Convection Regime Research Articles

Bejan's flow visualization of buoyancy-driven flow of a hydromagnetic Casson fluid from an isothermal wavy surface

The proposed work numerically investigates the buoyancy-driven flow of Casson fluid from a vertical wavy surface under the influence of a magnetic field and evaluates the underlying transport of heat in the free convective regime both quantitatively and qualitatively. Pertaining to this analysis, the primitive forms of coupled non-linear partial differential equations are evaluated with the help of an efficient and straightforward Crank–Nicolson implicit finite difference technique. By representing the graphical plots of flow velocity, temperature, and friction-factor coefficients, obtained for different physical parametric values pertinent to this analysis, we discuss the influence of the Hartmann number, surface waviness parameter, and rheological parameter of the fluid on the underlying thermohydrodynamics. In addition to the isotherm and streamline plots, by demonstrating the functional form, we discuss an extensive flow visualization technique that provides unique heat flow visualization for the wavy surface based on Bejan's heat function concept. The results indicate that for increasing the magnitude of the magnetic parameter, the evolution of isotherms, streamlines, and heatlines deviates periodically with a higher amplitude, which signifies the transfer of high heat energy as well as the efficient transport of heat in the convective regime.

Stationary Regimes of Electric Convection of a Low-Conductivity Fluid in the Case of Unipolar Charge Injection in a Constant Electric Field

Stationary Regimes of Electric Convection of a Low-Conductivity Fluid in the Case of Unipolar Charge Injection in a Constant Electric Field

Diurnal Cycle of Tropical Oceanic Mesoscale Cold Pools

AbstractTropical convection regimes range from deep organized to shallow convective systems. Mesoscale processes such as cold pools within tropical convective systems can play a significant role in the evolution of convection over land and open ocean. Although cold pools are widely observed, their diurnal properties are not well understood over tropical oceans and land. The oceanic cold pool identification metric applied herein uses the gradient feature (GF) technique and is compared with diurnally-resolved buoy-identified thermal cold pools. This study provides a first-ever diurnal climatology of GF number, area, and attributed TRMM 3B42 precipitation using a space-borne scatterometer (RapidScat). Buoy data over the Pacific, Atlantic, and Indian Ocean have been used to validate and examine the RapidScat-identified diurnal cycle of GF number and precipitation. Buoy-observed cold pool duration, precipitation, temperature, and wind speed is analyzed to understand the in situ cold pool properties over tropical oceans. GF- and buoy-observed cold pool number and precipitation exhibits a similar bimodal diurnal variability with a morning and afternoon maxima, thus establishing confidence in using GF as a proxy to observe cold pools over tropical oceans. The morning peak is attributed to cold pools associated with deep moist convection while the afternoon peak is related to shallower clouds in relatively drier environments resulting in smaller cold pools over global tropical oceans.

Parametric height influence analysis on thermal radiation and convection applied to real ovens

Due to their greater flexibility in heating and high productivity, continuous tunnel-type ovens have become the best option for industrial processes. The geometric optimization of ovens to better take advantage of the heat transfer mechanisms by convection and thermal radiation is increasingly researched; with the search for designs that combine lower fuel consumption, greater efficiency and competitiveness, and lower costs. In this sense, this work studied the influence of height on heat exchanges by radiation and convection and other flow parameters to define the best geometric height for the real oven under study. From the dimensions and real operating conditions of continuous tunnel-type ovens were built five numerical models of parametric variation, which were simulated with the free and open-source software OpenFOAM®. The turbulent forced convection regime was characterized in all models. The use of greater heights in the ovens increased and intensified the recirculation regions, reduced the rates of heat transfer by thermal radiation, and reduced the losses of heat by convection. The order of magnitude of heat exchanges by radiation proved to be much higher than heat exchanges by convection, confirming the results of the main references in the technical-scientific literature. It was concluded that the use of ovens with a lower height provides significant increases in the thermal radiation heat transfer rates.

