Mass Sink Research Articles

Thermo-solutal buoyancy-opposed free convection of a binary Ostwald–De Waele fluid inside a cavity having partially-active vertical walls

The present study investigates double-diffusive free convection of a non-Newtonian fluid inside a cavity having discretely active walls of source and sink for heat and solute mass. The Ostwald–De Waele model has been adopted for non-Newtonian fluid behavior. Five discrete heating and salting arrangements in the vertical walls are considered in the present analysis. A numerical solution is obtained using the control volume-based integration technique. The Modified Marker and Cell (MMAC) method is used for obtaining the numerical solution of the governing equations. A gradient-dependent consistent hybrid upwind scheme of second order (GDCHUSSO) is used for the discretization of the convective terms. The effect of the power-law index and the buoyancy ratio for different heating and salting arrangements has been analyzed to determine the heat and mass transfer characteristics. Interesting results of stability at a critical buoyancy ratio is noticed for dilatant fluids. The isotherm, isoconcentration and flow line contour maps are provided to bring the clarity in the understanding of flow, temperature and mass distributions for different arrangements. Graphical and tabular records of heat and mass transfer characteristics are provided for understanding the various effects of the pertinent parameters.

Analytical Solution of Onsager’s Pancake Equation with Mass Sources and Sinks

Of all mass, momentum, and energy sources and sinks in simulating the feed, withdrawals, and scoops in the flow field of a gas centrifuge by using Onsager’s pancake equation, the most complicated ones are mass sources/sinks, which impose very different boundary conditions from those by other sources/sinks. Therefore, so far an analytical solution has not been found for the flow field with a mass source/sink. Here a solution procedure is described in detail for solving the flow field in the presence of a mass source/sink, and an analytical solution is given. Numerical examples are also presented and analyzed.

Linear stability of piezoelectric-controlled discrete mechanical systems under nonconservative positional forces

Discrete models of piezoelectric controlled mechanical systems, subjected to follower forces, are derived in this paper. A targeted strategy of control is first discussed, leading to detect three novel linear controllers resembling, in the order, the Tuned Mass Damper with large and small mass, respectively, and Energy Sink, used in the literature for controlling linear and nonlinear oscillations. Here, however, and differently from those systems, coupling between the controller and the main structure is of gyroscopic type, whose magnitude is governed by an electro-mechanical parameter. Based on an eigenvalue sensitivity analysis, carried out via suitably selected perturbation methods, the effectiveness of the three controllers is investigated. As an application, the two-degree-of-freedom Ziegler column, undergoing Hopf bifurcation triggered by a follower force, equipped with piezoelectric controllers, is studied, and the different strategies proposed numerically compared.

The impact of forest architecture parameterization on GPP simulations

The presence of a forest strongly affects ecosystem fluxes by acting as a source or sink of mass and energy. The objective of this study was to investigate the influence of the vertical forest heterogeneity parameterization on gross primary production (GPP) simulations. To introduce a heterogeneity effect, a new method for the upscaling of the leaf level GPP is proposed. This upscaling method is based on the relationship between the leaf area index (LAI) and the leaf area density (LAD) profiles and the standard sun/shade leaf separation method. The effect of the crown shape and foliage distribution parameterization on the simulated GPP is confirmed in a comparison study between the proposed method and the standard sun/shade upscaling method. The observed values used in the comparison study are assimilated during the vegetation period on three distinguished forest eddy-covariance (EC) measurement sites chosen for the diversity of their morphological characteristics. The obtained results show (a) the sensitivity of the simulated GPP to the leaf area density profile, (b) the capability of the proposed scaling method to calculate the contribution of the different canopy layers to the entire canopy GPP, and (c) a better agreement with the observations of the simulated GPP with the proposed upscaling method compared with the standard sun/shade method.

CFD modelling and validation of wall condensation in the presence of non-condensable gases

The aim of this paper is to present and validate a mathematical model implemented in ANSYS CFD for the simulation of wall condensation in the presence of non-condensable substances. The model employs a mass sink at isothermal walls or conjugate heat transfer (CHT) domain interfaces where condensation takes place. The model was validated using the data reported by Ambrosini et al. (2008) and Kuhn et al. (1997).

