Thermal Stratification Research Articles

Predicting Breast Self-Examination Practices among Catholic Nuns in Tanzania's Lake Zone: A Health Belief Model Approach

Background: Early breast cancer detection is crucial for improving breast cancer outcomes. Breast Self-Examination (BSE) is a valuable tool that can empower Catholic nuns to take charge of their health, particularly in constrained resource countries like Tanzania. This study aims to use the Health Belief Model (HBM) to predict SBE practices among Catholic nuns in Tanzania's Lake Zone. Methods: This study employed a cross-sectional design whereby a total of 385 catholic nuns were enrolled to participate in the study. The Health Beliefs Model (HBM) guided the study of the interrelated variables related to self-breast examination. Data was collected using a self-administered questionnaire and analyzed using Stata version 18.0. Bivariate and multivariate logistic regression was used to test for association at a 5% significant level. Results: The prevalence of non-performance self-breast examination was 64.2% (95% CI, 59.1%-69.0%). The multivariate logistic regression revealed that nuns who are in the non-health field were more likely not to perform self-breast cancer examination with an AOR of 1.67 (95% CI, 1.02-2.73, p=0.041). Likewise, nuns with high barrier were more likely not to perform self-breast cancer examination with an AOR of 1.88 (95% CI, 1.17-3.02, p=0.009), also, an AOR of 2.25 (95% CI, 1.39-3.65, p=0.001) for nuns with low self-efficacy. Conclusion: The study revealed that the HBM constructs can predict the SBE behavior of catholic nuns. In addition, educational health interventions are highly recommended to enhance SBE practice among this special group.

Lake Champlain Zooplankton Community Dynamics Following an Extreme Flood Event

Lake Champlain, the sixth largest freshwater lake in the U.S., is a deep thermally stratified temperate zone lake system. Recently, flood events have impacted the Northeastern U.S. more frequently than in past decades, resulting in increased turbidity and other impacts in northern temperate lakes. This study represents an unprecedented analysis of the impacts of major spring and summer flooding in 2011 on Lake Champlain zooplankton communities. Few studies exist on flood impacts on lake systems, especially in natural un-impounded lakes. Our results illustrate the impacts of large-scale flooding on planktonic communities in deep stratified temperate lakes and the differential responses among species based on autecological traits. Community responses include flood-adapted increase during the flood event (Ceriodaphnia reticulata and Eubosmina coregoni), a flood-intolerant decline (Asplanchna spp. and Keratella cochlearis) or a delayed flood intolerance (Daphnia retrocurva, Mesocyclops edax, and Polyarthra spp.). Our results suggest that large, temperate lakes such as Lake Champlain will experience community shifts in zooplankton composition during future extreme flood events associated with climate change-related weather patterns in the Northeastern U.S.

Large eddy simulation of flow field and pollutant diffusion around buildings with different thermal stratifications

The main aim of the current article is to explore the influence of buildings on flow field structure in the surrounding area and analyze the dispersion of pollutants using the large eddy simulation (LES) model with varying Richardson numbers (Rib). The research findings indicate that the LES model effectively replicates the effect of buildings on flow field structures and pollutant dispersion, closely aligning with wind tunnel experiments (WTE) results. The impact of various Ribvalues on the flow field structure is not immediately evident within recirculation zone on building sheltered (x/H<1.0). However, the impact of thermal stratification becomes more pronounced farther downwind. In the Rib range of 0.22–0.47, significant changes occur in the vortex center position and length of the cavity region, with Rib=0.3 exhibiting a maximum cavity length of approximately 1.9 H, suggesting a potential critical Rib between 0.22 and 0.3. As Rib increases, the energy fluctuation in the high-frequency range rises, while low-frequency energy fluctuation remains comparatively smaller. Moreover, higher Rib values lead to increased pollutant concentration near the ground on building downwind side, making it more prone to pollutant accumulation at ground level.

