Search Results (433)

Vento Norte (VNOR; Portuguese for North Wind) is a downslope windstorm that develops over modest terrain in the central region of Rio Grande do Sul (RS), southern Brazil. The regional topography is characterized by an abrupt terrain transition with elevation differences of approximately 400–500 m. This atmospheric flow typically occurs during the cold season and is characterized by strong wind gusts, rapid warming, and drying of the planetary boundary layer (PBL). In this study, the performance of different PBL parameterization schemes in the Weather Research and Forecasting (WRF) model is assessed for simulating a VNOR event that occurred between 19 and 20 August 2021 in Santa Maria (SMA), RS. Five high-resolution numerical simulations were conducted using the Yonsei University (YSU), Asymmetric Convective Model version 2 (ACM2), Mellor–Yamada–Nakanishi–Niino level 2.5 (MYNN2.5), Quasi-Normal Scale Elimination (QNSE), and Three-Dimensional Turbulent Kinetic Energy (3DTKE) PBL schemes. Model results were evaluated against observations from a flux tower providing turbulence measurements, twice-daily radiosoundings, and hourly surface meteorological observations. Statistical metrics indicate that the MYNN2.5 scheme provided the most accurate representation of the nighttime stable boundary layer preceding the VNOR, as well as its onset and subsequent evolution. Although this study analyzes a single VNOR event and the results may be case-dependent, the overall performance of the MYNN2.5 scheme suggests that it is a promising option for the operational forecasting of VNOR events. These findings provide new insights into the ability of different PBL schemes to reproduce the mean boundary-layer structure and turbulence characteristics associated with downslope windstorms over modest terrain, contributing to the understanding of these events. Full article

The teaching of chaos and nonlinear dynamics remains a significant challenge in physics education, as these concepts are often introduced through abstract mathematical models that are difficult to visualize. In this work, we propose an experimental approach based on electro-optics in nematic liquid crystals as an effective and accessible platform for teaching these phenomena. In particular, the system exhibits a transition from ordered convective patterns to strongly disordered turbulent regimes, which can be directly observed in real time using simple optical techniques. This experimental framework enables students to explore key concepts of nonlinear physics, including instability thresholds, pattern formation, and the emergence of complex dynamical behavior. The transition occurs through the nucleation and growth of turbulent domains, facilitating the understanding of nonequilibrium dynamics. From a pedagogical perspective, the proposed experiment combines strong visual impact with experimental controllability and accessibility, making it suitable for undergraduate students in physics, mathematics, and engineering. Furthermore, the integration of AI-assisted analysis provides students with an accessible framework to process experimental data, identify dynamical regimes, and explore complex systems through novel data-driven methodologies. Full article

Entropy generation analysis provides a thermodynamic framework for quantifying irreversibility in thermal systems. However, most existing second-law studies rely on simplified boundary conditions and do not consider fully coupled conjugate heat transfer involving fluid convection, wall conduction, and external heat exchange. Consequently, thermodynamic assessments under realistic conditions remain limited. This study presents an entropy generation-based assessment of turbulent conjugate heat transfer in circular pipes by considering the combined effects of wall thickness ratio (0.02–0.08), wall thermal conductivity (0.2–400 W/m·K), and external convection (5–100 W/m²·K). A three-dimensional steady RANS-based conjugate heat transfer model is employed, and entropy generation is evaluated to quantify irreversibility within fluid and solid domains. The results indicate that wall-related thermal resistances significantly affect thermodynamic performance. Variations in wall conductivity lead to approximately 15–20% changes in total irreversibility, while increasing external convection from 5 to 20 W/m²·K results in up to 25–30% variation. Increasing wall thickness enhances conductive entropy generation, whereas higher Reynolds numbers increase overall irreversibility. These findings demonstrate that the Biot number is a key parameter governing irreversibility distribution. The results provide energy-efficient design insights for optimizing thermally coupled engineering systems under realistic operating conditions. Full article

In indoor thermal analyses, the effect of humid air as a radiatively participating medium that absorbs and emits energy is often neglected. This simplification can underestimate important values in the results. This study presents a numerical investigation of the humid air that participates radiatively in the conjugate heat and mass transfer convection into a room modeled as a two-dimensional square cavity with a semitransparent wall (glass). The governing equations for mass, momentum, energy, species transport, turbulence, and radiative heat transfer were solved using the Finite Volume Method and coupled with the SIMPLEC algorithm. Two scenarios were analyzed: a radiatively participating medium (RPM) and a non-participating medium (NPM), under two climatic conditions (hot and cold). Results show that considering the radiatively participating medium breaks the symmetric patterns observed in the case of NPM. The energy absorbed by humid air enhances turbulent viscosity, buoyant forces, and indoor temperature. Humid air absorbs approximately 30–32% of the incident energy entering the enclosure. Finally, a correlation for the average temperature is proposed. The results provide insight into the influence of radiatively participating humid air on indoor-like thermal behavior. The study focuses on the analysis of fundamental transport mechanisms. Full article

