Virtual Flux Research Articles

Validation of Virtual Flux Method for Forced Convection Flow

In this study, the virtual flux method (VFM) proposed by the authors was applied to the seamless calculation of heat fluid flow and heat conduction inside solid bodies. Flows both inside and around circular cylinders are calculated as examples. The estimated Nusselt numbers of the cylinder surfaces, which are calculated by the VFM, are compared with both numerical and experimental results obtained by other researchers. In order to show that the accuracy of the VFM is high, an exemplified seamless calculation of heat fluid flow and heat conduction inside a solid cylinder, which is an advantage of the VFM, are also performed.

Validation of Virtual Flux Method for Forced Convection Flows

This paper describes the validation of Virtual Flux Method (VFM) to forced convection flows. The method, one of Cartesian grid methods, can sharply capture boundaries. The method is also designed to calculate flows inside and outside boundaries seamlessly, while many Cartesian grid methods are not. This method is applied to forced convection flows around a circular cylinder and a pair of circular cylinders. The results are well compared with other available numerical results and experiments. Also, heat/fluid flow fields inside and outside a circular cylinder where a heater is placed at the center of the cylinder are seamlessly calculated. These results indicate the reliability of VFM for forced convection flows.

The role of surface freshwater flux boundary conditions in Arctic Ocean modelling

The role of surface freshwater flux boundary conditions in prognostic Arctic Ocean/sea-ice modelling is analysed, using a regional numerical model. Three different applications of freshwater flux formulations are evaluated. The standard formulation, which serves as a benchmark, takes surface volume fluxes due to precipitation, evaporation and river runoff into account. The total freshwater input to the Arctic Ocean by runoff, precipitation and Bering Strait inflow is approximately 6800 km 3 yr −1 in the model setup. The implementation of an Arctic river water tracer in the standard run enables the calculation of an average mean residence time of 14–15 years for river water in the Arctic halocline. The second formulation for surface freshwater fluxes neglects the volume input, which corresponds to applying ‘virtual salinity fluxes’. This simplification leads to a rapid salinity build-up in the upper layers of the Arctic Ocean and causes a substantial reduction of freshwater export through Fram Strait. The third formulation uses a constant reference salinity of 35 psu in the definition of the virtual salinity flux boundary condition. This approach results in hydrographic fields which are very similar to those from the standard run. Errors in circulation and freshwater transport are small and, for most applications, tolerable. Our results suggest that virtual salinity fluxes with fixed reference salinity are a reasonable approximation for Arctic Ocean models with horizontal resolution of order 100 km.

Active Filtering Function of Three-Phase PWM Boost Rectifier Under Different Line Voltage Conditions

Slight hardware and algorithm modifications as well as a higher power ratio of a three-phase pulsewidth-modulation (PWM) rectifier make compensation of neighboring nonlinear power load possible. The active filtering function enlarges the functionality of PWM rectifiers, which decreases the cost of additional installation of compensating equipment. It gives a chance to fulfill both shunt active filter (SAF) and PWM rectifier tasks in a multidrive system by one advanced converter. Thanks to the idea of virtual flux, the direct power control space-vector-modulated (DPC-SVM) and new synchronous double reference frame phase-locked loop approach, the control system is resistant to a majority of line voltage disturbances. This assures proper operation of the system for abnormal and failure grid conditions. Simulation and experimental results have proven excellent performance and verify the validity of the proposed system.

Flux profile modeling: Monte Carlo simulation and numerical computation

Spatial variations in flux are a result of the shape of the Knudsen cell crucible (K-cell) and the geometry of the growth chamber. In this article, a process simulation tool based on a phenomenological model is proposed and elaborated. Two different models based on physical principles are investigated. Model I includes the primary flux from the liquid surface and a secondary flux re-emitted from the crucible walls for the total flux at the platen. No intermolecular collisions are considered in this model. Since the investigated 10000 g SUMO crucible has a tapered region leading to the crucible opening (the smallest radii opening), molecules undergo extensive interwall and intermolecular collisions in that region. The simulation of intermolecular collisions requires considerable time and limits the usage of the modeling tool in the industry. Thus, the effect of intermolecular collisions is captured through the assumption of a virtual flux source (VFS) at the neck in model II. For the two models, a Monte Carlo approach and a numerical approach are employed for theoretical analysis. The Monte Carlo and numerical approaches as implemented are equivalent and produce essentially the same results. The results of model II from both approaches using cos2 θ distribution from the VFS best describes the experimental data for the SUMO 10000 g crucible. Further comparison with experimental data is necessary to validate the cos2 θ model.

