Mean Friction Velocity Research Articles

Effects of Wall Temperature on Scalar and Turbulence Statistics During Premixed Flame–Wall Interaction Within Turbulent Boundary Layers

Abstract Direct numerical simulations (DNS) have been utilised to investigate the impact of different thermal wall boundary conditions on premixed V-flames interacting with walls in a turbulent channel flow configuration. Two boundary conditions are considered: isothermal walls, where the wall temperature is set either equal to the unburned mixture temperature or an elevated temperature, and adiabatic walls. An increase in wall temperature has been found to decrease the minimum flame quenching distance and increase the maximum wall heat flux magnitude. The analysis reveals notable differences in mean behaviours of the progress variable and non-dimensional temperature in response to thermal boundary conditions. At the upstream of the flame–wall interaction location, higher mean friction velocity values are observed for the case with elevated wall temperature compared to the other cases. However, during flame–wall interaction, friction velocity values decrease for isothermal walls but initially rise before decreasing for adiabatic walls, persisting at levels surpassing isothermal conditions. For all thermal wall boundary conditions, the mean scalar dissipation rates of the progress variable and non-dimensional temperature exhibit a decreasing trend towards the wall. Notably, in the case of isothermal wall boundary condition, a higher scalar dissipation rate for the non-dimensional temperature is observed in comparison to the scalar dissipation rate for the progress variable. Thermal boundary condition also has a significant impact on Reynolds stress components, turbulent kinetic energy, and dissipation rates, showing the highest magnitudes with isothermal case with elevated wall temperature and the lowest magnitude for the isothermal wall with unburned gas temperature. The findings of the current analysis suggest that thermal boundary conditions can potentially significantly affect trubulence closures in the context of Reynolds averaged Navier–Stokes simulations of premixed flame–wall interaction.

Analysis of flux footprints in fragmented, heterogeneous croplands

An accurate quantification of fluxes from heterogeneous sites and further bifurcation into contributing homogeneous fluxes is an active field of research. Among such sites, fragmented croplands with varying surface roughness characteristics pose formidable challenges for footprint analysis. We conducted two flux monitoring experiments in fragmented croplands characterized by two dissimilar surfaces with objectives to: (i) evaluate the performance of two analytical footprint models in heterogeneous canopy considering aggregated roughness parameters and (ii) analyze the contribution of fluxes from individual surfaces under changing wind speed. A set of three eddy covariance (EC) towers (one each capturing the homogenous fluxes from individual surfaces and a third, high tower capturing the heterogeneous mixed fluxes) was used for method validation. High-quality EC fluxes that fulfill stationarity and internal turbulence tests were analyzed considering daytime, unstable conditions. In the first experiment, source area contribution from a surface is gradually reduced by progressive cut, and its effect on high-tower flux measurements is analyzed. Two footprint models (Kormann and Meixner ‘KM’; analytical solution to Lagrangian model ‘FFP’) with modified surface roughness parameters were applied under changing source area contributions. FFP model has consistently over predicted the footprints (RMSEFFP = 0.31 m−1, PBIASFFP = 19.00), whereas KM model prediction was gradually changed from over prediction to under prediction towards higher upwind distances (RMSEKM = 0.02 m−1, PBIASKM = 8.50). Sensitivity analysis revealed that the models are more sensitive to turbulent conditions than surface characteristics. This motivated to conduct the second experiment, where the fractional contribution of individual surfaces (α and β) to the heterogeneous fluxes measured by the high tower (T3) was estimated using the principle of superposition (FT3 = α FT1 + β FT2). Results showed that α and β are dynamic during daylight hours and strongly depend on mean wind speed (U) and friction velocity (u*). The contribution of fluxes from adjoining fields [1 − (α + β)] is significant beyond 80% isopleth. Our findings provide guidelines for future analysis of fluxes in heterogeneous, fragmented croplands.

