Turbulent Mixing Research Articles

LES of autoignition process in a turbulent droplet-laden mixing layer – impact of an evaporation model and discretisation method

This paper focuses on the numerical modelling of the autoignition process in an ethanol-air mixture within a turbulent droplet-laden temporally evolving mixing layer, utilising the Large Eddy Simulation (LES) method. We compare results obtained by implementing two notably distinct discretisation methods (second-order Total Variation Diminishing – TVD, and fifth-order Weighted Essentially Non-Oscillatory – WENO) in the species and energy transport equations. Additionally, we examine three widely recognised evaporation models (the ‘ D 2 ’ model, the Abramzon–Sirignano model, and a non-equilibrium model based on the Langmuir-Knudsen law). For modelling the combustion process, we employ the Eulerian Stochastic Field method and a laminar chemistry model. The simulations encompass two droplet diameter distributions, one with low and the other with significant diameter non-uniformity. Furthermore, nine distinct initial conditions are considered. Our findings indicate that adjusting the discretisation scheme or the evaporation model can yield simulation outcomes differing to a substantial degree, similar to the impact of modifying the actual properties of the spray droplets or their initial distribution. This is particularly pronounced in the case of sprays featuring a narrowed droplet diameter distribution. We found that the Abramzon–Sirignano model results in the most rapid autoignition. The TVD scheme contributes to a more even spatial distribution of the evaporated fuel, consequently facilitating quicker evaporation and faster autoignition. Using the WENO scheme generates higher fuel vapor gradients near the droplets, leading to less intense evaporation and slower fuel transport in space.

Development of a multi-layer network model for characterizing energy cascade behavior on turbulent mixing

Eddies of various sizes are visible to the naked eye in turbulent flow. Each eddy scale corresponds to a fraction of the total energy released by the turbulence cascade. Understanding the dynamic mechanism of the energy cascade is crucial to the study of turbulent mixing. In this paper, an energy cascade multi-layer network (ECMN) based on the complex network algorithm is proposed to investigate the spatio-temporal evolution of the energy cascade, covering both the inertial and dispersive ranges. The dynamic process of energy cascade is transformed into a topological structure based on the node definition and edge determination. The topological structure allows for the exploration of eddies interaction and chaotic energy transfer across scales. The model results show the intermittent and non-uniform nature of the energy cascade. Meanwhile, the scale gap found in the model verifies the fractal property of the energy evolution. We also found that scales of the generated eddies in energy cascade process are stochastic, and a synchronous energy cascade pattern is demonstrated according to the constructed framework. Furthermore, it provides a topological way to evaluate the contribution of large and small scale eddies. In addition, a network structure coefficient κ is proposed to evaluate the energy transfer strength. It agrees very well with the fluctuation of dissipation rates. All of this shows that the network model can effectively reveal the inhomogeneous properties of the energy cascade and quantify the turbulent mixing intensity based on the intermittent scale interaction. This also provides new insights into the study of fractal scales of nonlinear complex systems and the bridging of chaotic dynamics with topological frameworks.

MÔ PHỎNG XOÁY TÁCH RỜI TRONG NGHIÊN CỨU CÁC ĐẶC TRƯNG KHÍ ĐỘNG CỦA DÒNG CHẢY RỐI TRÊN VÙNG TƯƠNG TÁC

The turbulent mixing layer is a phenomenon that occurs and has many applications in aeronautical engineering. The study of mixing layer turbulence by the numerical method is suitable under domestic conditions. Two numerical simulation methods are used in most studies including, direct numerical simulation DNS and large eddy simulation LES, while there are very few studies using detached eddy simulation DES. In this article, the authors use numerical simulation methods evolving Reynolds average Navier-Stokes simulation RANS and detached eddy simulation DES to propose a numerical simulation model that allows to study of the aerodynamic characteristics of the turbulent flow interaction. RANS is used to evaluate grid independence. With the selected mesh, two models counting the DES-2D model and the DES-3D model are conducted. Processing velocity field data with Matlab code, the results show that the average flow behavior has closely followed the experimental results, while the turbulent kinetic energy still has a large error. POD and Q-criterion analysis has shown some large coherent structures in the interaction region.

