Turbulent Transport Processes Research Articles

Flow Field Characteristics at the Spanwise Edge of a Vegetative Canopy Model

This study investigates the complex flow field across a spanwise vegetative model canopy edge focusing on turbulent transport processes. Utilizing stereoscopic particle image velocimetry, the three velocity components were measured in wall-parallel planes at various elevations within canopy across the spanwise canopy edge. Conventional ensemble averaged results were contrasted with those obtained by conditionally averaged flow properties across instantaneous internal interfaces in the flow to understand their contribution to the ensemble average. The conditional average captured the strong gradients in mean velocities, Reynolds stresses, vorticity, swirling strength, and turbulent kinetic energy production across the dynamically changing instantaneous interface. In contrast, the conventional ensemble average smeared out the strong gradients. Small magnitudes of advective terms in the turbulent kinetic energy transport equation suggested weak secondary transverse flows in the present model canopy. The turbulent flow structure across the spanwise canopy edge was further investigated using Quadrant-Hole analysis for both averaging approaches. Conventional ensemble averaged results indicated a shift from sweep to ejection dominance when moving from canopy into the open patch, while the conditional average showed only sweep dominated transport. In contrast to a homogeneous canopy layout, below canopy height at the canopy edge, sweeps and ejections lose their dominance in vertical turbulent transport. The present results show that the dynamics of internal interfaces govern the ensemble averaged results and a possible implementation into existing models is proposed. The present results are expected to increase understanding of spanwise turbulent transport and aid in developing strategies to mitigate desertification.

Correction: Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study

Correction: Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study

Large eddy simulation of boundary layer combustion of hydrogen and hydrocarbon fuel films in a modeled scramjet combustor

Supersonic fuel film cooling is a promising way to simultaneously reduce the severe wall heat and friction load of the internal passage in a scramjet engine when operating at hypersonic conditions. Large eddy simulations were performed to investigate the cooling and wall friction characteristics of hydrogen and hydrocarbon films under inert and reactive circumstances. The results show that the essential difference of the turbulent state in the mixing layer contributes to the totally different behaviors of the cooling and wall friction reduction performances of the two fuel films. The turbulent transport processes between the hydrogen film and the mainstream are much weaker as compared to the case of hydrocarbon film, making inert hydrogen rather superior in cooling and friction reduction applications. Besides, the increase of wall temperature for hydrogen film under the inert case is mainly driven by the loss of hydrogen with high heat capacity instead of by direct heat addition. However, the film cooling performance severely deteriorates when the hydrogen film burns due to presence of severe heat release sources near the wall. On the other hand, combustion of hydrocarbon film in the boundary layer can remarkably improve its originally barely-satisfactory cooling and friction reduction performance to the level comparable to that of hydrogen film, due to the suppression of turbulent transport processes in the mixing layer and presence of heat absorption sources near the wall. Overall, the hydrogen film is more advantageous in friction reduction, while the hydrocarbon film is more suitable for cooling.

Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study

Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study

Stochastic modeling of multiple scalar mixing in a three-stream concentric coaxial jet based on one-dimensional turbulence

Modeling turbulent mixing is a standing challenge for nonpremixed chemically reacting flows. Key complications arise from the requirement to capture all relevant scales of the flow and the necessity to distinguish between turbulent advective transport and molecular diffusive transport processes. In addition, anisotropic mean shear, variable advection time scales, and the coexistence of turbulent and nonturbulent regions need to be represented. The fundamental issues at stake are addressed by investigating multi-scalar mixing in a three-stream coaxial jet with a map-based stochastic one-dimensional turbulence model. ODT provides full-scale resolution at affordable costs by a radical reduction of complexity compared to high-fidelity Navier–Stokes solvers. The approach is partly justified by an application of the boundary-layer approximation, but neglects fluctuating axial pressure gradients. It is demonstrated that low-order scalar statistics are reasonably but not fully captured. Despite this shortcoming, it is shown that the model is able to reproduce experimental state-space statistics of multi-stream multi-scalar mixing. The model therefore offers physics-compatible improvements in multi-stream mixing modeling despite some fundamental limitations that remain from unjustified assumptions.

