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Large eddy simulation of turbulent cavitating flow in a venturi nozzle is conducted. The fully compressible Favre-filtered Navier-Stokes equations are coupled with a homogeneous equilibrium cavitation model The dynamic Smagorinsky subgrid-scale turbulence model is employed to close the filtered nonlinear convection terms. The equations are numerically integrated in the context of a generalized curvilinear coordinate system to facilitate geometric complexities. A sixth-order compact finite difference scheme is employed for the Navier-Stokes equations with the AUSM + -up scheme to handle convective terms in the presence of large density gradients. The stiffness of the system due to the incompressibility of the liquid phase is addressed through an artificial increase in the Mach number. The simulation predicts the formation of a vapor cavity at the venturi throat with an irregular shedding of the small scale vapor structures near the turbulent cavity closure region. The vapor formation at the throat is observed to suppress the velocity fluctuations due to turbulence. The collapse of the vapor structures in the downstream region is a major source of vorticity production, resulting into formation of hairpin vortices. A detailed analysis of the vorticity transport equation shows a decrease in the vortex-stretching term due to cavitation. A substantial increase in the baroclinic torque is observed in the regions where the vapor structures collapse. A spectra of the pressure fluctuations in the far-field downstream region show an increase in the acoustic noise at high frequencies due to cavitation. [DOI: 10.1115/1.4001971]

It is shown that a quantitative relation exists between the stratospheric polar cap potential vorticity and the 100-hPa eddy heat flux. A difference in potential vorticity between years is found to be linearly related to the flux difference integrated over time, taking into account a decrease in relaxation time scale with height in the atmosphere. This relation (the PV--flux relation) is then applied to the 100-hPa flux difference between 2008/09 and the climatology (1989-2008) to obtain a prediction of the polar cap potential vorticity difference between the 2008/09 winter and the climatology. A prediction of the 2008/09 polar cap potential vorticity is obtained by adding this potential vorticity difference to the climatological potential vorticity. The observed polar cap potential vorticity for 2008/09 shows a large and abrupt change in the potential vorticity in midwinter, related to the occurrence of a major sudden stratospheric warming in January 2009; this is also captured by the potential vorticity predicted from the 100-hPa flux and the PV--flux relation. The results of the mean PV--flux relation show that, on average, about 50% of the interannual variability in the state of the Northern Hemisphere stratosphere is determined by the variations in the 100-hPa heat flux. This explained variance can be as large as 80% for more severe events, as demonstrated for the 2009 major warming. DOI: 10.1175/2010JAS3569.1

The sensitivity of the midlatitude storm track and eddy-driven wind to the sea surface temperature (SST) boundary forcing is studied over a wide range of perturbations using both simple and comprehensive general circulation models over aquaplanet lower boundary conditions. Under the single-jet circulation regime similar to the conditions of the present climate in the Northern Hemisphere winter or the Southern Hemisphere summer, the eddy-driven jet shifts monotonically poleward with both the global mean and the equator-to-pole gradient of the SST. The eddy-driven jet can have a reverse relationship to the gradient if it is well separated from the subtropical jet and Hadley cell boundary in a double-jet circulation regime. A simple scaling is put forward to interpret the simulated sensitivity of the storm-track/eddy-driven westerly wind position within the single-jet regime in both models. The rationale for the scaling is based on the notion that the wave activity flux can propagate horizontally away from the source region, resulting in a broader distribution of eddy potential vorticity (PV) flux in the upper troposphere than that of the flux in the opposite direction in the lower troposphere. As a consequence, the position of the maximum of the eddy-driven westerlies tends to be controlled by the profile of the relatively sharp-peaked low-level PV flux, which is dominated by the eddy heat flux component of the Eliassen-Palm (EP) flux. Thus, the position of the eddy-driven surface westerlies may be inferred from the vertical EP flux coming out of the lower troposphere. The vertical EP flux can be parameterized by a measure of baroclinicity, whose latitudinal variations show a linear relationship with the meridional displacement of the eddy-driven westerlies and the storm track. This relationship still holds well within the single-jet regime, even when only the variation of static stability is taken into consideration in estimating the baroclinicity (the temperature gradient component of which is fixed). To the extent that the static stability is deterministically constrained by and hence can be predicted from the given SST conditions through a moist scaling for the midlatitude stratification, one may, given SST perturbations, predict which way the storm track and eddy-driven wind should shift with respect to a chosen reference climate state. The resultant anomaly-wise scaling turns out to be valid for both the idealized and comprehensive models, regardless of the details in the model physics. By corollary, it can be argued that the poleward shift of storm track found in the global warming simulations by fully coupled climate models may be attributed, at least partially, to the increase in the subtropical and midlatitude static stability with global warming. DOI: 10.1175/2010JAS3477.1

Previous large-eddy simulations (LES) of stratocumulus-topped boundary layers have been exclusively set in marine environments. Boundary layer stratocumulus clouds are also prevalent over the continent but have not been simulated previously. A suite of LES runs was performed for a case of continental post-cold-frontal stratocumulus observed by the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF), located in northern Oklahoma. Comparison with fixed, ground-based sensors necessitated an Eulerian approach in which it was necessary to supply to the model estimates of synoptic-scale advection and vertical motion, particularly given the quickly evolving, baroclinic nature of the synoptic environment. Initial analyses from the Rapid Update Cycle model supplied estimates for these forcing terms. Turbulent statistics calculated from the LES results are consistent with large-eddy observations obtained from millimeter-wave cloud radar. The magnitude of turbulence is weaker than in typical marine stratocumulus, a result attributed to highly decoupled cloud and subcloud circulations associated with a deep layer of negative buoyancy flux arising from the entrainment of warm, free-tropospheric air. Model results are highly sensitive to variations in advection of temperature and moisture and much less sensitive to changes in synoptic-scale vertical velocity and surface fluxes. For this case, moisture and temperature advection, rather than entrainment, tend to be the governing factors in the analyzed cloud system maintenance and decay. Typical boundary layer entrainment scalings applied to this case do not perform very well, a result attributed to the highly decoupled nature of the circulation. Shear production is an important part of the turbulent kinetic energy budget. The dominance of advection provides an optimistic outlook for mesoscale, numerical weather prediction, and climate models because these classes of models represent these grid-scale processes better than they do subgrid-scale processes such as entrainment. DOI: 10.1175/2010JAS3467.1

Large-eddy simulations were carried out in order to numerically describe plasma-based control of turbulent boundary-layer separation. The configuration consisted of a flat-plate section, over which the boundary layer developed, followed by a curved convex rearward-facing ramp, corresponding to an experimental arrangement. A single dielectric-barrier-discharge plasma actuator was then employed to reduce the extent of the separated flow region. Solutions were obtained to the Navier--Stokes equations, which were augmented by source terms used to represent plasma-induced body forces imparted by the actuator on the fluid. A simple phenomenological model provided the electric field generated by the plasma, resulting in the body forces. The numerical method used a high- fidelity time-implicit scheme, employing domain decomposition in order to perform calculations on a parallel computing platform. Simulations were first conducted for an isolated actuator, which were used to optimize parameters inherent in the plasma model. Subsequent calculations were then carried out for the plate development section and for the plate/ramp combination, using both continuous and pulsed control. All simulations were compared with experimental measurements, which indicated that, although some deficiencies were apparent in the plasma model, they appeared to be adequate for use with large-eddy simulation in the exploration of plasma-based control for turbulent wall-bounded flows. DOI: 10.2514/1.J050014