Analysis of nitrogen migration and transformation in the typical deep and large reservoir of the Lancang River — Evidence from nitrogen and oxygen isotopes

The environmental effect resulting from dam construction and the formation of large reservoirs has long been a topic of significant concern. While current research at a macro scale provides a well understanding regarding the evolution of nitrogen nutrient status in river-reservoir ecosystems, the vertical migration and transformation characteristics of nitrogen in reservoirs under changing runoff remain less defined. This study centers on a typical deep reservoir along the Lancang River-Nuozhadu (NZD) Reservoir in April and December. The monitoring reveals a distinct vertical stratification phenomenon in the NZD Reservoir in the vertical profile in April, which can be categorized into a mixed layer (ML) (0–5 m), a thermocline layer (TL) (5–40 m), and an isothermal layer (IL) (>40 m) based on the water temperature. However, little stratification in the profile is occurred in December. The results of nitrogen and oxygen isotopes in April indicate that the biochemical process of nitrogen in the NZD Reservoir is predominantly driven by nitrification. However, different nitrogen biochemical processes are coupled in various stratified regions in the vertical direction. In the ML, simultaneous assimilation of ammonia nitrogen by phytoplankton occurs. In the TL, there is a process of oxidation and decomposition of particulate organic nitrogen due to a significant stratification effect and an extended hydraulic retention time. In the region of minimal dissolved oxygen (30–40 m), simultaneous denitrification may take place. In December, the main process is nitrogen assimilation in reservoir surface (ML). Statistical analysis of environmental factors highlights water temperature (T) and dissolved oxygen (DO) as crucial environmental factors influencing nitrogen migration and transformation within the reservoir. This study suggests that in deep reservoirs experiencing vertical thermal stratification, the characteristics of DO and nitrogen migration and transformation exhibit significant vertical spatial heterogeneity. This phenomenon may lead to diverse ecological environmental effects. Future efforts should focus on enhancing the fine monitoring of stratified water bodies in deep and large reservoirs and analyzing the ecological environmental effects of material cycling in greater detail.

Response of the algal-bacterial community to thermal stratification succession in a deep-water reservoir: Community structure, co-assembly patterns, and functional groups

Thermal stratification in lakes and reservoirs may intensify and become more persistent with global warming. Periodic thermal stratification is a naturally occurring phenomenon that indicates a transition in aquatic ecosystem homeostasis, which could lead to the deterioration of water quality and impaired aquatic communities. However, the responses of communities and associated nutrient cycling processes to periodic thermal stratification are still poorly understood. This study delved into the changes in water quality, algal-bacterial communities, and functional diversity influenced by thermal stratification succession, and their relationship with nutrient cycling. The results indicated that the apparent community dynamics were driven by environmental factors, with ammonium (NH4+) and nitrate (NO3−-N) being the most important factors that influenced the algal and bacterial community structure, respectively. Ecological niche widths were narrower during thermal stratification, exacerbating the antagonism of the communities, and stochastic processes dominated community assembly. Then, the complexities of the co-occurrence network decreased with succession. Algal community assembly became more deterministic, while bacterial assembly became more stochastic. Moreover, the roles of algal-bacterial multidiversity in nutrient cycling differed: bacterial diversity enhanced nutrient cycling, whereas algal diversity had the opposite effect. These findings broadened our understanding of microbial ecological mechanisms to environmental change and provided valuable ecological knowledge for securing water supplies in drinking water reservoirs.

Impacts of thermally stratified medium on transient convective heat transfer between co‐axial horizontal fixed pipes: Applications of the thermal stratification

