Clinical Medicine

Abstract When a solid inclined surface is submerged in a quiescent stratified fluid, the combined effects of buoyancy forces and diffusion generate an upward gravity flow along the slope. Thermally stratified ice-covered lakes remain in a nearly quiescent state and are potentially prone to this effect. We use three-dimensional hydrodynamic modeling to investigate the diffusion-gravity flow and its impact on lake-wide circulation in idealized ice-covered lakes. The qualitative characteristics of the boundary flow were adequately simulated by the model, supported by a good agreement with theoretical predictions. In enclosed lakes, the modeled diffusion-driven boundary flow generates residual circulation, which overturns the entire lake water column within 1 to 6 months, suggesting a significant contribution of this mechanism to heat and mass transport in lakes with long ice-covered seasons. When the insulation boundary condition is lifted and additional buoyancy is produced by heat flux from lake sediment, a counterflow emerges, resulting in a circulation pattern characterized by the superposition of two opposing boundary flows. At flux magnitudes exceeding one watt per square meter, the counterflow can entirely replace the diffusion-driven circulation. Due to the small magnitudes of these flows, the Coriolis effect substantially influences circulation, partially transforming radial flow into rotational lake-wide "gyres." The number and rotational direction of these gyres depend on the relative contribution of bottom heat flux. The results provide a framework for designing field studies in real lakes and investigating circulation effects on the transport of dissolved matter, such as nutrients, oxygen and greenhouse gases in ice-covered lakes.

Abstract Bubble curtains are widely used for different environmental applications relying on the generation of recirculation cells. Prior studies suggest that bubble curtains can modify the morphology of rivers and channels by keeping sediment in suspension, and as an alternative for sediment transport control. However, their use in coastal environments is still relatively unexplored. We investigate the interaction between the recirculation cell induced by a bubble curtain with incoming currents and waves. Laboratory experiments were carried out on a wave-current flume facility equipped with a bubble diffuser. Simulated cases considered different characteristics of the bubble curtain, water level, currents, and waves. Free-surface elevation and velocity were measured concurrently with high spatial resolution. Experimental observations show that the bubble curtain modifies wave- and current-induced velocity profiles. For unidirectional currents, as the flow approaches the curtain, the velocity magnitude decreases near the surface and increases near the bed due to the recirculation cell. In the presence of waves, the recirculation flow affects both the wave-induced velocity and asymmetry, both important parameters for sediment transport. Thus, synthetic particles were used as sediment proxy to investigate the role of the recirculation cell on the wave-induced near bed transport. Experimental results highlight the effects of both the bubble curtain recirculation cells and the structure of the diffuser itself in sediment deposition. These experiments reveal the potential of bubble curtains for modifying hydrodynamics and deposition patterns in coastal zones, and provide a novel data set which can be used for the calibration and validation of numerical models.

Abstract The evolution of bed shear stress in open-channel flow due to a sudden change in bed roughness was investigated experimentally for rough-to-smooth (RTS) and smooth-to-rough (STR) transitions. The velocity field was measured in the longitudinal-vertical plane from upstream to downstream using a Particle Image Velocimetry system. The bed shear stress was determined from the measured velocity profile and water depth using various methods. It was found that the variation of bed shear stress in gradually varied flow through a roughness transition was influenced by both flow depth and bottom roughness. In both RTS and STR transitions, the bed shear stress adjusted to the new bed condition almost immediately even though the velocity profile away from the bed was still evolving, but unlike external and close-conduit flows the bed shear stress in open-channel flows continued to evolve until the flow depth was uniform. It is shown that the evolution of bed shear stress in a STR transition is dependent on the choice of the displacement height on the rough bed, which affects the mixing length used to derive the logarithmic velocity profile and equivalent roughness. Bed shear stress variation consistent with published data was obtained when the $${k}_{s}/{d}_{90}$$ k s / d 90 ratio was determined as a function of the $$h/{d}_{90}$$ h / d 90 ratio, where $${k}_{s}$$ k s is the equivalent roughness height, $$h$$ h is the flow depth, and $${d}_{90}$$ d 90 is the grain diameter with 90% of finer particles.