Particle transport in turbulent square duct flows with a free surface

Direct numerical simulation combined with a one-way coupled Lagrangian particle tracking technique is employed to investigate dilute particle-laden turbulent flows in open square ducts with a free surface. The focus is on examining the influence of the mean cross-stream secondary flow on particle transport near the wall, free surface, and across the duct cross section. Based on the duct half-width and mean friction velocity, a shear Reynolds number of Reτ = 300 is considered, with the corresponding particle Stokes numbers ranging from St+ = 0.31 to 260. The results reveal that particle concentration near the sidewalls is lower than that near the bottom wall, and the minimum particle concentration is observed at the free surface. Along the bottom wall centerline orientated upward, particle concentration gradually decreases. An exception to this is in the vicinity of the free surface where a slight increase is observed for the heavier particles (St+ ≥ 25), and the amplitude of this increase gradually declines as the Stokes number increases. In the streamwise direction near the free surface, heavier particles tend to preferentially concentrate in regions where the instantaneous transverse secondary flow velocity is negative. As the Stokes number increases, the position of the maximum streamwise velocity for heavier particles is closer to the free surface, and the rotation centers of inner and outer secondary particle motions gradually disappear. The streamwise root mean square velocity for the lightest St+ = 0.31 particles is higher than that for particles with higher inertia in the middle region of the free surface.

Validation on the mean friction velocity of an atmospheric boundary layer flow reproduced by large-eddy simulation in terms of kinetic energy conservation

This study presents a validation of large-eddy simulation to reproduce normalized mean friction velocity in an atmospheric boundary layer flow in terms of kinetic energy conservation uncertainty. A primary finding of this study is that the normalised mean friction velocity of the atmospheric boundary layer is suggested to be insensitive to kinetic energy conservation errors. The present study approaches the sensitivity of the normalized friction velocity, which is one of the most fundamental statistics in the atmospheric boundary layer, to the uncertainty. The nearly complete conservation of kinetic energy in the present numerical framework is verified in an inviscid homogeneous isotropic fluctuation field. Then, the present analysis is applied to reproducing turbulent channel flows as direct numerical simulation and large-eddy simulation based on the previous studies before the present work analyses the present atmospheric boundary layer. The present analysis is then used to analyze the normalized friction velocity in the atmospheric boundary layer. The values of the friction velocity obtained in the previous study are in good agreement with that of the present study.

Wind tunnel test and CFD simulation of the near-roof wind speed and friction velocity on gable roofs

In this paper, wind tunnel tests and CFD (Computational Fluid Dynamics) simulations are conducted to measure and simulate the near-roof wind speed and friction velocity on gable roofs. By comparing the results of Irwin sensor wind tunnel tests with CFD simulations of different turbulence models, it is found that the Realizable k-ε turbulence model performs better. The CFD simulations and wind tunnel tests show that the mean near-roof wind speed and the mean friction velocity on the windward and leeward gable roofs have the same variation trend with the slope. Moreover, the mean near-roof wind speed and the mean friction velocity reach the maximum on the windward roof when the slope is 20°, the mean wind speed and the mean friction velocity on the leeward roof decrease with the increase of the slope. Finally, based on the snow transport rate on the windward roof with different slopes, it can be obtained that the snow transport rate on the windward gable roof with 20° is the largest among the four slopes selected in this paper. Meanwhile, when the roof slope is 20°, it is most likely to lead to the unbalanced snow load on the windward side and leeward side.

Database of extreme waves generated during the passage of a cold front in Rio Grande do Sul coast, southern Brazil

This datapaper supports the use of a database generated from wavefield simulations with the WAVEWATCH III model in waters off the coast of Rio Grande do Sul in the South Atlantic Ocean. In the WAVEWATCH III simulations, three domains are generated as a part of a numerical experiment to set up the best configuration. This database includes all input and output files for the two best-fit simulations. Bathymetry and wind files at 10 m above the surface are available as input files. The period of simulation and non-stationary wind data input corresponds to March 22-28, 2016. The date was chosen because it is related to the passage of a cold front through the area of interest. The different parameterizations used and with which good results were obtained in the simulations with the model are also described. The WAVEWATCH III output files contain the spatial and temporal distribution of the wavefield in the area of interest, as well as the outputs for point locations consistent with the location of on-site records. For the two best-fit domains, the following variables were obtained: mean wind speed (m s-1), sea-air temperature difference (°C), wave height (m), mean wavelength (m), mean wave period (s), mean wave direction (degrees), mean directional propagation (degrees) and friction velocity (m s-1). All these variables are provided in NetCDF format and will serve as a reference for future wave modeling work in the region, and the results will be able to be compared with those obtained in the database.

