Turbulent Transport Processes Research Articles

Large eddy simulations of pore-scale turbulent flows in porous media by the lattice Boltzmann method

To discuss turbulent transport processes inside porous media, high resolution large eddy simulations by the D3Q27 multiple relaxation time lattice Boltzmann method and their data analysis are performed. The focused porous medium structures are square rod arrays, staggered cube arrays, fractal cube arrays and open cell foam. Although the second moments appearing in the double averaged momentum equation in porous media are the macro-scale Reynolds stress, the micro-scale Reynolds stress and the dispersive covariance, it is found that the micro-scale Reynolds stress and the dispersive covariance become dominant in the present porous medium flows. It is also found that the turbulent anisotropy and the magnitudes of those second moments are totally different among the cases due to the porous structures. Their transport mechanisms are discussed through the budget terms of the transport equations.

On the velocity and pressure statistics during the flapping motion in a planar turbulent jet

The role of a large-scale coherent vortex structure in a planar jet, which is intermittently observed and called “flapping motion”, on the turbulent energy transport process is investigated experimentally. The experiment is performed by simultaneously measuring the two components of velocity and pressure in the self-preserving region of the jet. The probe for the simultaneous measurement of the velocity and pressure consists of an X-type hot-wire sensor and a static pressure probe. The measurement data are analyzed using a conditional sampling technique and the phase-averaging technique, on the basis of an intermittency function which shows whether the jet is now flapping or not. This intermittency function is obtained by the measurement results of the streamwise velocity fluctuation by means of the two I-type hot-wire sensors set in the self-preserving region of the jet with applying a continuous wavelet transform analysis to the data. The experimental results show that the phase-averaged velocity field during the flapping motion shows a good agreement with those obtained through the 23 points simultaneous measurement of the streamwise velocity in the previous studies. Further, the phase-averaged pressure field during the flapping motion indicates the existence of a large-scale coherent vortex structure, interpreted as a combination of the flapping and puffing motions in the self-preserving region of the jet. In addition, it is found that the production of the turbulent energy and its diffusion from the inner region to the outer region of the jet are enhanced by the flapping motion. In particular, this enhancement of the turbulent energy diffusion is caused by the combination of an increase of the turbulent diffusion to the outer region of the jet and a decrease of the pressure diffusion to the inner region of the jet.

A new method for measuring the imaginary part of the atmospheric refractive index structure parameter in the urban surface layer

Abstract. The atmospheric refractive index consists of both real and imaginary parts. The intensity of refractive index fluctuations is generally expressed as the refractive index structure parameter, with the real part reflecting the strength of atmospheric turbulence and the imaginary part reflecting absorption in the light path. A large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of the atmospheric refractive index, from which the sensible and latent heat fluxes can further be obtained, whereas the influence of the imaginary part is ignored or considered noise. In this theoretical analysis study, the relationship between logarithmic light intensity variance and the atmospheric refractive index structure parameter (ARISP), as well as that between the logarithmic light intensity structure function and the ARISP, is derived. Additionally, a simple expression for the imaginary part of the ARISP is obtained which can be conveniently used to determine the imaginary part of the ARISP from LAS measurements. Moreover, these relationships provide a new method for estimating the outer scale of turbulence. Light propagation experiments were performed in the urban surface layer, from which the imaginary part of the ARISP was calculated. The experimental results showed good agreement with the presented theory. The results also suggest that the imaginary part of the ARISP exhibits a different diurnal variation from that of the real part. For the wavelength of light used (0.62 μm), the variation of the imaginary part of the ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols.

Turbulent Transport Processes at Rough Surfaces with Geophysical Applications

In this numerical study we use the Lattice Boltzmann Method to investigate the effects of periodic and randomly rough surfaces on the turbulent transport of momentum and heat. A two-dimensional MPI code has been developed and validated against many test cases. We examine in some detail Rayleigh-Benárd convection in the presence of rough walls and study the effects of the wavelength and amplitude of the roughness distribution on the flow.