Lagrangian Chaotic Mixing in a Nonlinear Model of Resistive Drift-wave Turbulence in Tokamak Plasmas

In fusion plasma, numerical simulations are commonly employed to investigate the confinement properties of plasma in the bulk region of tokamaks. The modified Hasegawa-Wakatani (MHW) equations are used to model the behavior of the plasma, which enables us to understand the radial transport in two-dimensional numerical simulations of electrostatic resistive drift-wave turbulence. By utilizing the MHW equations, we have gained insights into the low-to-high confinement (L-H) transitions that occur spontaneously in the plasma when it moves from a low confinement stage, characterized by turbulent flow, to a turbulence-suppressed regime known as zonal flow. To investigate these transitions, we vary the value of a control parameter \(\alpha\), which is related to adiabaticity, in numerical simulations, and observe the transition between the two regimes. This simplified model of L-H transitions can provide valuable information for tokamaks. To identify the Lagrangian coherent structures (LCS) and to better characterize the chaotic mixing during the L-H transition, we computed the finite-time Lyapunov exponent (FTLE) of the calculated velocity field derived from the electrostatic potential. We further compared the statistics of the chaotic mixing of the two regimes. The results of our study offer insights into the turbulent transport processes in magnetic confinement fusion plasmas.

On the Lateral Entrainment Instability in the Inner Core Region of Tropical Cyclones

AbstractEntrainment of dry moat air with low equivalent potential temperature laterally into the eyewall and rainbands is a unique turbulent process in the inner‐core region of a tropical cyclone (TC). By analyzing in‐situ aircraft measurements collected by the reconnaissance flights that penetrated the eyewalls and rainbands of Hurricanes Rita (2005), Patricia (2015), Harvey (2017), and Michael (2018), as well as numerical simulations of Hurricanes Patricia (2015), and Michael (2018), we show that the moat air entrained into the eyewall and rainbands meets the instability criterion, and therefore, sinks unstably as a convective downdraft. The resultant positive buoyancy fluxes are an important source for the turbulent kinetic energy (TKE) in the eyewall and rainband clouds. This mechanism of TKE generation via lateral entrainment instability should be included in the TKE‐type turbulent mixing schemes for a better representation of turbulent transport processes in numerical forecasts of TCs.

Effects of combustion on the near-wall turbulence and performance for supersonic hydrogen film cooling using large eddy simulation

Supersonic film cooling using fuel on board is an effective way to simultaneously shield the huge heat and momentum flux transported from the mainstream to the wall in a scramjet engine. The self-ignition and combustion of the injected fuel film will significantly change the turbulent transport behavior in the boundary layer. To reveal the effects of the boundary layer combustion on the near-wall turbulence and wall fluxes, large eddy simulations (LES) of the Burrows–Kurkov supersonic combustion experiment using hydrogen as a film are performed based on the in-house solver scramjetFoam. The solver successfully captures the additional skin friction reduction phenomenon induced by the boundary layer combustion compared to other numerical works using LES in the public literature. The results reveal that further increased anisotropy of turbulence combined with the low-density region contributes to a remarkable suppression of turbulent transport processes in the wall-normal direction. The self-ignition point of the hydrogen film is found to oscillate back and forth in a span of 80 mm, which significantly enhances turbulence in the boundary layer. However, the increased turbulent fluctuating velocity is mainly concentrated in the streamwise direction, while the other two components are suppressed instead. The findings are also essential for improving engineering computations based on the Reynolds averaged simulation method.

Energetics of buoyancy-generated turbulent flows with active scalar: pure buoyant plume

A framework for the budget of Reynolds stress, temperature and density variations, turbulent mass flux and turbulent heat flux is presented for buoyancy-generated turbulent flows with a focus on turbulent plumes. The dynamical interactions between the turbulence generated from the velocity and thermodynamic fluctuations in thermal and heated gas plumes is studied. A large-eddy simulation tool has been used to simulate heavier and lighter than air plumes released from a circular heated source for high Reynolds number (Re). The budget equations are developed using a Favre-averaging approach. Both heated air plumes and heated gas plumes, namely, heated ${\rm SO}_2$ and heated ${\rm CH}_4$ plumes, are included in the study. The study focuses on addressing key questions – does the turbulence kinetic energy (TKE) spectrum follow the classical inertial $-5/3$ spectrum or is there a $-3$ buoyancy regime with a different scaling law? Fundamental pathways for generation of turbulence through velocity fluctuations, and temperature and density fluctuations are discussed. Analysis of the TKE and scalar variance budget equations has demonstrated that multiple mechanisms dominate the transport of Reynolds stresses, and in particular that correlations between the density and the velocity dominates the turbulence production mechanisms. Entrainment is connected to the turbulent transport and pressure-dilation processes.

Can evaporation from urban impervious surfaces be ignored?