Large-eddy simulation of flow over an open cavity corresponding to the experimental setup of Liu and Katz (2008, 'Cavitation Phenomena Occurring Due to Interaction of Shear Layer Vortices With the Trailing Corner of a Two-Dimensional Open Cavity,' Phys. Fluids, 20(4), p. 041702)is performed. The filtered, incompressible Navier--Stokes equations are solved using a co-located grid finite-volume solver with the dynamic Smagorinsky model for a subgrid-scale closure. The computational grid consists of around 7 x [10.sup.6] grid points with 3 x [10.sup.6] points clustered around the shear layer, and the boundary layer over the leading edge is resolved. The only input from the experimental data is the mean. velocity profile at the inlet condition. The mean flow is superimposed with turbulent velocity fluctuations generated by solving a forced periodic duct flow at a freestream Reynolds number. The flow statistics, including mean and rms velocity fields and pressure coefficients, are compared with the experimental data to show reasonable agreement. The dynamic interactions between traveling vortices in the shear layer and the trailing edge affect the value and location of the pressure minima. Cavitation inception is investigated using two approaches: (i) a discrete bubble model wherein the bubble dynamics is computed by solving the Rayleigh--Plesset and the bubble motion equations using an adaptive time-stepping procedure and (ii) a scalar transport model for the liquid volume fraction with source and sink terms for phase change. Large-eddy simulation, together with the cavitation models, predicts that inception occurs near the trailing edge similar to that observed in the experiments. The bubble transport model captures the subgrid dynamics of the vapor better, whereas the scalar model captures the large-scale features more accurately. A hybrid approach combining the bubble model with the scalar transport is needed to capture the broad range of scales observed in cavitation. [DOI: 10.1115/1.4002744]

A large-domain large-eddy simulation of a tropical deep convection system is used as a benchmark to derive and test a mixed subgrid-scale (SGS) scheme for scalar and momentum fluxes in cloud-resolving models (CRMs). The benchmark simulation resolves a broad range of scales ranging from mesoscale organizations, through gravity waves and individual clouds, down to energy-containing turbulent eddies. A spectral analysis shows that the vertical-velocity kinetic energy peaks at scales from hundreds of meters in the lower cloud layer to several kilometers higher up: these scales are typical grid sizes of today's CRMs. The analysis also shows that a significant portion of the scalar and momentum fluxes in the benchmark simulation are carried by motions smaller than several kilometers (i.e., smaller than a typical grid resolution of CRMs). The broad range of scales of the benchmark simulation is split into two components: filter scale (mimicking CRM resolvable scale) and subfilter scale (mimicking CRM SGS), using filter widths characteristic of a typical CRM grid spacing. The local relationship of the subfilter-scale fluxes to the filter-scale variables is examined. This leads to a mixed SGS scheme to represent the SGS fluxes of scalars and momentum in CRMs. A priori tests show that the mixed SGS scheme yields spatial distributions of subfilter-scale fluxes that correlate much better with those retrieved from the benchmark when compared with an eddy viscosity/diffusivity scheme that is commonly used in today's CRMs. DOI: 10.1175/2010JAS3565.1

This paper describes the numerical and experimental study of the control of the trajectory and spreading of an air-assisted water spray. The configuration is a coaxial set up where the liquid injected by the central tube is atomized by a surrounding high-speed annular air flow. This spray is actuated in its outlet plane using additional air jets to modify either its trajectory or its spreading angle (by adding swirl). Large eddy simulation is used jointly with experimental measurements (hot-wire anemometry for the air flow without spray and phase Doppler anemometry for the spray cases) to analyze the flow with and without control. Results show that 1) large eddy simulation of two-phase flows using a Lagrangian solver is a precise method to predict such flows, and 2) the spray jet can be effectively modified using simple actuators, suggesting that the same devices applied to fuel sprays in combustion chambers, would be an effective active control actuator. Large eddy simulation results also reveal the complex flow patterns and effects on droplet distribution induced by the actuators and appear as an excellent tool to complement and understand experimental data. DOI: 10.2514/1.J050416

More studies on the dynamics of marine stratus and stratocumulus clouds have been performed than comparable studies on continental stratocumulus. Therefore, to increase the number of observations of continental stratocumulus and to compare marine and continental stratocumulus to each other, the approach of large-eddy observation (LEO) was applied to a case of nocturnal continental stratocumulus observed over the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) in the central United States on 8 April 2006. The stratocumulus occurred in cold-air and dry-air advection behind a surface cold front. LEOs were obtained from millimeter-wavelength cloud radar and micropulse lidar, whereas traditional meteorological observations described the synoptic environment. This study focuses on a 9-h period of a predominantly nonprecipitating stratocumulus layer 250-400 m thick. A slight thinning of the cloud layer over time is consistent with dry-air advection. A deep layer of descent overlaid a shallower layer of ascent from the surface up to 800 rob, providing a mechanism for strengthening the inversion at cloud top. Time series of Doppler velocity indicate vertically coherent structures identifiable throughout much of the cloud layer. The magnitude of turbulence, as indicated by the variance of the vertical velocity, was weak relative to typical marine stratocumulus and to the one other case of continental stratocumulus in the literature. Conditional sampling of the eddy structures indicate that strong downdrafts were more prevalent than strong updrafts, and negative skewness of vertical velocity in the cloud implies an in-cloud circulation driven by longwave cooling at cloud top, similar to that in marine stratocumulus. DOI: 10.1175/2010JAS3389.1

A physically realistic roughness closure method for the simulation of turbulent open-channel flow over natural beds within the framework of large-eddy simulation (LES) is proposed. The description of bed roughness in LES is accomplished through a roughness geometry function together with forcing terms in the momentum equations. The major benefit of this method is that the roughness is generated from one physically measurable parameter, i.e., the mean grain diameter of the bed material. A series of flows over rough beds, for which mean flow and turbulence statistics are available from experiments, is simulated. Measured and computed values are compared to validate the proposed roughness closure approach. It is found that predicted streamwise velocity profiles, turbulence intensities, and turbulent shear stress profiles match the measured values fairly well. Furthermore, the effect of roughness on the overall flow resistance is predicted in reasonable agreement with experimental values. DOI: 10.1061/(ASCE)HY.1943-7900.0000236 CE Database subject headings: Channel flow; River beds; Turbulence; Bed roughness. Author keywords: LES; Rough bed; Turbulence; Roughness closure.

