Effects Of Turbulent Mixing Research Articles

Better calibration of cloud parameterizations and subgrid effects increases the fidelity of the E3SM Atmosphere Model version 1

Abstract. Realistic simulation of the Earth's mean-state climate remains a major challenge, and yet it is crucial for predicting the climate system in transition. Deficiencies in models' process representations, propagation of errors from one process to another, and associated compensating errors can often confound the interpretation and improvement of model simulations. These errors and biases can also lead to unrealistic climate projections and incorrect attribution of the physical mechanisms governing past and future climate change. Here we show that a significantly improved global atmospheric simulation can be achieved by focusing on the realism of process assumptions in cloud calibration and subgrid effects using the Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1). The calibration of clouds and subgrid effects informed by our understanding of physical mechanisms leads to significant improvements in clouds and precipitation climatology, reducing common and long-standing biases across cloud regimes in the model. The improved cloud fidelity in turn reduces biases in other aspects of the system. Furthermore, even though the recalibration does not change the global mean aerosol and total anthropogenic effective radiative forcings (ERFs), the sensitivity of clouds, precipitation, and surface temperature to aerosol perturbations is significantly reduced. This suggests that it is possible to achieve improvements to the historical evolution of surface temperature over EAMv1 and that precise knowledge of global mean ERFs is not enough to constrain historical or future climate change. Cloud feedbacks are also significantly reduced in the recalibrated model, suggesting that there would be a lower climate sensitivity when it is run as part of the fully coupled E3SM. This study also compares results from incremental changes to cloud microphysics, turbulent mixing, deep convection, and subgrid effects to understand how assumptions in the representation of these processes affect different aspects of the simulated atmosphere as well as its response to forcings. We conclude that the spectral composition and geographical distribution of the ERFs and cloud feedback, as well as the fidelity of the simulated base climate state, are important for constraining the climate in the past and future.

Effects of Turbulent Mixing and Orbitally Shaking on Cell Growth and Biomass Production in Active Fluids

Effects of Turbulent Mixing and Orbitally Shaking on Cell Growth and Biomass Production in Active Fluids

Oceanic Response to Tropical Cyclone Gonu (2007) in the Gulf of Oman and the Northern Arabian Sea: Estimating Depth of the Mixed Layer Using Satellite SST and Climatological Data

The category 5-equivalent tropical Cyclone Gonu (2007) was the strongest cyclone to enter the northern Arabian Sea and Gulf of Oman. The impact of this cyclone on the sea surface temperature (SST) cooling and deepening of the mixed layer was investigated herein using an optimally interpolated (OI) cloud-free sea surface temperature (SST) dataset, climatological profiles of water temperature, and data from Argo profilers. SST data showed a maximum cooling of 1.7–6.5 °C during 1–7 June 2007 over the study area, which is similar to that of slow- to medium-moving cyclones in previous studies. The oceanic heat budget equation with the assumptions of the dominant turbulent mixing effect was used to establish relationships between SST and mixed layer depth (MLD) for regions that were directly affected by cyclone-induced turbulent mixing. The relationships were applied to the SST maps from satellite to obtain maps of MLD for 1–7 June, when Gonu was over the study area. Comparing with the measured MLD from Argo data showed that this approach estimated the MLDs with an average error of 15%, which is an acceptable amount considering the convenience of this approach in estimating MLD and the simplifications applied in the heat budget equation. Some inconsistencies in calculating MLD were attributed to use of climatological temperature profiles that may not have appropriately represented the pre-cyclone conditions due to pre-existing cold/warm core eddies. Estimation of the diapycnal diffusion that quantified the turbulent mixing across the water column showed consistent temporal and spatial variations with the calculated MLDs.