The Mass Settling Flux of Suspended Particulate Matter in Venice Lagoon, Italy

Amos, C.L.; Kassem, H.; Bergamasco, A.; Sutherland, T.F., and Cloutier, D., 2021. The mass settling flux of suspended particulate matter in Venice Lagoon, Italy. Journal of Coastal Research, 37(6), 1099–1116. Coconut Creek (Florida), ISSN 0749-0208. A multidisciplinary study of the stability of the tidal flats of Venice Lagoon has provided field and laboratory data on the factors influencing the mass settling rates of material in suspension. This work was performed using two in situ benthic flumes (Sea Carousel and Mini Flume) in association with a wide range of physical and biological measurements undertaken during the summer of 1998 and the subsequent winter. Also, controlled experiments on erosion/sedimentation of prepared beds were carried out using Lab Carousel, a laboratory equivalent of Sea Carousel. Particle size and mass settling rates were found to be largely independent of suspended sediment concentration but strongly controlled by the antecedent bed shear stresses that led to the suspension. Results between the three flume types differed because of differences in the induced stress history created in each case. Comparable results were obtained by normalizing mass settling rate to the mean friction velocity of the flow during settling, i.e. the Rouse parameter () , and by use of the mean dimensionless particle diameter (D*). Results fell in line with results on carbonate and silica sands of the inlets of the lagoon. The mean particle diameter (df) varied in proportion to the applied shear stress and shear rate (G) suggesting that the suspended particles were eroded aggregates not floccules. The effective density of these aggregates was least (∼16 kgm–3) at the largest sizes (df > 1 × 10–4 m) and greatest (∼160–1600 kgm–3) at the smallest sizes (df < 1 × 10–4 m). The lack of an increase in df at low shear rates suggests that flocculation was not taking place. The mean deposition threshold (all experiments) was 0.68 Pa, which is less than the mean erosion threshold from these sites (0.78 Pa).

Mechanisms of particle preferential concentration induced by secondary motions in a dilute turbulent square duct flow

Particle-laden turbulent square duct flows at Reτ = 300 (based on the duct half-width and the mean friction velocity) are investigated using direct numerical simulation with one-way coupled Lagrangian particle tracking. Four particle-to-fluid density ratios are considered with the corresponding shear Stokes number St+ = 0.31, 25, 125, and 260. Particle motion is governed by drag, lift, added-mass, and pressure gradient forces. The main purpose of this work is to examine the effect of the turbulence-driven secondary flows on particle preferential accumulation and their dependence on the Stokes number. Results obtained indicate that the cross-stream secondary motions encourage inertial particles to accumulate preferentially in the duct corners, where the maximum of the cross-sectional particle concentration occurs. The extent of accumulation here is strongly dependent on the Stokes number, with the greatest accumulation found at St+ = 25. Interestingly, the maximum of the intensity of the secondary particle velocity along the corner bisector is also achieved at St+ = 25, whereas in the region adjacent to the wall, it is found to decrease with a particle Stokes number. Additionally, it is observed that the higher inertia particles are more easily trapped in the stagnation zone of secondary flows with low turbulence intensity in the corner region. In the near-wall region, the heavier particles (St+ ≥ 25) are prone to reside and form elongated clusters along the low-speed streamwise velocity streaks, with this trend less pronounced with the increasing Stokes number. Along the wall, away from the corner where the secondary motion is attenuated, particle accumulation is dominated by the near-wall coherent vortices. This phenomenon is further discussed using a region-based correlation analysis between the particle spatial distribution and local flow topology. An in-depth particle dynamic analysis determines that the average cross-sectional drag force resulting from the secondary flow is mainly responsible for the particle motion throughout the duct cross section, which tends to push particles away from the walls in the near-wall region but shows the exact opposite trend in the bulk flow region. Moreover, the pressure gradient force also plays an important role for low-inertia particles. As the Stokes number is increased, the lift force becomes progressively dominant in the viscous sublayer, acting to pull particles toward the corners and walls of the duct.