Response of Internal Wave‐Induced Turbulent Dissipation to ENSO in the Western Pacific Warm Pool

Response of Internal Wave‐Induced Turbulent Dissipation to ENSO in the Western Pacific Warm Pool

Revealing nitrate uptake and dispersion dynamics using high-frequency sensors and two-dimensional modeling in a large river system

Nitrate pollution of water bodies is a critical issue in many parts of the world because of its negative effects on aquatic ecosystem and human health. Effective management of pollution, such as the continuous or instantaneous release from point-sources, requires an understanding – with high spatial and temporal resolution – of how nitrate is dispersed and cycled within rivers. Nitrate sensing data show promise for this purpose, but their integration into numerical models is scarce; thus, questions remain regarding the necessary spatial grid size and temporal resolution required to resolve sensor readings. In this study, we developed an unsteady two-dimensional model to simulate nitrate transport, dispersal, and cycling along a 33-km stretch of the Kansas River (USA), following a strategic release of nitrogen from a decommissioned fertilizer plant. To validate modeled estimates of dispersion and uptake, we integrated 15-minute nitrate and temperature data from two aquatic sensors, one located proximal to the fertilizer release point and a second further downstream after complete lateral mixing. Model results at the site near to the contamination (0.4 km) were highly sensitive to river grid size and turbulent mixing, but insensitive to uptake. Results at the site far downstream of the contamination (31 km) were unaffected by grid size or mixing parameterization but were very sensitive to selection of uptake rate. High-frequency sensors allowed us to resolve diel variability in nitrate signals, which we incorporated into the model to improve performance and model realism. The 33-km study reach assimilated 14% of the total nitrate load in the river, or approximately half of what was contributed by the fertilizer release, during the two-month study period. Regarding nitrate cycling, modeled Cdiel/Cmax ranged from 0.04 to 0.11 whereas sensor observations showed much higher Cdiel/Cmax values of 0.11 to 0.25. Disagreements between data observations and model simulations in cycling are hypothesized to exist due to potential breakdown of the first-order rate kinetics. Together, our study shows the potential of combining numerical models and high-frequency data for a better understanding of the physical and biogeochemical processes that control nitrate dynamics in aquatic environments.

Prediction and comparison of aerodynamic performance and flow control for smooth and wavy tube banks in crossflow at subcritical Reynolds numbers using a 3D RANS approach

This paper investigates the effects of introducing wavy cylinders of appropriate wavelength into tube bundles for the passive control of flow-induced vibration. The Reynolds-averaged Navier–Stokes approach RANS was used to study the turbulent flow characteristics within staggered tube bundles having transverse and longitudinal pitch-to-diameter ratios of 3.8 and 2.1, respectively. Two wavy models are tested with different wavelength-to-mean diameter ratios, namely: Model-A with [Formula: see text] and Model-B with [Formula: see text], and their performances are compared with those of a smooth circular tube bundle Model-C ([Formula: see text]), in a range of subcritical Reynolds numbers [Formula: see text]. Mean velocity and pressure fields as well as aerodynamic coefficients are presented and compared. According to the obtained results, the wavy tube bundle with [Formula: see text] exhibits the best performance, with slightly smaller [Formula: see text] values than the smooth tube bundle. However, results show that the wavy model with [Formula: see text] is clearly the worst with an increase of [Formula: see text] up to 40%. Numerical outcomes also reveal that the interaction of neighboring rows and columns affects the fluctuation of [Formula: see text] values relative to each tube position. Moreover, [Formula: see text] evolutions depend significantly on the variation of Re as well, where drag reduction is more substantial by increasing Re. Thus, wavy-shaped tubes with optimum parameters could be more suitable for flow control by enhancing the secondary flow, turbulence and fluid mixing.

Generation mechanisms of SST anomalies associated with the canonical El Niño focusing on vertical mixing

Abstract The mean vertical advection of anomalous vertical temperature gradient is considered as the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most past studies had a residual term in their heat budget analysis and/or did not quantify the role of vertical mixing even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific. To quantitatively assess the importance of vertical mixing, a mixed layer heat budget analysis is performed using a hindcast simulation forced by daily mean atmospheric reanalysis data. It is found that when the mixed layer depth is defined as the depth at which potential density increases by 0.125 kg m−3 from the sea surface, the development of positive SST anomalies is predominantly governed by reductions in the cooling by vertical mixing, and their magnitude is much larger than those by vertical advection. The anomalous warming by vertical mixing may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the dominant negative feedbacks are attributed to both anomalous surface heat loss and anomalous deepening of the mixed layer that weakens warming by the mean surface heat flux.