Hurricane Eyewall Evolution in a Forced Shallow-Water Model

Abstract A forced shallow-water model is used to understand the role of diabatic and frictional effects in the generation, maintenance, and breakdown of the hurricane eyewall potential vorticity (PV) ring. Diabatic heating is parameterized as an annular mass sink of variable width and magnitude, and the nonlinear evolution of tropical storm–like vortices is examined under this forcing. Diabatic heating produces a strengthening and thinning PV ring in time due to the combined effects of the mass sink and radial PV advection by the induced divergent circulation. If the forcing makes the ring thin enough, then it can become dynamically unstable and break down into polygonal asymmetries or mesovortices. The onset of barotropic instability is marked by simultaneous drops in both the maximum instantaneous velocity and minimum pressure, consistent with unforced studies. However, in a sensitivity test where the heating is proportional to the relative vorticity, universal intensification occurs during barotropic instability, consistent with a recent observational study. Friction is shown to help stabilize the PV ring by reducing the eyewall PV and the unstable-mode barotropic growth rate. The radial location and structure of the heating is shown to be of critical importance for intensity variability. While it is well known that it is critical to heat in the inertially stable region inside the radius of maximum winds to spin up the hurricane vortex, these results demonstrate the additional importance of having the net heating as close as possible to the center of the storm, partially explaining why tropical cyclones with very small eyes can rapidly intensify to high peak intensities.

A theory of the MJO horizontal scale

AbstractHere we ask, what controls the horizontal scale of the Madden‐Julian Oscillation, i.e., what controls its zonal wave number k? We present a new one‐dimensional (1D) β‐plane model that successfully simulates the MJO with the same governing mechanism as the 2D shallow water model of Yang and Ingersoll (2013). Convection is parameterized as a short‐duration localized mass source that is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. Both models tend toward a statistically steady state—a state of radiative‐convective equilibrium, not just on a global scale but also on the scale of each MJO event. This gives k ~ (Sc/c)1/2, where Sc is the spatial‐temporal frequency of convection events and c is the Kelvin wave speed. We offer this scaling as a prediction of how the MJO would respond to climate change.

Enhancement of modified solar still integrated with external condenser using nanofluids: An experimental approach

The distilled water productivity of the single basin solar still is very limited. In this context, the design modification of a single basin solar still has been investigated to improve the solar still performance through increasing the productivity of distilled water. The experimental attempts are made to enhance the solar still productivity by using nanofluids and also by integrating the still basin with external condenser. The used nanofluid is the suspended nanosized solid particles of aluminum-oxide in water. Nanofluids change the transport properties, heat transfer characteristics and evaporative properties of the water. Nanofluids are expected to exhibit superior evaporation rate compared with conventional water. The effect of adding external condenser to the still basin is to decrease the heat loss by convection from water to glass as the condenser acts as an additional and effective heat and mass sink. So, the effect of drawn vapor at different speeds was investigated. The results show that integrating the solar still with external condenser increases the distillate water yield by about 53.2%. And using nanofluids improves the solar still water productivity by about 116%, when the still integrated with the external condenser.

Local Source Based CFD Modeling of Effusion Cooling Holes: Validation and Application to an Actual Combustor Test Case

State-of-the-art liner cooling technology for modern combustion chambers is represented by effusion cooling (or full-coverage film cooling). Effusion is a very efficient cooling strategy typically based on the use of several inclined small diameter cylindrical holes, where liner temperature is controlled by the combined protective effect of coolant film and heat removal through forced convection inside each hole. A CFD-based thermal analysis of such components implies a significant computational cost if the cooling holes are included in the simulations; therefore many efforts have been made to develop lower order approaches aiming at reducing the number of mesh elements. The simplest approach models the set of holes as a uniform coolant injection, but it does not allow an accurate assessment of the interaction between hot gas and coolant. Therefore higher order models have been developed, such as those based on localized mass sources in the region of hole discharge. The model presented in this paper replaces the effusion hole with a mass sink on the cold side of the plate, a mass source on the hot side, whereas convective cooling within the perforation is accounted for with a heat sink. The innovative aspect of the work is represented by the automatic calculation of the mass flow through each hole, obtained by a run time estimation of isentropic mass flow with probe points, while the discharge coefficients are calculated at run time through an in-house developed correlation. In the same manner, the heat sink is calculated from a Nusselt number correlation available in literature for short length holes. The methodology has been applied to experimental test cases of effusion cooling plates and compared to numerical results obtained through a CFD analysis including the cooling holes, showing a good agreement. A comparison between numerical results and experimental data was performed on an actual combustor as well, in order to prove the feasibility of the procedure.