Rotating Horizontal Convection

Global differences of temperature and buoyancy flux at the ocean surface are responsible for small-scale convection at high latitudes, global overturning, and the top-to-bottom density difference in the oceans. With planetary rotation the convection also contributes to the large-scale horizontal, geostrophic circulation, and it crucially involves a 3D linkage between the geostrophic circulation and vertical overturning. The governing dynamics of such a surface-forced convective flow are fundamentally different from Rayleigh–Bénard convection, and the role of buoyancy forcing in the oceans is poorly understood. Geostrophic balance adds to the constraints on transport in horizontal convection, as illustrated by experiments, theoretical scaling, and turbulence-resolving simulations for closed (mid-latitude) basins and an annulus or reentrant zonal (circumpolar) channel. In these geometries, buoyancy drives either horizontal mid-latitude gyre recirculations or a strong Antarctic Circumpolar Current, respectively, in addition to overturning. At large Rayleigh numbers the release of available potential energy by convection leads to turbulent mixing with a mixing efficiency approaching unity. Turbulence-resolving models are also revealing the relative roles of wind stress and buoyancy when there is mixed forcing, and in future work they need to include the effects of turbulent mixing due to energy input from tides.

Ignition and flame stabilisation of primary reference fuel sprays at engine-relevant conditions

This study aims to investigate the underlying processes governing ignition and flame stabilisation in compression-ignition (CI) engine-relevant conditions. The experiments feature a canonical configuration with a single fuel jet injected into a constant-volume combustion chamber. Primary reference fuels (PRFs), including PRF100 (neat iso-octane, a gasoline surrogate), PRF80 (a blend of 80 vol.% iso-octane and 20 vol.% n-heptane) and PRF0 (neat n-heptane, a diesel surrogate), were tested to simulate changes in fuel ignition quality inside a quiescent steady environment with an ambient density of 22.8 kg/m3 and an O2 concentration of 15 vol.%. The ambient gas temperatures were controlled at 1150 K (PRF100), 1120 K (PRF80) and 900 K (PRF0), in order to adapt to the fuel reactivity so that a constant ignition delay of 1.15 ms can be achieved for all blends. This approach was employed in order to substantially reduce the effect of fuel-oxidiser mixing prior to ignition while highlighting the effect of fuel chemistry on the ignition process and flame evolution. Under the test conditions of this study, optical imaging reveals that the blends with higher iso-octane content exhibit a faster spreading of combustion after ignition and establish a steady lifted flame that is closer to the nozzle. Imaging by CH2O-PLIF indicates that blends with higher iso-octane content produce CH2O that is distributed across larger portions of the jet at an earlier timing when compared to neat n-heptane that shows a propagating first-stage ignition through the fuel jet. Supporting unsteady flamelet calculations are presented to investigate the effect of chemistry and turbulent mixing. The flamelet calculations agree qualitatively in several respects to the experiments, especially in the spatial and temporal trends for CH2O production and consumption. Synthesis of the flamelet and experimental results suggests that for the iso-octane-containing fuels, CH2O is formed via single-stage ignition reactions rather than exhibiting the typical two-stage ignition behaviour which is found in the pure n-heptane fuel case. Furthermore, the flamelet calculations suggest high-temperature ignition occurs first in lean mixtures in the iso-octane-containing fuel cases, but in rich mixtures for the PRF0 case. If autoignition is the mode of flame stabilisation, this provides an explanation for why the PRF100 and PRF80 cases stabilise further upstream, since lean mixtures have longer residence times, experience lower scalar dissipation rate, and may be more likely to be exposed to a supporting peripheral reservoir of hot products, should one exist. Overall, this study provides insights into the roles of fuel chemistry and turbulent mixing on the ignition and combustion behaviour of PRFs under engine-relevant conditions.

A Case Study of the Thermohaline Fine-scale Structure of the Sub-Arctic Front

ABSTRACT This paper analyzes the characteristics of the thermohaline fine-scale structure of the sub-Arctic front (SAF) in the Kuroshio–Oyashio confluence region using temperature, salinity, and current data obtained from surveys in June 2016. Thermohaline intrusion is found to be prevalent in the sea area of the SAF, occurring predominantly in frontal zones with depths less than 200 m and in North Pacific intermediate water (NPIW) layers with depths greater than 350 m. The intrusion in the SAF zones with depths less than 200 m has a higher intensity but smaller dimensions in the NPIW. According to our analysis, the thermohaline intrusion in the SAF sea zone is mainly attributable to the joint effect of double diffusion and flow instability. At depths of 0–350 m, the thermohaline intrusion is vulnerable to the influences of salt fingering, diffusive convection, and symmetric instability. At depths deeper than 350 m, diffusive convection is significant. In addition, shear instability might be a contributor. Considering that the effect of turbulent mixing is primarily governed by symmetric instability in the SAF sea areas, the flow instability characterized by a dominant contribution from symmetric instabilities might exert a more significant influence on thermohaline intrusion. The more distinct thermohaline staircases at layers deeper than 350 m, which are mainly produced by diffusive convection, have larger vertical scales than the staircases in other layers.