Detecting the role of Stokes drift under typhoon condition by a fully coupled wave-current model

The impacts of Stokes drift and sea-state-dependent Langmuir turbulence (LT) on the three-dimensional ocean response to a tropical cyclone in the Bohai Sea are studied through two-way coupled wave-current simulations. The Stokes drift is calculated from the simulated wave spectrum of the wave model, Simulating Waves Nearshore (SWAN), and then input to the Princeton Ocean Model with the generalized coordinate system (POMgcs) to represent the Langmuir effect. The Langmuir circulation is included in the vertical mixing of the ocean model by adding the Stokes drift to the shear of the vertical mean current and by including LT enhancements to the Mellor-Yamada 2.5 turbulent closure submodel. Simulations are assessed through the case study of Typhoon Masta in 2005 with a set of diagnostic experiments that incorporated different terms of Stokes production (SP) respectively. It is shown that with the consideration of SP, a deeper mixed layer, an enhanced vertical mixing coefficient KMS, and a more accurate representation of the vertical temperature distribution could be derived. Moreover, the effect of LT in elevating the turbulence mixing is stronger than that of Coriolis Stokes force (CSF) and Craik-Leibovich vortex force (CLVF). LT has a greater influence on the vertical mixing during typhoon than that in normal weather.

Mooring observations of tidal currents in the Jailolo Strait of the Indonesian seas

The vertical structure of internal tidal currents in the Jailolo Strait of the Indonesian seas is studied using the year-long mooring observations with two Acoustic Doppler Current Profiler instruments looking upward and downward, respectively, coving the full depth of the strait. The barotropic tides are extracted from the full-depth current profile measurements, showing the dominant M2, K1, and O1 tidal currents with amplitudes of 13.5 cm s−1, 11.0 cm s−1 and 7.6 cm s−1, in nearly reciprocating movement in the direction of 34.5°, 63.1°, and 75.2° clockwise from due north. The internal tidal currents in the Jailolo Strait contributing to about 50% of the total kinetic energy are found to be dominated by the low baroclinic modes, with M2 tide as the most dominant tidal constituent. High-frequency internal tides of D3 and D4 are observed, with baroclinic structure similar to that of M2 tides, and are suggested to be stimulated by nonlinear processes. The complicated vertical structure of tidal currents in the Jailolo Strait is disclosed for the first time in history. Its potential impact on vertical turbulent mixing suggests its importance in global ocean circulation and climate.

Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.

A numerical investigation of the thermal characteristics of supercritical carbon dioxide (sCO2) flowing through mini serpentine channels

In this study, we numerically investigate the heat transfer characteristics of CO2 during phase transitions in its supercritical state in serpentine channels. Four different inclination angles are applied to three different radii of curvature flow channels with the same diameter of 2 mm. The supercritical state is maintained by assuming that the mass flux is approximately 191 kg/m2s, the inlet temperature is 298.15 K, and the operating pressure is 7.65 MPa. To ensure a consistent total heat input, different heat flux values are applied to the three different cases, eliminating the potential impact of the increased surface area on the heat transfer performance and allowing a clearer investigation of the geometric effects within the same temperature range. In a channel with a constant radius of curvature, the centrifugal force has a dominant influence on the heat flow compared to gravitational buoyancy, showing a flow phenomenon independent of the angle. In contrast, in a flow channel with a varying radius of curvature, the gravitational buoyancy fluctuates depending on the local area. As a result, the ratio of the centrifugal force to the gravitational buoyancy decreases, causing more variations with changes in the inclination angle. As the centrifugal force contributes significantly to turbulent mixing, the average heat transfer coefficient is highest in the flow channel with the shortest wavelength. As the wavelength increases, the ratio of centrifugal force to buoyancy begins to affect density stratification, especially in gas-like regions, leading to buoyancy. The local heat transfer coefficient at points around the circumference of the cross-section affected by buoyancy and in the film is analyzed, which is closely related to the centrifugal buoyancy and gravitational buoyancy, and their ratio. To express this influence in a correlation equation, a new Nu correlation with an error of less than 25 % is proposed using De, which combines the centripetal force and Reynolds number, ratio of centrifugal buoyancy and gravitational buoyancy, Ф, and Pr as.Nu=0.849De0.541∅-0.154Pr-0.160