This study numerically investigates the aerodynamic and thermal interactions between a full-scale metro train and the surrounding airflow within a confined tunnel environment using steady-state Reynolds-averaged Navier–Stokes (RANS) simulations. The six-car train, with a total length of 108 m and a cross-sectional area of 5.97 m², operates in a tunnel with a 9.83 square meter cross-section, resulting in a high blockage ratio of approximately 60 percent. The Shear Stress Transport (SST) k–ω turbulence model and a high-resolution finite-volume mesh comprising over 8.5 million elements were employed to capture detailed near-wall phenomena. Six representative motion scenarios were analyzed, including early acceleration, peak cruising, and deceleration phases, with realistic thermal boundary conditions applied by assigning the tunnel air temperature as 29.2 °C and the train surface temperature as 35.0 °C. Velocity, pressure, temperature, and turbulence kinetic energy distributions were extracted from both longitudinal and cross-sectional planes. In addition to visual contour assessments, pointwise and spatially averaged field data were examined to quantify the development of airflow structures, pressure distribution, and thermal behavior. The results reveal speed-dependent aerodynamic resistance, pronounced recirculation and stagnation zones around the train nose and tail, and variations in convective heat transfer rates that evolve with train velocity. These findings provide insights into tunnel ventilation design and thermal management for underground metro operations, representing a novel integration of full-scale computational fluid dynamics (CFD) with thermal characterization under realistic conditions. Full article

Langmuir turbulence is a key and common process in the ocean surface boundary layer, playing a major role in vertical mixing, heat flux, and material transport. However, because direct simulation of Langmuir turbulence demands considerable computational resources, parameterizations within established schemes like the K-profile parameterization (KPP) offer a practical alternative for representing its effects in ocean and climate models. However, Langmuir turbulence parameterizations based on KPP may overestimate vertical mixing when convection is significant. To address this, we introduce a dynamic weighting factor, based on characteristic velocity scales, to balance the contributions of convective and Langmuir turbulence. The improved scheme shows a significant enhancement in performance, especially under strong convective conditions. We compare and evaluate the new parameterization schemes against other widely used schemes in three typical scenarios. Additionally, we validate it using large-eddy simulation results and field observation data. Our enhanced mixing scheme is highly competitive and performs robustly under a variety of conditions. Full article

Urbanization and climate change intensify urban heat islands and air pollution; therefore, street canyon building planning that accounts for road orientation, shading, thermal environment, and ventilation is crucial. This study uses numerical simulations to investigate how non-uniform wall and road heating affects airflow and pollutant dispersion in street canyons under varying Richardson numbers (Ri) and heating scenarios (windward wall, leeward wall, road surface). The results indicate that large wall–atmosphere temperature differences combined with low incoming wind speed (high Ri) make thermal buoyancy a dominant control on canyon flow and pollutant transport. Heating of the leeward wall and road surface enhances ventilation and pollutant removal (prominently when the Ri ≥ 0.49), whereas heating of the windward wall suppresses dispersion and increases concentrations (prominently when the Ri ≥ 0.12). For a north–south street, diurnal solar heating produces strong micro-environmental contrasts. With easterly winds, morning heating of the windward wall elevates pollutant levels, while afternoon heating of the leeward wall promotes dispersion and lowers concentrations. Specifically, compared with the isothermal condition, the turbulent exchange rate at the top of the street canyon is enhanced to 1.71~6.86 times, while the convective exchange rate is suppressed to 58%~83% in the morning and enhanced to 1.21~1.92 times. These findings suggest that urban planning should limit windward wall temperature rises via shading and greening; thus, single-sided sidewalk and greening layouts on the windward side are recommended. Full article

Classical eddy viscosity models add a viscosity term with a turbulent viscosity coefficient developed beginning with the Kolmogorov–Prandtl parameterization. Approximations of unknown accuracy of the unknown mixing lengths and turbulent kinetic energy are typically constructed by solving associated systems of nonlinear convection–diffusion-reaction equations with nonlinear boundary conditions. These often over-diffuse, so additional fixes are added such as wall laws, or different approximations are used in different regions (which must also be specified). Alternately, one can solve an ensemble of NSEs with perturbed data, compute the ensemble mean and fluctuation, and simply directly compute the turbulent viscosity parameterization. This idea is recent. From previous work it seems to be of a lower complexity and greater accuracy. It also produces parameterizations with the correct near-wall asymptotic behavior. The question then arises: Does this ensemble eddy viscosity approach over-diffuse solutions? This question is addressed herein. Full article