Simple Direct Power Control of Three-Phase PWM Rectifier Using Space-Vector Modulation (DPC-SVM)

This paper proposes a novel and simple direct power control of three-phase pulsewidth-modulated (PWM) rectifiers with constant switching frequency using space-vector modulation (DPC-SVM). The active and reactive powers are used as the pulse width modulated (PWM) control variables instead of the three-phase line currents being used. Moreover, line voltage sensors are replaced by a virtual flux estimator. The theoretical principle of this method is discussed. The steady-state and dynamic results of DPC-SVM that illustrate the operation and performance of the proposed system are presented. It is shown that DPC-SVM exhibits several features, such as a simple algorithm, good dynamic response, constant switching frequency, and particularly it provides sinusoidal line current when supply voltage is not ideal. Results have proven excellent performance and verify the validity of the proposed system.

Simulation around Arbitrary Shape Body with Virtual Flux Method on Cartesian Grid

Simulation around Arbitrary Shape Body with Virtual Flux Method on Cartesian Grid

1312 仮想流束法による、直交格子上での任意形状物体周りの流れ場の計算(GS-1.5 流れの数値解析(2))

1312 仮想流束法による、直交格子上での任意形状物体周りの流れ場の計算(GS-1.5 流れの数値解析(2))

Theory of 3D reconnection and coronal heating heating

Magnetic reconnection is a prime candidate for heating coronae. We here summarise recent developments in the theory of 31) reconnection and the different ways in which it is thought to heat the solar corona. At 3D null points reconnection may occur by either spine, fan or separator reconnection. In absence of null points, reconnection of two flux tubes is completely different from 21) reconnection, since in general in 3D a flux-conserving velocity w does not exist. Instead two flux tubes split into four parts, which ‘flip’ via ‘virtual flux tubes’ to form four flux tubes. Heating in the corona has been proposed to occur by: driven reconnection at X-ray bright points; binary reconnection due to the coronal interaction of a pair of opposite-polarity magnetic sources; separator reconnection due to a higher-order interaction; braiding; and coronal tectonics due to the formation and dissipation of current sheets at myriads of separatix surfaces that thread the corona and separate the flux coming from the many different sources in the photosphere.

High resolution spectroscopy of the 12Lambda B hypernucleus produced by the (e,e'K+) reaction.

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the (e,e'K+) reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be approximately 900 keV for the (12)(Lambda)B spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

A vector potential and exact flux through surfaces using Lagrange and Stokes stream functions

A vector potential is obtained for two–dimensional flow fields using the Lagrange stream function, and for axisymmetric flow fields using the Stokes stream function. The flux through surfaces bounded by straight–line segments with arbitrary orientation is obtained using the line integral of the tangential component of the vector potential along the boundary of the surface. Integration of the Lagrange stream function results for surfaces in two–dimensional flow; a Cauchy integral of the Stokes stream function follows for axisymmetric flow. The process of assembling vector potentials and developing closed–form expressions to provide exact flux through surfaces is illustrated for a partially penetrating well with uniform flow in a horizontal aquifer. A virtual flux is generated through vortex sheets where the Lagrange stream function is discontinuous, and through singular vortex lines where the Stokes stream function is non–zero on the axis of symmetry. This virtual flux must be accounted for to obtain exact flux. The unifying framework presented here to assemble vector potentials obtained using Lagrange and Stokes stream functions and to compute flux is applicable to three–dimensional divergence–free, steady flow fields that can be decomposed into two–dimensional and axisymmetric components.