Effect of the free-stream orientation on mixed convective flow past a porous cylinder

Numerical simulations are conducted for steady flow striking a porous circular cylinder from various orientations in the mixed convection regime over wide ranges of the Reynolds number (5 ≤ Re ≤ 40), Darcy number (10−6 ≤ Da ≤ 10−2), and Richardson number (0 ≤ Ri < 7). The free-stream orientation is defined as an angle (α) between the directions of the free stream and the gravitational acceleration, varying from 90° to 180°. For 90° ≤ α < 180°, the recirculating wakes present an asymmetric structure under thermal buoyancy, which is more obvious for relatively permeable cases. The impact of α on this asymmetry is inextricably dependent on Re, Ri, and Da. Diverse wake structures are identified owing to the interaction between the free-stream orientation and the strength of thermal buoyancy. Two or three regimes with various wake behaviours are distinguished, including the two-recirculating-wake regime, the single-recirculating-wake regime, and the no-recirculating-wake regime. Thermal buoyancy suppresses the development of the recirculating wakes. Less thermal energy is required for the complete suppression of the recirculating wakes at smaller Re and larger Da. The heat transfer rate in terms of the average Nusselt number (Nuave) is enhanced when α increases, and the stimulative effect of thermal buoyancy on heat transfer at constant Re and Da is more pronounced at a higher α.

Phase change dynamics in a cylinder containing hybrid nanofluid and phase change material subjected to a rotating inner disk

In this numerical study, the phase change dynamics of a 3D cylinder containing hybrid nanofluid and phase change material (PCM) is investigated with a finite element solver. The PCM consists of spherical encapsulated paraffin wax, and the flow is under the forced convection regime. The dynamic features of the phase change process are studied for different values of the Reynolds number (between Re=100 and 300), the rotational Reynolds number of the inner disk (Rew=0 and 300), and the size of the rotating disk (length between 0.1L and 0.55L; height between 0.001H2 and 0.4H2). The flow dynamics and separated flow regions are found to be greatly influenced by the rotational speed and size of the inner disk. As Re is increased, the difference between the transition times at different rotational disk speeds decreases. At Re=100, a 21% reduction in the phase transition time is observed when the inner disk rotates at the highest speed as compared to the motionless case. Up to a 26% variation in the phase transition time occurs when the size of the inner rotating disk is varied. A 5 input-1 output feed-forward artificial neural network is applied to achieve fast and reliable predictions of the phase change dynamics. This study shows that introducing rotational effects can have a profound effect on the phase change dynamics of a hybrid nanofluid system containing phase change material.

Influence of High Frequency Vertical Vibrations on Convective Regimes in a Closed Cavity at Normal and Low Gravity Conditions

Influence of High Frequency Vertical Vibrations on Convective Regimes in a Closed Cavity at Normal and Low Gravity Conditions

Double-diffusive mixed convection in rectangular cavities filled with non-Newtonian fluids