Major contribution of diatom resting spores to vertical flux in the sub-polar North Atlantic

The mass sinking of phytoplankton cells following blooms is an important source of carbon to the ocean's interior, with some species contributing more to the flux of particulate organic carbon (POC) than others. During the 2008 North Atlantic Bloom Experiment in the Iceland Basin, we examined plankton community composition from surface waters and from sediment traps at depths down to 750m. Samples collected with neutrally buoyant Lagrangian sediment traps captured a major flux event. Diatoms comprised ≥99% of cell flux into the sediment traps, with vegetative cells and resting spores of the genus Chaetoceros contributing 50–95% of cell flux. Resting spores of one species, identified as Chaetoceros aff. diadema, were dominant, comprising 35–92% of cell flux. The flux of resting spores ranged from 2 to 63mgCm−2day−1 and was significantly correlated with POC flux (p=0.003). Over the course of 10 days, the flux of resting spores increased by 26 fold, suggesting that the cells sank en masse, possibly in aggregates. In contrast, vegetative cells of C. aff. diadema sampled from surface waters during the period preceding the flux event generally comprised <1% of the diatom community and never exceeded 5.2%. Resting spores of C. aff. diadema were rarely observed in surface waters but their concentrations increased with depth (to 200m) below the mixed layer. This increase in resting spore abundance, coupled with increased dissolved silicic acid concentrations at depth, suggest that the morphological changes associated with spore formation may have occurred in the mesopelagic zone, while cells were sinking. The values of variable fluorescence (Fv/Fm) measured on sediment trap material dominated by resting spores were among the highest values measured in the study area at any depth. This, in combination with the rapid germination of resting spores in ship-board incubations, suggests that vegetative cells were not physiologically stressed during spore formation. The degradation-resistant, heavily silicified resting spore valves explain the high relative contribution of C. aff. diadema resting spores to total plankton carbon at depth. These data emphasize the ephemeral nature of organic carbon flux events in the open ocean and highlight how non-dominant species and transient life stages can contribute more to carbon flux than their more abundant counterparts.

Triggered Convection, Gravity Waves, and the MJO: A Shallow-Water Model

Abstract The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Despite its primary importance, a generally accepted theory that accounts for fundamental features of the MJO, including its propagation speed, planetary horizontal scale, multiscale features, and quadrupole structures, remains elusive. In this study, the authors use a shallow-water model to simulate the MJO. In this model, convection is parameterized as a short-duration localized mass source and is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. The following MJO-like signals are observed in the simulations: 1) slow eastward-propagating large-scale disturbances, which show up as low-frequency, low-wavenumber features with eastward propagation in the spectral domain, 2) multiscale structures in the time–longitude (Hovmöller) domain, and 3) quadrupole vortex structures in the longitude–latitude (map view) domain. The authors propose that the simulated MJO signal is an interference pattern of westward and eastward inertia–gravity (WIG and EIG) waves. Its propagation speed is half of the speed difference between the WIG and EIG waves. The horizontal scale of its large-scale envelope is determined by the bandwidth of the excited waves, and the bandwidth is controlled by the number density of convection events. In this model, convection events trigger other convection events, thereby aggregating into large-scale structures, but there is no feedback of the large-scale structures onto the convection events. The results suggest that the MJO is not so much a low-frequency wave, in which convection acts as a quasi-equilibrium adjustment, but is more a pattern of high-frequency waves that interact directly with the convection.

