Convection Field Research Articles

Shape determination for prescribing flow velocity in unsteady natural convection fields

Shape determination for prescribing flow velocity in unsteady natural convection fields

Assessing Ionospheric Convection Estimates From Coherent Scatter From the Radio Aurora

AbstractCoherent radar backscatter spectra from auroral E region plasma irregularities can be measured with great precision and accuracy. The Doppler spectra obtained from those measurements can be used to estimate auroral convection patterns using an empirical model. These estimates are consistent with rocket, optical, and incoherent scatter radar data, but direct comparison between the different data sets is not straightforward. In order to establish the quality of the empirical model, alternative approaches are then needed. In this work, we use the electrostatic nature of the convection electric field to assess the physical consistency of the empirical model: The model will be satisfactory to the extent that the convection patterns derived from them are incompressible. The incompressibility criteria were assessed for the estimated convection patterns obtained from the substorm of 20 December 2015. Two different approaches were used for this assessment. First, an electrostatic potential was fitted to the convection pattern produced by the empirical model. A good fit implies that there exists an incompressible convection pattern consistent with the Doppler spectra measurements. Second, the uncertainties on the Doppler spectral estimates where propagated through the empirical model to calculate the expected uncertainties on the divergence of the convection field obtained directly from the empirical model. The convection field will be incompressible if its divergence lies within the uncertainty estimates. Both approaches indicate that the evaluated convection patterns are consistently incompressible.

Cloud Cells by Data of Satellite Measurements and Convective Energy Balance on Invasion of Cold Air into the Atmosphere over the Black Sea

This paper considers a typical case of winter cold air invasion into the Black Sea region in January 2010. Using the WRF numerical model, the intensive convection over the sea on January 25 has been reproduced and the simulation results have been verified using satellite data on cloudiness, cloud top temperature, and specific humidity. It has been shown that, according to both the modeling results and the satellite data, mixed convection over the sea occurred but with the prevalence of cells playing the main part in the mixing in the atmospheric boundary layer (ABL). The cloud top height has been determined both by satellite data and by simulation results; it has been shown that the model reproduces the observed increase in the cloud top height over the sea with the distance from the coast. Spectral analysis of the vertical convective moisture flux fields has shown that the model correctly reproduced the horizontal size of cloud structures near the shore. It was found from the modeling results and satellite pictures that the horizontal size of convective cells over the sea increased downwind. The main components of the convective kinetic energy balance in the atmospheric boundary layer have been considered: energy generation due to pressure pulsations and work of the buoyancy force, as well as the decrease in energy due to turbulent diffusion and convective advection. It has been shown how these quantities varied with height and with the distance from the shore.

Shallow Convective Cloud Field Lifetime as a Key Factor for Evaluating Aerosol Effects

SummaryClouds control much of the Earth's energy and water budgets. Aerosols, suspended in the atmosphere, interact with clouds and affect their properties. Recent studies have suggested that the aerosol effect on warm convective cloud systems evolve in time and eventually approach a steady state for which the overall effects of aerosols can be considered negligible. Using numerical simulations, it was estimated that the time needed for such cloud fields to approach this state is >24 hr. These results suggest that the typical cloud field lifetime is an important parameter in determining the total aerosol effect. Here, analyzing satellite observations and reanalysis data (with the aid of numerical simulations), we show that the characteristic timescale of warm convective cloud fields is less than 12 hr. Such a timescale implies that these clouds should be regarded as transient-state phenomena and therefore can be highly susceptible to changes in aerosol properties.

Deconvolution-based stabilization of the incompressible Navier–Stokes equations

The numerical simulation of the incompressible Navier–Stokes equations may suffer from instabilities due to the energy cascades activating small-scale dynamics even when high-frequency components are not in the initial conditions. The energy cascade is generally triggered by the non-linear convective term. However, the process can be triggered by particular geometries, such that the instabilities occur even with relatively low Reynolds numbers. While instabilities for high convective fields are well known and investigated (for instance, in the framework of the Variational Multiresolution formulation designed by T.J. Hughes and his collaborators), numerical stabilization of low-convection instabilities is less investigated. In this paper, we present a novel method where the backbone of classical stabilizations is merged with a localization of the potentially unstable regions by means of a deconvolution-filter indicator inspired by the Large Eddie Simulation (LES) turbulence modeling advocated by W. Layton and his collaborators. We introduce the method for steady incompressible Navier–Stokes problems, we motivate the design of the method in two different variants, the Streamline-diffusion one and the strongly consistent one. We provide some analysis of the approach and numerical results proving the improved performances in comparison with classical schemes.