Mediterranean tropical-like cyclones (MTLCs), commonly referred to as Medicanes, are high-impact weather systems characterized by complex interactions between baroclinic forcing and tropical-like processes. Despite growing interest, their vertical structures and dynamical regimes remain incompletely understood. In this study, high-resolution Weather Research and Forecasting (WRF) simulations at 1 km resolution are used to investigate the structure and evolution of two dynamically contrasting MTLCs: Ianos (2020) and Qendresa (2014). The analysis focuses on the temporal evolution of kinetic energy and turbulent dissipation as well as on the three-dimensional organization of wind and temperature fields during representative phases of the cyclone life cycle. The results reveal pronounced differences between the two events, with Ianos exhibiting a compact, vertically coherent, convection-dominated structure and Qendresa showing a wider, more asymmetric, and less stationary organization influenced by baroclinic processes. A comparative framework with the ERA5 reanalysis is employed to contextualize cyclone intensity, with ERA5 used as a dynamically consistent large-scale reference rather than as an observational benchmark. Overall, the study highlights the importance of vertical structure and boundary-layer processes in shaping Mediterranean tropical-like cyclones and demonstrates the added value of high-resolution numerical simulations for distinguishing between different dynamical regimes. Full article

(1) Theoretical studies have shown that large-scale vorticity generates a divergent-type helicity flux associated with small-scale magnetic fluctuations. Similar to the $\alpha$-effect, this mechanism breaks the equatorial reflection symmetry of magnetic fluctuations in stellar convection zones. This contribution has been termed the new Vishniac flux (hereafter NV flux). (2) Methods: We employ a mean-field dynamo model to investigate the influence of the NV flux on solar-type dynamos. (3) Results: We find that the NV flux leads to an enhancement of the dynamo efficiency for the turbulent generation of the large-scale poloidal magnetic field in the Sun. The dynamical impact of the NV flux on the evolution of the magnetic field results in a concentration of dynamo waves toward the equatorial region. Using numerical simulations of the mean-field dynamo, we compare the helicity production rates arising from different turbulent dynamo mechanisms, namely the $\alpha$-effect and the NV flux. The model demonstrates that the new dynamo source associated with large-scale vorticity and small-scale dynamo action leads to an amplification of poloidal field generation in the polar regions near the top of the dynamo domain. (4) Conclusions: Any fluctuating magnetic activity arising within the differentially rotating stellar convection zone can serve as an additional source for the generation of the large-scale poloidal magnetic field of a star. Full article

Research on the frequency spectra of velocity and pressure fluctuations in turbulent channel flow is central to applications in petroleum engineering, including pipeline transport efficiency, erosion prediction, and flow-induced vibration in wellbores and surface facilities. Direct numerical simulation at high Reynolds numbers remains prohibitively expensive, motivating the use of resolvent analysis as a computationally efficient alternative. The resolvent analysis, formulated from the linearized Navier–Stokes equations, relies on appropriate modeling of stochastic forcing. In this work, we demonstrate that the conventional white-in-time stochastic forcing model exhibits fundamental deficiencies in predicting velocity and pressure statistics. Specifically, it fails to reproduce the correct two-point correlation of the wall-normal velocity, leading to inaccurate predictions of the rapid pressure spectrum. Moreover, it does not capture the correct wall-normal distribution of the forcing divergence, resulting in erroneous predictions of the slow pressure component. More fundamentally, we rigorously show that a linear convection–diffusion system driven by white-in-time stochastic forcing possesses an infinite frequency bandwidth, which implies unphysical vanishing Taylor time microscales for velocity fluctuations. These results highlight intrinsic limitations of white-in-time forcing and demonstrate the necessity of adopting colored-in-time stochastic forcing models to obtain physically consistent spectral predictions. Full article

An experimental investigation was conducted to evaluate the thermal conductivity (TC) and local heat-transfer coefficients (LHTCs) of nanofluids containing alumina (Al₂O₃), hematite (Fe₂O₃), and copper oxide (CuO) nanoparticles dispersed in deionized water. A newly developed non-invasive LHTC probe was integrated into the inner wall of the test section to enable direct quantification of interfacial heat-transfer performance. The measurements were conducted under laminar and turbulent flow conditons across Reynolds numbers ranging from 1000 to 10,000. The selected nanoparticles were chosen based on their high intrinsic thermal conductivity, cost effectiveness, and, in the case of Fe₂O₃, magnetic recoverability. The nanoparticles enhanced both TC and LHTCs through improved thermophysical propoerties and possible interfacial effects. Maximum TC enhancements of 19%, 21%, and 25% were achieved for Al₂O₃/distilled water (DW), Fe₂O₃/DW, and CuO/DW nanofluids, respectively, at 0.05 vol% and 55 °C, while the corresponding LHTC enhancements reached 44%, 50%, and 53%. Under turbulent flow, CuO/DW exhibited the highest heat-transfer performance, attributed to a 25% increase in TC and corresponding improvement in connective heat transfer. Since the boundary-layer thickness exceeded the nanoparticle diameter (30 nm), nanoparticles penetrated the interfacial film, inducing localized micro-convection and catalytic micro-mixing, which intensified interfacial heat transport. The experimentally determined Nusselt numbers showed strong agreement with the Xuan–Qiang correlation at 55 °C, suggesting that the nanoparticle volume fraction governs the catalytic interfacial heat-transfer mechanism. Full article