IDDES simulation of hydrogen-fueled supersonic combustion using flamelet modeling

A turbulent combustion modeling method combining the improved delayed detached eddy simulation (IDDES) model and the flamelet/progress variable (FPV) model is developed and then applied in the hybrid RANS/LES simulation of two canonical configurations for hydrogen-fueled supersonic combustors, namely the DLR supersonic combustor and Gamba model combustor. Results of both cases show good agreement with experiment, validating the accuracy of this method in both two- and three-dimensional combustion simulations. Moreover, the results suggest that the IDDES model is capable of capturing large scale vortex structures, and verify that the FPV model is applicable under supersonic conditions. Meanwhile, the k-ω SST model combined with the FPV model is applied in both cases. It is found that the application of the k-ω SST model leads to larger deviations compared with that of the IDDES model. Further analysis indicates that large scale vortex structures can influence the distribution of control variables of the FPV model, which further affects the species mixing and reaction progress. Therefore, the simulation of turbulent transport process has a significant effect on the performance of the FPV model.

Experimental verification of the turbulence models for the pressure diffusion process in a turbulent plane jet

We performed simultaneous measurement of the three velocity components and the pressure in a plane turbulent jet as well as the verification of turbulence models related to the pressure diffusion process, such as the gradient-diffusion model and the model for the rapid and slow terms of the pressure diffusion term. The results show that the gradient-diffusion model developed in the previous studies are valid in the region where the turbulent intensity and the turbulent/non-turbulent intermittency are high and the production of the turbulent energy is dominant to the other processes such as the convection and diffusion of the turbulent energy in the turbulent energy budget. In addition, the pressure diffusion of the turbulent energy cannot be modeled accurately by using only the slow term, and its accuracy increases by considering both rapid and slow terms in the model. These results indicate that the modeling of the pressure diffusion process using only the slow term has a certain risk for causing a misunderstanding of the turbulent energy transport process.

Analysis of Turbulence Energy Spectrum by Using Particle Image Velocimetry

Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by Kang et al. (2003) has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus.

A contribution to solve the atmospheric diffusion equation with eddy diffusivity depending on source distance

An integral solution of the atmospheric diffusion equation considering wind speed as a function of vertical height and eddy diffusivity as a function of both downwind distance from the source and vertical height is presented. The near-source dispersion problem is investigated comparing a vertical eddy diffusivity function of distance from the source against their asymptotic limit. The results suggest that the inclusion of the memory effect as modeled by Taylor's theory, improves the description of the turbulent transport process of atmospheric effluent released by a low continuous source in convective conditions.

Astrophysical Hydromagnetic Turbulence

Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich--Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new large-scale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.

AEROSOL VARIABILITY OBSERVED WITH RPAS

Abstract. To observe the origin, vertical and horizontal distribution and variability of aerosol particles, and especially ultrafine particles recently formed, we plan to employ the remotely piloted aircraft system (RPAS) Carolo-P360 "ALADINA" of TU Braunschweig. The goal of the presented project is to investigate the vertical and horizontal distribution, transport and small-scale variability of aerosol particles in the atmospheric boundary layer using RPAS. Two additional RPAS of type MASC of Tübingen University equipped with turbulence instrumentation add the opportunity to study the interaction of the aerosol concentration with turbulent transport and exchange processes of the surface and the atmosphere. The combination of different flight patterns of the three RPAS allows new insights in atmospheric boundary layer processes. Currently, the different aerosol sensors are miniaturized at the Leibniz Institute for Tropospheric Research, Leipzig and together with the TU Braunschweig adapted to fit into the RPAS. Moreover, an additional meteorological payload for measuring temperature, humidity and turbulence properties is constructed by Tübingen University. Two condensation particle counters determine the total aerosol number with a different lower detection threshold in order to investigate the horizontal and vertical aerosol variability and new particle formation (aerosol particles of some nm diameter). Further the aerosol size distribution in the range from about 0.300 to ~5 μm is given by an optical particle counter.

Preliminary Results of the NORCOWE Direct Covariance Flux System for Ship based Measurements

To cover the increased demand of renewable energy the development of offshore wind turbines that can be placed in deep water has started. Model results from Sullivan et al. [2] indicate that the ocean has the ability to interact with the lower part of the Marine Atmospheric Boundary Layer (MABL). This leads to turbulent horizontal and vertical motions of the air that potentially reduces turbine lifetime and energy reliability. This paper gives a description of the Norwegian Center for Offshore Wind Energy (NORCOWE) direct covariance flux system. Consisting of a sonic anemometer, a state-of-the-art inertial measurement unit (IMU) and a data logger the system can easily be mounted on all floating platforms, e.g. ships or buoys and operates autonomously. This will in the future enable researchers to investigate turbulent motion and turbulent momentum transport processes in the MABL in more detail.