• Double-diffusive mixed convection in a double-lid driven rectangular cavity filled with non-Newtonian fluids is investigated analytically and numerically. • Analytical solution based on the parallel flow assumption agrees perfectly with the numerical solution for a wide range of governing parameters. • Mixed convection parameter is established to separate the dominance zones of natural, mixed and forced convective regimes for pseudo-plastic, Newtonian and dilatant fluids. • Increasing thermal Rayleigh number or Peclet number enhances fluid circulation and heat and mass transfer, by strengthening the buoyancy force and shear effect, respectively. • Increasing Lewis number/buoyancy ratio promotes the contribution of forced/natural regime. Also, forced convection affects strongly the rheological behavior of fluids as it abolishes its effect on flow characteristics observed in natural regime. The present study investigates numerically and analytically double-diffusive mixed convection in a closed rectangular cavity filled with non-Newtonian power-law fluid and subjected to uniform heat and mass fluxes from its vertical short sides, while the horizontal ones are insulated and impermeable and moving in opposite directions. The finite volume method is used to solve governing equations numerically, while the analytical solution is developed based on the parallel flow assumption valid in the case of a shallow cavity. A good agreement is found between both solutions for a wide range of governing parameters, namely, thermal Rayleigh number Ra T , Peclet number Pe , Lewis number Le , buoyancy ratio N , and power-law behavior index n . The effects of mentioned governing parameters on flow intensity and heat and mass transfer rates were illustrated and discussed in terms of the stream function Ψ, the average Nusselt number N u ¯ , the average Sherwood number S h ¯ , streamlines, isotherms, and iso-concentrations along with velocity, temperature, and concentration profiles. A mixed convection parameter is introduced to separate the zones where natural, mixed, and forced convective regimes dominate the convection, allowing to examine the effects of governing parameters for each separated regime. As a result, the ratios Ra T / Pe 2.6 , Ra T / Pe 3.0 and Ra T / Pe 3.4 are found to outline the limit between the predominance regions of the three convective regimes for n = 0.6, n = 1.0, and n = 1.4, respectively. It is found that increasing thermal Rayleigh number or Peclet number enhances heat and mass transfer, as the first strengthen the buoyancy force responsible for natural convection while the second promotes the contribution of shear effect accountable for forced convection. As for Le and N , interesting results are found, where increasing Le boost the contribution of forced convection while buoyancy ratio N produces an opposite effect, as increasing it leads to a more pronounced effect of natural regime on the convection. On the other hand, and while it is common knowledge that decreasing the power law-index enhances flow characteristics in the natural regime, the forced convection is found to affect strongly the rheological behavior of non-Newtonian fluids as it cancels the effect of n on flow characteristics.

Microphysical features of typhoon and non-typhoon rainfall observed in Taiwan, an island in the northwestern Pacific

Abstract. Information about the raindrop size distribution (RSD) is vital for comprehending the precipitation microphysics, improving the rainfall estimation algorithms, and appraising the rainfall erosivity. Previous research has revealed that the RSD exhibits diversity with geographical location and weather type, which leads to the assessment of the region and weather-specific RSDs. Based on long-term (2004 to 2016) disdrometer measurements in northern Taiwan, this study attempts to demonstrate the RSD aspects of summer seasons that were bifurcated into two weather conditions, namely typhoon (TY) and non-typhoon (NTY) rainfall. The results show a higher concentration of small drops and a lower concentration of large-sized drops in TY compared to NTY rainfall, and this behavior persisted even after characterizing the RSDs into different rainfall rate classes. RSDs expressed in gamma parameters show higher mass-weighted mean diameter (Dm) and lower normalized intercept parameter (Nw) values in NTY than TY rainfall. Moreover, sorting these two weather conditions (TY and NTY rainfall) into stratiform and convective regimes revealed a larger Dm in NTY than in TY rainfall. The RSD empirical relations used in the valuation of rainfall rate (Z–R, Dm–R, and Nw–R) and rainfall kinetic energy (KE–R and KE–Dm) were enumerated for TY and NTY rainfall, and they exhibited profound diversity between these two weather conditions. Attributions of RSD variability between the TY and NTY rainfall to the thermodynamical and microphysical processes are elucidated with the aid of reanalysis, remote sensing, and ground-based data sets.

Augmentation in heat transfer for a hybrid nanofluid flow over a roughened surface

Heat transfer characteristics of mixed convective flow of a hybrid nanofluid along a wavy surface are investigated. Dimensionless equations of the problem are solved utilizing finite difference method. Numerical results revealed that for increasing the amplitude-wavelength ratio, magnetic field strength and volume fractions of Cu and Al2O3 nanoparticles, the thermal boundary-layer increases. Skin friction coefficient and local Nusselt number considerably increase owing to the increase of the volume fractions of nanoparticles and magnetic field parameter. Heat transfer for a flat surface is remarkably higher than that for a wavy surface except around the crests of it. Whatever the forced, mixed or free convection regimes, skin friction coefficient and local Nusselt number are almost linearly correlated with the volume fraction of nanoparticles. Moreover, the Cu nanoparticles provide higher heat transfer than the Al2O3 nanoparticles. It is recognized that the local Nusselt number calculated on 5% volume fraction of Cu nanoparticles is higher by 0.5%, 1.0% and 7.7% respectively for forced, mixed and free convection processes, however, when the volume fraction of nanoparticles is 8% the corresponding increments are 0.9%, 1.6% and 13.3%.