Feasibility of particle image velocimetry for low-speed unconventional vapor flows

This work assesses the feasibility of the planar PIV technique to study the characteristics of a siloxane vapor D4. Titanium dioxide (TiO2) seeding particles were used to track the motion around a rotating disk in a low speed flow. Vector fields of natural convection (NC) and a superposition of NC and rotating flow were selected as exemplary cases. The particles were capable of tracing the flow since the calculated Stokes number St is 6.5×10-5. The quality of the experimental data is assessed by means of particle seeding density and particle image Signal to Noise ratio (S/N). The final results are deemed acceptable for an accurate assessment of the flow field. Rejected outliers are below 2.3% and the relative uncertainties corresponding to the average velocity fields are below 1%.

Numerical Simulation Based on High Pressure Jet Mud Impact Technology

Since crude oil is composed of colloidal solution, solid hydrocarbon, asphaltene, and fine sediment dispersal, the problem of sludge deposition will occur after the long-term operation of the underground crude oil storage. The high shear rate produced by the jet has a suction effect on the sludge. Based on the advantages of groundwater sealing reservoirs, this study uses CFD software to study the jet flow of underground crude oil reservoirs, and analyzes the effects of the flow on the sludge and the changes of the flow with time. The research shows that the jet stream has impact and entrainment in the convection field, and the jet at a certain speed can make the flow field circulate as a whole, and dissolve the sludge at the bottom of the reservoir. The energy of submerged jet decays rapidly in the process of advance.

A nonlinear parabolic equation with drift term

We consider weak solutions to a nonlinear evolution diffusion–convection equation with an unbounded convection field. We provide existence and uniqueness results as well as the asymptotic behaviour of solutions.

Numerical simulation of conjugate turbulent mixed convection in an open cavity: Evaluation of different wall heat conduction models

The flow structure and temperature field of the conjugate turbulent mixed convection in an open cavity were investigated numerically. Results including the local and average temperatures as well as Nusselt number were obtained and compared for the Reynolds number of 1200, the Rayleigh numbers of 1.25 × 1012 ≤ Ra ≤ 2.5 × 1013, the thermal conductivity ratios of 1 ≤ ≤ 1 × 105, and the reduced wall thickness of 0.05 ≤ δ ≤ 0.5. The conjugate model as well as two commonly used simplified heat conduction models, that is, one-dimensional and lumped-parameter methods, were used to solve the issue of heat transfer within the solid walls. The effects of the buoyancy, thermal conductance ratio, and reduced thickness on the errors in predicting the dimensionless temperature profile and Nusselt number using two simplified wall conduction models are discussed in detail. The results show that the one-dimensional method accurately predicts the flow field and temperature distribution for all cases considered. The lumped-parameter method shows a lower temperature distribution but a closer average Nusselt number with the conjugate method. The errors in the simplified models also increase with the increasing buoyancy force.

Organization and Oscillations in Simulated Shallow Convective Clouds

AbstractPhysical insights into processes governing temporal organization and evolution of cloud fields are of great importance for climate research. Here using large eddy simulations with a bin microphysics scheme, we show that warm convective cloud fields exhibit oscillations with two distinct periods (~10 and ~90 min, for the case studied here). The shorter period dominates the nonprecipitating phase, and the longer period is related to the precipitating phase. We show that rain processes affect the domain's thermodynamics, hence forcing the field into a low‐frequency recharge‐discharge cycle of developing cloudiness followed by precipitation‐driven depletion. The end result of precipitation is stabilization of the lower atmosphere by warming of the cloudy layer (due to latent heat release) and cooling of the subcloud layer (by rain evaporation, creating cold pools). As the thermodynamic instability weakens, so does the cloudiness, and the rain ceases. During the nonprecipitating phase of the cycle, surface fluxes destabilize the boundary layer until the next precipitation cycle. Under conditions that do not allow development of precipitation (e.g., high aerosol loading), high‐frequency oscillations dominate the cloud field. Clouds penetrating the stable inversion layer trigger gravity waves with a typical period of ~10 min. In return, the gravity waves modulate the clouds in the field by modifying the vertical velocity, temperature, and humidity fields. Subsequently, as the polluted nonprecipitating simulations evolve, the thermodynamic instability increases and the cloudy layer deepens until precipitation forms, shifting the oscillations from high to low frequency. The organization of cold pools and the spatial scale related to these oscillations are explored.