Effects of the circumferential casing groove configuration on aerothermal performance of a transonic turbine stage

In order to control tip leakage flow and improve the tip aerothermal performance, a circumferential casing groove (CG) configuration is proposed. Taking the typical transonic turbine stage TTM (Thermal Turbomachinery and Machine Dynamics) as the research object, the effects of CG with different heights (b), initial locations (IL) and widths (w) on the aerodynamic loss of the turbine stage and heat transfer characteristics of the tip region are investigated numerically. Results indicate that due to the introduction of CG, the vortex system topology in the tip region changes remarkably. Tip leakage vortex (TLV) is separated into two parts, with TLV-1 being entrained and dissipated by upper passage vortex (UPV) and TLV-2 rebuilding behind the groove and extending to the trailing edge. The interaction between TLV and UPV changes with the development of both. Also, complicated flow-thermal interplays lead to the change of thermal load in the tip region, which attributes to the difference of local turbulence mixing intensity. More quantitatively, a CG with the groove of b = 3.0 g, IL = 0.20 Cax and w = 0.05 Cax can increase ηTS by 0.38%, and reduce the area-averaged heat transfer coefficient (HTC) by 5.28% and 1.49% for the tip and casing surface, respectively, with a slight increase of area-averaged HTC on the blade by 0.29%. Overall, CG with appropriate geometrical parameters can improve aerothermal performance of the turbine tip significantly, without any serious increase of thermal load on the blade.

Influence of side dilution jets on swirling and non-swirling pulverised biomass turbulent annular flows

The use of pulverised biomass in non-premixed combustion systems, featuring turbulent annular flows (swirling, non-swirling) with downstream air dilution jets, is of immense practical importance. The overall combustion performance and emissions heavily rely on the underlying flow dynamics, turbulence and dispersion behaviour of the pulverised biomass particles. Despite the practical significance of side dilution jets in aiding complete combustion and controlling pollutants, the fundamental understanding of side dilution jets influence on the biomass particle velocity field, turbulence, and dispersion characteristics is still lacking. However, the presence of a multi-phase (solid-gas) flow field, highly turbulent annular flows, swirl, and cross-wise turbulent dilution jets poses significant challenges to these investigations. This work employs a combination of two-dimensional planar Particle Image Velocimetry (PIV) experiments and three-dimensional multi-phase numerical modelling to resolve the influence of turbulent side dilution jets (Red = 18,000 and 27,000) on the particle flow and dispersion characteristics of turbulent annular jets equipped with a central pulverised biomass-laden jet. The numerical modelling uses Reynolds stress model (RSM) and discrete phase model (DPM) for the solution of gas and dispersed phases, respectively. Walnut flour based raw pulverised biomass is injected at a certain particle loading ratio (Φp=0.33 and 0.49) through a turbulent central jet (Rj = 4500) into non-swirling (S = 0) and swirling (S = 0.3) turbulent confined annular flows (Res = 35,500 and 17,800). Comprehensive Constant Temperature Anemometry (CTA) experiments are conducted to obtain well-defined boundary conditions for experiments as well as numerical simulations. Prior to numerical predictions, the models are duly validated against the experimental data.Results reveal that the characteristic flow feature of side dilution jets, i.e., peripheral recirculation zones (PRZ) significantly entrains pulverised biomass particles from the turbulent particle-laden (central jet) annular flows (both swirling and non-swirling) and alters the particle flow and turbulence characteristics. Under non-swirling conditions (S = 0), side dilution jets reduce mean particle axial velocity by 22.4% and increase turbulent fluctuating velocity by 52.6%. These figures elevate to 32.5% and 100% respectively for swirling conditions (S = 0.3), depicting enhanced turbulent mixing between solid particles and the gas phase. In terms of particle dispersion, side dilution jets significantly enhance lateral dispersion of biomass particles, particularly under swirling flow conditions. In the PRZ region, side dilution jets led to a 108% and 279% increase in normalised particle count for non-swirling and swirling cases, respectively. These findings suggest that swirl doubles the particle dispersion in the PRZ. Furthermore, a 50% increase in Red enables side dilution jets to entrain more biomass particles and disperse them into the PRZ. Conversely, a 50% increase in ṁp leads to a significant increase in particle volume fraction along the central axis of the nozzle.