ISOGEOMETRIC DIVERGENCE-CONFORMING B-SPLINES FOR THE DARCY–STOKES–BRINKMAN EQUATIONS

We develop divergence-conforming B-spline discretizations for the numerical solution of the Darcy–Stokes–Brinkman equations. These discretizations are motivated by the recent theory of isogeometric discrete differential forms and may be interpreted as smooth generalizations of Raviart–Thomas elements. The new discretizations are (at least) patchwise C0 and can be directly utilized in the Galerkin solution of Darcy–Stokes–Brinkman flow for single-patch configurations. When applied to incompressible flows, these discretizations produce pointwise divergence-free velocity fields and hence exactly satisfy mass conservation. In the presence of no-slip boundary conditions and multi-patch geometries, the discontinuous Galerkin framework is invoked to enforce tangential continuity without upsetting the conservation or stability properties of the method across patch boundaries. Furthermore, as no-slip boundary conditions are enforced weakly, the method automatically defaults to a compatible discretization of Darcy flow in the limit of vanishing viscosity. The proposed discretizations are extended to general mapped geometries using divergence-preserving transformations. For sufficiently regular single-patch solutions, we prove a priori error estimates which are optimal for the discrete velocity field and suboptimal, by one order, for the discrete pressure field. Our estimates are in addition robust with respect to the parameters of the Darcy–Stokes–Brinkman problem. We present a comprehensive suite of numerical experiments which indicate optimal convergence rates for both the discrete velocity and pressure fields for general configurations, suggesting that our a priori estimates may be conservative. The focus of this paper is strictly on incompressible flows, but our theoretical results naturally extend to flows characterized by mass sources and sinks.

Hot accretion flow with ordered magnetic field, outflow, and saturated conduction

The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A∗ and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction.

The Numerical Simulation of the Shell Side Flow and Heat Transfer for 600MW Steam Turbine Condenser

Detailed prediction of steam flow field and heat transfer process is significant for the condensers. The flow and heat transfer performance of the condenser of 600MW power unit is numerical simulated. A model of porous media with distributed resistance and mass sink is used to simulate the function of the tube bundle. The equations including the continuous, momentum and air concentration are numerically solved using the finite control-volume integration method and SIMPLE algorithm. The distribution of steam velocity, pressure, heat transfer coefficient and air concentration are obtained and analyzed. On the basis of results, the condenser is evaluated.

An implicit ghost-cell immersed boundary method for simulations of moving body problems with control of spurious force oscillations

A fully-implicit ghost-cell immersed boundary method for simulations of flow over complex moving bodies on a Cartesian grid is presented. The present immersed boundary method is highly capable of controlling the generation of spurious force oscillations on the surface of a moving body, thereby producing an accurate and stable solution. Spurious force oscillations on the surface of an immersed moving body are reduced by alleviating spatial and temporal discontinuities in the pressure and velocity fields across non-grid conforming immersed boundaries. A sharp-interface ghost-cell immersed-boundary method is coupled with a mass source and sink algorithm to improve the conservation of mass across non-grid conforming immersed boundaries. To facilitate the control for the temporal discontinuity in the flow field due to a motion of an immersed body, a fully-implicit time-integration scheme is employed. A novel backward time-integration scheme is developed to effectively treat multiple layers of fresh cells generated by a motion of an immersed body at a high CFL number condition. The present backward time-integration scheme allows to impose more accurate and stable velocity vectors on fresh cells than those interpolated. The effectiveness of the present fully-implicit ghost-cell immersed boundary method coupled with a mass source and sink algorithm for reducing spurious force oscillations during simulations of moving body problems is demonstrated in a number of test cases.

GK Per (Nova Persei 1901):HUBBLE SPACE TELESCOPEIMAGERY AND SPECTROSCOPY OF THE EJECTA, AND FIRST SPECTRUM OF THE JET-LIKE FEATURE

We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), revealing hundreds of cometary-like structures. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with the Helix nebula). Some of the structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. A detailed expansion map of the nova's ejecta was created by comparing HST images taken in successive years. WFPC2 narrowband images and STIS spectra demonstrate that the physical conditions in the ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor the ejecta is < 1%, and the nova ejecta mass must be less than $10^{-4} \Msun$. A few of the nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [NII], [OII], [OIII], and [SII], this feature is probably a shock due to ejected material running into stationary ISM, slowly moving ejecta from a previous nova episode, or circum-binary matter present before 1901. An upper limit to the mass of the jet is of order a few times $10^{-6} \Msun$. The jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.