Urban evapotranspiration is an important component linking the urban hydrological cycle and energy balance. Previous studies on urban evapotranspiration (ET) mainly focused on evaporation from the soil, vegetation, and water surfaces. However, the urban Ei process was overlooked mainly due to its low amount. In this study, the urban Ei was determined by eddy correlation (EC) observations combined with a flux footprint model and urban land use information for two urban EC sites. The evaluation of flux data quality indicated that more than 76 % of observed flux data can be used for Ei analysis at the two stations during rainy periods. The daily urban Ei exhibited high intermittent temporal patterns. The Ei value was 0 during the non-rainy period and peaked on the following 1–2 days after the rainy event, then gradually decreased to 0. The average daily Ei accounted for 11–14 % of the total ET for the two EC stations during the rainy period. Further analysis indicated that urban Ei significantly altered the urban energy balance and turbulent transport processes. The urban Ei reduced sensible heat flux (H) and Bowen ratio (BR) during the rainfall period, thereby playing a vital role in mitigating urban heat land effect. Moreover, urban Ei improved the turbulent transport efficiency of latent heat flux (rwq) and restored turbulent transport similarity in an urban area. The temporal characteristics of urban Ei exhibited a large discrepancy among different impervious surface materials, which mainly depend on the water retention and thermodynamic properties of the impervious surface. Gravel had the best water retention capability and resulted in the highest spatially averaged Ei, while the concrete surface had the best thermal storage capability and resulted in the longest duration time of Ei. The spatial average of urban Ei and its duration time were positively correlated with the water retention and heat storage capabilities of impervious surface materials. This study gained insight into urban Ei, including its determination method, temporal characteristics, controlling factors, and impacts of urban Ei on the regional energy balance and turbulent transport process.

Adaptive thermal image velocimetry of spatial wind movement on landscapes using near-target infrared cameras

Abstract. Thermal image velocimetry (TIV) is a near-target remote sensing technique for estimating two-dimensional (2D) near-surface wind velocity based on spatio-temporal displacement of fluctuations in surface brightness temperature captured by an infrared camera. The addition of an automated parameterization and the combination of ensemble TIV results into one output made the method more suitable to changing meteorological conditions and less sensitive to noise stemming from the airborne sensor platform. Three field campaigns were carried out to evaluate the algorithm over turf, dry grass, and wheat stubble. The derived velocities were validated with independently acquired observations from fine-wire thermocouples and sonic anemometers. It was found that the TIV technique correctly derives atmospheric flow patterns close to the ground. Moreover, the modified method resolves wind speed statistics close to the surface at a higher resolution than the traditional measurement methods. Adaptive thermal image velocimetry (A-TIV) is capable of providing contactless spatial information about near-surface atmospheric motion and can help to be a useful tool in researching turbulent transport processes close to the ground.

Convective Updraft and Downdraft Characteristics of Continental Mesoscale Convective Systems in the Model Gray Zone

AbstractThe “gray zone” of convective modeling is defined as the range of horizontal grid spacings (Δx) at which turbulent transport processes are only partially resolved by the dynamics of the numerical model. This zone typically covers Δx from a few kilometers to several hundred meters, wherein the realistic representation of convective cloud processes can be challenging. This study characterizes the convective draft behaviors at multiple Δx across the gray zone and determines the appropriate Δx that can reliably capture these salient convective properties. We perform an ensemble of idealized simulations of mesoscale convective systems (MCS) using the Weather Research and Forecasting model at various Δx from 4 km to 250 m over the central U.S. An evaluation of key MCS kinematic properties is constrained using unique, long‐term vertical velocity estimates obtained by radar wind profilers deployed by the Department of Energy Atmospheric Radiation Measurement user facility. MCS simulations for all Δx tested overestimate (underestimate) the probabilities of convective updrafts (downdrafts) compared to the observations. In terms of the convective draft intensity, finer‐Δx models overestimate the updraft intensity, while the opposite is found for downdrafts. Moving from Δx = 4 km to 250 m, downdrafts become stronger and more frequent especially at middle and upper levels, attributed to additional drag from an increasing graupel frequency and compensation of enhanced updraft velocity. Simulated draft characteristics, including core size, intensity, and probability of occurrence, exhibit pronounced changes at Δx greater than 500 m, but suggest less sensitivity when Δx is reduced below 500 m.

Investigation of heat transfer and pressure drop for a multiple‐started ribbed pipe using large‐eddy simulation

AbstractTurbulent heat transport phenomena in multiple‐started helically ribbed pipes are investigated. Such structures are applied to enhance heat transfer in various technical systems. A large‐eddy simulation (LES) approach is used to model the turbulent flow field. The simulation results for heat transfer and pressure loss are in good agreement with available experimental data and the simulation model is successfully validated for complex surface geometries. For a better understanding of the impact of the wall structures on the turbulent transport processes, local profiles of the relevant flow variables values are investigated. Thus, the specific mechanism of the heat transfer enhancement can be explained and a knowledge‐based optimization of innovative structures is possible.