The ability of eddy-viscosity models to simulate the turbulent wake produced by cross-flow separation over a curved body of revolution is assessed. The results obtained using the standard k - [omega] model show excessive levels of turbulent kinetic energy k in the vicinity of the stagnation point at the nose of the body. Additionally, high levels of k are observed throughout the wake. Enforcing laminar flow upstream of the nose (which replicates the experimental apparatus more accurately) gives more accurate estimates of k throughout the flowfield. A stress limiter in the form of Durbin's T-limit modification for eddy-viscosity models is implemented for the k - [omega] model, and its effect on the computed surface pressures, skin friction, and surface flow features is assessed. Additionally, the effect of the T-limit modification on both the mean flow and the turbulent flow quantities within the wake is also examined. The use of the T-limit modification gives significant improvements in predicted levels of turbulent kinetic energy and Reynolds stresses within the wake. However, predicted values of skin friction in regions of attached flow become up to 50% greater than the experimental values when the T-limit is used. This is due to higher values of near-wall turbulence being created with the T-limit. [DOI: 10.1115/1.4001967]

An exact diagnostic formalism for finite-amplitude eddy--mean flow interaction is developed for barotropic and quasigeostrophic baroclinic flows on the beta plane. Based on the advection--diffusion--reaction equation for potential vorticity (PV), the formalism quantifies both advective and diffusive contributions to the mean flow modification by eddies, of which the latter is the focus of the present article. The present theory adopts a hybrid Eulerian--Lagrangian-mean description of the flow and defines finite-amplitude wave activity in terms of the areal displacement of PV contours from zonal symmetry. Unlike previous formalisms, wave activity is readily calculable from data and the local Eliassen---Palm relation does not involve cubic or higher-order terms in eddy amplitude. This leads to a natural finite-amplitude extension to the local nonacceleration theorem, as well as the global stability theorems, in the inviscid and unforced limit. The formalism incorporates mixing with effective diffusivity of PV, and the diffusive flux of PV is shown to be a sink of wave activity. The relationship between the advective and diffusive fluxes of PV and its implications for parameterization are discussed in the context of wave activity budget. If all momentum associated with wave activity were returned to the zonal-mean flow, a balanced eddy-free flow would ensue. It is shown that this hypothetical flow [u.sub.REF] is unaffected by the advective PV flux and is driven solely by the diffusive PV flux and forcing. For this reason, [u.sub.REF], rather than the zonal-mean flow, is proposed as a diagnostic for the diffusive mean-flow modification. The formalism is applied to a freely decaying beta-plane turbulence to evaluate the contribution of the diffusive PV flux to the jet formation. DOI: 10.1175/2010JAS3432.1

We study time and length scales of eddy dispersion in 100 [micro]m i.d. capillary silica monoliths. First, the monolith's macropore morphology was visualized over complete column cross sections by confocal laser scanning microscopy, revealing a wall region with large voids (with a lateral dimension of up to ~ 15 [micro]m) and a homogeneous core region. A hulk segment from the core region was then physically reconstructed to receive a 60 x 12 x 12 [micro]m matrix consisting of ~3 x [10.sup.8] cubic voxels of 30 nm edge length for direct numerical simulations of fluid flow by the lattice-Boltzmann method and convective-diffusive mass transport by a random-walk particle-tracking technique on a high-performance computing platform. Pore-scale dispersion was analyzed in detail using the generalized Giddings equation. Eddy dispersion contributions originating in the bulk macropore heterogeneity were quantified and correlated with structural features of the monolith. To complement the simulation results, column scale dispersion was investigated by analysis of chromatographic plate heights. We found a much smaller bulk dispersion than generally assumed for silica monoliths (plate heights of ~2 pm over a wide velocity range), promising excellent separation efficiency also at high flow velocities (~1 cm/s). This potential is not realized by the capillary monolith in chromatographic practice because of the wall defect. 10.1021/ac101393b

Baroclinic eddy equilibration under a Northern Hemisphere-like seasonal forcing is studied using a modified multilayer quasigeostrophic channel model to investigate the widely used 'quick baroclinic eddy equilibration' assumption and to understand to what extent baroclinic adjustment can be applied to interpret the midlatitude climate. Under a slowly varying seasonal forcing, the eddy and mean flow seasonal behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in summer, a mainly dynamically determined eddy spinup time interval starting in midfall and lasting less than one month, and a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring, with a mainly external forcing determined spindown time interval for eddy activity from late winter to late spring. The baroclinic adjustment can be clearly observed from late fall to late spring. The sensitivity study of the eddy equilibration to the time scale of the external forcing indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycles one year and longer. In spite of the strong seasonality of the eddy activity, similar to the observations, a robust potential vorticity (PV) structure is still observed through all the seasons. However, it is found that baroclinic eddy is not the only candidate mechanism to maintain the robust PV structure. The role of the boundary layer thermal forcing and the moist convection in maintaining the lower-level PV structure is discussed. The adjustment and the vertical variation of the lower-level stratification play an important role in all of these mechanisms. DOI: 10.1175/2010JAS3392.1

There are three distinct processes by which upward-propagating gravity waves influence the large-scale dynamics and energetics of the middle atmosphere: (i) nonlocalized transport of momentum through wave propagation in three dimensions that remotely redistributes atmospheric momentum in both zonal and meridional directions from wave generation to wave dissipation regions; (ii) localized diffusive transport of momentum, heat, and tracers due to mixing induced by wave breaking; and (iii) localized transport of heat by perturbing wave structures due to dissipation that redistributes the thermal energy within a finite domain. These effects become most significant for breaking waves when momentum drag, eddy diffusion, and wave heating-- the 'breaking trinity'--are all imposed on the background state. This paper develops a 3D parameterization scheme that self-consistently includes the breaking trinity in large-scale numerical models. The 3D parameterization scheme is developed based on the general relationship between the wave action flux and the subgrid-scale momentum and heat fluxes developed by Zhu in 1987 and a mapping approximation between the wave source spectrum and momentum deposition distribution developed by Alexander and Dunkerton in 1999. For a set of given input wind and temperature profiles at each model grid, the parameterization scheme outputs the vertical profiles of the subgrid-scale force terms together with the eddy diffusion coefficients in the momentum and energy equations for a 3D background flow. DOI: 10.1175/2010JAS3302.1

A two-layer quasigeostrophic model is used to study the effect of lower boundary Ekman pumping on the energetics of baroclinic waves. Although the direct impact of the Ekman pumping is to damp the total eddy energy, either the eddy available potential energy (EAPE) or the eddy kinetic energy (EKE), individually, can grow because of the Ekman pumping. Growth of EAPE is favored if the phase difference between the upper and lower wave fields is less than a quarter wavelength, and EKE is favored if the phase difference is greater than a quarter wavelength. A numerical model calculation shows that the EAPE growth occurs directly through the Ekman pumping and that the increased EAPE can in turn lead to further growth by strengthening the baroclinic energy conversion from zonal available potential energy to the EAPE. Through this indirect effect, the Ekman pumping can increase the net production of total eddy energy. DOI: 10.1175/2010JAS3295.1

Synoptic eddy--mean flow interaction has been recognized as one of the key sources for extratropical low-frequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and low-frequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability. DOI: 10.1175/2009JAS3185.1