Viscoelasticity and cell jamming state transition

Although collective cell migration (CCM) is a highly coordinated migratory mode, perturbations in the form of jamming state transitions and vice versa often occur even in 2D. These perturbations are involved in various biological processes, such as embryogenesis, wound healing, and cancer invasion. CCM induces accumulation of cell residual stress, which has a feedback impact to cell packing density. Density-mediated change of cell mobility influences the state of viscoelasticity of multicellular systems and on that base the jamming state transition. Although a good comprehension of how cells collectively migrate by following molecular rules has been generated, the impact of cellular rearrangements on cell viscoelasticity remains less understood, thus considering that the density-driven evolution of viscoelasticity caused by reduction of cell mobility could result in a powerful tool in order to address the contribution of cell jamming state transition in CCM and help to understand this important but still a controversial topic. In this work a review of existing literature in CCM modeling is given along with an assortment of published experimental findings, in order to invite experimentalists to test the given theoretical considerations in multicellular systems. In addition, five viscoelastic states gained within three regimes: (1) convective regime, (2) conductive regime, and (3) damped-conductive regime, which were discussed with special emphasis of jamming and unjamming states.

A Physically Based Definition of Convectively Generated Density Currents: Detection and Characterization in Convection‐Permitting Simulations

AbstractIn this study, a conceptual model to define convective density currents is proposed. Based on theory, observations and modeling studies, we define convective density currents as 3D coherent structures with an anomalously cold core, an adjacent wind gust, and a vertical structure made of two layers: a well‐mixed one near the surface and a stratified one above. With this definition, a methodology is proposed to identify and label individual convective density currents in convection‐permitting simulations. The method is applied to four distinct cloud scenes taken from a convection‐permitting simulation. Our methodology reveals new dynamic, thermodynamic, and geometric features related to the density currents’ imprint on the planetary boundary layer. The method is found to be (i) relevant in distinct convective regimes, (ii) relevant in land and oceanic situations, and (iii) adapted to both Cloud Resolving Models and Large Eddy Simulations. It also provides proxies such as the number, the spatial coverage, the mean radius, and the mean velocity of convective density currents, from which a detailed analysis of their role in the life‐cycle and spatial organization of convection could be performed in the near future.

Numerical Investigation into the Gas Production from Hydrate Deposit under Various Thermal Stimulation Modes in a Multi-Well System in Qilian Mountain.

The primary objective of this study was to investigate the energy recovery performance of the permafrost hydrate deposit in the Qilian Mountain at site DK-2 using depressurization combined with thermal injection by the approach of numerical simulation. A novel multi-well system with five horizontal wells was applied for large-scale hydrate mining. The external heat is provided by means of water injection, wellbore heating, or the combinations of them through the central horizontal well, while the fluids are extracted outside from the other four production wells under constant depressurization conditions. The injected water can carry the heat into the hydrate deposit with a faster rate by thermal convection regime, while it also raises the local pressure obviously, which results in a strong prohibition effect on hydrate decomposition in the region close to the central well. The water production rate is always controllable when using the multi-well system. No gas seepage is observed in the reservoir due to the resistance of the undissociated hydrate. Compared with hot water injection, the electric heating combined with normal temperature water flooding basically shows the same promotion effect on gas recovery. Although the hydrate regeneration is more severe in the case of pure electric heating, the external heat can be more efficiently assimilated by gas hydrate, and the efficiency of gas production is best compared with the cases involving water injection. Thus, pure wellbore heating without water injection would be more suitable for hydrate development in deposits characterized by low-permeability conditions.