This study aims to employ a generative adversarial network (GAN) for the creation of flow fields in turbulent round jets, with a subsequent assessment of their performance. A ground-truth dataset to train a GAN model is obtained using a three-dimensional large eddy simulation. The reliability of the large eddy simulation dataset is validated against experimental data from laboratory experiments. The performance of GAN is assessed using temporally and azimuthally averaged flow quantities, e.g., temporal-azimuthal averaged velocity and turbulent kinetic energy. The normalized root mean square errors between the flow fields generated by GAN and the ground truth flow fields are 0.46% for the averaged velocity and 3.08% for the turbulent kinetic energy. Utilizing GAN, the time required to generate flow fields, encompassing both GAN training and simulation, is reduced by an impressive 99.7% compared to the duration associated with LES simulations.

Factors such as different building shapes have an important impact on the transmission of explosive tear gas, and directly affect its simulation effect. For the study of its propagation characteristics, this paper adopts the finite element analysis and the numerical simulation method of FLUENT based on the discrete phase model (DPM), and uses the wind tunnel test to check the reliability of the numerical simulation. This paper discusses the effects of wind direction, aspect ratio and the height difference of buildings on the smoke diffusion characteristics of street canyons. The results show that as the street becomes wider, the diffusion area and airflow flux would increase, which is not conducive to the formation of high-concentration smoke areas. When the upstream windward building is higher, it can protect the downstream tear gas to a certain extent, reducing the impact of wind. When the wind speed V0 = 1 m/s, the reflux velocity is low and the smoke particles can maintain a good retention state. The highest concentration of smoke at point A of the breathing area at a wind speed of 1 m/s is 4.6 times the highest concentration at that point at a wind speed of 3 m/s, and 47.3 times the highest concentration at that point at a wind speed of 5 m/s. The study found that the influence of various working conditions on the diffusion of smoke particles can be explained by the changes of airflow field.

The study of dam break flow (DBF) is essential as it is associated with losses to lives and properties. In this study, 2-D DBF is numerically investigated using a mesh-free Lagrangian approach, i.e., the smoothed particle hydrodynamics (SPH) technique by writing a code. The concept of cylindrical columns of water particles and the variable smoothing length technique is adopted to estimate the shock waves in the channel and floodplains. The evolution of the flow field in terms of flow depth and velocity in channel transitions is obtained. The model performance is evaluated by comparing the simulated results with analytical and experimental data in the literature. Several numerical examples considered in the study accurately model the mixed regimes of flow with moving fronts without any special numerical treatment.

Numerical simulations were carried out by using Large Eddy Simulation in a corridor fire with a ceiling-duct to investigate the smoke thermal stratification effect on natural ventilation performance. It was studied the influence of the geometric duct variations on the smoke layer stability based on the stratification parameter and the flow field behavior in the corridor. Results show that the mixing effect and the plug-holing increase with the duct section and the duct height increasing. The strong vertical inertial force could break the stratification stability, and thus the smoke layer descends to near ground resulting in poor natural ventilation performance. The modified parameter showed that the stratification becomes unstable in case of $$S_{mod} { } \le 1.1$$ and a new correlation is predicted for the design of the smoke layer stratification.

To acquire knowledge of the surface storage effects in the flow separation zone of a T-shaped open-channel confluence, independent Large Eddy Simulations are performed of the flushing with fresh water of a downstream branch which is initially uniformly contaminated by a passive scalar. Based on the ensemble averaged concentration, the spatial distributions of the local flushing lag and the local flushing time are determined. The flushing lag is much smaller than the flushing time, except in the maximum velocity zone, where both timescales are of the same small magnitude. For the local flushing time, the separation zone shear layer forms a transition between the high values in the flow separation zone and the low values in the maximum velocity zone. Inside the separation zone, the highest flushing times occur in a small zone near the downstream junction corner. Delineations in the time-averaged velocity field of the separation zone or its proxies, the recirculation zone and the reverse flow zone, are also assessed as pragmatic means to find the zone with the highest local flushing times. Finally, a regional flushing time and a residence time distribution were also determined on the basis of performed simulations.