The objective of the current work is to shed light on simulating the flow features of nonpremixed flame stabilized by a circular-disk bluff-body with large eddy simulation technique. Two subgrid scale (SGS) models (Smagorinsky and Germano), combined with a constrained chemical equilibrium model, are applied to simulate this turbulent flame. Validation is made through the particle image velocimetry measurements. The comparison between the numerical simulation and experimental data shows that both models perform well and reproduce most of the significant features of the bluff-body flame, while the Germano SGS model performs better in prediction of turbulent fluctuations. These investigations show that it is possible to describe such flows with relatively simple turbulence and combustion models with moderate grids. [DOI: 10.1115/1.3205029] Keywords: large eddy simulation, bluff-body, nonpremixed combustion

In gas and steam turbine applications a common approach to prevent the blades from high cycle fatigue failures due to high vibration amplitudes is the usage of friction damping elements. Besides the intended amplitude reduction this procedure also features some possibly unwanted side effects like a shift in resonance frequencies due to stiffening effects caused by the contact. Thus, as an alternative an eddy current based noncontacting damping concept for the application in turbomachinery is investigated. In this paper two different types of eddy current dampers are considered. Theoretical models for both are established by applying electromagnetic-mechanical theory. The theoretical models are compared with forced response measurements that are performed at a stationary test rig. [DOI: 10.1115/1.3205032]

Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which thermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state. The latitude [[phi].sub.0] of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time to study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are compared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations for all [[phi].sub.0] approach the nearly inviscid, angular-momentum-conserving limit, despite the presence of finite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting simulations, the Hadley circulations undergo a regime transition as [[phi].sub.0] is increased, from an equinox regime (small [[phi].sub.0]) in which eddy momentum fluxes strongly influence both Hadley cells to a solstice regime (large [[phi].sub.0]) in which the cross-equatorial winter Hadley cell more closely approaches the angular-momentum-conserving limit. In axisymmetric time-dependent simulations, the Hadley cells undergo transitions between a linear equinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddy-permitting simulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in the axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddy-permitting simulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum budget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the eddy-permitting simulations, a mean-flow feedback--involving the upper-level zonal winds, the lower-level temperature gradient, and the poleward boundary of the cross-equatorial Hadley cell--makes it possible for the circulation fields to change at the transition more rapidly than can be explained by the steady-state response to the thermal forcing. However, the regime transitions in the axisymmetric simulations are less sharp than those in the eddy-permitting simulations because eddy--mean flow feedbacks in the eddy-permitting simulations additionally sharpen the transitions. DOI: 10.1175/2009JAS3294.1

Large-eddy simulations (LES) and Reynolds-averaged Navier--Stokes (RANS) calculations of the flow in two asymmetric three-dimensional diffusers were performed. The setup was chosen to match an existing experiment with separation. Both diffusers possess the same expansion ratio but differ in performance. The aim of the present study is to find the least expensive method to reliably and with reasonable accuracy account for the impact of the change in geometry. RANS calculations failed to predict both the extent and location of the separation. In contrast, LES with wall-functions delivered results within the accuracy of the experimental data. [DOI: 10.1115/1.4001009]

Reynolds-averaged and hybrid Reynolds-averaged/large-eddy simulations have been applied to a supersonic coaxial jet flow experiment. The experiment was designed to study compressible mixing flow phenomenon under conditions that are representative of those encountered in scram jet combustors. The experiment used either helium or argon as the inner jet nozzle fluid, and the outer jet nozzle fluid consisted of laboratory air. The inner and outer nozzles were designed and operated to produce nearly pressure-matched Math 1.8 flow conditions at the jet exit. The purpose of the computational effort was to assess the state of the art for each modeling approach and to use the hybrid Reynolds-averaged/large-eddy simulations to gather insight into the deficiencies of the Reynolds-averaged closure models. The Reynolds-averaged simulations displayed a strong sensitivity to choice of turbulent Schmidt number. The initial value chosen for this parameter resulted in an overpredictiou of the mixing layer spreading rate for the helium case, but the opposite trend was observed when argon was used as the injectant. A larger turbulent Schmidt number greatly improved the comparison of the results with measurements for the helium simulations, but variations in the Schmidt number did not improve the argon comparisons. The hybrid Reynolds-averaged/large-eddy simulations also overpredicted the mixing layer spreading rate for the helium ease, while underpredicting the rate of mixing when argon was used as the injectant. The primary reason conjectured for the discrepancy between the hybrid simulation results and the measurements centered around issues related to the transition from a Reynolds- averaged state to one with resolved turbulent content. Improvements to the inflow conditions were suggested as a remedy to this dilemma. Second-order turbulence statistics were also compared with their modeled Reynolds-averaged counterparts to evaluate the effectiveness of common turbulence closure assumptions. DOI: 10.2514/1.43771

In this study, large-eddy simulation is used to analyze supersonic flow, mixing, and combustion in a supersonic combustor equipped with a two-stage fuel injector strut. The present study focuses on mixing, ignition, and flame stabilization and the degree of detail required by the reaction mechanism in the large-eddy simulation model framework. An explicit large-eddy simulation model, using a mixed subgrid model and a partially stirred reactor turbulence-chemistry interaction model, is used in an unstructured finite volume setting. The model, and its components, has been carefully validated in a large number of other studies. To bestow further validation and to provide supplementary information about the physics of mixing and supersonic combustion, experimental data from the National Aerospace Laboratory of Japan's supersonic combustor, equipped with the two-stage strut injector and connected to ONERA's vitiation air heater, are employed. The large-eddy simulation predictions are compared with the experimental centerline wall pressure distribution and the planar laser-induced fluorescence imaging of hydroxide--ion radicals distributions in several cross sections of the combustor, showing excellent qualitative and quantitative agreements. The large-eddy simulation results are furthermore used to elucidate the complicated flow, mixing, and combustion physics imposed by the multi-injector two-stage injector strut. The importance of the combustion chemistry appears weaker than expected but with the one-step mechanism resulting in a too early ignition (caused by local shock wave heating) and a more stable flame, as compared with the more detailed two-and seven-step mechanisms. DOI: 10.2514/1.43746

In 2001, the authors presented a higher-order mass-flux model called 'assumed distributions with higher-order closure' (ADHOC 1), which represents the large eddies of the planetary boundary layer (PBL) in terms of an assumed joint distribution of the vertical velocity and scalars. In a subsequent version (ADHOC 2) the authors incorporated vertical momentum fluxes and second moments involving pressure perturbations into the framework. These versions of ADHOC, as well as all other higher-order closure models, are not suitable for use in large-scale models because of the high vertical and temporal resolution that is required. This high resolution is needed mainly because higher-order closure (HOC) models must resolve discontinuities at the PBL top, which can occur anywhere on a model's Eulerian vertical grid. This paper reports the development of ADHOC 3, in which the computational cost of the model is reduced by introducing the PBL depth as an explicit prognostic variable. ADHOC 3 uses a stretched vertical coordinate that is attached to the PBL top. The discontinuous jumps at the PBL top are 'hidden' in the layer edge that represents the PBL top. This new HOC model can use much coarser vertical resolution and a longer time step and is thus suitable for use in large-scale models. To predict the PBL depth, an entrainment parameterization is needed. In the development of the model, the authors have been led to a new view of the old problem of entrainment parameterization. The relatively detailed information available in the HOC model is used to parameterize the entrainment rate. The present approach thus borrows ideas from mixed-layer modeling to create a new, more economical type of HOC model that is better suited for use as a parameterization in large-scale models. DOI: 10.1175/2009JAS3205.1

