Friction Velocity Research Articles

Modulation of turbulent Couette flow with vortex cavitation in a minimal flow unit

Modulation of turbulent flow by cavitation in fluid machinery can cause vibrations, noise, and erosion. In this study, we confirm the cavitation phenomenon and observe its characteristics to predict the flow and control it accordingly. We perform a direct numerical simulation of the turbulent Couette flow of water with vortex cavitation using a cavitation model to predict phase change based on pressure distribution. In this simulation, we investigate the characteristics of the local interaction between turbulence vortices and cavitation and the global modulation of the turbulent flow, i.e. mean velocity and wall friction coefficient. We observe that a cavity is generated where a low-pressure region is created in the centre of the turbulence vortex; the growth of the cavity weakens the vortex and reduces the intensity of the turbulence. Further, the vortex becomes stronger as the cavity contracts; this phenomenon occurs repeatedly in a turbulent flow field with vortex cavitation. In a turbulent flow field with vortex cavitation, mechanical oscillations can occur spontaneously. In addition, we found that the turbulence vortex weakened by cavitation regenerates around the cavity. The unsteady phenomenon of the turbulence vortex cavitation repeatedly grows and decays monotonically; however, it does not necessarily repeat these spatially in the same manner. The spatial characteristics of the turbulence structure are different from those observed in single-phase turbulent flow.

The structure of turbulence in rotating rough-channel flows

Direct numerical simulation (DNS) of rib-roughened turbulent channel flow rotating about its spanwise axis, by Narasimhamurthy and Andersson [Turbulence statistics in a rotating ribbed channel. International Journal of Heat and Fluid Flow 51, 29–41. (2015)], is revisited to seek complementary insights into the combined effects of roughness and Coriolis force on the turbulence. Flow in the channel was maintained at a friction Reynolds number, Reτ=400 and the non-inertial reference frame was rotated at different speeds quantified by the dimensionless rotation number, Ro=0, 2 and 6. Both the aforementioned parameters are based on the friction velocity uτ and half-height of the channel h. The channel walls were symmetrically mounted with transverse square ribs with cross-section of side k=0.1h and pitch λ=8k. Rotation causes preferential aligning of the near-wall vortical structures. The Taylor-Görtler–like roll-cells similar to those found in the rotating smooth-channel flows, survive the presence of the transverse ribs, but exhibit transient behavior. Increased transport of turbulent kinetic energy from the pressure side at higher Ro is evident from the variation of the vertical transport velocity Vk. The rotational production rates assume increasingly significant roles in distributing the kinetic energy in different directions. Anisotropy invariant maps and Taylor microscales show that the structure of turbulence is affected by rotation in a significant manner.

Bir Yarı-düzlemde Yüzeyaltı Termoelastik Temas Gerilmelerinin Sıcaklık Bağımlı Özellikler Kullanılarak Belirlenmesi

Contact mechanics problem between a rigid punch and a homogenous half-plane is examined considering frictional heat generation. Friction between the sliding rigid punch and the surface of the half-plane leads to a frictional heat which directly flows towards the half-plane material without a loss, and it changes the material’s thermoelastic properties. Calculation of subsurface stresses is crucial in the aspect of mechanical design of components since most of failures arise from fatigue and fracture at regions where subsurface stresses reach higher levels. In order to solve the problem, an iterative algorithm is developed based on the finite element method. Steady state subsurface contact stresses are obtained once the frictional heat on the contact surface reaches equilibrium. Subsurface stresses are calculated for different values of punch sliding velocity and coefficient of friction. It is observed that difference between subsurface contact stresses calculated based on temperature dependent and temperature independent properties is remarkable. Higher values of punch velocity and coefficient of friction leads to greater amount of heat generation, and percent difference between stresses reaches significant level especially near the contact surface. The utilization of temperature dependent material properties provides better approximation in assessing fatigue and fracture behavior of machine parts subjected to frictional contact with heat generation.