Resolution-Dependence of Extreme Wind Speed Projections in the Great Lakes Region

Abstract The effect of anthropogenic climate change on extreme near-surface wind speeds is uncertain. Observed trends are weak and difficult to disentangle from internal variability, and model projections disagree on the sign and magnitude of trends. Standard coarse-resolution climate models represent fine structures of relevant physical phenomena such as extratropical cyclones (ETCs), upper-level jet streaks, surface energy fluxes, and land surface variability less skillfully than their high-resolution counterparts. Here we use simulations with the NCAR Community Earth System Model with both uniform (110 km) resolution and the variable resolution configuration (VR-CESM-SONT, 110 km to 7 km), to study the effect of refined spatial resolution on projections of extreme strong and weak wind speeds in the Great Lakes region under end-of-century RCP8.5 forcing. The variable-resolution configuration projects strengthening of strong-wind events in the refined region with the opposite occurring in the uniform-resolution simulation. The two configurations provide consistent changes to synoptic scale circulations associated with high-wind events. However, only the variable resolution configuration projects weaker static stability, enhanced turbulent vertical mixing, and consequentially enhanced surface wind speeds, because boundary layer dynamics are better captured in the refined region. Both models project increased frequency of extreme weak winds, though only VR-CESM-SONT resolves the cold-season inversions and summertime high temperatures associated with stagnant wind events. The identifiable mechanism of the changes to strong winds in VR-CESM-SONT provides confidence in its projections and demonstrates the value of enhanced spatial resolution for the study of extreme winds under climate change.

Sensitivity of boundary layer features to depth-dependent baroclinic pressure gradient and turbulent mixing in an ocean of finite depth

The present numerical study builds on Ekman (1905)’s work in surface boundary layer and extends the boundary value problem to overcome some of its limitations. Previous studies addressed model’s limitations by assuming that deviations from observations are usually ascribed to different eddy viscosity shapes, but seldom to the presence of baroclinic pressure gradients and shallow seas, which are the mainstays of this work. Improved solutions in the ocean boundary layer are obtained considering both depth-dependent wind-induced eddy viscosity and horizontal density gradients, ranging from well-mixed to highly-stratified conditions in a finite-depth ocean. High-order numerical solutions extend those in previous analytical and numerical works in the literature and widens the parameter space analyzed. Remarkably, the current profiles are obtained without ambiguity as a truly superposition of a geostrophic and a ageostrophic terms. Results indicate that, for a vertically-uniform eddy viscosity without density gradients and in shallow waters, currents are practically aligned with wind. As depth increases, misalignment between currents and wind increases and the complexity of the vertical structure increases. At large depths, Ekman’s values are attained, i.e., deflection angles relative to wind direction, θW, are θS−θW=−45∘ at the surface, where the current is maximum, and θT−θW=−90° for the depth-integrated transport (negative for deflections to the right in the Northern Hemisphere). These features remain regardless of the magnitude of the eddy-viscosity. For non-uniform eddy viscosity, θS−θW decreases from −45∘ up to −90∘ from low to high stratification level, respectively, whereas θT−θW is rather insensitive (θT−θW≈−90∘). Contrary to wind effects, the presence of the only vertically-uniform density gradient forcing, with constant eddy viscosity, deflects the surface angle θS−θD=+45∘ relative to the density-gradient direction, θD, in deep waters. Maximum currents no longer occur at the surface in this case. For non-uniform density-gradient profiles, current magnitude decreases overall while θS−θD≈+45∘ as long as the gradient affects the entire surface boundary layer. The deflection angle θT−θD∼+95∘ remains less sensitive to changes in density-gradient profile.

A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18b

While small Neptune-like planets are among the most abundant exoplanets, our understanding of their atmospheric structure and dynamics remains sparse. In particular, many unknowns remain regarding the way moist convection works in these atmospheres, where condensable species are heavier than the non-condensable background gas. While it has been predicted that moist convection could cease above some threshold abundance of these condensable species, this prediction is based on simple linear analysis and relies on some strong assumptions regarding the saturation of the atmosphere. To investigate this issue, we developed a 3D cloud-resolving model for hydrogen-dominated atmospheres with large amounts of condensable species and applied it to a prototypical temperate Neptune-like planet --- K2-18\,b. Our model confirms the inhibition of moist convection above a critical abundance of condensable vapor and the onset of a stably stratified layer in the atmosphere of such planets, which leads to much hotter deep atmospheres and interiors. Our 3D simulations further provide quantitative estimates of the turbulent mixing in this stable layer, which is a key driver of the cycling of condensables in the atmosphere. This allowed us to build a very simple, yet realistic, 1D model that captures the most salient features of the structure of Neptune-like atmospheres. Our qualitative findings on the behavior of moist convection in hydrogen atmospheres go beyond temperate planets and should also apply to regions where iron and silicates condense in the deep interior of hydrogen-dominated planets. Finally, we used our model to investigate the likelihood of a liquid ocean beneath an H$_2$-dominated atmosphere on K2-18\,b. We find that the planet would need to have a very high albedo ($ to sustain a liquid ocean. However, due to the spectral type of the star, the amount of aerosol scattering that would be needed to provide such a high albedo is inconsistent with the latest observational data.