This study explores the feasibility and validity of using topology optimization (TO) to design the internal cooling of an airfoil-like geometry approximating a turbine guide vane. A conjugate heat transfer approach where the fluid flow physics are coupled with a convection–diffusion model for heat transfer is used in the TO. The objective is to minimize the maximum temperature on the outer surface of the vane with a constraint on the mass flow of the internal coolant. Two different flow models are investigated for the TO process: the Stokes model and the Reynolds-Averaged Navier–Stokes (RANS) equations with a simple zero-equation turbulence model. Velocity and temperature fields in topology-optimized designs are then compared to conventional conjugate heat transfer analyses performed on post-processed designs with body-fitted meshes and those using the shear stress transport (SST) RANS turbulence model. Designs obtained with the Stokes model exhibit different flow trajectories and mixing, while the use of RANS equations improves predictions but introduces uncertainties due to turbulence modeling limitations, particularly in the presence of flow separation. Thus, considering these limitations, the findings suggest that a simple flow model, such as Stokes in TO, with a comparatively low computational cost, can yield useful design concepts. However, the simplifications in the governing equations and their impact on physics should be considered carefully, and further aerothermal validation is required. Thus, the study findings, along with advances in robust meshing, enhance the practicality of topology optimization for industrial applications. Full article

The increasing demand for compact and high-performance thermal management systems in the industrial and energy sectors has renewed interest in plate-type heat exchangers for high heat-flux dissipation. These exchangers offer high surface-area-to-volume ratios, modular architecture, and scalable construction, making them suitable for applications requiring advanced cooling within restricted space. This study presents a structured thermo-hydraulic design framework for compact plate heat exchangers operating under fixed wall-temperature boundary conditions. The framework integrates geometric scaling, surface-morphology variation, and multi-parameter performance evaluation to assess the balance between convective enhancement and hydraulic losses. Water at 25 °C serves as the working fluid due to its favorable thermophysical properties and economic viability. A constant wall temperature of 100 °C is applied as a fixed boundary condition to provide a consistent thermal driving potential for comparing different geometries in a range of industrially relevant operating regimes. Three primary design variables are examined: (i) a baseline flat-plate configuration used to establish the fundamental flow–thermal response; (ii) systematic variation of inter-plate spacing to characterize the hydraulic–thermal tradeoff; and (iii) surface-morphology variation using chevron and sinusoidal corrugations to enhance convection through secondary flow generation and boundary-layer modulation. The key performance metrics include wall heat flux, overall heat-transfer coefficient, thermal resistance, and pressure-drop penalty. These indicators are evaluated to identify configurations that are thermally effective and hydraulically feasible. The results show that an inter-plate spacing of 7 mm provides a favorable balance between confinement and convective enhancement under the present operating conditions. Sinusoidal corrugations yield the most favorable thermo-hydraulic performance (PEC $\approx 1.30$) while maintaining low frictional losses. The proposed framework provides a transferable physics-based methodology for comparative assessment and early-stage design of compact heat exchangers under fixed pumping-power constraints. The approach is broadly applicable to renewable-energy systems and compact thermal management in industrial applications. Full article

Complex systems—ranging from biological organisms to turbulent fluids—exhibit multiscale heterogeneity and intermittency that traditional, differentiable calculus fails to adequately capture. Therefore, we propose a mathematical framework for analyzing complex system dynamics by assimilating the trajectories of structural units to continuous but non-differentiable multifractal curves. Utilizing the scale covariance principle, the authors recast the conservation of momentum as a geodesic equation within a multifractal space. This approach naturally separates the complex velocity field into differentiable and non-differentiable scale resolutions, where the balance of multifractal acceleration, convection, and dissipation is parametrized by a singularity spectrum f(α). We also discuss broad interdisciplinary implications, because, in our opinion, non-differentiability can enhance predictive capabilities in various fields such as oncology, pharmacology, and geophysics. Full article