Derivation of an eddy diffusivity coefficient depending on source distance for a shear dominated planetary boundary layer

In this study an integral and an algebraic formulation for the eddy diffusivities in a shear driven planetary boundary layer are derived for pollutant dispersion applications. The expressions depend on the turbulence properties and on the distance from the source. They are based on the turbulent kinetic energy spectra, Taylor’s statistical diffusion theory and measured turbulent characteristics during intense wind events. The good agreement between the algebraic and the integral formulation for the eddy diffusivities corroborate the hypothesis that using an algebraic formulation as a surrogate for the eddy diffusivities in the neutral planetary boundary layer is valid. As a consequence, the vertical eddy diffusivity provided by the algebraic formulation and its asymptotic limit for large time (diffusion time much larger than the Lagrangian integral time scale), were introduced into an analytical air pollution model and validated against data from the classic Prairie Grass project. A statistical analysis, employing specific indices shows that the results are in good agreement with the observations. Furthermore, this study suggests that the inclusion of the memory effect, which is important in regions near to a continuous point source, improves the description of the turbulent transport process of atmospheric contaminants. Therefore, the major finding of this paper is the necessity of including the downwind distance-dependent eddy diffusivity for low continuous point sources in air quality modeling studies.

On the effects of free-stream turbulence on axisymmetric disc wakes

Wind tunnel experiments have been used to study the effects of free-stream turbulence on the axisymmetric wake behind a disc. The disc and its wake were introduced to various turbulent streams having various levels of turbulence intensity and length scale. It was found that the presence of free-stream turbulence enhances the body’s drag and hence wake momentum deficit, if it is of sufficient strength, changes the far wake’s decay rate and prevents the appearance of self-similarity. The external turbulence causes a significant transformation in the wake’s turbulence structure. This gradually evolves towards the character of the free-stream turbulence itself and thus is characterised by much weaker turbulence (cross-stream) transport processes and a consequent dominance of shear stress production, which acts to maintain the shear stress and mean velocity profiles.

An analysis of the impact of convective parameter sensitivity on simulated global atmospheric CO distributions

[1] In an effort to better understand how uncertainty in simulated convection propagates into simulations of global trace gas distributions, we have constructed an eight-member ensemble of simulations using NASA's Goddard Earth Observing System Version 5 (GEOS-5) general circulation model (GCM). The ensemble was created by perturbing parameters in the model's moist physics schemes found to strongly influence the magnitude of convective mass flux. Globally, ensemble spreads in column CO are typically small (less than 4% of the mean column value) and, in many areas, are not significantly different from internal model variability. The largest ensemble spreads are found near source regions and outflow pathways. At the majority of remote surface monitoring sites, the annual mean ensemble spread is less than 5%, indicating that these locations, which are often the basis of inversion studies, are relatively insensitive to uncertainty in the representation of convection. We also examine in greater detail two simulations in which the magnitude of convective mass flux is significantly altered. Changes to convective parameters strongly influence grid-scale vertical and turbulent transport processes in addition to convective mass flux. Despite large differences in the magnitude of convective mass fluxes, this compensating behavior by other model processes results in comparable atmospheric residence times in the two simulations and largely similar global CO distributions. The results indicate that convective mass flux is strongly related to other vertical transport processes in a GCM and cannot be viewed as entirely separate. Future studies of the role of convective transport need to consider the relationship between convective and total mass flux.