(Invited) Microfluidics for Biomedical Devices By Two Examples

The need and the tremendous value of “very small” machines and devices were realized in late 1980’s. In the following decade, the indispensable utility of micro- or even nano-level transport phenomena in these machines was recognized, leading to the start of research and development in microflows. Researchers started investigating from a point where they deemed relevant body of knowledge on macroflows stood. Over the ensuing years, there has been significant amount of resource investment to, for example, the understanding of how the underlying physics differed as compared to macroflows and, just as importantly, what existing transport phenomena tools would be applicable. It was quickly observed that what significantly changed were the relative effects of, for example, very low Reynolds numbers, the domination of surface and interface tension, and even the continuum assumption. Much happened since then, hence, today we enjoy the ever ubiquities micro- and nano-fluidic devices this body of research and development have produced. The author took interest in this field in the early 2000’s. This presentation will briefly sample a few of our earlier studies which varied from “design optimization of a micro synthetic jet actuator to control microflow separation,” to “electrodiffusiophoretic motion of charged spherical particle in a nanopore.” The emphasis, however, will be on our current research on two biomedical devices, using in silico approach and collaborating with adjoining experimentations: (i) a convective polymerase chain reactor (cPCR); (ii) microfluidics-aided collagen fiber (MCF) fabrication. Our goal in the first project is to develop a methodology for a portable and affordable c-PCR design intended for point-of-care use. Computational fluid and heat transfer models are developed to simulate the prerequisite convective transport fields. The search space for optimality is determined based on a specified range of the selected design variables. The convection-diffusion equations are then recast to determine and minimize the time-to-double the reagent (in our case, the double-stranded DNA) by the candidate design. For the MCF device, we are developing a new microfluidic methodology to fabricate advanced telocollagen fibers. The ultimate goal is to obtain an advanced microfluidic-based telocollagen manufacturing technology suitable to produce ligament and tendon repair and replacement grafts. Our specific goal is to provide the in silico guidance to the fabrication of collagen fibrils, which are much finer and better axially aligned, since prior research has shown that the mechanical strength of such fibers would be more favorable in achieving their purpose.

Magneto-thermo hydrodynamic peristaltic flow of Eyring-Powell nanofluid in asymmetric channel

Abstract This research is devoted to the peristaltic flow of Eyring-Powell nanofluid in an asymmetric channel. Robins-type (convective) boundary conditions are employed in the presence of mixed convection and magnetic field. The basic equations of Eyring-Powell nanofluid are modeled in wave frame of reference. Long wavelength and low Reynolds number approach is utilized. Numerical solution of the governing problem is computed and analyzed. The effects of various parameters of interest on the velocity, pressure rise, concentration and temperature are discussed and illustrated graphically. Brownian motion parameter and thermophoresis parameter facilitates the increase in temperature of fluid. Biot numbers serve to reduce the temperature at channel walls.

Computer simulations of Jupiter’s deep internal dynamics help interpret what Juno sees

We describe computer simulations of thermal convection and magnetic field generation in Jupiter's deep interior: that is, its convective dynamo. Results from three different simulations highlight the importance of including the dynamics in the very deep interior, although much of the convection and field generation seems to be confined to the upper part of the interior. A long-debated question is to what depth do Jupiter's zonal winds extend below its surface. Our simulations suggest that, if global latitudinally banded patterns in Jupiter's near-surface magnetic and gravity fields were detected by Juno, NASA's orbiting spacecraft at Jupiter [Bolton S, et al. (2017) Science 356:821-825], they would provide evidence for Jupiter's zonal winds extending deep below the surface. One of our simulations has also maintained, for a couple simulated years, a deep axisymmetric inertial wave, with properties at the surface that depend on the size of the model's small rocky core. If such a wave was detected on Jupiter's surface, its latitudes and oscillation frequency would provide evidence for the existence and size of Jupiter's rocky core.