Effect of turbulence-driven secondary flow on particle preferential concentration and clustering in turbulent square duct flows

In this study, the preferential concentration and clustering of inertial particles in fully developed turbulent square duct flows are studied using large eddy simulations combined with Lagrangian approach, where the Reynolds number is equal to Reτ=600 (based on the mean friction velocity and duct full height), and the particle Stokes number ranges from 0.0007 to 1.16. The results obtained for duct flows are compared with those for channel flows under the same working conditions. Then, the effect of the secondary flow on the particle concentration in duct flows is investigated. The equation of particle motion is governed by the drag force, lift force, added mass force, pressure gradient force, and gravity. The inter-phase interaction that was considered includes one-way and two-way coupling. The simulations of a single phase are verified and in good agreement with the available literature data. For the discrete phase, particles in the duct flow are found to be more dispersed in the vertical direction compared with the channel flow. In near-wall regions, a small fraction of particles tends to accumulate in duct corners, forming stable particle streaks under the effect of the secondary flow. Meanwhile, most particles are likely to reside preferentially in the low-speed flow regions and form elongated particle streaks steadily in the middle region of duct or channel floors. The Voronoi diagram analysis shows that the near-wall secondary flows in the square duct could cause particle clusters to transfer from regions of high to low concentration, and this trend increases with particle size. In addition, two-way coupling is found to enhance the near-wall particle accumulation and to promote particles to form more elongated streaks than one-way coupling. Finally, the mechanism responsible for the particle preferential concentration in turbulent square duct flows is determined.

Global Impacts of Subseasonal (&lt;60 Day) Wind Variability on Ocean Surface Stress, Buoyancy Flux, and Mixed Layer Depth

AbstractSubseasonal surface wind variability strongly impacts the annual mean and subseasonal turbulent atmospheric surface fluxes. However, the impacts of subseasonal wind variability on the ocean are not fully understood. Here, we quantify the sensitivity of the ocean surface stress (𝛕), buoyancy flux (B), and mixed layer depth (MLD) to subseasonal wind variability in both a one‐dimensional (1‐D) vertical column model and a three‐dimensional (3‐D) global mesoscale‐resolving ocean/sea ice model. The winds are smoothed by time filtering the pseudo‐stresses, so the mean stress is approximately unchanged, and some important surface flux feedbacks are retained. The 1‐D results quantify the sensitivities to wind variability at different time scales from 120 days to 1 day at a few sites. The 3‐D results quantify the sensitivities to wind variability shorter than 60 days at all locations, and comparisons between 1‐D and 3‐D results highlight the importance of 3‐D ocean dynamics. Globally, subseasonal winds explain virtually all of subseasonal 𝛕 variance, about half of subseasonal B variance but only a quarter of subseasonal MLD variance. Subseasonal winds also explain about a fifth of the annual mean MLD and a similar and spatially correlated fraction of the mean friction velocity, where ρsw is the density of seawater. Hence, the subseasonal MLD variance is relatively insensitive to subseasonal winds despite their strong impact on local B and 𝛕 variability, but the mean MLD is relatively sensitive to subseasonal winds to the extent that they modify the mean u*, and both of these sensitivities are modified by 3‐D ocean dynamics.

Wind Turbulence Intensity Characteristics at 10m Above Ground Along the Cotonou Coast, Benin

The characteristics of the wind turbulence intensity that are essential to know before installing a wind turbine at a site were investigated along the coast of Cotonou in Benin. The average speed, direction, roughness length, friction velocity, turbulence intensity and relationship between the roughness and wind turbulence intensity were evaluated as well. Using the estimators derived from a simple isotropic Gaussian model of turbulent wind fluctuations, we proposed modified models for estimating the turbulence intensity of wind components. Wind speed and direction data recorded at 10 m above ground level from 2011 to 2014 during the first Compact of the Millennium Challenge Account (MCA) in Benin were utilized. The results obtained indicated that the annual average roughness length is evaluated at 1.25×10-4 m, and the annual mean friction velocity is equal to 0.41 m.s-1. Peak values of the turbulence intensity vary from 0.3 to 0.6 except during the months of January, April, July, August and September. The high values obtained could jeopardize the production of wind energy during these months. The correlation between the turbulence intensity and roughness length ranging from 0.75 in January to 0.94 in August revealed that these two parameters are linked by an increasing linear function. Finally, modified formulations of the longitudinal and transversal wind turbulence intensity developed from the van den Hurk and de Bruin model and based on the best-fitting approach were proposed. The error estimators (MAE; RMSE) computed to validate these modified models vary respectively from (0.0099; 0.0141) to (0.0614; 0.0890).