Errors in electromagnetic flow meter with magnetic particles

Magnetic admixtures change the magnitude and distribution of the magnetic field and virtual current in active fluid volume of electromagnetic flow meter, and measurement error appears. The value of error depends on the concentration, permeability and electric conductivity of magnetic admixtures and on the form of particles. The principles of error formation are investigated assuming that in the fluid with uniform properties there may be a small concentration of spherical particles, whose magnetic and electric properties are different from that of the fluid. An ideal electromagnetic flow meter with a rectangular duct and infinitely conductive large electrodes is assumed. The flat velocity profile is supposed. Measurement error is evaluated using the analogy between electrostatic, magnetic and electric current fields. The main causes of measurement error are examined. There is a deviation of densities of virtual current and magnetic flux inside particles, a deviation of mean values of magnetic flux and virtual current densities outside particles and a distortion of magnetic and virtual current fields in surroundings of particles. The magnetic particles with electric conductivity considerably greater than fluid conductivity are especially dangerous. A relative error in this case may exceed volume concentration of particles six times. For non-magnetic particles measurement error is absent independent of their conductivity.

Vertical Structure Of Turbulence In Offshore Flow During Rasex

The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Riso Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study. The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface. With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow.

Long-Term Observations of the Dynamics of the Continental Planetary Boundary Layer

Time series of mixed layer depth, zi, and stable boundary layer height from March through October of 1998 are derived from a 915-MHz boundary layer profiling radar and CO 2 mixing ratio measured from a 447-m tower in northern Wisconsin. Mixed layer depths from the profiler are in good agreement with radiosonde measurements. Maximum zi occurs in May, coincident with the maximum daytime surface sensible heat flux. Incoming radiation is higher in June and July, but a greater proportion is converted to latent heat by photosynthesizing vegetation. An empirical relationship between zi and the square root of the cumulative surface virtual potential temperature flux is obtained ( r 2 5 0.98) allowing estimates of zi from measurements of virtual potential temperature flux under certain conditions. In fair-weather conditions the residual mixed layer top was observed by the profiler on several nights each month. The synoptic mean vertical velocity (subsidence rate) is estimated from the temporal evolution of the residual mixed layer height during the night. The influence of subsidence on the evolution of the mixed, stable, and residual layers is discussed. The CO 2 jump across the inversion at night is also estimated from the tower measurements.

Virtual-flux-based direct power control of three-phase PWM rectifiers

In this paper, direct power control (DPC) of three-phase pulsewidth-modulated rectifiers without line voltage sensors is presented. The new system is based on virtual flux (VF) estimation. Theoretical principles of this method are discussed. The steady-state and dynamic behavior of VF-DPC are presented that illustrate the operation and performance of the proposed system compared to a conventional DPC method. Both strategies are also investigated under unbalance and predistorted grid. It is shown that the VF-DPC exhibits several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal. Test results show the excellent performance of the proposed system.

Salt conservation, free surface, and varying levels: A new formulation for ocean general circulation models

In order to clarify the link between ocean salt content (OSC) conservation and the freshwater flux formulation in ocean general circulation models (OGCMs), a varying level thickness, nonlinear free surface version of the OPA is presented. Linear/nonlinear free surface equations are solved using an original approach based on an explicit damping of fast external gravity waves. The method leaves both the potential vorticity equation and the equilibrium state unchanged. Its numerics and cost are quite similar to those of implicit schemes. When nonlinearities are kept, a variable first level thickness is required. Its discretization is determined by volume and energy constraints. The OSC conservation depends on the surface kinematic equation used. Four formulations are presented: (1) virtual salt flux (fixed ocean volume and no volume flux), (2) natural (fixed ocean volume and volume flux), (3) linear free surface (fixed volume and volume flux computed from a linear free surface equation), and (4) assumption free (variable volume computed from a nonlinear free surface equation). Their impact is illustrated in 25 year low‐resolution global OGCM simulations. In all cases the first‐order ocean response is quite similar, as the concentration‐dilution effect always exists. Formulations 4 and 2 ensure a strict conservation of the OSC. Nevertheless, the difference in formulation 3 is not strong enough to play a significant role: the conservation is almost perfect. Only formulation 1 neglects fresh water‐driven surface pumping. This mainly modifies the sea surface salinity of the ocean basin where river runoffs are strong. No significant difference is found between the other formulations as a large time step dampens high‐frequency free surface motion. The best compromise for climate is the linear free surface formulation. It allows a nearly exact OSC conservation, introduces the fresh water‐driven pumping, and runs faster than all the other formulations.