Multiscale Modeling of Flow in Rod Bundles: From Direct Numerical Simulation and Subchannel to Coarse-Mesh CFD

Fast and accurate evaluation of flow and heat transfer phenomena in rod bundles is a problem of long-standing interest in nuclear engineering. Computational fluid dynamics (CFD) can provide accurate but relatively time-intensive estimates, such that simulations of very long transients with high spatial detail are infeasible. On the other hand, subchannel codes require relatively low computational time and can provide pin-level estimates, but have substantial empiricism in evaluating aspects such as crossflows and turbulent mixing coefficients. A multiscale method (SC+) for bridging this accuracy/speed gap, based on the Subchannel CFD (SubChCFD) method of Liu et al. [Nucl. Eng. Design, Vol. 355, paper 110318 (2019)], is demonstrated and developed here with a focus on a 5 × 5 square rod bundle geometry. The method has been newly implemented into the commercial code STAR-CCM+ and benchmarked against Liu et al.’s data. The 5 × 5 geometry was chosen in part due to the availability of direct numerical simulation (DNS) data at a relevant Reynolds number of a similar configuration for comparison. The SC+ results are variously compared against results from DNS, large eddy simulation, wall-resolved Reynolds-averaged Navier-Stokes, and coarse-mesh CFD methods. As an expansion to the original SubChCFD approach, a simple Hi2Lo approach is demonstrated using the DNS data as a correction to the original friction factor correlations employed. This is verified to improve the predictions. Additional test cases with geometric perturbations are pursued, illustrating the flexibility of SC+. The potential of this method for modeling the narrow gap vortex instability, which would represent an advancement over standard subchannel approaches, is also assessed. The method is expanded to include transverse flow losses, which was found to improve the results for modeling the gap instability. Initial extensions of SC+ for hexagonal rod bundles are also presented; some inaccuracies for the coarsest meshes prompted a detailed investigation of the mesh convergence behavior of the method. Geometric correction factors were devised that provided substantial improvement on these very coarse meshes, improving the prospects of SC+ for wider usage. Future work plans are to expand the methodology to wire-wrapped rod bundles and to implement the method into Pronghorn, with a unified pipeline via the Cardinal wrapper between the codes NekRS, Pronghorn, and BISON, to solve fuel performance problems of direct interest to industry.

Studies on parameterizations of turbulent mixing in the deep ocean

Studies on parameterizations of turbulent mixing in the deep ocean

How turbulence spreading improves power handling in quiescent high confinement fusion plasmas

Viable magnetic fusion devices necessitate combining good confinement with effective power flux handling. A major concern for ITER, and devices beyond, is the divertor heat load width, which sets peak boundary heat loads on the plasma-facing materials. Current estimates of the heat flux width are narrow for future reactors. Here, we demonstrate how pedestal turbulence can expand into, or entrain, the stable scrape-off-layer and so broaden the heat flux width beyond these neoclassical predictions. Employing combined theoretical, computational, and experimental approaches, we focus on quiescent high confinement discharges on the DIII-D tokamak, but the results are of broader significance. Our findings uncover common trends in the edge turbulence intensity flux, the pressure perturbation skewness, and the turbulence mixing length, which together determine the heat flux width. This research demonstrates the physics of scrape-off-layer broadening by turbulence and highlights the promise of a turbulent pedestal for successful core-edge integration in ITER and future fusion devices.

Characterizing turbulent non-premixed flame structure and pollutant formation of cracked ammonia jet flames using simultaneous NH and NO PLIF

The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N2O. The effect of turbulent disturbances on N2O formation needs to be considered in turbulent CAJFs because N2O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control.