Effect of grid resolution on large eddy simulation of wall-bounded turbulence

The effect of grid resolution on large eddy simulation (LES) of wall-bounded turbulent flow is investigated. A channel flow simulation campaign involving systematic variation of the streamwise ($\Delta x$) and spanwise ($\Delta z$) grid resolution is used for this purpose. The main friction-velocity based Reynolds number investigated is 300. Near the walls, the grid cell size is determined by the frictional scaling, $\Delta x^+$ and $\Delta z^+$, and strongly anisotropic cells, with first $\Delta y^+ \sim 1$, thus aiming for wall-resolving LES. Results are compared to direct numerical simulations (DNS) and several quality measures are investigated, including the error in the predicted mean friction velocity and the error in cross-channel profiles of flow statistics. To reduce the total number of channel flow simulations, techniques from the framework of uncertainty quantification (UQ) are employed. In particular, generalized polynomial chaos expansion (gPCE) is used to create meta models for the errors over the allowed parameter ranges. The differing behavior of the different quality measures is demonstrated and analyzed. It is shown that friction velocity, and profiles of velocity and the Reynolds stress tensor, are most sensitive to $\Delta z^+$, while the error in the turbulent kinetic energy is mostly influenced by $\Delta x^+$. Recommendations for grid resolution requirements are given, together with quantification of the resulting predictive accuracy. The sensitivity of the results to subgrid-scale (SGS) model and varying Reynolds number is also investigated. All simulations are carried out with second-order accurate finite-volume based solver. The choice of numerical methods and SGS model is expected to influence the conclusions, but it is emphasized that the proposed methodology, involving gPCE, can be applied to other modeling approaches as well.

LES and wind tunnel test on friction velocity on roof surfaces

Snow saltation is governed by the shear stress from wind on the snow surface, in form of the friction velocity. In most previous studies, the friction velocity was measured or estimated by wind velocity at a certain height using the log law, which is suitable for the fully developed wind field of an open area. However, the applicability of such method to roofs has not been tested. In this study, Large Eddy Simulation (LES) of wind flow around a flat roof was performed to study the flow field characteristics above a flat roof and the results were compared with those from the wind tunnel measurement. Owing to the flow separation and the reversed flow above the roof, the log law is observed to exist only on the leeward part of the roof; therefore, measurement based on the log law is not universally suitable to estimate the friction velocity on roofs. The comparison of the simulation results against experimental data shows that LES reasonably reproduces the flow field around the roof and can be an alternative approach to estimate the friction velocity on building roofs. The fluctuating friction velocity is found to be unneglectable compared with the mean friction velocity and should be considered in the estimation of the snow transport on roofs.

On the effect of an anisotropy-resolving subgrid-scale model on large eddy simulation predictions of turbulent open channel flow with wall roughness

ABSTRACTA large eddy simulation (LES) was conducted of turbulent flow in a channel with a rough wall on one side and a free surface on the other by adopting an anisotropy-resolving subgrid-scale (SGS) model. A shear Reynolds number of Reτ = 395 was used based on the mean friction velocity and channel height. To investigate the grid dependency of the LES results caused by the SGS model, three grid resolutions were tested under the same definition of a roughness shape by using the immersed boundary method. The results obtained were compared with direct numerical simulation data with and without the wall roughness and those without the extra anisotropic term. The primary focus was on how the present anisotropic SGS model with coarser grid resolutions can properly provide the effects of roughness on the mean velocity and turbulent stresses, leading to a considerable reduction of the computational cost of LES.