Polarization and magnetization in collisional and turbulent transport processes

Expressions of polarization and magnetization in magnetically confined plasmas are derived, which include full expansions in the gyroradius to treat effects of both equilibrium and microscopic electromagnetic turbulence. Using the obtained expressions, densities and flows of particles are related to those of gyrocenters. To the first order in the normalized gyroradius expansion, the mean part of the particle flow is given by the sum of the gyrocenter flow and the magnetization flow, which corresponds to the so-called magnetization law in drift kinetics, while the turbulent part contains the polarization flow as well. Collisions make an additional contribution to the second-order particle flow. The mean particle flux across the magnetic surface is of the second-order, and it contains classical, neoclassical, and turbulent transport processes. The Lagrangian variational principle is used to derive the gyrokinetic Poisson and Ampère equations, which properly include mean and turbulent parts so as to be useful for full-f global electromagnetic gyrokinetic simulations. It is found that the second-order Lagrangian term given by the inner product of the turbulent vector potential and the drift velocity consisting of the curvature drift and the ∇B drift should be retained in order for the derived Ampère equation to correctly include the diamagnetic current, which is necessary especially for the full-f high-beta plasma simulations. The turbulent parts of these gyrokinetic Poisson and Ampère equations are confirmed to agree with the results derived from the WKB representation in earlier works.

Wind Fetch and Direction Effects on Langmuir Turbulence in a Coastal Ocean

AbstractMixing processes in the upper ocean play a key role in transferring heat, momentum, and matter in the ocean. These mixing processes are significantly enhanced by wave‐driven Langmuir turbulence (LT). Based on a paired analysis of observations and simulations, this study investigates wind fetch and direction effects on LT at a coastal site south of the island Martha’s Vineyard (MA, USA). Our results demonstrate that LT is strongly influenced by wind fetch and direction in coastal oceans, both of which contribute to controlling turbulent coastal transport processes. For northerly offshore winds, land limits the wind fetch and wave development, whereas southerly winds are associated with practically infinite fetch. Observed and simulated two‐dimensional wave height spectra reveal persistent southerly swell and substantially more developed wind‐driven waves from the south. For oblique offshore winds, waves develop more strongly in the alongshore direction with less limited fetch, resulting in significant wind and wave misalignments. Observations of coherent near‐surface crosswind velocities indicate that LT is only present for sufficiently developed waves. The fetch‐limited northerly winds inhibit wave developments and the formation of LT. In addition to limited fetch, strong wind–wave misalignments prevent LT development. Although energetic and persistent, swell waves do not substantially influence LT activity during the observation period because these relatively long swell waves are associated with small Stokes drift shear. These observational results agree well with turbulence‐resolving large eddy simulations (LESs) based on the wave‐averaged Navier–Stokes equation, validating the LES approach to coastal LT in the complex wind and wave conditions.

Large-scale volumetric flow studies on transport of aerosol particles using a breathing human model with and without face protections

Exhalation of small aerosol particle droplets and their airborne transport, dispersion, and (local) accumulation in closed rooms have been identified as the main pathways for direct and indirect respiratory virus transmission from person to person, for example, for severe acute respiratory syndrome coronavirus-2 or measles. Therefore, understanding airborne transport mechanisms of aerosol particles inside closed populated rooms is an important key factor for assessing and optimizing various mitigation strategies. Unsteady flow features, which are typically evolving in such mixed convection flow scenarios, govern the respective particle transport properties. Experimental and numerical methods that enable capturing the related broad range of scales in such internal flows over many cubic meters in order to provide reliable data for the adaptation of proper mitigation measures (distances, masks, shields, air purifiers, ventilation systems, etc.) are required. In the present work, we show results of a large-scale, three-dimensional Lagrangian particle tracking (LPT) experiment, which has been performed in a 12-m3 generic test room capturing up to 3 × 106 long-lived and nearly neutrally buoyant helium-filled soap bubbles (HFSBs) with a mean diameter of dHFSB ∼370 μm as (almost) passive tracers. HFSBs are used as fluid mechanical replacements for small aerosol particles dP < 5 μm, which allow to resolve the Lagrangian transport properties and related unsteady flow field inside the whole room around a cyclically breathing thermal manikin with and without mouth-nose-masks and shields applied. Six high-resolution complementary metal-oxide semiconductor streaming cameras, a large array of powerful pulsed light emitting diodes, and the variable-time step Shake-The-Box LPT algorithm have been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around a seated and breathing human model.

