclinic and barotropic terms and into zonal mean and eddy components, and was applied to the 6‐hourly ERA‐Interim reanalysis for the period of 1979–2016. The obtained results show that the positive NAO phase is preceded by a significant increase of synoptic baroclinic eddy activity. The eddy available potential energy is converted into kinetic energy and transferred to barotropic synoptic eddies. Then, the kinetic energy is transferred upscale into the barotropic planetary waves, which produce the NAO pattern. Immediately after the positive NAO peak, there is a significant reduction of baroclinic activity, and no indication of an anomalous increase of the global‐mean baroclinic eddy activity in the 10‐day period after the peak was found. At longer lags, between 10 and 20 days, the kinetic energy of barotropic eddies shows positive anomalies consistent with the eddy–zonal mean flow feedback. We conclude that the synoptic baroclinic eddy activity forces the NAO variability, and no modulating role of the NAO in the global‐mean baroclinic eddy activity (storminess) occurs.