Numerical Simulations of a Freely Falling Rigid Sphere in Bounded and Unbounded Water Domains

Abstract Numerical studies were conducted on the hydrodynamics of a freely falling rigid sphere in bounded and unbounded water domains to investigate the drag coefficient, normalized velocity, pressure coefficient, and skin friction coefficient as a function of dimensionless time. The bounded domain was simulated by bringing the cylindrical water container’s wall closer to the impacting rigid sphere and linking it to the blockage ratio (BR), defined as the ratio of the projection area of a freely falling sphere to that of the cross-sectional area of the cylindrical water container. Six cases of bounded domains (BR = 1%, 25%, 45%, 55%, 65%, and 75%) were studied. However, the unbounded domain was considered with a BR of 0.01%. In addition, the k–ω shear stress transport (SST) turbulence model was used, and the computed results of the bounded domain were compared with those of other studies on unbounded domains. In the case of the bounded domain, which has a higher value of BR, a substantial reduction in normalized velocity and an increase in the drag coefficient were found. Moreover, the bounded domain yielded a significant increase in the pressure coefficient when the sphere was half-submerged; however, an insignificant effect was found on the skin friction coefficient. In the case of the unbounded domain, a significant reduction in the normalized velocity occurred with a decrease in the Reynold number (Re), whereas the drag coefficient increases with a decrease in Reynolds number.

Variations in Wave Slope and Momentum Flux From Wave‐Current Interactions in the Tropical Trade Winds

AbstractObservations from six Lagrangian Surface Wave Instrument Float with Tracking drifters in January‐February 2020 in the northwestern tropical Atlantic during the Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign are used to evaluate the influence of wave‐current interactions on wave slope and momentum flux. At observed wind speeds of 4–12 ms−1, wave mean square slopes are positively correlated with wind speed. Wave‐relative surface currents varied significantly, from opposing the wave direction at 0.24 ms−1 to following the waves at 0.47 ms−1. Wave slopes are 5%–10% higher when surface currents oppose the waves compared to when currents strongly follow the waves, consistent with a conservation of wave energy flux across gradients in currents. Assuming an equilibrium frequency range in the wave spectrum, wave slope is proportional to wind friction velocity and momentum flux. The observed variation in wave slope equates to a 10%–20% variation in momentum flux over the range of observed wind speeds (4–12 ms−1), with larger variations at higher winds. At wind speeds over 8 ms−1, momentum flux varies by at least 6% more than the variation expected from current‐relative winds alone, and suggests that wave‐current interactions can generate significant spatial and temporal variability in momentum fluxes in this region of prevailing trade winds. Results and data from this study motivate the continued development of fully coupled atmosphere‐ocean‐wave models.

Topographic perturbation of turbulent boundary layers by low‐angle, early‐stage aeolian dunes

AbstractDecimeter‐scale early‐stage aeolian bedforms represent topographic features that differ notably from their mature dune counterparts, with nascent forms exhibiting more gently sloping lee sides and a reverse asymmetry in their flow‐parallel bed profile compared to mature dunes. Flow associated with the development of these “protodunes,” wherein the crest gradually shifts downstream towards its mature state, was investigated by studying the perturbation of the turbulent boundary layer over a succession of representative bedforms. Rigid, three‐dimensional models were studied in a refractive‐index‐matched experimental flume that enabled near‐surface quantification of mean velocities and Reynolds stresses using particle‐image velocimetry in wall‐normal and wall‐parallel measurement planes. Data indicate strong, topographically induced flow perturbations over the protodunes, to a similar relative degree to that found over mature dunes, despite their low‐angled slopes. The shape of the crest is found to be an important factor in the development of flow perturbations, and only in the case with the flattest crest was maximal speed‐up of flow, and reduction in turbulent stresses, found to occur upstream of the crest. Investigation of the log‐linearity of the boundary layer profile over the stoss sides showed that, although the profile is strongly perturbed, a log‐linear region exists, but is shifted vertically. A streamwise trend in friction velocity is thus present, showing a behavior similar to the trends in mean velocity. Analysis of the growth of the internal boundary layer on the dune stoss sides, beginning at the toe region, reveals a similar development for all dune shapes, despite clear differences in mean velocity and turbulent stress perturbations in their toe regions. The data presented herein provide the first documentation of flow over morphologies broadly characteristic of subtle, low‐angle, aeolian protodunes, and indicate key areas where further study is required to yield a more complete quantitative understanding of flow–form–transport couplings that govern their morphodynamics.