Renormalization group method for the turbulent mass transport two-equation model

Turbulent mass transport is an essential phenomenon in chemical reactors. A two-equation model of turbulent mass transport has been proposed for less numerical simulation error in previous work. However, there are still some model constants that have undetermined values introduced by empirical modeling rules. In the present paper, the renormalization group (RNG) method is applied to the turbulent mass transport process. Through the RNG procedure, the small scale turbulent components in Navier-Stokes and convection diffusion equations can be eliminated and combined into large scale sources. The corresponding model constants are derived and applied to a grid turbulent diffusion simulation case. In this paper, the grid turbulence diffusion experiment with laser induced fluorescence (LIF) is carried out to verify the simulation results. A satisfactory agreement of concentration distribution between simulation and experiment indicates that the model constants obtained by the RNG method are reliable.

Sensitivity of large eddy simulations of tropical cyclone to sub-grid scale mixing parameterization

The surface wind structure and vertical turbulent transport processes in the eyewall of hurricane Isabel (2003) are investigated using six large-eddy simulations (LESs) with different horizontal grid spacing and three-dimensional (3D) sub-grid scale (SGS) turbulent mixing models and a convection permitting simulation that uses a coarser grid spacing and one-dimensional vertical turbulent mixing scheme. The mean radius-height distribution of storm tangential wind and radial flow, vertical velocity structure, and turbulent kinetic energy and momentum fluxes in the boundary layer generated by LESs are consistent with those derived from historical dropsonde composites, Doppler radar, and aircraft measurements. Unlike the convection permitting simulation that produces storm wind fields lacking small-scale disturbances, all LESs are able to produce sub-kilometer and kilometer scale eddy circulations in the eyewall. The inter-LES differences generally reduce with the decrease of model grid spacing. At 100-m horizontal grid spacing, the vertical momentum fluxes induced by the model-resolved eddies and the associated eddy exchange coefficients in the eyewall simulated by the LESs with different 3D SGS mixing schemes are fairly consistent. Although with uncertainties, the decomposition in terms of eddy scales suggests that sub-kilometer eddies are mainly responsible for the vertical turbulent transport within the boundary layer (~1 km depth following the conventional definition) whereas eddies greater than 1 km become the dominant contributors to the vertical momentum transport above the boundary layer in the eyewall. The strong dependence of vertical turbulent transport on eddy scales suggests that the vertical turbulent mixing parameterization in mesoscale simulations of tropical cyclones is ultimately a scale-sensitive problem.

Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB

Exhalation of small aerosol droplets and their transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathway for indirect or airborne respiratory virus transmission from person to person, e.g. for SARS-CoV 2 or measles (Morawska and Cao 2020). Understanding airborne transport mechanisms of viruses via small bio-aerosol particles inside closed populated rooms is an important key factor for optimizing various mitigation strategies (Morawska et al. 2020), which can play an important role for damping the infection dynamics of any future and the ongoing present pandemic scenario, which unfortunately, is still threatening due to the spreading of several SARS-CoV2 variants of concern, e.g. delta (Kupferschmidt and Wadman 2021). Therefore, a large-scale 3D Lagrangian Particle Tracking experiment using up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of ~ 370 µm as passive tracers in a 12 m³ generic test room has been performed, which allows to fully resolve the Lagrangian transport properties and flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs have been used and the Shake-The-Box (STB) (Schanz et al. 2016) Lagrangian particle tracking algorithm has been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around seated breathing persons.

A Non‐Gradient Model of Turbulent Gas Fluxes Over Land Surfaces

AbstractA non‐gradient model is formulated for estimating gas fluxes using single‐level time‐series data of near‐surface gas concentration over land surfaces. When the vertical turbulent transport process in the atmospheric surface layer is described by a one‐dimensional diffusion equation, a gas flux is expressed as a weighted integral of the time‐history of single‐level gas concentration. The eddy‐diffusivity may be parameterized as a function of sensible heat flux based on the Monin‐Obukhov similarity theory without explicit dependence on wind speed and surface roughness. Sensible heat flux may be estimated from net radiation and surface temperature using the maximum entropy production model. Case studies at six sites with diverse vegetation covers, geographic and climatic conditions at sub‐daily and seasonal scales demonstrate the model's capability of simulating water vapor and CO2fluxes using fewer inputs than other models. The proposed method provides an alternative modeling tool for the study of water and carbon cycles over vegetated land surfaces.