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Intermittent turbulence in general refers to a brief turbulent burst, which is the main mechanism of scalar diffusion in the stable boundary layer (SBL). The impacts of intermittent turbulence on a radiation fog were investigated based on the measurements at a 255-m meteorological tower and the Weather Research and Forecasting model. Observational results showed that intermittent turbulence inhibited fog formation. As intermittent turbulence weakened, radiation fog formed in the SBL. During the fog development and maturity stage, intermittent turbulence at the high levels promoted the vertical development of fog. However, the downward propagation of intermittent turbulence did not reach the surface. Low intermittent strength of turbulence and weak turbulent mixing at 40 m indicated that there was a barrier layer hindering the transmission up and down. The barrier effect led to explosively reinforced fog at the surface. Intermittent turbulence is not considered in the original Yonsei University (YSU) scheme, leading to the underestimation of the simulated turbulent diffusion coefficient ( k m ). The average ratio of observed k m to simulated k m was 4.30 during the fog episode. Thus, three sensitivity experiments – a double k m , a quadruple k m from the original YSU scheme, and an updated YSU scheme – were designed to study the contributions of the increase in k m to fog evolution. The results showed that the increase in k m can improve the simulation of fog-top height and correct the onset timing of fog. Thus, an improvement in the original YSU scheme is necessary for a reasonable description of intermittent turbulence.

This work studies the effect of the daily dynamics of turbulent processes on the daily dynamics of the electric field in the surface air layer. During simulation, the coefficient of turbulent diffusion within the electrode layer is specified as a stationary function of altitude in view of hydrodynamic concepts. A mathematical model of the dynamics of the electric field intensity in the surface air layer in the case of a turbulent electrode effect is presented. The main equation of the model is the equation of the total current in the surface layer, which has been derived in the approximation of strong turbulent mixing and describes the electrodynamics of the surface layer under the combined action of local and global current generators. The work examines the non-stationary nature of turbulent exchange in order to confirm the previously ascertained effects in the daily dynamics of the electric field strength in the surface air layer under stationary turbulence. To describe the daily dynamics of turbulent processes, gradient measurements in high-altitude conditions of the Elbrus region were used. Processing of the measurement data enables deriving the time dependence of the turbulent diffusion coefficient from the solution of the total current equation. Taking into account this dependence, the expression for the daily dynamics of the field strength was refined. Time shifts of the daily extremes, a change in their amplitude, and the appearance of additional extremes depending on the electric field strength have been established. All these effects are comparable to the global unitary variation and increase with the electric field strength. The results can be useful for solving a number of applied geophysical problems, in particular, monitoring the electric field of the atmosphere and analyzing atmospheric-electrical measurement data.

An analytical solution to the problem of the convective-diffusive transport of suspended matter from dredging in a river system is presented within the framework of the model of a point source in the flux. Calculations of the distribution of suspended matter concentration and silt height at the river bottom for various coefficients of turbulent diffusion are carried out. A good agreement of the distributions of suspended matter concentration with the results of numerical solution and experimental data is shown.

The general objective of this project is to develop an accurate combustion and radiation modeling framework for high-fidelity large eddy simulations (LES) of well-controlled turbulent laboratory-scale fires for which the fuel composition and fuel oxidation chemistry are known. This modeling framework is aimed at providing a solid basis for the development and validation of engineering-level models used in simulations of real-world fire problems for which the sources of fuel are diverse, complex, and in many cases, poorly characterized. The combustion model features a library of flamelet solutions corresponding to one-dimensional, steady, laminar, counterflow diffusion flames simulated with specialized software, a chemical kinetic mechanism and an equi-diffusive molecular transport model (i.e., unity Lewis numbers). Two different flamelet libraries are considered here: a first library generated with a solver called libOpenSMOKE and a detailed chemical kinetic mechanism developed for C1-C3 combustion chemistry and a second library generated with a solver called FlameMaster and the GRI-Mech v3.0 chemical kinetic mechanism developed for methane combustion chemistry. The radiation model features a banded Weighted-Sum-of-Gray-Gases model but (so far) no description of subgrid-scale turbulence-radiation interactions (TRI). This modeling framework is incorporated into a LES solver developed by FM Global and called FireFOAM, and is evaluated in simulations of a two-dimensional, plane, buoyancy-driven, methane-air, turbulent diffusion flame experimentally studied at the University of Maryland. The configuration corresponds to an intermediate validation step in our model development strategy without the complications of flame extinction. The flame structure is characterized by new micro-thermocouple measurements of the temporal mean and root-mean-square gas temperatures. Comparisons between simulated and measured temperatures show significant discrepancies that are explained by the large values of the width of the presumed probability density function (PDF) representing subgrid-scale variations of mixture fraction and by the absence of a model for subgrid-scale TRI.