Contribution of Tore Supra in preparation of ITER

Tore Supra routinely addresses the physics and technology of very long-duration plasma discharges, thus bringing precious information on critical issues of long pulse operation of ITER. A new ITER relevant lower hybrid current drive (LHCD) launcher has allowed coupling to the plasma a power level of 2.7 MW for 78 s, corresponding to a power density close to the design value foreseen for an ITER LHCD system. In accordance with the expectations, long distance (10 cm) power coupling has been obtained. Successive stationary states of the plasma current profile have been controlled in real-time featuring (i) control of sawteeth with varying plasma parameters, (ii) obtaining and sustaining a ‘hot core’ plasma regime, (iii) recovery from a voluntarily triggered deleterious magnetohydrodynamic regime. The scrape-off layer (SOL) parameters and power deposition have been documented during L-mode ramp-up phase, a crucial point for ITER before the X-point formation. Disruption mitigation studies have been conducted with massive gas injection, evidencing the difference between He and Ar and the possible role of the q = 2 surface in limiting the gas penetration. ICRF assisted wall conditioning in the presence of magnetic field has been investigated, culminating in the demonstration that this conditioning scheme allows one to recover normal operation after disruptions. The effect of the magnetic field ripple on the intrinsic plasma rotation has been studied, showing the competition between turbulent transport processes and ripple toroidal friction. During dedicated dimensionless experiments, the effect of varying the collisionality on turbulence wavenumber spectra has been documented, giving new insight into the turbulence mechanism. Turbulence measurements have also allowed quantitatively comparing experimental results with predictions by 5D gyrokinetic codes: numerical results simultaneously match the magnitude of effective heat diffusivity, rms values of density fluctuations and wavenumber spectra. A clear correlation between electron temperature gradient and impurity transport in the very core of the plasma has been observed, strongly suggesting the existence of a threshold above which transport is dominated by turbulent electron modes. Dynamics of edge turbulent fluctuations has been studied by correlating data from fast imaging cameras and Langmuir probes, yielding a coherent picture of transport processes involved in the SOL.Corrections were made to this article on 6 January 2012. Some of the letters in the text were missing.

Modeling the vegetation–atmosphere carbon dioxide and water vapor interactions along a controlled CO 2 gradient

Ecosystem functioning is intimately linked to its physical environment by complex two-way interactions. These two-way interactions arise because vegetation both responds to the external environment and actively regulates its micro-environment. By altering stomatal aperture, and therefore the transpiration rate, plants modify soil moisture and atmospheric humidity and these same physical variables, in return, modify stomatal conductance. Relationships between biotic and abiotic components are particularly strong in closed, managed environments such as greenhouses and growth chambers, which are used extensively to investigate ecosystem responses to climatic drivers. Model-assisted designs that account for the physiological dynamics governing two-way interactions between biotic and abiotic components are absent from many ecological studies. Here, a general model of the vegetation–atmosphere system in closed environments is proposed. The model accounts for the linked carbon–water physiology, the turbulent transport processes, and the energy and radiative transfer within the vegetation. Leaf gas exchange is modeled using a carbon gain optimization approach that is coupled to leaf energy balance. The turbulent transport within the canopy is modeled in two-dimensions using first-order closure principles. The model is applied to the Lysimeter CO 2 Gradient (LYCOG) facility, wherein a continuous gradient of atmospheric CO 2 is maintained on grassland assemblages using an elongated chamber where the micro-climate is regulated by variation in air flow rates. The model is employed to investigate how species composition, climatic conditions, and the imposed air flow rate affect the CO 2 concentration gradient within the LYCOG and the canopy micro-climate. The sensitivity of the model to key physiological and climatic parameters allows it to be used not only to manage current experiments, but also to formulate novel ecological hypotheses (e.g., by modeling climatic regimes not currently employed in LYCOG) and suggest alternative experimental designs and operational strategies for such facilities.

Summary of the 19th International Toki Conference - Advanced Physics in Plasma and Fusion Research

This article summarizes advanced physics in plasma and fusion research, which was covered by the 19th International Toki Conference (ITC). Summary is made from the viewpoint of progress of physics of nonequilibrium properties of plasmas, putting an emphasis upon the theme ‘Knowledge must be developed into understanding’. The aspects of advanced physics in this ITC was assessed in the issues: (i) advance in knowledge of confinement, (ii) multi-scale turbulent structure formation, (iii) transport processes, continued (nonlocal transport, statistical theory), (iv) advanced diagnostics, (v) reconnection, magnetic islands and MHD activities, (vi) plasma atomic molecular physics, (vii) integrated modelling, (viii) problem definitions for reactor studies, (ix) solar, space and astrophysical plasmas, (x) nano-bio plasmas, and (xi) further extensions. This report summarizes achievements in plasma physics with the perspective of 20 years of National Institute of Fusion Science after its inauguration.