Super Residual Circulation: a new perspective on ocean vertical heat transport

AbstractOcean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities to diagnose circulation. Using quasi-steady state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of Super-Residual Transport (SRT), which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies, but excludes small-scale three dimensional turbulent mixing effect, to construct a new overturning circulation – the ‘Super Residual Circulation’ (SRC).We find that in the coarse resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11 Sv circulation which transports heat upward. The SRC’s upward heat transport is ~2 times larger in a finer horizontal resolution (0.1°) version of ACCESS, suggesting a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into super-residual processes, as the SRC elucidates the pathways in temperature and depth space along which watermass transformation occurs.

Diagnosing Subsurface Vertical Velocities from High-Resolution Sea Surface Fields

AbstractUsing the extended “interior + surface quasigeostrophic” method from the 2019 study by Liu et al. (hereafter L19), subsurface density and horizontal velocities can be reconstructed from sea surface buoyancy and surface height. This study explores the potential of L19 for diagnosing the upper-ocean vertical velocity w field from high-resolution surface information, employing the 1/30° horizontal resolution OFES model output. Specifically, we employ the L19-reconstructed density and horizontal velocity fields in a diabatic version of the omega equation that incorporates a simplified parameterization for turbulent vertical mixing. The w diagnosis is evaluated against OFES output in the Kuroshio Extension region of the North Pacific, and the result indicates that the L19 method constitutes an effective framework. Statistically, the OFES-simulated and L19-diagnosed w fields have a 2-yr-averaged spatial correlation of 0.42–0.51 within the mixed layer and 0.51–0.67 throughout the 1000-m upper ocean below the mixed layer. Including the diabatic turbulent mixing effect has improved the w diagnoses inside the mixed layer, particularly for the cold-season days with the largest correlation improvement reaching 0.31. Our encouraging results suggest that the L19 method can be applied to the high-resolution sea surface height data from the forthcoming Surface Water and Ocean Topography (SWOT) satellite mission for reconstructing 3D hydrodynamic conditions of the upper ocean.

Turbulent flame-wall interactions for flames diluted by hot combustion products

Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Nitrogen Oxides (NOx) in industrial applications. In gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products is investigated using direct numerical simulation. A three-Dimensional (3D) turbulent V-flame in a channel with isothermal hot and cold walls is simulated.It is shown that a main reaction zone in the central region between two walls supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. The cold wall leads to a longer flame, thereby having disproportionately large contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localises the near-wall CO distribution. High mean CO mass fraction develops in the free-stream while a high magnitude of the peak RMS CO mass fraction is present closer to the wall. It is also shown that one-dimensional flame solutions can reasonably describe the changes of CO mass fraction as a function of temperature in the free-stream region and some parts of the near-wall region but not close to the wall. Turbulent mixing and diffusion effects contribute to this deviation. The results highlight the complexities involved in CO modelling for diluted flames and set a benchmark for future work.

Study on Dust Turbulence-Chemical Agglomeration for Electrostatic Precipitation Technology

Due to the turbulent mixing between agglomeration agent and dust, the number of collisions per unit time between particles and droplets increases with the particle size in the process of chemical agglomeration, and the agglomeration efficiency increases accordingly. Turbulent agglomeration can promote the agglomeration effect of subsequent chemical agglomeration by strengthening the collision between particles and agglomerant droplets. The author used chemical agglomeration and turbulent mixing to cooperate in order to improve the efficiency of dust removal. Turbulent mixing can promote chemical agglomeration from agglomeration effect and dust removal efficiency, which can greatly improve electric dust removal technologyTurbulent mixing is the most intense at the inlet box position, and the agglomeration effect is the best. Turbulent mixing synergistic effect has an effect on dust removal efficiency. Compared with the three curves, it can be seen that the dust removal efficiency increases rapidly with the increase of agglomeration concentration, the curve trend changes obviously. The dust removal efficiency can reach 98.36 % and it is the highest in the middle section when the wind speed is greater than 11.2m/s. Through the experiment, the turbulent mixing and chemical coagulation method has a good application prospect in the electrostatic precipitators.