AbstractThermal stratification improves coaxial pipe systems’ efficiency and stability. Thermal stratification enables accurate temperature maintenance, reduces thermal stress, optimizes heat transmission performance, and minimizes usage of energy to guarantee the system's long‐term performance. The main aim of the current study is to investigate the impacts of thermal stratification on buoyancy force flow and thermal transmission between coaxial fixed pipes. In the present research, the applications of thermally stratified medium on transient convective heat transfer between two coaxial fixed pipes are studied. A two‐dimensional mathematical formulation in terms of mutually nonlinear partial differential equations is used to analyze the unsteady flow and temperature field between the co‐axial pipes, when the internal pipe is uniformly heated and the outer wall of the external pipe is placed at infinity from the surface of the inner fixed pipe. Flow is assumed along the axial direction of the internal pipe and stationary boundary condition is assumed at the surface of the inner pipe. The coupled equations of the simulated model are solved numerically by applying the Implicit Finite Difference Technique. The computed outcomes in the form of geometrical interpretation are highlighted by using the technically advanced software TECHPLOT‐360. Comprehensive detail of the obtained results for the non‐dimensional parameters included in the flow formulation is predicted for steady state velocity, temperature distribution, time‐dependent surface shearness and time‐dependent energy shearness in results and discussion section of the manuscript. The emphasis is placed on the thermal stratification parameter in the above mentioned chief quantities. From the obtained results, it is predicted that the fluid flow pattern and thermal distribution are both reduced for rising values of the thermal stratification parameter S = 0.001, 0.03, 0.05, and 0.07. Minimum flow and thermal profile are observed at S = 0.07. Further, the amplitude of the time‐dependent surface shearness is uniformly distributed throughout the medium and the amplitude of the time‐dependent energy shearness is reduced effectively for S = 1.0, 5.0, and 10.0.

Investigation of heat transfer enhancement and thermal stratification phenomenon on supercritical hydrogen fuel flow

Thermal stratification is one of the major issues in rectangular regenerative cooling channels because of the asymmetric heating and limited turbulent diffusion of thermal energy of the fuel. As a result, the heat transfer performance is decreased. This paper uses Ansys fluent software to construct a novel cooling structure that combines wavy wall surfaces with cylindrical fins. In addition to this new configuration, this study also examined three other configurations (straight-smooth, cylindrical-ribs, and wavy-smooth) and compared them with each other. This structure aims to efficiently enhance hydrogen coolant flow and heat transfer characteristics while reducing the extent of the non-uniform distribution issue. The results show that the new structure’s Nusselt number and maximum wall temperature are enhanced by 46.89% and 22.71%, respectively, compared to the straight-smooth structure; in contrast, the friction factor increased only by 3.64% over the smooth channel. The novel design has the best thermal performance factor compared to the other three designs, which is improved by 43.33% over the smooth channel. Among all configurations, the new structure is the best fluid temperature distribution configuration, which shows an optimal distribution of the coolant temperatures between layers; thus, the stratification phenomena decreased significantly.

POD–ANN as digital twins for surge line thermal stratification

This paper describes a hybrid proper orthogonal decomposition (POD) and artificial neural network (ANN) strategy to construct digital twins of a pressurizer surge line under thermal stratification conditions. The one-way coupled conjugate heat transfer and thermal stress analysis was conducted by use of parametric modeling and the introduction of the inverse distance weighted interpolation for the grid mapping, which allows for the mapped grids to have the same number of nodes regardless of variations of surge line geometries. A snapshot-based POD was utilized to obtain truncated lower-order modes and the full-order system response was projected onto these modes with reduced state coefficients. Then the ANN was employed to establish a surrogate model between the five chosen design variables of interest and the reduced state coefficients, resulting in a surrogate-assisted digital twin for a pressurizer surge line. Prediction of fluid–structure interface temperature and thermal stress distribution was thus achieved in an in-line real-time manner for a wide range of parameter variations. We publicly share all code implementations, and we believe that our efforts open a door for the digital twining of thermo–fluid–structure interaction problems

Unsteady flow past an impulsively started infinite vertical plate in presence of thermal stratification and chemical reaction

Unsteady flow past an impulsively started infinite vertical plate in presence of thermal stratification and chemical reaction

Tracing Holocene temperatures and human impact in a Greenlandic Lake: Novel insights from hyperspectral imaging and lipid biomarkers