Large eddy simulation (LES) is employed in this paper to model the axial flow along a circular array of rods with a focus on anisotropic large-scale turbulence. The circular array consists of four whole rods and eight half rods, with a pitch-to-diameter ratio of 1.08. A dynamic Smagorinsky model with SIMPLE coupling method and a bounded central difference scheme are used to reduce numerical errors. The high demands for computations of the three-dimensional turbulent flows are afforded through parallel processing and utilization of 20 processors. The numerical results obtained using LES are compared with independent experimental data available in the literature; good agreement is achieved. The LES model was developed to accurately predict (i) the dependence of turbulence intensity and dominant frequency on the gap size and (ii) the turbulence structure in different directions. [DOI: 10.1115/1.4000574] Keywords: complex flow, large eddy simulation, turbulence

This paper reports the results of numerical investigations of swirling turbulent jet flows by large-eddy simulation. Reynolds-averaged Navier-Stokes simulations are used to generate mean inflow data. Fluctuations are added to the profiles of the means to produce instantaneous inflow data. The instantaneous inflow data are correlated such that the characteristic length and time scales of inflowing turbulent eddies are consistent with the corresponding Reynolds- averaged Navier-Stokes profiles imposed at the inlet. The resulting large-eddy simulation method is validated against experimental data of both swirling and nonswirling turbulent jets. The application of the validated large- eddy simulation method to studies of the mechanism of swirl effects shows the following. Swirl breaks apart the typical ring structures of nonswirling turbulent jets into two modes: a helical mode and streamwise braid structures. The interaction of these two modes generates unorganized turbulence that enhances the turbulent mixing. An analysis of the mixing enhancement by swirl shows a significant increase of the turbulent mixing efficiency of scalars with the swirl number (of up to 21.7% for the cases considered). DOI: 10.2514/1.43813

Separated flow past a hump in a turbulent boundary layer is studied numerically using detached-eddy simulation (DES), zonal detached-eddy simulation (ZDES), delayed detached-eddy simulation (DDES), and Reynolds-averaged Navier-Stokes (RANS) modeling. The geometry is smooth so the separation point is a function of the flow solution. Comparisons to experimental data show that RANS with the Spalart-Allmaras turbulence model predicts the mean-field statistics well. The ZDES and DDES methods perform better than the DES formulation and are comparable to RANS in most statistics. Analyses motivate that modeled-stress depletion near the separation point contributes to differences observed in the DES variants. The order of accuracy of the flow solver ACUSOLVE is also documented. [DOI: 10.1115/1.4000376] Keywords: detached-eddy simulation, delayed detached-eddy simulation, zonal detached-eddy simulation, modeled-stress depletion, boundary-layer shielding, ACUSOLVE, order of accuracy

A thickened-flame (TF) modeling approach is combined with a large eddy simulation (LES) methodology to model premixed combustion, and the accuracy of these model predictions is evaluated by comparing with the piloted premixed stoichiometric methane-air flame data of Chen et al. (1996, 'The Detailed Flame Structure of Highly Stretched Turbulent Premixed Methane-Air Flames,' Combust. Flame, 107, pp. 233-244) at a Reynolds number Re = 24,000. In the TF model, the flame front is artificially thickened to resolve it on the computational LES grid and the reaction rates are specified using reduced chemistry. The response of the thickened-flame to turbulence is taken care of by incorporating an efficiency function in the governing equations. The efficiency function depends on the characteristics of the local turbulence and on the characteristics of the premixed flame such as laminar flame speed and thickness. Three variants of the TF model are examined: the original thickened-flame model, the power-law flame-wrinkling model, and the dynamically modified TF model. Reasonable agreement is found when comparing predictions with the experimental data and with computations reported using a probability distribution function modeling approach. The results of the TF model are in better agreement with data when compared with the predictions of the G-equation approach. [DOI: 10.1115/1.3094021] Keywords: thickened-flame, flame-wrinkling, turbulence, gas turbine emission, large eddy simulation

As a first step toward predicting rough-wall boundary-layer noise, the sound radiated from a single hemispherical roughness element and a pair of roughness elements in a turbulent boundary layer at [Re.sub.[theta]] = 7500 is investigated. The roughness height is 3.6% of the boundary-layer thickness, or 95 wall units. The flowfield is obtained from large-eddy simulation, and the results are validated against experimental measurements. Acoustic calculations are performed based on the Curle--Powell integral solution to the Lighthill equation for an acoustically compact hemisphere. The sound radiation is dominated by unsteady drag dipoles and their images in the wall. It is found that the spanwise dipole, which has been overlooked in previous studies of roughness noise, is of larger or similar strength compared with the streamwise dipole, and the viscous contribution to the drag dipoles is negligible in comparison with the pressure contribution. Important flow features contributing to sound radiation are identified by examining the unsteady surface-pressure field and the surrounding flow structures. Pressure fluctuations are strongest on the upstream part of the hemispheric surface near the base due to impingement of incoming turbulent eddies and their interaction with horseshoe vortices. On the back surface of the hemisphere, pressure fluctuations are relatively weak, indicating that shear-layer separation and vortex shedding do not produce significant self-noise from the hemisphere. In the case with two hemispheres, the wake of the upstream hemisphere is found to significantly enhance sound radiation from the downstream hemisphere, particularly in the streamwise direction and at high frequencies. DOI: 10.2514/1.42343

Large-eddy simulations (LESs) of flow past a circular cylinder in the vicinity of a flat plate have been carried out for three different gap-to-diameter (G / D) ratios of 0.25, 0.5, and 1.0 (where G signifies the gap between the flat plate and the cylinder, and D signifies the cylinder diameter)following the experiment of Price et al. (2002, 'Flow Visualization Around a Circular Cylinder Near to a Plane Wall,' J. Fluids Struct., 16, pp. 175-191). The flow visualization along with turbulent statistics are presented for a Reynolds number of Re = 1440 (based on D and the inlet free-stream velocity [U.sub.[infinity]]). The three-dimensional time-dependent, incompressible Navier-Stokes equations are solved using a symmetry-preserving finite-difference scheme of second-order spatial and temporal accuracy. The immersed-boundary method is employed to impose the no-slip boundary, condition at the cylinder surface. An attempt is made to understand the physics of flow involving interactions of shear layers shed from the cylinder and the wall boundary layer. Present LES reveals the shear layer instability and formation of small-scale eddies apart from their mutual interactions with the boundary layer. It has been observed that G/ D ratio has a large influence on the modification of wake dynamics and evolution of the wall boundary layer. For a low gap ratio, it is difficult to identify the boundary layer because of its strong interactions with the shear layers; however; a rapid transition to turbulence of the boundary layer, which is similar to bypass transition, is observed for a large gap ratio. [DOI: 10.1115/1.3176982] Keywords: wake-boundary layer interactions, shear layer transition, LES, IB method

Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield and near-field acoustics for various total pressure ratios from overexpanded to underexpanded jet flow conditions have been investigated. The location and spacing of the shock cells are in good agreement with experimental data and previous theoretical results. The velocity profiles are also in good agreement with data from experimental measurements. A Mach disk is observed immediately downstream of the nozzle exit for overexpanded jet conditions with nozzle pressure ratios much lower than the fully expanded value. It is found that this type of nozzle with a sharp turning throat does not have a shock- free condition for supersonic jet flows. The near-field intensities of pressure fluctuations show wavy structures for cases in which screech tones are observed. The large-eddy simulations predictions of the near-field noise intensities show good agreement with those obtained from experimental measurements. This good agreement shows that large-eddy simulations and measurements can play complementary roles in the investigation of the noise generation from supersonic jet flows. DOI: 10.2514/1.43281

For the aim of computing compressible turbulent flowfield involving shock waves, an implicit large eddy simulation (LES) code has been developed based on the idea of monotonically integrated LES. We employ the weighted compact nonlinear scheme (WCNS) not only for capturing possible shock waves but also for attaining highly' accurate resolution required for implicit LES. In order to show that WCNS is a proper choice for implicit LES, a two-dimensional homogeneous turbulence is first obtained by solving the Navier--Stokes equations for incompressible flow. We compare the inertial range in the computed energy spectrum with that obtained by the direct numerical simulation (DNS) and also those given by the different LES approaches. We then obtain the same homogeneous turbulence by solving the equations for compressible flow. It is shown that the present implicit LES can reproduce the inertial range in the energy spectrum given by DNS fairly well. A truncation of energy spectrum occurs naturally at high wavenumber limit indicating that dissipative effect is included properly in the present approach. A linear stability analysis for WCNS indicates that the third order interpolation determined in the upwind stencil introduces a large amount of numerical viscosity to stabilize the scheme, but the same interpolation makes the scheme weakly unstable for waves satisfying k[DELTA]x [approximately equal to] 1. This weak instability results in a slight increase in the energy spectrum at high wavenumber limit. In the computed result of homogeneous turbulence, a fair correlation is shown to exist between the locations where the magnitude of [nabla] X [omega] becomes large and where the weighted combination of the third order interpolations in WCNS deviates from the optimum ratio to increase the amount of numerical viscosity. Therefore. the numerical viscosity involved in WCNS becomes large only at the locations where the subgrid-scale viscosity can arise in ordinary LES. This suggests the reason why the present implicit LES code using WCNS can resolve turbulent flowfield reasonably well. [DOI: 10.1115/1.3077141] Keywords: implicit large eddy simulation, weighted compact nonlinear scheme, two-dimensional homogeneous turbulence

Turbulence modeling techniques are compared for the simulation of low speed indoor air flow in a simple room. The effect of inlet turbulence intensity on the flow field is investigated using the constant coefficient large eddy simulation (LES) model with uniform mean inlet conditions at several levels of inlet turbulence intensities. The results show significant differences between the simulations with laminar inflow conditions and those in which turbulence was introduced at the inlet. For simulations with turbulent inlet conditions, it is noticed that the jet transitions to a state of fully developed turbulence wherein the dynamics of the flow become nearly insensitive to any further increase in the level of inlet turbulence. For laminar flow conditions, it is seen that the jet slowly spreads and mixes with the quiescent room air. As a result, the jet reaches a fully developed turbulent state further away from the inlet relative to the simulations with inlet turbulence. The effect of using experimental inlet profiles is also investigated. It is seen that, close to the inlet, the flow is sensitive to the inflow details, whereas further away from the inlet, these effects become less pronounced. The results from the constant coefficient and the dynamic LES models are compared. The most noticeable differences in the flow occur at the locations where the subgrid-scale's contribution to the turbulent kinetic energy is highest. Finally, the results from the dynamic LES and the k-[epsilon] models are compared. It is found that there are significant differences between the two models for the zero inlet turbulence limit where the flow is most probably transitional in nature and turbulence has not yet reached a fully developed state. It is seen that in the laminar inflow case the k-[epsilon] model predicts a fully turbulent jet very close to the inlet and thus fails to capture the slow development of the jet found in LES. Accordingly, the k-[epsilon] model results are nearly insensitive to the level of inlet turbulence especially far from the origin of the flow. It is also seen that for cases with nonzero inlet turbulence level, the k-[epsilon] model predicts the general features of the mean flow reasonably well; however, the k-[epsilon] model overpredicts the jet spreading rate and the turbulent kinetic energy close to the inlet. Furthermore, the k-[epsilon] model under predicts the turbulence level near the corner of the ceiling as it fails to capture the complicated mean velocity and turbulent kinetic energy, most likely because of the highly intermittent flow pattern found there in LES. [DOI: 10.1115/1.3112386]

This paper presents an analysis of an original structure of induction heater using superconducting coils. With the proposed structure, two operating modes are possible to heat the aluminum billet. Either the billet is at standstill and placed within a strong rotating magnetic field (around 3.5 T) of low frequency (around 1 Hz), or the billet rotates at low angular speed (around 6 rad/s) within a strong dc magnetic field (around 3.5 T). The analysis is based on the analytical solution of two-dimensional diffusion equations for the magnetic vector potential. A closed-form expression is derived for the eddy-current power loss in the billet. This analytical expression can be used as a tool for design optimization of the induction heater. The analytical results are compared with those obtained by finite-element software. The study of the temperature field in the billet shows that a homogeneous temperature profile is obtained when a strong magnetic field of low frequency (rotating speed or ac current pulsation) is used. Index Terms--Analytical solution, eddy current, induction heater, superconducting coils, temperature.

Anticipating that large eddy simulations will increasingly become the future engineering tool for research, development, and design, it is deemed necessary to formulate some quality assessment measures that can be used to judge the resolution of turbulent scales and the accuracy of predictions. In this context some new and refined measures are proposed and compared with those already published by the authors in the common literature. These measures involve (a) fraction of the total turbulent kinetic energy, (b) relative grid size with respect to Kolmogorov or Taylor scales, and (c) relative effective subgrid/numerical viscosity with respect to molecular viscosity. In addition, an attempt is made to segregate the contributions from numerical and modeling errors. Proposed measures are applied to various test cases and validated against fully resolved large eddy simulation and/or direct numerical simulation whenever possible. [DOI: 10.1115/1.3059703] Keywords: quality assessment measures, LES, modeling and numerical errors, LES quality

The purpose of the research reported in this paper was to investigate the aerodynamic response of a double-deck bus in gusty winds using a detached-eddy simulation (DES). The bus was subjected to three different scenarios of wind gusts: gust in a wind tunnel gust in a natural wind, and gust behind the exit of a tunnel. The proposed scenarios of gusts are in the time domain and take into account the dynamic behavior of natural winds. The Reynolds number of the flow, based on the time-averaged speed of the side wind and a reference length of [square root of 0.1] m, was 1.3 X [10.sup.6]. Detailed transient responses of the aerodynamic coefficients and flow structures were investigated. Good agreement was found between the DES results and the available experimental data. A comparison between the influence of the different gust scenarios on the aerodynamic coefficients shows that the gust behind the exit from a tunnel has a stronger influence on the aerodynamics than the other gust scenarios. Moreover, the influence of the gusts on the time history of aerodynamic coefficients is found to be limited to the period of the gust. [DOI: 10.1115/1.3054288] Keywords: DES, side wind, vehicle aerodynamics, flow structures, unsteady flow

Eddy currents induced in steel are analyzed by the boundary element method. As the periodic electromagnetic quantities in the steel are distorted due to the nonlinear magnetization property, they are expressed by a Fourier series, whose fundamental and harmonic waves are determined by solving the corresponding integral equations with the surface magnetic fields given as the boundary values. In this paper, we introduce a new combined integration scheme that enables efficient, volume-mesh-less treatment of the practical problems typically characterized by the geometrical singularities like edges and corners. Using this procedure, the internal and surface electromagnetic fields are obtained numerically one after another by applying an adaptive equation technique to obtain the boundary values. Index Terms--Adaptive equation technique, boundary element method (BEM), integral equation, nonlinear eddy current analysis.