Hydrogen sulfide emissions from a midwestern manure slurry storage basin.

Hydrogen sulfide (H2 S) emissions from midwestern U.S. dairy lagoons are episodic and seasonal. Emissions were determined using an inverse diffusion model in conjunction with measured upwind and downwind line-averaged H2 S concentrations and turbulence. Mean daily H2 S emissions from manure stored in earthen basins was 0.97μg m-2 s-1 (σ=1.35μg m-2 s-1 ). Mean live animal basis daily emission from the basins was 1.1g d-1 hd-1 (0.84g d-1 AU-1 ). Daily emission was modeled using the van't Hoff function with air temperature as a surrogate for slurry surface temperature and a linear function of friction velocity. The mean standard error of estimate of the model was 1.8μg m-2 s-1 (2.0g d-1 hd-1 , 1.6g d-1 AU-1 ) and accounted for 60% of emissions variability. H2 S emissions were enhanced for short periods during the year when the stored slurry was loaded onto trucks for removal. Emissions from the basins were not statistically different as barn manure handling changed from flushing to scraping. More measurements are needed to verify annual emissions estimates for these manure slurry storage basins and derive emission factors for these slurry storage systems.

The near-surface turbulent kinetic energy characteristics under the different convection regimes at four towers with contrasting underlying surfaces

Turbulence kinetic energy (TKE) is widely used to characterize the spatio-temporal changes of atmospheric turbulence. The objective of this study is to investigate the near-surface TKE characteristics under the different convection regimes classified by the ratio of the buoyancy term (BP) to the shear term (SP) based on the sonic anemometer data measured at four boundary layer towers in China with contrasting underlying surfaces. The annual average diurnal variations of TKE and the related variables (BP, SP, friction velocity u∗, and wind speed u¯) at the four towers are analyzed. Large differences exist in each variable between sites, leading to convection regimes differing by site. TKE varies significantly (up to 243%) with the convection regimes. Although the percentage of given convection regime varies greatly between sites, the results from the four towers show that TKE initially increases from the unstable to neutral atmospheric stratification conditions, and decreases from the neutral to stable conditions. Note that the ratio between BP and SP (BP/SP) is closely related to near-surface atmospheric stability. Except for BP, the other three variables (SP, u∗, and u¯) have similar trends with TKE. The buoyancy term tends to decrease as BP/SP decreases. The correlation coefficients (R) between TKE and the related variables vary up to 0.33 (67%) with different convective regimes, whereas the variation trends of R between TKE and a given variable (BP and SP) are broadly similar at the four towers. This study indicates that the variation trend of TKE and its associations with BP and SP seem reliable under the different convection regimes. Our findings advance the understanding of near-surface turbulence and demonstrate that the different convection regimes should be considered when investigating TKE characteristics.