Motivated by the need for compact descriptions of the evolution of non-classical wakes behind yawed wind turbines, we develop an analytical model to predict the shape of curled wakes. Interest in such modelling arises due to the potential of wake steering as a strategy for mitigating power reduction and unsteady loading of downstream turbines in wind farms. We first estimate the distribution of the shed vorticity at the wake edge due to both yaw offset and rotating blades. By considering the wake edge as an ideally thin vortex sheet, we describe its evolution in time moving with the flow. Vortex sheet equations are solved using a power series expansion method, and an approximate solution for the wake shape is obtained. The vortex sheet time evolution is then mapped into a spatial evolution by using a convection velocity. Apart from the wake shape, the lateral deflection of the wake including ground effects is modelled. Our results show that there exists a universal solution for the shape of curled wakes if suitable dimensionless variables are employed. For the case of turbulent boundary layer inflow, the decay of vortex sheet circulation due to turbulent diffusion is included. Finally, we modify the Gaussian wake model by incorporating the predicted shape and deflection of the curled wake, so that we can calculate the wake profiles behind yawed turbines. Model predictions are validated against large-eddy simulations and laboratory experiments for turbines with various operating conditions.

In January 2013, February 2014, December 2015 and December 2016 to 10 January 2017, 12 persistent heavy aerosol pollution episodes (HPEs) occurred in Beijing, which received special attention from the public. During the HPEs, the precise cause of PM2.5 explosive growth (mass concentration at least doubled in several hours to 10 h) is uncertain. Here, we analyzed and estimated relative contributions of boundary-layer meteorological factors to such growth, using ground and vertical meteorological data. Beijing HPEs are generally characterized by the transport stage (TS), whose aerosol pollution formation is primarily caused by pollutants transported from the south of Beijing, and the cumulative stage (CS), in which the cumulative explosive growth of PM2.5 mass is dominated by stable atmospheric stratification characteristics of southerly slight or calm winds, near-ground anomalous inversion, and moisture accumulation. During the CSs, observed southerly weak winds facilitate local pollutant accumulation by minimizing horizontal pollutant diffusion. Established by TSs, elevated PM2.5 levels scatter more solar radiation back to space to reduce near-ground temperature, which very likely causes anomalous inversion. This surface cooling by PM2.5 decreases near-ground saturation vapor pressure and increases relative humidity significantly; the inversion subsequently reduces vertical turbulent diffusion and boundary-layer height to trap pollutants and accumulate water vapor. Appreciable near-ground moisture accumulation (relative humidity> 80 %) would further enhance aerosol hygroscopic growth and accelerate liquid-phase and heterogeneous reactions, in which incompletely quantified chemical mechanisms need more investigation. The positive meteorological feedback noted on PM2.5 mass explains over 70 % of cumulative explosive growth.

Turbulent entrainment at the turbulent/non-turbulent interface (TNTI) plays an important role in understanding the turbulent diffusion. While entrainment in fully developed canonical turbulent flows has been extensively studied, the evolution of entrainment in spatially developing flows remains poorly understood. In this work, characteristics of entrainment and the effect of vortices on entrainment of the shear layer separated from a wall-mounted fence are studied by the experiment in a water channel. The shedding vortex experiences a series of stages, including generation, growth, deformation and breakdown into smaller vortices. With the development of the flow, entrainment varies correspondingly. The prograde vortex near the TNTI is found to suppress entrainment but have little effect on the detrainment process, while the retrograde vortex promotes entrainment and suppresses detrainment as well. Consequently, the local entrainment velocity is decreased by the prograde vortex and increased more significantly by the retrograde vortex. Along the streamwise direction, the time-mean entrainment velocity is smallest where the prograde vortex is strongest in the vortex deformation stage. However, the largest time-mean entrainment velocity is located where the enstrophy gradient near the TNTI is greatest after reattachment, rather than where the retrograde vortex is strongest shortly after the breakdown of the shedding vortex, because the scarcity of retrograde vortices in the vicinity of the TNTI makes their long-time cumulative contribution not as significant as their local enhancement. The present study reveals how entrainment evolves in the separated and reattaching flow, and improves our understanding of the effect of vortices on entrainment.