Estabilidade e estrutura da turbulência sob a influência de jatos de baixos níveis noturnos no sudoeste da amazônia

RESUMO O objetivo deste trabalho foi avaliar como a ocorrência de Jatos de Baixos Níveis (JBNs) noturnos pode influenciar a estrutura da turbulência e a estabilidade atmosférica à superfície. Utilizando-se dados coletados por radiossondagem e pelo sistema de covariância de vórtices turbulentos durante a campanha WetAMC-LBA, foram definidos três regimes de estabilidade atmosférica à superfície: fracamente estável; transição e muito estável. Relacionando estes regimes e a estrutura da turbulência com a ocorrência de JBNs fortes, fracos-tipo 1 (ocorrem acima de 500 m) e fracos-tipo 2 (ocorrem até 500 m), observou-se que cerca de 22% dos casos de JBNs fortes estiveram dentro do regime fracamente estável, enquanto que apenas aproximadamente 3% dos casos de jatos fracos (tipo 1 e tipo 2) encontraram-se neste regime. Outro resultado interessante é que nos casos de JBNs fracos tipo 1, o maior percentual dos pontos encontraram-se dentro do regime muito estável (aproximadamente 54%). Durante a atividade de JBNs fortes a média da velocidade de fricção e da energia cinética turbulenta foi de 0,09 ms-1 e 0,13 m2s-2, respectivamente. Para JBNs fracos-tipo 1 estas variáveis apresentaram valores de 0,04 ms-1 e 0,02 m2s-2, enquanto que para JBNs fracos-tipo 2 os valores atingiram 0,06 ms-1 e 0,03 m2s-2 . Estes resultados sugerem que os JBNs com velocidades suficientemente altas e dependendo da altura em que ocorrem, podem aumentar a turbulência e introduzir fraca estabilidade atmosférica à superfície.

Large Eddy Simulation of Turbulent Heat Transfer in Curved-Pipe Flow

In the present investigation, turbulent heat transfer in fully developed curved-pipe flow has been studied by using large eddy simulation (LES). We consider a fully developed turbulent curved-pipe flow with axially uniform wall heat flux. The friction Reynolds number under consideration is Reτ = 1000 based on the mean friction velocity and the pipe radius, and the Prandtl number (Pr) is 0.71. To investigate the effects of wall curvature on turbulent flow and heat transfer, we varied the nondimensionalized curvature (δ) from 0.01 to 0.1. Dynamic subgrid-scale models for turbulent subgrid-scale stresses and heat fluxes were employed to close the governing equations. To elucidate the secondary flow structures due to the pipe curvature and their effect on the heat transfer, the mean quantities and various turbulence statistics of the flow and temperature fields are presented, and compared with those of the straight-pipe flow. The friction factor and the mean Nusselt number computed in the present study are in good agreement with the experimental results currently available in the literature. We also present turbulence intensities, skewness and flatness factors of temperature fluctuations, and cross-correlations of velocity and temperature fluctuations. In addition, we report the results of an octant analysis to clarify the correlation between near-wall turbulence structures and temperature fluctuation in the vicinity of the pipe wall. Based on our results, we attempt to clarify the effects of the pipe curvature on turbulent heat transfer. Our LES provides researchers and engineers with useful data to understand the heat-transfer mechanisms in turbulent curved-pipe flow, which has numerous applications in engineering.