Effect analysis on power coefficient enhancement of a convective wind energy collecting device in the expressway

The purpose of this paper is to study the equipment parameters of expressway convective wind energy collection devices and determine the optimal structural parameters by numerical methods. Firstly, the characteristics of convection wind energy and flow field on the expressway are analyzed and the principle of wind energy utilization is determined. Based on traditional resistance wind turbines, two different blades of the device are designed and developed. Some structural parameters (such as blade height and the number of blades) of the device are discussed based on the flow field characteristics. Secondly, the device is numerically simulated using Star CCM+ software, and the optimal structural parameters are determined based on the power coefficient. Thirdly, the proposed numerical analysis method is verified and indirect verification is used to determine the most suitable turbulence model and grid independence based on the similarity principle. Based on the above research results, the experimental verification is used to determine the high reliability of the simulation model. Finally, the simulation results between the Rectangular turbine and the Savonius turbine are compared and obtained. The results show that the device is useful for the wind energy usage, wind energy conversion and the wind resistance reduction of the opposite vehicle. At the same time, when the inlet wind speed is 15 m/s, the tip speed ratio (TSR) is 1.4, the device is rectangular turbine (RT), the height of blade is 1.55 m, the number of blades is 4, and the maximum power coefficient Cp is 0.363.

Solar cycle variation of the venus magnetic barrier

Although there is no intrinsic magnetic field at Venus, the convected interplanetary magnetic field piles up to form an induced magnetosphere around the planetary ionosphere. Previous investigations show that the magnetic barrier, the part of the induced magnetosphere in the dayside inner magnetosheath, can act as an effective obstacle to the solar wind during solar maximum, and the magnetic barrier can stop the solar wind even during solar minimum. In this study, we perform a comprehensive statistical study of the magnetic barrier near the terminator during almost a complete solar cycle by using Venus Express magnetic data. The magnetic barrier configuration is located at the dayside even near the terminator and a hemispheric asymmetry exists during the whole solar cycle. We also demonstrate that the general magnetic barrier configuration is controlled by the interplanetary magnetic field orientation and solar cycle dependent. The magnetic barrier under IMF quasi-perpendicular to the solar wind flow is stronger than quasi-parallel to the solar wind flow during the solar cycle, and this difference becomes larger with the increase in solar activity.

Analysis of stochastic bifurcations with phase portraits.

We propose a method to obtain phase portraits for stochastic systems. Starting from the Fokker-Planck equation, we separate the dynamics into a convective and a diffusive part. We show that stable and unstable fixed points of the convective field correspond to maxima and minima of the stationary probability distribution if the probability current vanishes at these points. Stochastic phase portraits, which are vector plots of the convective field, therefore indicate the extrema of the stationary distribution and can be used to identify stochastic bifurcations that change the number and stability of these extrema. We show that limit cycles in stochastic phase portraits can indicate ridges of the probability distribution, and we identify a novel type of stochastic bifurcation, where the probability maximum moves to the edge of the system through a gap between the two nullclines of the convective field.

A case study investigation into the effects of spatially dependent convection coefficients on the fatigue response of a power plant header component

Societal and environmental pressures are forcing thermal power plant operators to deviate greatly from the generation strategies of the past. The application of high frequency start up/shut down/partial load cycles to components that may well be outside their design life makes research into the long-term integrity of at risk assets paramount. Decoupled thermal/mechanical analyses have been used in the literature to estimate anisothermal fatigue in header components, with convective boundary conditions typically assumed for internal surfaces in order to determine heat fluxes and hence a temperature field. In reality, convective coefficients are heavily dependent upon local velocity profiles. In the present work, computational fluid dynamics is used in order to better approximate the steam flow in a real power plant header, leading to a convection coefficient field that is used to solve the thermal problem. Anisothermal fatigue analysis is finally conducted using a Chaboche type model. The results of computational fluid dynamics have illustrated that heat transfer coefficient values can vary (spatially) by a factor of 5.49 over the internal header wall, with noticeable hot spots in the wake of the stub penetrations. Peak differences of 6.47% in accumulated plastic strain levels have been observed between simulations conducted with constant (simplified) and variable (computational fluid dynamics derived) thermal boundary conditions.