Global warming particularly impacts terrestrial and aquatic ecosystems in the Arctic. To constrain the sensitivity of Arctic lakes and make meaningful predictions about future change under global warming, we need to examine their response to previous warm phases. Lake sediments from Greenland's deglaciated area offer valuable archives to investigate past climate variability and associated lake changes.Here, we applied hyperspectral imaging and lipid biomarker thermometry to a Holocene-length sediment record from Lake 578 in the Eastern Settlement of the Norse (61.08° N, 45.62° W; ∼155 m a.s.l) to investigate local temperature, productivity, and anoxia histories. We calibrated branched glycerol dialkyl glycerol tetraethers (brGDGTs) with summer mean water temperatures (SMWT) using a previously published site-specific calibration and analyzed pigment fluxes based on hyperspectral imaging. Notably, the anoxia reconstructions were corroborated with two independent proxies (GDGT-0/Crenarchaeol and bacterio pheophytins). We investigated the lake's environmental history and identified periods of significant change by employing generalized additive models (GAMs).Our results reveal significant transitions in Lake 578 driven both by natural climate shifts and anthropogenic impacts. During the early Holocene, low SMWT and productivity coupled with high anoxia suggest strong seasonality and prolonged inverted thermal stratification, possibly enhanced by extended ice cover. The mid-Holocene showed higher SMWT and productivity along with low anoxia, indicating a dimictic lake system. The early Holocene temperature rise lagged that of to the Northern Hemisphere, but closely followed the Atlantic-Fennoscandian stack. The Holocene Thermal Maximum (7.5–4.5 cal ka BP) aligns with other regional reconstructions. After 3 cal ka BP, we observed a Neoglacial cooling characterized by increased anoxia and reduced temperatures due to enhanced stratification. At around 1.0 cal ka BP, Lake 578 saw a surge in productivity and anoxia, which we attribute to land use and lake damming by the Norse. Despite a post-Norse decline in productivity and disappearance of anoxia, the lake never reverted to its pre-Norse state, with modern sheep farming further intensifying productivity in recent decades. While early Holocene anoxia resulted from natural cold temperature stratification, anoxia during the Norse period was anthropogenically induced.This research underscores the value of integrating lipid biomarkers with hyperspectral imaging for detailed reconstructions of changes within Arctic lakes. It provides crucial insights for anticipating the ecologic and climatic resilience of Arctic lakes to ongoing global warming and anthropogenic influence.

Environmental Variables Outpace Biotic Interactions in Shaping a Phytoplankton Community

We evaluated the main environmental factors (abiotic and biotic) driving the phytoplankton community in a shallow tropical reservoir located in an environmentally protected area. Phytoplankton samples were collected from the surface and bottom of the reservoir. The phytoplankton samples were later identified at the species level, and the species were further assigned to morphology-based functional groups (MBFGs). Zooplankton were sampled through vertical haul, communities were identified to species level, and functional diversity was estimated based on community-weighted means (CWM). Phytoplankton MBFGs IV, V, and VI contributed the most to the biomass under high light availability coupled with low nutrient availability. Potentially toxic cyanobacteria from MBFG III were observed during thermal stratification. Hydraulic mixing plays a crucial role in reducing the phytoplankton biomass during the warmer/rainy season. Cyclopoid copepods accounted for more than 83% of the zooplankton biomass. There was a weak but significant effect of zooplankton functional diversity on phytoplankton functional diversity, mainly because of the dominance of small zooplankton. Altogether, our findings suggest that environmental filtering plays a greater role than zooplankton grazing in phytoplankton community structure in this shallow tropical reservoir.

The flow regimes of two parallel-connected chambers respectively with a mechanical extractor and a buoyancy source

This research studies the ventilation flow patterns and density stratification in two parallel-connected chambers which respectively have a mechanical extractor and a localized buoyancy source. Two parallel-connected chambers are denoted as the forced and extracted chambers. The forced chamber having a continuous localized buoyancy source is located upstream, and the extracted chamber having a mechanical extractor is located downstream. Two chambers are interconnected by an internal connection opening and are connected to the exterior in a parallel manner through their own connection openings. The effects of the adjoining chamber having a mechanical extractor on the ventilation flow patterns and density stratification in the space are investigated in this study. The flow driving forces include the buoyant and inertial forces in this ventilated space. The buoyant force results from the buoyancy source, and the inertial force is due to the mechanical extractor. This research uses the theoretical analysis approach and the laboratory reduced-scale model to investigate this problem. This paper focuses on the ventilation flow patterns in the steady state and discusses different flow regimes due to the adjoining extracted chamber effect. The internal connection opening level and orientation between two chambers are discussed in this study. Three flow regimes, the small (or reduced), transition, and large (or enhanced) flow regimes, are found in two parallel-connected chambers and there is a range of critical flow rates to determine the flow regime when the internal connection opening is located in a partition wall as investigated in this research. If the internal connection opening is located in a horizontal ceiling, a clear-cut critical flow rate is expected and only two flow regimes exist. This study discusses the characteristics of different flow regimes in the space of two parallel-connected chambers. The paper presents three derived theoretical models and analogous salt-bath experimental results for three flow regimes, and comparisons between them show reasonable agreement.