Centennial oscillations in an ocean-ice coupled model

Irregular centennial oscillations, with a spectral peak at 106 years, were obtained from an ocean-ice coupled model for the North Atlantic with realistic coastline and bottom topography. The model's thermohaline circulation is forced by mixed boundary conditions, i.e., a Haney-type relaxation condition for temperature, but an equivalent virtual salt flux condition for salinity. All forcing fields are taken from the observed monthly mean climatological wind stress and buoyancy fluxes. The oscillations appeared in the form of a surface—intensified tripole in both the sea surface temperature and salinity fields located in the vicinity of the Labrador Sea. The oscillations involve a delicate interplay between heat and fresh water advection by meridional overturning circulation, horizontal gyres, vertical convection, and the seasonal cycle. The oscillations are primarily controlled by the salinity component of the circulation; however, sea ice plays a minor role in driving the oscillations observed in the model. On the other hand, a regular seasonal cycle in the forcing fields is an important ingredient for the centennial oscillations.

The Relative Influence of Diapycnal Mixing and Hydrologic Forcing on the Stability of the Thermohaline Circulation*

Abstract Scaling analysis of the oceanic thermohaline circulation has been done under two types of surface boundary conditions: (i) Under “relaxation” conditions (sea surface temperature and salinity are relaxed to prescribed values), there is a two-thirds power law dependence of the meridional overturning (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (ii) Under “mixed” boundary conditions (temperature is relaxed to prescribed values and a virtual salt flux condition is used for salinity), multiple equilibria become possible. For a given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing: for each diapycnal diffusivity K, there is a threshold freshwater flux Ec = CK2/3 (C is a constant) below which three modes are possible with one stable thermal mode, one unstable thermal mode, and a stable hali...

Entrainment results from the Flatland boundary layer experiments

A primary objective of the 1995 and 1996 Flatland boundary layer experiments, known as Flatland95 and Flatland96, was to measure and characterize entrainment at the top of the convective boundary layer. The experiments took place in the area near the Flatland Atmospheric Observatory near Champaign‐Urbana, Illinois, in August‐September 1995 and June‐August 1996. The site is interesting because it is extraordinarily flat, has uniform land use, and is situated in a prime agricultural area. Measurements in the entrainment zone are difficult to make due to the time and space scales involved. We will present entrainment estimates derived from budget calculations with data from UHF wind profiling radars and from radiosondes. The results demonstrate that the remote sensing instruments produce results comparable to radiosondes and have significant advantages for boundary layer studies. Surface flux measurements are also used in the calculations. Direct heating by shortwave radiation absorbed by aerosols in the boundary layer is found to be an important component of the boundary layer heat budgets. The entrainment virtual temperature flux and the ratio of entrainment to surface flux found from the budget calculations are somewhat larger than expected. Advection of warm air, which is not accounted for in the budget calculations, is probably a factor in some periods but may not be significant in the full data set. For the full data set, the mean entrainment velocity found from the heat budget is 0.03±0.01 m s−1, slightly less than the mean rate of change of the boundary layer height. The mean entrainment ratio AR is 0.51±0.12 and the median is 0.43, comparable to results from some other studies in comparable conditions.

Observed Lagrangian Transition of Stratocumulus into Cumulus during ASTEX: Mean State and Turbulence Structure

Aircraft measurements made during the ‘‘First Lagrangian’’ of the Atlantic Stratocumulus Transition Experiment (ASTEX) between 12 and 14 June 1992 are presented. During this Lagrangian experiment an air mass was followed that was advected southward by the mean wind. Five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below. From the horizontal aircraft legs the boundary layer mean structure, microphysics, turbulence structure, and entrainment were analyzed. The vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds. During the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli, the total water flux, the virtual potential temperature flux, and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights. To analyze the cloud-top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top. These jumps were compared with cloud-top entrainment instability criteria discussed in the literature. It is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus‐cumulus transition.