An experimental Lagrangian study based on particle tracking velocimetry has been completed in an incompressible turbulent round water jet freely spreading into water. The jet is seeded with tracers only through the nozzle: inhomogeneous seeding called nozzle seeding. The Lagrangian flow tagged by these tracers therefore does not contain any contribution from particles entrained into the jet from the quiescent surrounding fluid. The mean velocity field of the nozzle seeded flow, $\\langle \\boldsymbol {U}_{\\boldsymbol {\\varphi }} \\rangle$, is found to be essentially indistinguishable from the global mean velocity field of the jet, $\\langle \\boldsymbol {U} \\rangle$, for the axial velocity while significant deviations are found for the radial velocity. This results in an effective compressibility of the nozzle seeded flow for which $\\boldsymbol {\\nabla }\\boldsymbol {\\cdot } \\langle \\boldsymbol {U}_{\\boldsymbol {\\varphi }} \\rangle \\neq 0$ even though the global background flow is fully incompressible. By using mass conservation and self-similarity, we quantitatively explain the modified radial velocity profile and analytically express the missing contribution associated with entrained fluid particles. By considering a classical advection–diffusion description, we explicitly connect turbulent diffusion of mass (through the turbulent diffusivity $K_T$) and momentum (through the turbulent viscosity $\\nu _T$) to entrainment. This results in new practical relations to experimentally determine the non-uniform spatial profiles of $K_T$ and $\\nu _T$ (and hence of the turbulent Prandtl number $\\sigma _T = \\nu _T/K_T$) from simple measurements of the mean tracer concentration and axial velocity profiles. Overall, the proposed approach based on nozzle seeded flow gives new experimental and theoretical elements for a better comprehension of turbulent diffusion and entrainment in turbulent jets.

In particle-laden turbulent wall flows, transport of particles towards solid walls is phenomenologically thought to be governed by the wall-normal turbulence intensity supporting the underlying particle–eddy interactions that are usually modelled by a combination of turbophoresis and turbulent diffusion. We estimate the turbophoretic and turbulent diffusive coefficients as a function of wall-normal coordinate directly from a generated direct numerical simulation (DNS) database of low volume fraction point particles in a turbulent pipe flow. These coefficients are then used in an advection–diffusion equation to estimate the particle concentration as a function of wall-normal distance and time, with favourable comparison against DNS for smaller Stokes number ($St^+$) particles suggesting a limitation of the common gradient diffusion hypothesis for larger $St^+$ particles. Using DNS we explore the non-trivial effects of $St^+$, pipe wall condition (particle absorbing or elastic) as well as the influence of drag and lift force on the velocity and particle statistics giving rise to different particle concentrations. We then appraise various Eulerian-based models of turbophoretic and turbulent diffusive coefficients and, finally, use physical insights from Lagrangian correlation times, conditional quadrant analysis and flow topology to shed further light on the particle transport as a function of various parameters and the limits of gradient diffusion hypothesis.

Turbulent motions enhance the diffusion of large-scale flows and temperature gradients. Such diffusion is often parameterized by coefficients of turbulent viscosity ($\\nu _{t}$) and turbulent thermal diffusivity ($\\chi _{t}$) that are analogous to their microscopic counterparts. We compute the turbulent diffusion coefficients by imposing sinusoidal large-scale velocity and temperature gradients on a turbulent flow and measuring the response of the system. We also confirm our results using experiments where the imposed gradients are allowed to decay. To achieve this, we use weakly compressible three-dimensional hydrodynamic simulations of isotropically forced homogeneous turbulence. We find that the turbulent viscosity and thermal diffusion, as well as their ratio the turbulent Prandtl number, $\\textit {Pr}_{t} = \\nu _{t}/\\chi _{t}$, approach asymptotic values at sufficiently high Reynolds and Péclet numbers. We also do not find a significant dependence of $\\textit {Pr}_{t}$ on the microscopic Prandtl number $\\textit {Pr} = \\nu /\\chi$. These findings are in stark contrast to results from the $k{-}\\epsilon$ model, which suggests that $\\textit {Pr}_{t}$ increases monotonically with decreasing $\\textit {Pr}$. The current results are relevant for the ongoing debate on, for example, the nature of the turbulent flows in the very-low-$\\textit {Pr}$ regimes of stellar convection zones.