Theoretical aspects of upscale error growth on the mesoscales: Idealized numerical simulations

The mechanism for the upscale growth of small errors through the atmospheric mesoscales has not been conclusively identified, but geostrophic adjustment in response to diabatically generated motions such as cumulus convection is a plausible candidate. In a companion paper, an analytic solution of the linearized, hydrostatic Boussinesq equations to an impulsive, localized heat source that mimics the effect of latent heating within a convective cloud on an unperturbed, rotating environment is found. Three characteristics of the solution are shown to be potentially useful for identifying the geostrophic adjustment process in numerical simulations. The predictions relate to the horizontal gravity wave speed, the Rossby number and the quantitative relationship between a precipitation anomaly and the balanced flow response (i.e. large‐scale vorticity).Here these predictions are tested in the framework of error growth experiments in idealized numerical simulations of a convective cloud field. Three different rotation rates are employed in order to identify the geostrophic adjustment mechanism and allow a quantitative comparison with the predictions of the analytic model. The gravity wave speed estimated from the simulations resembles the theoretical value and is independent of the Coriolis parameter, as predicted. The Rossby number resulting from the proposed scaling of temporal and spatial coordinates features a unique shape and the vorticity diagnostic agrees quantitatively with the analytical predictions. Based on these findings, it is proposed that upscale error growth through the atmospheric mesoscales is governed by the geostrophic adjustment process.

Mixing in a co‐current upflow bubble column reactors with and without internals

AbstractLiquid phase mixing is a phenomenon that results mainly due to convective and turbulent flow fields, which are generated by hydrodynamic interactions between the gas and liquid phases within a continuous co‐current upflow bubble column reactor. The extent of liquid phase mixing is usually quantified through the mixing time, or the axial dispersion coefficient. In the present work, the computational fluid dynamics (CFD) simulations for mixing and RTD in a continuous bubble column (with and without internals) are performed by using OpenFOAM 2.3.1. The superficial gas velocities were 0.014, 0.088, and 0.221 m/s and the superficial liquid velocities were 0.005 and 0.014 m/s. The simulations have been performed for three different configurations of the bubble column, that is, (a) an open bubble column, (b) a column with one vertical central rod of 36 mm diameter, (c) a column with the same central rod and four vertical additional rods of 12 mm diameter. The effects of superficial gas and liquid velocities and column internals were investigated on liquid phase mixing and the axial dispersion coefficient. Comparisons have been made between the experimental measurements and the CFD simulations.

The thermal and hydrodynamic behaviour of confined, normally impinging laminar slot jets

Jet impingement cooling is an effective means of inducing high convective heat transfer coefficients for applications such as the cooling of gas turbine surfaces, drying of textiles, and the tempering of metal and glass. Slot jets can be used for applications which require cooling over a line or strip, and these can be realized at the micro-scale if size is constrained. In this paper the application of these jets is of specific interest for high-density photonic integrated circuits (PICs), which can generate device-level heat fluxes as high as 1kW/cm2. In this context, an understanding of the thermal and hydrodynamic behaviour of low Reynolds number, submerged, confined and normally impinging slot jets is currently unavailable in the literature and would be beneficial for the design of micro-scale cooling systems. This investigation utilized the isoflux heated foil technique and Particle-Image Velocimetry (PIV), to measure the single-phase convective heat transfer and velocity fields associated with confined liquid slot jets. The aspect ratios of the jets were varied from 1 to 8, with a constant hydraulic diameter, in order to determine the influence of aspect ratio on the measured parameters. The study was carried out over a jet exit Reynolds number range of 100–500, and a fixed confinement ratio of H/Dh=1. The results showed that, enhancements of up to 68% in area-averaged heat transfer could be achieved through increasing slot jet aspect ratio, with a corresponding 12% reduction in head loss coefficient. The appearance of off-center peaks in the velocity and Nusselt number distributions were also observed. These peaks were concluded to be as a result of the stagnation zone fluid dynamics and local flow entrainment.