Abstract Desalination discharges are commonly in the form of inclined negatively buoyant jets (INBJs). Numerical predictions of INBJs remain a challenge. While accurate large eddy simulations (LES) of INBJs have become available very recently, they are substantially more resource intensive. Reynolds-averaged Navier–Stokes (RANS) modelling is potentially more efficient and can be more readily applied in practice. However, existing RANS simulations show substantial error when compared with experimental measurements. In this study, RANS simulations of 45° INBJs are performed with a dynamic turbulent Schmidt number (DTSN) approach. This new approach involves extracting turbulent Schmidt number ( $$S{c}_{t}$$ S c t ) profiles in the INBJs from recently published LES data that have been validated by experiments. Detailed cross-sectional $$S{c}_{t}$$ S c t profiles in INBJs are reported here for the first time. The relationship between $$S{c}_{t}$$ S c t and a local mean flow parameter is also determined from the LES data. RANS simulations are then performed—with $$S{c}_{t}$$ S c t being allowed to change dynamically during the simulation according to the pre-determined relationship. The results show that the DTSN approach improved the overall predictive capabilities of the RANS model to a limited extent. However, significant issues remain in terms of the models’ ability to predict dilutions in the descending portion of the flow. Importantly, the DTSN simulations demonstrate that the model predictions are sensitive to the determination of the relationship between $$S{c}_{t}$$ S c t and the local flow parameter. Further improvements in the DTSN approach are therefore possible with refinement to the characterisation of this relationship. Based on a discussion of the present and recent literature describing RANS simulations of INBJs, the authors encourage a more cautious interpretation of the current predictive capabilities of RANS simulations in the context of INBJs.

Abstract A mixing layer (ML) forms when two streams of different speeds or densities merge. MLs are ubiquitous in nature and can be often observed in the atmosphere, ocean, rivers, canals, lakes and reservoirs. This review paper focuses on the turbulent MLs developing in open-channel flows when the vertical size of the ML is smaller than its streamwise and spanwise dimensions. Such MLs are referred to as shallow MLs (SMLs). The SMLs often involve large-scale features such as quasi-two-dimensional coherent structures with a bed-normal axis, streamwise-oriented vortices, secondary currents, gravity currents, and bed-induced turbulent structures such as large- and very-large-scale motions. Considering various types of SMLs, we distinguish SMLs driven by (i) spanwise inhomogeneity of hydrodynamic parameters, (ii) lateral changes in flow resistance, and (iii) spanwise heterogeneity in fluid density. As SMLs and associated flow structures largely control transverse exchanges of various substances (e.g., sediments, pollutants, nutrients) and heat, the mixing of substances and thermal mixing are also addressed. Then, commonalities and differences among the various types of SMLs are identified. The paper is concluded with suggestions on future research efforts for advancing the knowledge on SMLs and capabilities for their predictions and control.

The confluence of rivers is a very common phenomenon; a large number of works have been devoted to its study. Quite often, confluent rivers are characterized by significantly different water compositions. If the flow velocity in the confluence zone is sufficiently low, then the formation of stratification of water masses is possible, which significantly affects the nature of the hydrodynamics of the phenomenon under consideration. In this case, as a rule, only one factor determining density stratification is considered. In this work, the influence of two factors: temperature and water salinity on the formation of density stratification of water masses is considered using an example of the confluence of the Sylva and Chusovaya Rivers, which are backed up by the Kama hydroelectric station. The influence of these two factors on the formation of layered structures is shown. Based on a combined set of field observations and computational experiments, the possibility of the formation of three-layer structure is shown where water masses with increased mineralization occupy the central part of the flow in depth. The novelty of the study is associated with a situation in which an equal contribution of two independent factors is realized: temperature and water salinity, which leads to a more complex distribution pattern of the consumer properties of water by depth.

Bubble plumes in density stratified liquids induce a narrowly closed convective circulation around them. This localized dynamic process limits the application of bubble plumes to ventilate large aquatic systems experiencing water quality issues, such as reservoirs, lakes, and coastal waters. However, choosing a suitable bubbling condition, i.e. bubble size and gas flow rate, bubble plumes can drive long-range horizontal density currents and ultimately cause the global destratification of the water body. We experimentally investigate how a bubble plume creates a density current that supports the destratification of a strongly two-layer stratified 2 m-long flume. The result leads to the establishment of a mathematical model for predicting the suitable range of the gas flow rate and the time required for the entire destratification of a reservoir. Furthermore, a real-scale experiment in a stratified dam lake of 600 m in horizontal longest length allowed us to confirm the robustness of the model and demonstrate its applicability to aquatic environments.