Electromagnetic fields due to excitation coils in boreholes containing cracks have been calculated using a boundary element method formulated in terms of a scalar decomposition of the field. By using a Green's kernel that ensures continuity of the tangential electric and magnetic field at the cylindrical surface, it is only necessary to render the flaw region in a discrete form. The effect of an ideal crack, one with zero opening but acting as a perfect barrier to eddy currents, is here represented by a current dipole layer orientated normal to the crack surface. An integral equation determines the source density on the crack and a numerical solution is found using boundary elements. The coil impedance change due to the crack is then computed using an expression based on a reciprocity principle. Index Terms--Boundary elements, cracks, eddy current, integral equation, nondestructive evaluation.

This paper reports a large-scale eddy-current analysis of conductive frame of large-capacity inverter. The eddy-current loss obtained by the electromagnetic field analysis is used as a heat source for the thermal analysis. Then, the suitable configuration of the frame for the suppression of temperature increase is investigated and designed. The hybrid finite-element and boundary-element (FE-BE) method is very suitable for the analysis of a complicated-shaped frame because it does not require mesh division for a free space and can easily treat eddy current. For the reduction of large computational costs, the fast multipole method (FMM) is introduced. Furthermore, in order to improve the convergence characteristic of iterative methods for system matrix, we develop the application of the IDR(s) method to the minor iterative preconditioning technique (MIP) and evaluate the performance. Finally, some numerical results that demonstrate the effectiveness of the developed method are presented. Index Terms--Coupled electromagnetic-thermal analysis, eddy-current, fast multipole method, hybrid finite-element and boundary-element method, IDR(s), large-scale analysis.

This paper describes a series of 6-day large eddy simulations of a deep, sometimes drizzling stratocumulus-topped boundary layer based on forcings from the East Pacific Investigation of Climate (EPIC) 2001 field campaign. The base simulation was found to reproduce the observed mean boundary layer properties quite well. The diurnal cycle of liquid water path was also well captured, although good agreement appears to result partially from compensating errors in the diurnal cycles of cloud base and cloud top due to overentrainment around midday. At all times of the day, entrainment is found to be proportional to the vertically integrated buoyancy flux. Model stratification matches observations well; turbulence profiles suggest that the boundary layer is always at least somewhat decoupled. Model drizzle appears to be too sensitive to liquid water path and subcloud evaporation appears to be too weak. Removing the diurnal cycle of subsidence had little effect on simulated liquid water path. Simulations with changed droplet concentration and drizzle susceptibility showed large liquid water path differences at night, but differences were quite small at midday. Droplet concentration also had a significant impact on entrainment, primarily through droplet sedimentation feedback rather than through drizzle processes.

This paper focuses on the modeling and prediction of core losses in nonoriented magnetic materials of electrical machines. The aim is to investigate the accuracy, efficiency, and stability of certain models, including the commonly used and the advanced ones, and to discuss their advantages and disadvantages when they are implemented in the finite-element method (FEM). It is shown in the paper that the traditional technique based on the loss separation theory can efficiently produce reasonable results in specific operation conditions but can, on the other hand, over- or underestimate the core losses in other circumstances. The advanced model based on solving the one-dimensional (l-D) Maxwell equations can give accurate results for the prediction of core losses in a lamination strip, but its accuracy, stability, and computational burden are put under scrutiny when it is applied to the prediction of core losses in an electrical machine. A third technique, referred to as the hybrid model, which captures the advantages of the traditional and advanced techniques and merges them into one, has been found to be the best compromise. The principal aim of the hybrid model is to avoid the numerical procedure of the 1-D Maxwell equations while maintaining relatively accurate predictions with a reasonable computational burden. A comparative investigation has been conducted for the three core-loss models that have been incorporated into the 2-D FEM analysis of a 37-kW induction motor on which experiments were carried out for comparisons. Index Terms--Core loss, dynamic hysteresis, eddy currents, excess loss, finite-element method (FEM), harmonics, rotating electrical machines, soft magnetic materials, time-stepping.

Three-dimensional large-eddy simulations of flows past cavities are performed by solving a generalized lattice-Boltzmann equation, which is based on multiple relaxation times. To incorporate subgrid-scale turbulence effects in this equation, its hydrodynamic relaxation time scales are modified by the eddy viscosity Smagorinsky model, which is modulated near walls by the van Driest damping function. The relaxation times for kinetic modes are adjusted to enhance its numerical stability at higher Reynolds numbers. Parallel computations of subsonic compressible flow over cavities of different configurations (shallow and deep, with and without a splitter plate) are performed to investigate serf-sustained resonant flow instabilities and generation of tones. Computed flow patterns and flow oscillation frequencies in the shear-layer region for these problems are found to be in good agreement with previous computational simulations and experiments. When possible, detailed comparisons of the components of the root-mean-square velocity fluctuations have also been made, and reasonably good agreement with data from a previous computational study has been found. The computational approach exhibited near-linear parallel scalability on a parallel cluster. DOI: 10.2514/1.38278

Rotation and curvature (RC) effects on turbulence are expected to impact losses and flow structure in turbomachines. This paper examines two recent eddy-viscosity-model corrections devised to account for these effects: the Spalart and Shur (1997, 'On the Sensitization of Turbulence Models to Rotation and Curvature,' Aerosp. Sci. Technol., 1(5), pp. 297-302) correction to the model of Spalart and Allmaras (1994, 'A One-Equation Turbulence Model for Aerodynamic Flows,' Rech. Aerosp., I, pp. 5-21) and the correction of Cazalbou et al. (2005, 'Two-Equation Modeling of Turbulent Rotating Flows,' Phys. Fluids., 17, p. 055110) to the (k, [epsilon]) model. The method of verification and validation is applied to assess the impact of these corrections on the computation of a centrifugal-compressor test case. First, a review of RC effects on turbulence as they apply to centrifugal compressors is made. The two corrected models are then presented. Second, the Radiver open test case (Ziegler K. U., Gallus, H. E., and Niehuis R., 2003, 'A Study on Impeller Diffuser Interaction Part 1: Influence on the Performance,' ASME J. Turbomach, 125, pp. 173-182) is used as a basis for the assessment of the two corrections. After a physical-consistency analysis, the Richardson extrapolation is applied to quantify the numerical errors involved in all the calculations. Finally, experimental data are used to perform validation for both global and local predictions. The consistency analysis shows that both corrections lead to significant changes in the turbulent field, in perfect agreement with the underlying theoretical considerations. The uncertainty analysis shows that the predictions of the global performances are more sensitive to grid refinement than they are to RC turbulence modeling. However, the opposite conclusion is drawn with regard to the prediction of some local flow properties: Improvements are obtained with the RC corrections, the best results being observed for the RC-corrected (k, [epsilon]) model. [DOI: 10.1115/1.2953231]