The Role of Tide and Wind in Modulating Density Stratification in the Pearl River Estuary during the Dry Season

Density stratification plays a crucial role in estuarine hydrodynamics and material transport. In this study, we utilized a well-calibrated numerical model to investigate the stratification processes and underlying mechanisms in the dynamically wide Pearl River Estuary (PRE). In the upper estuary, longitudinal straining governs stratification, enhancing it during ebb tide and reducing it during flood tide. The Coriolis force becomes significant in the lower estuary due to the increased basin width, causing seaward freshwater to be confined to the West Shoal, where a pronounced transverse salinity gradient forms. Interacting with lateral current shear, density stratification is most pronounced in this region. The prevailing northeasterly wind creates a mixed layer near the surface, shifting stratification to the middle layer of the water column in the upper estuary. Wind stirring reduces stratification throughout the estuary. Under the wind’s influence, the seaward outflow is confined to a narrower region and shifts westward, resulting in the most apparent stratification occurring on the West Shoal of the PRE due to lateral straining. These findings on the evolution of freshwater pathways and their role in modulating density stratification have significant implications for other wide estuaries, such as Delaware Bay and the La Plata-Parana estuary.

Momentum induced by steam injection into a subcooled pool

Boiling Water Reactors (BWR) and Advanced Pressurized Water Reactors (APWR) often use spargers to release steam from the primary coolant system into a pool with subcooled water to prevent containment overpressure. Direct contact Condensation (DCC) creates sources of mass, heat, and momentum determined by the condensation regimes in a subcooled pool. Thermal stratification can develop in the pool if buoyancy forces created by the heat source dominate the momentum source. Only part of the stratified pool volume can be used as the heat sink which is a safety concern. Modeling of steam injection and its effect on a large pool is computationally expensive due to the considerable spatial and temporal scale differences between the steam-water interface dynamics and global pool circulation. To enable the prediction of realistic plant transients, Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were proposed. These models aim to calculate the time-averaged integral effects of the steam injection on the pool without resolving the dynamics of the interface and DCC phenomena.This work aims to develop the empirical correlations that predict the time-averaged effective momentum induced by steam injection into a subcooled pool. The experimental data were collected in a Separate Effect Facility (SEF-POOL) at LUT, Finland. The force acting on the injection pipe was measured in SEF-POOL to estimate the effective momentum rate created by steam injection. The condensation regime coefficient C, defined as the ratio of effective momentum rate to the theoretical momentum rate of injected steam, is presented as a function of Jakob and Mach numbers. We found that the pitch-to-diameter ratios (P/D) had a significant effect on both the effective (time averaged) momentum rate and instantaneous forces measured in the low Jakob number region, which might be attributed to the interactions between neighboring nozzles.

The causes of algal blooms exist significant scale effect in tributary of the Three Gorges Reservoir