Hysteresis and Surface Shear Stresses During Snow-Particle Aeolian Transportation

Surface shear stresses produced by wind and particle collision play a key role in aerodynamic entrainment and splash processes. The fluid shear stress at the surface during aeolian transport has been researched for decades; however, the equilibrium property reported in the literature, numerical simulations, and experiments is inconsistent. To discuss this discrepancy, this study investigates fluid and particle shear stresses at the surface during the aeolian transport of snow particles using a two-dimensional random-flight model of drifting snow. The simulations are performed for various friction velocities on a loose snow bed. By varying the wind conditions in stages, the transport hysteresis is confirmed, and the impact threshold is estimated from the particle transport rate (0.206,hbox {m,s}^{-1}). The friction velocity at the surface during transport decreases marginally with an increase in wind speed caused by the impact threshold, revealing that our results do not contradict Owen’s second hypothesis. The total shear stress, which is calculated by summing the fluid and particle shear stresses, is vertically uniform in the equilibrium state; thus, the increase in the particle shear stress decreases the fluid shear stress at the surface. The equilibrium property of the fluid shear stress near the surface changes significantly with height (from a decreasing trend to an increasing trend) because the particle shear stress decreases rapidly in the height range of 1–10 mm. Our findings suggest that it is difficult to accurately measure the fluid shear stress in the surface vicinity using anemometers, and a new methodology is needed.

Surface Layer Drag Coefficient at Different Radius Ranges in Tropical Cyclones

Using dropsonde data and a flux-profile method, this study investigates the drag coefficient (Cd)–wind speed relationship within different radius ranges. The results show a systematic decrease of friction velocity u* from the range of R/RMW > 1.05 to that of R/RMW < 0.95 (R is the radial location of a dropsonde profile, and RMW is the radius of maximum wind), and the reduction is 5~25% for different wind speeds. Further, within the ranges of either R/RMW > 1.05 or R/RMW < 1.05, a clear feature of “roll-off” at about 35 m s−1 can be obtained. However, the roll feature becomes vague in the ranges of R/RMW < 0.95, R/RMW < 0.85, and R/RMW < 0.75, indicating the TC dynamics within and near RMW play a role in affecting the flux-profile relationship. Even more, Cd of R < 0.75RMW deviates significantly from the Cd of R < 0.85RMW and R < 0.95RMW, while the deviation between R < 0.85RMW and R < 0.95RMW is much smaller. Especially when 10 m winds exceed 40 m s−1, u* of R < 0.75RMW is significantly larger than that of R < 0.85RMW. This phenomenon is also linked to the TC dynamics (e.g., the large radial gradients of winds and the drastic vertical variation of the bulk Richardson number), but the speculation needs to be verified in future study.

Outer turbulent boundary layer similarities for different 2D surface roughnesses at matched Reynolds number

The outer layer similarity in zero pressure gradient (ZPG) boundary layers developing over different geometries of 2D roughness elements is assessed, using single hotwire anemometry. Two types of 2D roughness are used: circular rods and sinewave surfaces with two different heights, k = 1.6, and 2.4 mm, and three different streamwise spacings, i.e. sx=8k,12k, and 16k. These roughnesses cover a range of ratios of the boundary layer thickness (δ) to the roughness height (k) from δ/k = 21 to 45. As expected, all roughnesses caused a downward shift on the wall-unit normalised streamwise mean velocity profiles compared with smooth wall profiles, with a maximum shift observed for rods with a spacing of sx=8k, while the minimum shift is noticed for a sinewave surface with a spacing of sx=16k. The defect velocity profiles collapse entirely for all smooth and rough wall flows when normalised by the friction velocity. It was found that the shape factor, H is a suitable scaling parameter for improving the collapse of the data when using the diagnostic plot. The inner peak of the turbulence intensity profiles for the sinewave roughness is reduced gradually with increasing Reynolds number while the turbulent boundary layer (TBL) develops from a transitionally to a fully rough regime. Meanwhile, this inner peak disappears completely for all rod roughnesses, as the TBL is always in a fully rough regime, and the profiles exhibit only an outer peak, located at a wall-normal location y/δ≈0.06. The results suggest that Townsends similarity hypothesis for 2D surface roughness is relatively well approximated in the outer region of the flow as reflected by the collapse of the distributions of velocity defect, turbulence intensity, skewness, and flatness when scaled with δ.