Effects of subgrid-scale horizontal turbulent mixing on a simulated convective storm at kilometer-scale resolutions

At kilometer-scale resolutions, the most energetic turbulent eddies associated with deep moist convection is partially resolved and partially subgrid scale (SGS). Therefore, SGS turbulent fluxes still play an important role in the transport of heat, momentum and other scalars. In contrast to its vertical counterpart, the horizontal SGS turbulent transport is often overlooked due to its relative insignificance at mesoscale grid spacings. This study investigates the effects of SGS horizontal turbulent mixing on a simulated real convective storm at kilometer-scale grid spacings. The SGS horizontal fluxes are parameterized by the widely-used 2D Smagorinsky model. Sensitivity studies are performed by varying the horizontal mixing length of the model, and are conducted on 3 km, 1 km and 333 m grids. Simulation results are evaluated against precipitation products and radar observations, and inter-compared with respect to both model resolution and SGS horizontal mixing. The comparison focuses on precipitation statistics as well as storm organization and morphology. The simulated precipitation is found to be most sensitive to SGS horizontal mixing at 3 km spacings, while the influence is much reduced at 333 m resolution. But for storm structure, SGS horizontal mixing remains important even at 333 m resolution. For practical numerical weather prediction purposes, this study suggests that in the kilometer-scale range, coarse resolution results could be made to resemble fine resolution results by optimizing the amount of SGS horizontal mixing. However, not all aspects of the coarse resolution simulations can be improved by tuning SGS horizontal mixing alone.

Numerical study of the mixing dynamics of trans- and supercritical coaxial jets

Characterization of the transcritical coaxial injectors, accounting for the geometrical features and thermodynamics nonlinearities, is of both practical and fundamental importance. In the present study, the interactions and effects of turbulent mixing and pseudo-boiling phenomena are investigated. To do this, the mixing dynamics of bi-shear jets injected under trans- and supercritical conditions has been investigated numerically using the large-eddy simulation technique. The numerical framework provides real-gas thermodynamics and transport properties, using the Peng–Robinson equation-of-state and Chung’s models, respectively. The obtained flow quantities are in good agreement with the pervious experimental and numerical results. Investigations show that formation of the highly stratified mixing layer and thermal-shield, a layer of large isobaric specific heat encompassing the transcritical jet, influences the transcritical mixing layer evolution. However, significant interactions of turbulent coherent structures and real-gas thermodynamics nonlinearities, referred to as the Turbulence-Thermodynamics Interaction (TTI), can locally destruct the thermal-shield and make alternate regions of gas-like and liquid-like fluids, appearing as finger-like structures on the jet interface. Therefore, it can be speculated that the initial stage of the mixing of a transcritical jet is controlled by the pseudo-boiling phenomenon that can produce strong TTI. Furthermore, local distortions of the thermal-shield provide opportunities for prominent activation of the baroclinic-torque and volume-dilatation, which, in turn, can facilitate the hydrodynamic instabilities. The growth of these instabilities disintegrates the transcritical jet’s core and results in shedding of dense lumps from the core. In addition, results indicate that nearly throughout the flow field, the baroclinic-torque to a great extent affects the turbulent-kinetic-energy.

Numerical study of the pseudo-boiling phenomenon in the transcritical liquid oxygen/gaseous hydrogen flame

The interactions and effects of turbulent mixing, pseudo-boiling phenomena, and chemical reaction heat release on the combustion of cryogenic liquid oxygen and gaseous hydrogen under supercritical pressure conditions are investigated using RANS simulations. Comparisons of the present numerical simulation results with available experimental data reveal a reasonably good prediction of a supercritical axial shear hydrogen-oxygen flame using the standard k-ε turbulence model and the eddy dissipation concept combustion model with a 23 reaction steps kinetics for H2-O2 reaction. The present simulation qualitatively reproduced oxygen injection and its reaction with the co-flowing hydrogen, which is characterized by rapid flame expansion, downstream flame propagation, and expansion induced flow recirculation. Several turbulence models were used for numerical simulations. It is shown that the selection of an appropriate turbulence model for transcritical reacting flows is crucial and far more important than for subcritical reacting flows. It is indicated that the pseudo-boiling phenomena is the main reason for the considerable differences between the turbulence models in a transcritical flame. Also, it is demonstrated that the liquid oxygen core disappears faster in a non-reacting flow than in a reacting flow. The shear layer in the non-reacting flow is much stronger than reacting case; providing a large transfer of energy from the outer layer to the inner layer. At the supercritical injection conditions, the difference between the turbulence models is much less than the transcritical injection conditions.