Accuracy and reliability of large-eddy simulation data in a really complex industrial geometry are invesigated. An original methodology based on a response surface for LES data is introduced. This surrogate model for the full LES problem is built using the Kriging technique, which enables a low-cost optimal linear interpolation of a restricted set of large-eddy simulation (LES) solutions. Therefore, it can be used in most realistic industrial applications. Using this surrogate model it is shown that (i) optimal sets of simulation parameters (subgrid model constant and artificial viscosity parameter in the present case) can be found; (ii) optimal values, as expected, depend on the cost functional to be minimized. Here, a realistic approach, which takes into account experimental data sparseness, is introduced. It is observed that minimization of the error evaluated using a too small subset of reference data may yield a global deterioration of the results. [DOI: 10.1115/1.2829602] Keywords: kriging, surrogate model large-eddy simulation, multiobjective optimization, error estimation

Granular materials exhibit unusual kinds of behavior, including pattern formations during the shaking of the granular materials; the characteristics of these various patterns are not well understood. Vertically shaken granular materials undergo a transition to convective motion that can result in the formation of bubbles. A detailed overview is presented of collective processes in gas-particle flows that are useful for developing a simplified model for molecular dynamic type simulations of dense gas-particle flows. The governing equations of the gas phase are solved using large eddy simulation technique. The particle motion is predicted by a Lagrangian method. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles. Interparticle normal and tangential contact forces are calculated using a generalized Hertzian model The other forces that are taken into account are gravitational and drag force resulting from velocity difference with the surrounding gas. A simulation of gas-particle flow is performed for predicting the flow dynamics of dense mixtures of gas and particles in a vertical, pentagonal, prism shaped, cylindrical container. The base wall of the container is subjected to sinusoidal oscillation in the vertical direction that spans to the bottom of the container. The model predicts the formation of oscillon type structures on the free surface. In addition, the incomplete structures are observed. Interpretations are proposed for the formation of the structures, which highlights the role played by the surrounding gas in dynamics of the shaken particles. [DOI: 10.1115/1.2409358] Keywords: localized structures, dense granular flows, vibrations, large eddy simulations, pattern formations, viscoelastic solids

The dynamical equations of atmospheric flow are written using potential vorticity as the meridional coordinate and potential temperature as the vertical coordinate. Within this system, the atmosphere is divided into undulating tubes bounded by isentropic and constant potential vorticity surfaces, and, under adiabatic and frictionless conditions, the air moves through the tubes without penetrating through the walls. A model that uses this system of coordinates incorporates as built-in both the dry convective adjustment and barotropic adjustment processes, which prevent the folding of isentropic and potential vorticity surfaces but crudely represent the effects of interactions with the mean flow. The Eliassen--Palm flux vector in this frame of reference yields a new interpretation of the eddy momentum transport. Eddy momentum exchange is through the form drag created by pressure forces exerted on the potential vorticity-potential temperature tubes.

Basic studies on the eddy current examination of defects in Inconel, a typical nickel-base alloy used in the reactor vessel, pressurizer, and core internal of nuclear power plants, are carried out. The detecting and sizing capability of the eddy current method is investigated through analytical and experimental approaches. Probe's detectability is numerically evaluated, and appropriate probe and examination conditions are correspondingly selected. The numerical signal calculation and crack reconstruction approach is confirmed in terms of the study of the eddy current examination of electrodischarge machining notches in Inconel base metal, and further applied to eddy current examination of fatigue cracks in Inconel welds. The profiles of fatigue cracks are reconstructed using eddy current testing signals. Crack depths estimated by eddy current reconstruction agree well with that of ultrasonic testing and are consistent with the crack depths revealed from destructive testing. The research presented in this paper shows that by choosing a proper testing situation, eddy current examination is feasible for detecting and sizing of surface-breaking cracks in Inconel welds. [DOI: 10.1115/1.2435718] Keywords: eddy current examination, fatigue crack, Inconel weld, reconstruction

This study combines the experimental measurements with large-eddy simulation (LES) data of a neutral planetary boundary layer (PBL) documented by a 60-m lower instrumented with eight sonic anemometers, and a high-resolution Doppler lidar during the 1999 Cooperative Atmospheric and Surface Exchange Study (CASES-99) experiment. The target of the paper is (i) to investigate the multiscale nature of the turbulent eddies in the surface layer (SL), (ii) to explain the existence of a -1 power law in the velocity fluctuation spectra, and (iii) to investigate the different nature of turbulence in the two sublayers within the SL, which are the eddy surface layer (ESL; lower sublayer of the SL lying between the surface and about 20-m height) and the shear surface layer (SSL; lying between the ESL top and the SL top). The sonic anemometers and Doppler lidar prove to be proper instruments for LES validation, and in particular, the Doppler lidar because of its ability to map near-surface eddies. This study shows the different nature of turbulence in the ESL and the SSL in terms of organized eddies, velocity fluctuation spectra, and second-order moment statistics. If quantitative agreement is found in the SSL between the LES and the measurements, only qualitative similarity is found in the ESL due to the subgrid-scale model, indicating that the LES captures part of the physics of the ESL. The LES helps explain the origin of the -1 power-law spectral subrange evidence in the velocity fluctuation spectra computed in the SL using the CASES-99 dataset: strong interaction between the mean flow and the fluctuating vorticities is found in the SL, and turbulent production is found to be larger than turbulent energy transfer for [k.sub.1]z> 1 ([k.sub.1] being the longitudinal wavenumber and z the height), which is the condition of the -1 power-law existence.

A parameterization for surface roughness and blending height at regional scales, under neutral atmospheric stability, is studied and tested. The analysis is based on a suite of large-eddy simulations (LES) over surfaces with varying roughness height and multiple variability scales. The LES are based on the scale-dependent Lagrangian dynamic subgrid-scale model, and the surface roughnesses at the ground are imposed using the rough-wall logarithmic law. Several patterns of roughness distribution are considered, including random tiling of patches with a wide distribution of length scales. An integral length scale, based on the one-dimensional structure function of the spatially variable roughness height, is used to define the characteristic surface variability scale, which is a critical input in many regional parameterization schemes. Properties of the simulated flow are discussed with special emphasis on the turbulence properties over patches of unequal roughness. The simulations are then used to assess a generalized form of the parameterization for the blending height and the equivalent surface roughness at regional scales that has been developed earlier for regular patterns of surface roughness (regular stripes). The results are also compared with other parameterizations proposed in the literature. Good agreement is found between the simulations and the regional-scale parameterization for the surface roughness and the blending height when this parameterization is combined with the characteristic surface variability scale proposed in this paper.

A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.