A numerical model for snow drifting simulations on flat roofs using Lagrangian approach

A numerical model based on Lagrangian description is developed in this study to predict wind-induced snow drifting on flat roofs, which fully considers the key physical processes of snow particle saltation. A scaled experiment of snow redistributions on the model roof was carried out in the wind tunnel for the verification and evaluation of different numerical methods selected from related literature. The optimal model is determined by comparing the predicted transport rate on the roof with estimations from empirical formulas and by comparing simulated snow distributions with experimental data. Characteristics of the saltation layer above the flat roof surface are then presented in detail based on the developed model. The rebound particles account for approximately 60% of the total transported particles above the roof, indicating the dominant role of rebound in the lift-off process for saltating snow particles. The developing snow transport along the roof span leads to the increase of surface erosion flux caused by the splash entrainment with downwind distance. The splash entrainment could thus contribute considerably to unsaturated snow transport on the flat roof, which is also found to be influenced by the approaching wind velocity and threshold friction velocity to varying degrees.

The effect of inlet turbulence on the quiescent core of turbulent channel flow

The impact of inlet turbulence on the structure of turbulent channel flow is investigated using particle image velocimetry. Streamwise–wall-normal plane measurements are performed in a channel, where different turbulence intensities were generated at the inlet with an active grid. Four cases are tested with matched centreline mean velocities, while the centreline turbulence intensities ranged from 3.7 % for the reference case, up to 6.4 %. The friction velocity is found to be approximately constant with varying centreline turbulence intensities, resulting in a matched friction Reynolds number of $Re_\tau \approx 770$ for all cases, which contrasts with similar experiments performed in a zero-pressure-gradient boundary layer. The log region remains intact for all cases. The so-called quiescent core of the turbulent channel flow is also investigated. In addition to increased core discontinuity, the increased fluctuations of the streamwise velocity give rise to new core states, which differ from the conventional ones in their characteristic velocity. They are associated with a bulk of low- or high-momentum fluid passing through the measurement domain, and their occurrence increases with turbulence intensity. Tracking the core boundaries indicates an overall tendency of the core to move closer to the wall for increased inlet turbulence intensities, resulting in an increased core thickness. Moreover, it is found that the low-momentum cores generally reside closer to the wall compared with the ordinary cores and appear to be thicker than them, whereas the opposite, i.e. residing farther from the wall and being thinner, is true for the high-momentum cores.

Coarsening Dynamics of 2D Subaqueous Dunes

AbstractFluid flow over an initially flat granular bed leads to the formation of a surface‐wave instability. The sediment bed profile coarsens and increases in amplitude and wavelength as disturbances develop from ripples into dunes. We perform experiments and numerical simulations to quantify both the temporal evolution of bed properties and the relationship between the initial growth rate and the friction velocity u∗. Experimentally, we study underwater bedforms originating from a thin horizontal particle layer in a narrow and counter‐rotating annular flume. We investigate the role of flow speed, flow depth and initial bed thickness on dune evolution. Bedforms evolve from small, irregular disturbances on the bed surface to rapidly growing connected terraces (2D equivalent of transverse dunes) before splitting into discrete dunes. Throughout much of this process, growth is controlled by dune collisions which are observed to result in either coalescence or ejection (mass exchange). We quantify the coarsening process by tracking the temporal evolution of the bed amplitude and wavelength. Additionally, we perform Large Eddy Simulations (LES) of the fluid flow inside the flume to relate the experimental conditions to u∗. By combining the experimental observations with the LES results, we find that the initial dune growth rate scales approximately as . These results can motivate models of finite‐amplitude dune growth from thin sediment layers that are important in both natural and industrial settings.