The effects of vertical mixing on nonlinear Kelvin waves

Abstract

Impact of a medicane on the oceanic surface layer from a coupled, kilometre-scale simulation

Abstract. A kilometre-scale coupled ocean–atmosphere numerical simulation is used to study the impact of the 7 November 2014 medicane on the oceanic upper layer. The processes at play are elucidated through analyses of the tendency terms for temperature and salinity in the oceanic mixed layer. While comparable by its maximum wind speed to a Category 1 tropical cyclone, the medicane results in a substantially weaker cooling. As in weak to moderate tropical cyclones, the dominant contribution to the surface cooling is the surface heat fluxes with secondary effects from the turbulent mixing and lateral advection. Upper-layer salinity decreases due to heavy precipitation that overcompensates the salinizing effect of evaporation and turbulent mixing. The upper-layer evolution is marked by several features believed to be typical of Mediterranean cyclones. First, strong, convective rain occurring at the beginning of the event builds a marked salinity barrier layer. As a consequence, the action of surface forcing is favoured and the turbulent mixing dampened with a net increase in the surface cooling as a result. Second, due to colder surface temperature and weaker stratification, a cyclonic eddy is marked by a weaker cooling opposite to what is usually observed in tropical cyclones. Third, the strong dynamics of the Strait of Sicily enhance the role of the lateral advection in the cooling and warming processes of the mixed layer.

Light Scattering in a Turbulent Cloud: Simulations to Explore Cloud-Chamber Experiments

Radiative transfer through clouds can be impacted by variations in particle number size distribution, but also in particle spatial distribution. Due to turbulent mixing and inertial effects, spatial correlations often exist, even on scales reaching the cloud droplet separation distance. The resulting clusters and voids within the droplet field can lead to deviations from exponential extinction. Prior work has numerically investigated these departures from exponential attenuation in absorptive and scattering media; this work takes a step towards determining the feasibility of detecting departures from exponential behavior due to spatial correlation in turbulent clouds generated in a laboratory setting. Large Eddy Simulation (LES) is used to mimic turbulent mixing clouds generated in a laboratory convection cloud chamber. Light propagation through the resulting polydisperse and spatially correlated particle fields is explored via Monte Carlo ray tracing simulations. The key finding is that both mean radiative flux and standard deviation about the mean differ when correlations exist, suggesting that an experiment using a laboratory convection cloud chamber could be designed to investigate non-exponential behavior. Total forward flux is largely unchanged (due to scattering being highly forward-dominant for the size parameters considered), allowing it to be used for conditional sampling based on optical thickness. Direct and diffuse forward flux means are modified by approximately one standard deviation. Standard deviations of diffuse forward and backward fluxes are strongly enhanced, suggesting that fluctuations in the scattered light are a more sensitive metric to consider. The results also suggest the possibility that measurements of radiative transfer could be used to infer the strength and scales of correlations in a turbulent cloud, indicating entrainment and mixing effects.