Characteristics of coherent vortical structures in turbulent flows over progressive surface waves

Vortical structures in turbulence over progressive surface waves are studied using the data from direct numerical simulation of a stress-driven turbulent Couette flow above a waving surface. Instantaneous flow field and its evolution, vorticity statistics, and conditionally averaged flow field with various sampling methods are examined. Unique vortical structures are identified, which are found to be strongly dependent on the wave motion. For a slow wave (with a small value of wave age c/u∗=2; here c is the phase speed of the wave and u∗ is the friction velocity), the vortical structures are characterized by reversed horseshoe vortices and quasistreamwise vortices. The former is concentrated above the wave trough and is associated with sweep events there; the latter has high intensity over the windward face of the wave and is associated with ejection events. Relative to the waveform, the coherent vortical structures propagate in the downstream direction. Vortex turning and vortex stretching play an important role in the vortex transformation and evolution processes. For an intermediate wave (c/u∗=14) and a fast wave (c/u∗=25), the dominant vortical structure is bent quasistreamwise vortices, which are predominantly horizontal but have a distinctive downward bending in their upstream ends near the wave trough. The vortices are found to propagate in the upstream direction with respect to the waveform. The above-wave coherent vortices identified in this study are found to play an important role in the turbulent transport process.

The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study

Terrestrial ecosystems are characterized by a wide range of canopy vegetation density, which is known to affect turbulent transport processes across the canopy–atmosphere interface. In the presence of a dense and horizontally homogeneous canopy, the canopy sublayer has been described as resembling a plane mixing layer. At the other extreme, where the canopy is essentially absent, the canopy sublayer is typically assumed to be similar to a turbulent boundary layer over a rough surface. However, it remains unclear how the canopy turbulence changes from boundary-layer-like to mixing-layer-like as the vegetation density increases. We use large-eddy simulation to study five different vegetation densities varying from an extremely sparse canopy to an extremely dense canopy. This investigation draws on the study of flow statistics as well as large-scale coherent turbulent structures within the canopy sublayer. The coherent structures are identified through the use of proper orthogonal decomposition. The results of skewness of velocity components and characteristic length scales suggest that, as the vegetation density increases, the canopy turbulence gradually undergoes a transition from resembling a rough-wall boundary layer to being similar to a mixing layer. As demonstrated by others, we found that the coherent structures within the canopy sublayer consist of a strong sweep/ejection motion framed by a counter-rotating vortex pair with elliptical cross-sections. As the canopy becomes denser, these important structures are shown here to be more elevated. Vegetation density does not appear to have a significant effect on the percentage of the total turbulent kinetic energy that is represented by the coherent structures.

Importance of the subgrid-scale turbulent moist process: Cloud distribution in global cloud-resolving simulations

This study investigated the turbulent transport process within the nonhydrostatic icosahedral atmospheric model (NICAM), a global cloud-resolving model (GCRM), with particular focus on the spatial reproducibility of cloud characteristics in NICAM simulations. A turbulent closure model was applied, based on level 2 of the model developed by Nakanishi and Niino ([Nakanishi, M., and Niino, H., 2006: An improved Mellor–Yamada level-3 model: its numerical stability and application to a regional prediction of advection fog. Boundary-Layer Meteorol., 119, 397–407.]; a revised version of the original Mellor–Yamada model), together with a subgrid-scale condensation process. NICAM simulations were conducted for boreal summer of 2004 using mesh sizes of about 14 km and 7 km. Simulated cloud amounts and radiative budget were compared with observed data. The results confirmed an improvement in the spatial distribution of low clouds that develop in offshore regions of subtropical continents compared to past NICAM experiments ([Iga, S., Tomita, H., Tsushima, Y., and Satoh, M., 2007: Climatology of a nonhydrostatic global model with explicit cloud process. Geophys. Res. Lett., 34, L22814, doi:10.1029/2007/GL031048.]). A sensitivity study of subgrid-scale clouds in the turbulent closure scheme revealed that the turbulent transport process modulated by the subgrid-scale cloud strongly controls not only the low-cloud amount but also mid- and high-cloud amounts. This indicates that parameterization of unresolvable subgrid-scale clouds remains an important component of cloud behavior in GCRMs.