Unsteady Magnetohydrodynamic Convective Flow of a Nanoliquid via a Radially Stretched Riga Area via Optimal Homotopy Analysis Method

This article presents the investigation of the mechanism of mass and heat transport of an unsteady incompressible electrically conducting viscous convective nanoliquid through a radially stretching Riga plate with viscous dissipation, velocity slip, Brownian motion, Thermophoresis parameter, and first-order chemical reaction. The optimal homotopy analysis technique has been utilized to compute the solutions for the differential equations system after using the similarity variables to the governing flow equations. For physical clarification, the acquired outcomes are illustrated graphically and also in the form of tables. These graphs and tables are used to exhibit physical quantities, namely, Sherwood number, skin friction coefficient, concentration, Nusselt number, temperature and velocity. One of the significant outcomes of the current scrutiny is velocity profile improves for rising values of Hartman number, and this investigation finds several applications in various fields of automobile industries and engineering.

Investigating threshold wind velocity for movement of sparsely distributed gravels in a wind tunnel: Effect of surface coarseness

In this study, wind tunnel experiments were conducted to determine the relationship between the texture of road surfaces and the threshold wind velocity that causes the movement of sparsely distributed gravels. We proposed a method that uses analogous particles having lower densities compared to actual gravel, considering the threshold velocity for gravel is too large to imitate in a typical wind tunnel. We first conducted a preliminary experiment, which confirmed that the threshold velocity for denser prototype particles can be estimated from the density ratios of the analogous and prototype particles. Furthermore, a main experiment was conducted to analyze different road surface textures using pumice stones, as their particle density is less than that of gravels. The results showed that coarser road surfaces exhibited larger threshold velocities for the gravel movement. Quantitatively, the coarsest asphalt concrete exhibited a threshold velocity that was 2.5 times larger than that of the smoothest surface (cement concrete). However, surface coarseness was dependent on the particle size, whereas the effect of particles trapped by the gaps in surfaces was more dominant than the fluid force acting on the particles of a relatively coarse surface. This force balance is reversed for a smooth surface, which indicates the possibility of determining threshold friction velocity from the particle size and surface texture coarseness.

Frictional and Lithological Controls on Shallow Slow Slip at the Northern Hikurangi Margin

AbstractSlow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide‐hold‐slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 × 10−9 m/s) to ∼1 mm/s (10−3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a‐b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of ∼0.3 µm/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay‐rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1–10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs.

Numerical analysis of a second-grade fuzzy hybrid nanofluid flow and heat transfer over a permeable stretching/shrinking sheet

In this work, the heat transfer features and stagnation point flow of Magnetohydrodynamics (MHD) hybrid second-grade nanofluid through a convectively heated permeable shrinking/stretching sheet is reported. The purpose of the present investigation is to consider hybrid nanofluids comprising of Alumina left( {{text{Al}}_{{2}} {text{O}}_{{3}} } right) and Copper left( {{text{Cu}}} right) nanoparticles within the Sodium Alginate (SA) as a host fluid for boosting the heat transfer rate. Also, the effects of free convection, viscous dissipation, heat source/sink, and nonlinear thermal radiation are considered. The converted nonlinear coupled fuzzy differential equations (FDEs) with the help of triangular fuzzy numbers (TFNs) are solved using the numerical scheme bvp4c. The numerical results are acquired for various engineering parameters to study the Nusselt number, skin friction coefficient, velocity, and temperature distribution through figures and tables. For the validation, the current numerical results were found to be good as compared to existing results in limiting cases. It is also inspected by this work that with the enhancement of the volume fraction of nanoparticles, the heat transfer rate also increases. So, it may be taken as a fuzzy parameter for a better understanding of fuzzy variables. For the comparison, the volume fraction of nanofluids and hybrid nanofluid are said to be TFN [0, 0.1, 0.2]. In the end, we can see that fuzzy triangular membership functions (MFs) have not only helped to overcome the computational cost but also given better accuracy than the existent results. Finding from fuzzy MFs, the performance of hybrid nanofluids is better than nanofluids.