Autoignition of an n-heptane jet in a confined turbulent hot coflow of air

The autoignition of a continuous, single jet of pure liquid n-heptane injected concentrically and axisymmetrically from a water-cooled circular nozzle into a confined turbulent hot coflow (CTHC) of air at atmospheric pressure has been investigated experimentally at air temperatures up to 1150 K and velocities up to 40 m/s. The aim of this work was to examine the emergence of liquid-fuel autoignition in the presence of flow, mixture and phase inhomogeneities, to which end, the velocity, temperature and fuel-droplet fields inside the CTHC reactor were characterized in a series of dedicated measurement campaigns. Distinct phenomena were identified concerning the emergence of various regimes: no autoignition, random spots, and continuous flame. In the random spots regime, autoignition appeared in the form of well-defined, discrete localized spots occurring randomly within the reactor, similar to observations in a similar apparatus with gaseous fuels (Markides, 2005; Markides and Mastorakos, 2005, 2011; Markides et al., 2007). High-speed optical measurements of these random spots were made from which the autoignition locations/lengths were measured, and then used to infer average autoignition delay, or residence, times from injection based on the bulk air velocity. An increase in the air temperature moved the region of autoigniting spots closer to the injector nozzle, thus decreasing the autoignition length and also decreasing the autoignition delay time. Generally, autoignition moved downstream with increasing bulk air velocity, but the delay times decreased contrary to the aforementioned earlier work with pre-vaporized n-heptane in this geometry. Of interest is the finding that at the highest investigated air velocities, the autoignition length decreased as the air velocity increased, which again deviates from the same earlier work with vaporized n-heptane. Furthermore, higher liquid injection velocities also resulted in increased autoignition lengths and times. The results from this work suggest that the turbulent inhomogeneous autoignition of liquid fuels cannot be directly predicted from chemical delay times of equivalent homogeneous systems, or even from knowledge of the turbulent inhomogeneous autoignition of single-phase (gaseous) fuels in the same reactor system (flow geometry, fuel, conditions, etc.), and that the effects of evaporation and turbulent mixing must be accounted for. The data can assist the development of advanced turbulent and multiphase reacting flow models.

Earth’s Polar Night Boundary Layer as an Analog for Dark Side Inversions on Synchronously Rotating Terrestrial Exoplanets

Abstract A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free atmosphere. Here we analyze data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogs on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called “mesoscale slope winds,” which have horizontal scales of 10–100 s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models.

A study on atmospheric turbulence structure and intermittency during heavy haze pollution in the Beijing area

In this study, the characteristics of turbulence transport and intermittency and the evolutionary mechanisms were studied in different pollution stages of heavy haze weather from December 2016 to January 2017 in the Beijing area using the method developed by Ren et al. (2019) as the automatic identification of atmospheric spectral gaps and the reconstruction of atmospheric turbulence sequences. The results reveal that turbulence intermittency is the strongest in the cumulative stage (CS) of heavy haze weather, followed by in the transport stage (TS), and it is the weakest in the dissipation stage (DS). During the development and accumulation of haze pollution, buoyancy contributes negatively to turbulent kinetic energy (TKE), and horizontal wind speed is low. The classical turbulent motion is often affected by submesoscale motion. As a result, the calculation results of turbulence parameters are affected by submesoscale motion, which causes intensified turbulence intermittency. During the dissipation of pollution, the downward momentum transfer induced by low-level jets provides kinetic energy for turbulent motion in the near surface layer. The turbulent mixing effect is enhanced, and intermittency is weakened. Due to the intermittency of atmospheric turbulence, turbulence parameters calculated from the original fluctuation of meteorological elements may be overestimated. The overestimation of turbulence parameters in the CS is the strongest, followed by the TS, and the DS is the weakest. The overestimation of turbulent fluxes results in an overestimation of atmospheric dissipation capability that may cause an underestimation of pollutant concentrations in the numerical simulations of air quality.

Effect of turbulent motions at different length scales on turbulent premixed swirl-stabilised flame topology

Three-dimensional direct numerical simulation data of H2-air turbulent swirling premixed combustion at two different swirl numbers are analysed to investigate the local reaction zone morphology and its relation with local turbulent motions at different length scales. The effect of small scale turbulent mixing on local flames is investigated, and the results have shown that the contribution of microscale turbulent diffusivity on the local flamelet is insignificant, although there is some evidence of flame thinning for the higher swirl number case. The flame morphology such as high-level convolution and interacting flames, on the other hand, shows greater influence on local flamelets, suggesting the importance of local reaction zone topology on overall combustion processes. The local reaction zones are analysed by using the shapefinders to quantify their topology. Although the shapefinders showed various local reaction zone shapes consisting of “pancakes” and “tubes” and intermissive intense reaction zone distributions, the smallest characteristic length scale shows that the local reaction zones are thin. Finally, the relationship between these local reaction zone topology and turbulent motions at different sizes were discussed. The local reaction zone topology has a direct relation with Taylor microscale, integral length scale and their associated velocity scale, whereas almost no correlation is observed with Kolmogorov length scale, in the presence of inhomogeneous turbulence and strong mean shears. The present results suggest the importance of Taylor microscale on flame surface topology, which is often understated in turbulent combustion modelling frameworks.