Explore the vast range of titles available.

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.

This study establishes an experimental platform consisting of an adjustable inclined surface and a cross-slope wind system. Turbulent diffusion flames are investigated by examining the variation characteristics of flame morphology under slope angles of 10–40°, cross-slope wind velocities of 0.8–2.0 m/s, and heat release rates of 15.38–61.50 kW. The results show that variations in slope angle change the components of buoyancy in the normal and tangential directions. The normal component influences the lifting of the flame perpendicularly to the slope, while the tangential component, together with differences in air entrainment on both sides of the flame, promotes flame inclination and spreading along the slope surface. The cross-slope wind enhances the horizontal stretching and attachment tendency of the flame through inertial shear, while simultaneously suppressing flame height and its development along the slope. The coupled effects of these factors cause the flame morphology to gradually transition from a nearly vertical state to an attached state. Based on dimensionless analysis, empirical correlations of flame morphology parameters are established by introducing the cross-slope wind Froude number, dimensionless heat release rate, the density ratio of propane to air, and a slope function. Within the experimental range of this study, the data under various conditions show good collapse and correlation under the selected dimensionless parameters.

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.

AbstractA new approach to the problem of determining the density of emission fluxes of anthropogenic impurities from distributed urban sources by the rate of growth of the integral content of impurities in the vertical column of the atmosphere in the morning hours is proposed. The method is based on the use of a singularly perturbed reaction–diffusion model describing the vertical distribution of an admixture (carbon monoxide) over a city, in combination with atmospheric CO measurements over Moscow. The vertical profile of the turbulent diffusion coefficient was calculated from the measurement data at the Ostankino television tower, and the vertical profiles of the CO concentration for different seasons were reconstructed. The average annual CO emissions from the entire territory of Moscow were calculated using model data. The reliability of the obtained emission values is confirmed by comparisons with the emission inventory data.

AbstractBuoyant diffusion flame is one of the main forms of undesirable source release. Hazardous fires in industrial accidents that cause heavy casualties and environmental damages are often accompanied by burning of multiple diffusion flames. To avoid and minimize damage and control energy release, efforts are being done to evaluate the fire hazard of multiple fires. This paper presents an experimental study on criteria of flame merging and interaction of two unequal propane gas fires. The dimensions of two square burners were different and fixed at 10 and 15 cm. The heat release rate of each burner varied from 10.8 to 64.8 kW and the burner spacing changed from 0 cm to 60 cm. The results showed that as the spacing increases, the interaction of two unequal flames presents fully merging, intermittent merging and non-merging states. The flame heights and tilt angles of two flames are different due to asymmetric air entrainment. The flame merging probability based on the statistical flame shape is proposed to determine the flame merging extent. Based on dimensional analysis method, a model for flame merging probability is proposed, which shows that spacing has the greatest influence on flame merging, followed by the heat release rates of burners. According to the proposed model, the safety distance between two discrete combustibles can be estimated, which helps to plan the layout and spacing of discrete fuels.

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.

The streaming instability (SI) is a leading candidate for planetesimal formation, which can concentrate solids through two-way aerodynamic interactions with the gas. The resulting concentrations can become sufficiently dense to collapse under particle self-gravity, forming planetesimals. Previous studies have carried out large parameter surveys to establish the critical particle to gas surface density ratio (Z), above which SI-induced concentration triggers planetesimal formation. The threshold Z depends on the dimensionless stopping time (τ s , a proxy for dust size). However, these studies neglected both particle self-gravity and external turbulence. Here, we perform 3D stratified shearing box simulations with both particle self-gravity and turbulent forcing, which we characterize via a turbulent diffusion parameter, α D. We find that forced turbulence, at amplitudes plausibly present in some protoplanetary disks, can increase the threshold Z by up to an order of magnitude. For example, for τ s = 0.01, planetesimal formation occurs when Z ≳ 0.06, ≳0.1, and ≳0.2 at α D = 10−4, 10−3.5, and 10−3, respectively. We provide a single fit to the critical Z required for the SI to work as a function of α D and τ s (although limited to the range τ s = 0.01–0.1). Our simulations also show that planetesimal formation requires a mid-plane particle-to-gas density ratio that exceeds unity, with the critical value being largely insensitive to α D. Finally, we provide an estimation of particle scale height that accounts for both particle feedback and external turbulence.

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.