Energy Balance Closure Problem over a Tropical Seasonal Rainforest in Xishuangbanna, Southwest China: Role of Latent Heat Flux

The unresolved energy-unclosed problem in micrometeorology refers to the fact that the sum of turbulent fluxes (sensible and latent heat fluxes, Hs and LE) monitored by eddy covariance (EC) methods tends to be lower than the available energy (net radiation (Rn), soil heat flux (G), and heat storage (S)). The lack of energy balance closure (EBC) increases evapotranspiration-measurement uncertainty. Using EC data from Xishuangbanna, a Southeast Asian tropical seasonal rainforest, we analyzed the energy distribution and closure based on micrometeorological features. We found that: (1) the EBC in the rainy season exceeds that in other seasons and that the seasonal moisture content, frictional wind velocity (u*), and LE contribute to the high seasonal variability in EBC; (2) the annual closure is approximately 65%, and energy non-closure is influenced by turbulence intensity and atmospheric stability. When the atmospheric state is unstable to near neutral, u* is greatest, and EBC can reach nearly 80%. (3) energy is mainly allocated to LE, and energy non-closure leads to LE underestimation, especially in the foggy-cool and hot-dry seasons. (4) Heat storage and large time-scale flux effects on EBC were excluded. The causes of energy non-closure in the tropical calm zone need further investigation.

Forest–atmosphere exchange of reactive nitrogen in a remote region – Part I: Measuring temporal dynamics

Abstract. Long-term dry deposition flux measurements of reactive nitrogen based on the eddy covariance or the aerodynamic gradient method are scarce. Due to the large diversity of reactive nitrogen compounds and high technical requirements for the measuring devices, simultaneous measurements of individual reactive nitrogen compounds are not affordable. Hence, we examined the exchange patterns of total reactive nitrogen (ΣNr) and determined annual dry deposition budgets based on measured data at a mixed forest exposed to low air pollution levels located in the Bavarian Forest National Park (NPBW), Germany. Flux measurements of ΣNr were carried out with the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD) for 2.5 years. The average ΣNr concentration was 3.1 µg N m−3. Denuder measurements with DELTA samplers and chemiluminescence measurements of nitrogen oxides (NOx) have shown that NOx has the highest contribution to ΣNr (∼51.4 %), followed by ammonia (NH3) (∼20.0 %), ammonium (NH4+) (∼15.3 %), nitrate NO3- (∼7.0 %), and nitric acid (HNO3) (∼6.3 %). Only slight seasonal changes were found in the ΣNr concentration level, whereas a seasonal pattern was observed for the contribution of NH3 and NOx. NH3 showed highest contributions to ΣNr in spring and summer, NOx in autumn and winter. We observed deposition fluxes at the measurement site with median fluxes ranging from −15 to −5 ngNm-2s-1 (negative fluxes indicate deposition). Median deposition velocities ranged from 0.2 to 0.5 cm s−1. In general, highest deposition velocities were recorded during high solar radiation, in particular from May to September. Our results suggest that seasonal changes in composition of ΣNr, global radiation (Rg), and other drivers correlated with Rg were most likely influencing the deposition velocity (vd). We found that from May to September higher temperatures, lower relative humidity, and dry leaf surfaces increase vd of ΣNr. At the measurement site, ΣNr concentration did not emerge as a driver for the ΣNrvd. No significant influence of temperature, humidity, friction velocity, or wind speed on ΣNr fluxes when using the mean-diurnal-variation (MDV) approach for filling gaps of up to 5 days was found. Remaining gaps were replaced by a monthly average of the specific half-hourly value. From June 2016 to May 2017 and June 2017 to May 2018, we estimated dry deposition sums of 3.8 and 4.0 kgNha-1a-1, respectively. Adding results from the wet deposition measurements, we determined 12.2 and 10.9 kgNha-1a-1 as total nitrogen deposition in the 2 years of observation. This work encompasses (one of) the first long-term flux measurements of ΣNr using novel measurements techniques for estimating annual nitrogen dry deposition to a remote forest ecosystem.