Correlations for heat transfer characteristics of the onset of nucleate boiling in submerged jet impingement

The submerged jet impingement boiling is one of the most effective heat transfer methods in dissipating both transient and steady concentrated heat loads. However, studies on the submerged jet impingement boiling still lack experimental data, especially for the heat transfer characteristics of the onset of nucleate boiling (ONB). Therefore, an experimental study of a submerged circular jet impinging on a polished copper surface is conducted in the subcooled water. The experiments investigate the steady-state heat transfer characteristics at the stagnation point from single phase convection to partial nucleate boiling. The effects of jet Reynolds number, liquid subcooling, and nozzle diameter on the onset of nucleate boiling are systematically studied. The experimental results show that the single phase convective heat transfer can be enhanced by increasing jet velocity at a fixed nozzle diameter or decreasing nozzle diameter under the same jet Reynolds number. The liquid subcooling seems to have negligible effect on the single phase heat transfer coefficient. For the ONB point, it is found that the ONB wall superheat is not influenced by liquid subcooling or jet parameters. The ONB heat flux, on the other hand, is found to depend on the experimental parameters. Increasing the jet Reynolds number and liquid subcooling, or decreasing the nozzle diameter could broaden the single phase convective regime and delay the onset of nucleate boiling, in terms of heat flux. Empirical correlations are proposed for predicting the stagnation point heat transfer coefficient, heat flux, and wall superheat at the ONB point. The comparison between the predicted values and the experimental data indicates that the empirical correlations are able to predict the heat transfer characteristics of the onset of nucleate boiling within an acceptable accuracy under the applicable range.

Evaluation of the Causes of Wet‐Season Dry Biases Over Amazonia in CAM5

AbstractThis study investigates the causes of pronounced low precipitation bias over Amazonia in the Community Atmosphere Model version 5 (CAM5), a common feature in many global climate models. Our analysis is based on a suite of 3‐day long hindcasts starting every day at 00Z from 1997 to 2012 and an AMIP simulation for the same period. The Amazonia dry bias appears by the second day in the hindcasts and is very robust for all the seasons with the largest bias magnitude during the wet season (December–February). The bias pattern and magnitude do not change much during different dynamical wind regimes on sub‐seasonal time scales. We further classify the diurnal cycle of precipitation near the LBA sites from observations and hindcasts into three convective regimes: no precipitation, late afternoon deep convection, and nighttime deep convection. CAM5 can only simulate the late afternoon convective regime and completely fails to simulate the nighttime convection, which is mostly from propagating convective systems originating from remote locations. CAM5 mainly underestimates precipitation in the late afternoon and nighttime convective regimes, which occur during ∼67% of wet season days and account for ∼75% of accumulated precipitation amount in observations. The persistent warm temperature bias and slightly higher moisture below 850 mb likely trigger deep convection too frequently, resulting in an earlier but weaker rainfall peak in the diurnal cycle. Furthermore, shallow convection may not effectively transport moisture from boundary layer to the free atmosphere, which also leads to weaker deep convection events.

Convective Cloud Regimes From a Classification of Object‐Based CloudSat Observations Over Asian‐Australian Monsoon Areas

AbstractThe present study objectively classified the convective cloud objects detected by the space‐borne CloudSat radar over the Asian‐Australian monsoon region using the hierarchical agglomerative clustering algorithm. Based on key properties representing the morphological features and convective intensity of the systems, five distinct convective cloud regimes were derived. The unique Coastal‐Intense (CI) regime exhibits the most expansive horizontal scales (&gt;1,000 km), high convective strength, the strongest cloud radiative effects, the highest probability of extreme rainfall, and a significant coupling with the sharp onset of the Asian summer monsoon circulation. Second, the Coastal regime illustrates smaller but also highly organized coastal convections, with the strongest convective strength. Less than 10% of the systems in the CI and Coastal regimes overlap with the tropical cyclones. The rest three regimes mark the less organized convection at various life cycle stages mainly over the land areas, with small seasonal variation in their occurrence.