Viscous and resistive accretion flows with radially self-similar outflows

The existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamics (MHD) simulations. In this paper, we study outflows of accretion flows in the presence of resistivity and toroidal magnetic field. The mechanism of energy dissipation in the flow is assumed to be the viscosity and the magnetic diffusivity due to turbulence in the accretion flow. It is also assumed that the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and $\alpha$-prescription is used for them. The influence of outflow emanating from accretion disc is considered as a sink for mass, angular momentum and energy. The self-similar method is used to solve the integrated equations that govern the behavior of the accretion flow in the presence of outflow. The solutions represent the disc which rotates faster and becomes cooler for stronger outflows. Moreover, by adding the magnetic diffusivity, the surface density and rotational velocity decrease, while the radial velocity and temperature increase. The study of present model with the magnitude of magnetic field implies that the disc rotates and accretes faster and becomes hotter, while the surface density decreases. The disc thickness increases by adding the magnetic field or resistivity, while it becomes thinner for more losses of mass and energy due to the outflows.

Surface Flux Modeling for Air Quality Applications

For many gasses and aerosols, dry deposition is an important sink of atmospheric mass. Dry deposition fluxes are also important sources of pollutants to terrestrial and aquatic ecosystems. The surface fluxes of some gases, such as ammonia, mercury, and certain volatile organic compounds, can be upward into the air as well as downward to the surface and therefore should be modeled as bi-directional fluxes. Model parameterizations of dry deposition in air quality models have been represented by simple electrical resistance analogs for almost 30 years. Uncertainties in surface flux modeling in global to mesoscale models are being slowly reduced as more field measurements provide constraints on parameterizations. However, at the same time, more chemical species are being added to surface flux models as air quality models are expanded to include more complex chemistry and are being applied to a wider array of environmental issues. Since surface flux measurements of many of these chemicals are still lacking, resistances are usually parameterized using simple scaling by water or lipid solubility and reactivity. Advances in recent years have included bi-directional flux algorithms that require a shift from pre-computation of deposition velocities to fully integrated surface flux calculations within air quality models. Improved modeling of the stomatal component of chemical surface fluxes has resulted from improved evapotranspiration modeling in land surface models and closer integration between meteorology and air quality models. Satellite-derived land use characterization and vegetation products and indices are improving model representation of spatial and temporal variations in surface flux processes. This review describes the current state of chemical dry deposition modeling, recent progress in bi-directional flux modeling, synergistic model development research with field measurements, and coupling with meteorological land surface models.

Gas stratification break-up by a vertical jet: Simulations using the GOTHIC code

Abstract The capability assessment of three-dimensional computational tools to predict the erosion and the break-up of stratified conditions that can build-up in a containment through the release of hydrogen during an early phase of a hypothetical severe accident is the focus of intense research worldwide. In conjunction with the OECD SETH-2 project, the GOTHIC code is assessed against experiments in which mass and/or heat sources or sinks cause mixing. This paper reports on simulation results of selected experiments where the initial helium stratification in a vessel is eroded by a vertical jet originating from an injection below the initial density interface. A 3-D coarse mesh, as well as various finer 2-D meshes, is used to simulate the evolution of the helium distribution generated by jets having different initial momentum. In general, the 3-D calculations predict too slow mixing in the vessel and a reasonable agreement between calculated and measured gas concentrations can only be achieved with a sufficiently fine mesh. These results can be explained by comparing the calculated velocity field with that measured using the PIV technique, which also provides valuable insight into the mechanisms of the interaction between the jet and the density interface.

Effects of mass source/sink at the western boundary on the wind-driven gyres: Implications for the ventilation of the north pacific intermediate layer through convection in the Okhotsk Sea and tidal mixing at the Kuril Straits

A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven and mass-driven circulations. First, net mass input induces a “barotropic” mode inter-gyre flow along the western boundary through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre. Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary. Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region. Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport (Q/T Sv max ), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input rate into the lower layer to that in total (Q 2/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to the β effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with respect to Q/T Sv max and Q 2/Q are also presented.