Algal blooms of rivers becomes a global issue, while there is controversy over the attribution of blooms due to the differences of research scales. This study utilized random forest and spatiotemporal geographically weighted models to explore the triggers and mechanisms of blooms from the different spatial resolutions (river, local and grid scales) in tributary bay of the Three Gorges reservoir based on Sentinel-2 data and Environmental Fluid Dynamics Code model. The results indicated that blooms in the Daning River were more severe during the spring and summer, with the average chlorophyll about 10.15 ug/L and 9.55 ug/L. The high-risk areas primarily located in the middle reaches (10.65 ug/L). The causes of blooms existed significant scaling effect. The water temperature recognized at the river scale was not a sufficient condition for blooms in the early stage of drainage. Compared with temperature, the relative importance of Zme ranked in the top three, which highlighted the influences of Zme at the local and grid scales. The macro-analysis identified the importance of water level fluctuations during drainage stage and the low level period, with ranking in the top two. The effects indirectly altered Zme and Zm, thus highlighting triggering effect of Zme and Zm at local scales. During the stage, thermal stratification and water circulation patterns determined the heterogeneity of blooms. The lag time between chlorophyll and these key factors ranged from 1 to 5 days. This diagnostic to the spatiotemporal scale contributes to understand of the formation mechanism of algal blooms in the tributary bay of the Three Gorges reservoir, providing reference for early warning and prevention strategy.

A sequence of weather‐driven hydrodynamic events stimulates the formation of harmful algal blooms on an oligotrophic lake

AbstractHarmful algal blooms (HABs) are common in many eutrophic lakes and frequently associated with nutrient excesses, warm waters, and calm conditions. While HABs can also occur in oligotrophic waterbodies, bloom‐stimulating factors remain elusive for these ecosystems. Here, using a high‐frequency sensor platform supported by hydrodynamic modeling, we document a clearly linked sequence of three hydrodynamic events that immediately preceded each HAB event on Skaneateles Lake, NY during 2018. HABs were first preceded by interactions between internal waves in the thermal structure of the lake with the northern slope of the lakebed. These interactions disturbed sediments resulting in upwelling of nutrient‐rich sediment and possibly cyanobacterial cells into the lower water column. Next, moderate winds stimulated Langmuir circulation which facilitated the transport of material from the lower water column to well‐lit surface waters. Finally, the sequence culminated with a calm wind period, which is often classically associated with surface HAB formation. While individual steps in this sequence occurred independently and repeatedly throughout the year, the complete sequence occurred only eight times, immediately preceding each of the eight documented HABs. We empirically derive thresholds for and timing of the sequence to independently predict HAB occurrence and elevated cyanobacteria levels in the lake. The discovery of this specific and repeated event sequence and the identified thresholds provides the potential for forecasting of HABs days in advance. Ongoing climate change impacts, including increasing water column thermal stratification and changing wind speeds may alter the frequency, timing, and duration of this sequence and consequent HAB formation.

The similitude of indoor airflow in natural ventilation for a reduced-scale model: Investigation of nonisothermal flow fields by RANS simulation

Reduced-scale experiments and simulations are important approaches in natural ventilation research, and the similarity requirement is fundamental for generalising the flow characteristics obtained from reduced-to full-scale conditions. However, the similarity requirement of a nonisothermal natural ventilation flow in a reduced-scale model poses additional challenges because of the reduced approaching flow, which can potentially result in Reynolds dependence issues. This study investigated the Reynolds number (Re) independence of indoor airflow in natural ventilation under isothermal and nonisothermal conditions using computational fluid dynamics (CFD) with Reynolds-averaged Navier–Stokes. A wind tunnel experiment was first conducted to validate the accuracy of the CFD using a reduced-scale model. Indoor airflow fields characterised by the same Archimedes number (Ar) but with varying approaching wind velocities and temperatures were compared between the full-scale and 1/10 reduced-scale simulations. The dimensionless ventilation rate showed the least dependence on the Re number, while the temperature field was very sensitive to the Re number, especially in the near-wall region. However, the temperature field on the ventilation pathway is much less dependent on the Re number, the deviation of which is less than 10 % compared to the full-scale simulation. The temperature distribution in the reduced-scale simulation exhibits a thermal stratification pattern similar to that in the full-scale simulation.