Realizing the ultimate scaling in convection turbulence by spatially decoupling the thermal and viscous boundary layers

Abstract

ABOUT SPECIFIC FEATURES OF CONVECTION IN COMPRESSED GAS

Stability characteristics of static Rayleigh - Benard convection regime in a compressible, viscous and heat-conducting gas are investigated by means of numerical simulation. It is shown that, depending on the height of convection region and the magnitude of temperature gradient, various convection modes are realized - isobaric, adiabatic, and superadiabatic. The conditions under which convection develops at stable stratification are determined. A diagram of convection modes is plotted depending on the height of the region and the magnitude of temperature gradient.

Is a more physical representation of aerosol activation needed for simulations of fog?

Abstract. Aerosols play a crucial role in the fog life cycle, as they determine the droplet number concentration and hence droplet size, which in turn controls both the fog's optical thickness and lifespan. Detailed aerosol-microphysics schemes which accurately represent droplet formation and growth are unsuitable for weather forecasting and climate models, as the computational power required to calculate droplet formation would dominate the treatment of the rest of the physics in the model. A simple method to account for droplet formation is the use of an aerosol activation scheme, which parameterises the droplet number concentration based on a change in supersaturation at a given time. Traditionally, aerosol activation parameterisation schemes were designed for convective clouds and assume that supersaturation is reached through adiabatic lifting, with many imposing a minimum vertical velocity (e.g. 0.1 m s−1) to account for the unresolved subgrid ascent. In radiation fog, the measured updraughts during initial formation are often insignificant, with radiative cooling being the dominant process leading to saturation. As a result, there is a risk that many aerosol activation schemes will overpredict the initial fog droplet number concentration, which in turn may result in the fog transitioning to an optically thick layer too rapidly. This paper presents a more physically based aerosol activation scheme that can account for a change in saturation due to non-adiabatic processes. Using an offline model, our results show that the equivalent cooling rate associated with the minimum updraught velocity threshold assumption can overpredict the droplet number by up to 70 % in comparison to a typical cooling rate found in fog formation. The new scheme has been implemented in the Met Office Natural Environment Research Council (NERC) Cloud (MONC) large eddy simulation (LES) model and tested using observations of a radiation fog case study based in Cardington, UK. The results in this work show that using a more physically based method of aerosol activation leads to the calculation of a more appropriate cloud droplet number. As a result, there is a slower transition to an optically thick (well-mixed) fog that is more in line with observations. The results shown in this paper demonstrate the importance of aerosol activation representation in fog modelling and the impact that the cloud droplet number has on processes linked to the formation and development of radiation fog. Unlike the previous parameterisation for aerosol activation, the revised scheme is suitable to simulate aerosol activation in both fog and convective cloud regimes.

Contrasting Cloud Regimes and Associated Rainfall over the South Asian and East Asian Monsoon Regions

AbstractThe Asian monsoon has large spatial and temporal variabilities in winds and precipitation. This study reveals that the Asian monsoon also exhibits pronounced regional differences in cloud regimes and cloud–rainfall relationship at a wide range of time scales from diurnal to seasonal to interannual. Over South (East) Asia, the convectively active regime of deep convection (CD) occurs frequently in June–September (March–September) with a late-afternoon peak (morning feature). The intermediate mixture (IM) regime over South Asia mainly occurs in summer and maximizes near noon. It develops as CD at late afternoon and dissipates as convective cirrus (CC) after midnight, showing a life cycle of thermal convection in response to solar radiation. Over East Asia, IM is dominant in cold seasons with a small diurnal cycle, indicating a prevalence of midlevel stratiform clouds. Further analyses show that CD and CC contribute 80%–90% of the rainfall amount and most of the intense rainfall in the two key regions. The CD-related rainfall also accounts for the pronounced diurnal cycles of summer rainfall with a late-afternoon peak (morning feature) over northern India (Southeast China). The afternoon CD-related rainfall mainly results from thermal convection under the moderate humidity but warm conditions particularly over northern India, while the morning CD-related rainfall over Southeast China is more related to the processes with high humidity. The CD/CC-related rainfall also exhibits large interannual variations that explain ~90% of the interannual variance of summer rainfall. The interannual variations of CD/CC occurrence are positively correlated with the moist southerlies and induced convergence, especially over Southeast China, suggesting a close relationship between cloud regimes and monsoon activities.