Scale-by-scale energy budget in a turbulent boundary layer over a rough wall

Hot-wire velocity measurements are carried out in a turbulent boundary layer over a rough wall consisting of transverse circular rods, with a ratio of 8 between the spacing (w) of two consecutive rods and the rod height (k). The pressure distribution around the roughness element is used to accurately measure the mean friction velocity (Uτ) and the error in the origin. It is found that Uτ remained practically constant in the streamwise direction suggesting that the boundary layer over this surface is evolving in a self-similar manner. This is further corroborated by the similarity observed at all scales of motion, in the region 0.2⩽y/δ⩽0.6, as reflected in the constancy of Reynolds number (Rλ) based on Taylor’s microscale and the collapse of Kolmogorov normalized velocity spectra at all wavenumbers.A scale-by-scale budget for the second-order structure function 〈(δu)2〉 (δu=u(x+r)-u(x), where u is the fluctuating streamwise velocity component and r is the longitudinal separation) is carried out to investigate the energy distribution amongst different scales in the boundary layer. It is found that while the small scales are controlled by the viscosity, intermediate scales over which the transfer of energy (or 〈(δu)3〉) is important are affected by mechanisms induced by the large-scale inhomogeneities in the flow, such as production, advection and turbulent diffusion. For example, there are non-negligible contributions from the large-scale inhomogeneity to the budget at scales of the order of λ, the Taylor microscale, in the region of the boundary layer extending from y/δ=0.2 to 0.6 (δ is the boundary layer thickness).

Parsing the variability in CH4 flux at a spatially heterogeneous wetland: Integrating multiple eddy covariance towers with high‐resolution flux footprint analysis

AbstractRestored wetlands are a complex mosaic of open water and new and old emergent vegetation patches, where multiple environmental and biological drivers contribute to the measured heterogeneity in methane (CH4) flux. In this analysis, we replicated the measurements of CH4 flux using the eddy covariance technique at three tower locations within the same wetland site to parse the spatiotemporal variability in CH4 flux contributed by large‐scale seasonal variations in climate and phenology and short‐term variations in flux footprint movement over a mosaic of vegetation and open water. Using a hierarchical statistical model accounting for site‐level environmental effects, tower‐level footprint and biological effects, and temporal autocorrelation, we partitioned the key drivers of the daily CH4 flux variability among the three replicated towers. The daily mean air temperature and mean friction velocity, a measure of momentum transfer, explained a significant variability in CH4 flux across the three towers, and the abundance and spatial aggregation of vegetation in the flux footprint along with the daily gross primary productivity explained much of the tower‐level variability. This statistical model captured 67% of the total variance in the daily integrated growing season CH4 fluxes at this site, which bridged an order of magnitude from 80 to 480 mg C m−2 d−1 during the measurement period from 10 May 2012 to 24 October 2012.

Turbulence spectra measured during fire front passage

Four field experiments were conducted over various fuel and terrain to investigate turbulence generation during the passage of wildland fire fronts. Our results indicate an increase in horizontal mean winds and friction velocity, horizontal and vertical velocity variances as well as a decreased degree of anisotropy in TKE during fire front passage (FFP) due to fire-induced winds. Vertical velocity and temperature variances observed during FFP approached the local free convection prediction when represented as a function of stability parameter z/L under very unstable conditions. The results of our wavelet spectral analysis show increased energy in velocity and temperature spectra at high frequency during FFP for all four cases; we hypothesize this is caused by the shedding of small eddies generated from the fire front. Additionally, spectral energy of velocity components at low frequencies may be affected by cross-flow intensity, topography, presence of canopy layer, and degree of fire–atmosphere coupling. When the velocity spectra are normalized using the friction velocity u* following Monin–Obukhov scaling, the velocity spectra observed during the FFP collapsed into a fairly narrow band in the inertial subrange, suggesting that as far as inertial range is concerned, the friction velocity u* is a valid scaling parameter that can be used for wildfire application. When the temperature spectra are normalized by T*, the temperature spectra observed during the FFP did not show any systematic behaviors predicted by the similarity scaling due to the extreme surface heating environment of fires.

J054024 遷移レイノルズ数域チャネル流の乱流斑点成長過程の研究

Long-term growth of a turbulent spot is examined numerically in plane Poiseuille flow at low Reynolds numbers that are characterized by subcritical intermittent turbulent flow. The turbulent spot is generated by vortex pairs with a finite spatial size and a magnitude strong enough to induce transition. In a range of the friction Reynolds number from 56 to 64, the turbulent stripe pattern occurs inside the spot in a late stage of development, while the spot itself grows in a form of V shape and is accompanied by two leading-edge branches and successive waves. The propagation speeds with respect to the leading and trailing edges of the spot are found to be 0.86U_c and 15u_T (U_c, laminar centerline velocity; u_r, mean friction velocity), respectively, exhibiting their independence on the Reynolds number.