The prevalence of vertical transmission of human immunodeficiency virus and associated factors among exposed infants in Eastern Lake zone and Southern Highland of Tanzania: a cross-sectional study

Background Globally, the rate of antiretroviral therapy coverage for pregnant women living with human immunodeficiency virus (HIV) increased by 38% between 2010 and 2015 but only by 2% between 2016 and 2020. Objectives We aimed to determine the prevalence of vertical transmission of HIV among infants from mothers living with HIV and associated factors in the Eastern Lake Zone and Southern Highland of Tanzania from January to December 2022. Methods This retrospective cross-sectional study extracted data from the Open Laboratory Data Repository database collected from January to December 2022 at 93 health facilities. A total of 1,411 infants exposed to HIV from the Mbeya (851), Songwe (304), and Mara regions (256) were enrolled. Results The prevalence for vertical transmission of HIV was 2.48% (35/1411). We observed a non-significant difference in the prevalence of vertical transmission in children whose first test was done below six weeks of life (1.89%) and other age groups (2.52-2.62%) (p < 0.917). Children not given antiretroviral prophylaxis had eleven times higher odds of acquiring infection (AOR 11.39, 95% CI: 3.61–35.97). Mothers who were not on ART during pregnancy had three times the odds of transmitting HIV to their infants (AOR 3.03, 95%CI: 0.91–10.15). Conclusions We found a low prevalence of vertical transmission of HIV compared to previous studies done in Tanzania. The use of ART prophylaxis for infants exposed to HIV is significantly associated with the low rate of HIV transmission.

Investigating the influence of stepped roofs on wind dynamics using large eddy simulation

This study investigates airflow and turbulence within varying roof configurations in urban boundary layers by using the parallelized large-eddy simulation model (PALM). Key findings reveal stark contrasts in airflow between stepped and flat roof arrangements, with the former presenting dual counter-rotating vortices and enhanced vertical motion in one street, while the latter exhibited a primary vortex and several secondary vortices. Additionally, normalized turbulence kinetic energy (TKE) was markedly lower in flat roof configurations, with stepped roofs fostering more significant turbulence and small-scale turbulent motion. This variability intensified with decreasing step height and quantity. Vertical profiles further disclosed considerable velocity disturbances and larger Reynolds numbers in one street of stepped roofs, affirming the roof’s influence on turbulence generation and transport. Dispersive stress also displayed differing characteristics in various canyons. Turbulent momentum transport through eddies significantly differs between the windward and leeward streets of flat and stepped roofs. Further exploration into density stratification effects and scalar transport in stepped roofs would enhance practical implications.

Coupled modeling and simulation of tank self-pressurization and thermal stratification

Numerical analysis of thermo-fluid dynamics for cryogenic propellant storage primarily consists of nodal and computational fluid dynamics (CFD) modeling approaches. While nodal approaches prioritize faster computation over fidelity, CFD approaches promise accuracy and fidelity at the expense of significant computational resources. This paper presents an efficient coupling methodology developed to facilitate the simulation of a long-term self-pressurization process of a cryogenic propellant tank as well as associated thermal stratification phenomenon under both normal and reduced gravity conditions. Key highlight of the methodology is the development of a coupling scheme based on a domain decomposition approach that separates the tank into two computational regions at the liquid-vapor interface. SINDA/FLUINT, the nodal code, is utilized to simulate the liquid region, while ANSYS Fluent, the CFD code, handles the vapor region. A data exchange algorithm was developed to compute required boundary conditions for each region using the local thermodynamic properties of assumed infinitely thin interface. The coupling between the nodal and CFD codes was demonstrated by simulating a self-pressurization process in a small size tank using hydrogen as the working fluid. A sensitivity study on the grid sizing and coupling time step was conducted to determine appropriate spatial and ensure a divergent-free explicit data exchange time step size. Using the coupling approach, two numerical case studies were performed to study tank self-pressurization by considering two initial hydrogen fill levels. The effectiveness and efficiency of the coupling methodology was assessed by comparing the temperature and pressure evolution results of the coupled simulation with those obtained solely from the CFD simulation. Results showed that the coupling scheme and data exchange algorithm were successfully implemented, and the coupled simulation agreed well with the CFD simulations. In addition, a stratification number was used to characterize the transient dynamic development of the thermal stratification. Lastly, the computational costs associated with both approaches were compared. Significant reductions in simulation time were achieved for both cases.