Low Turbulence Conditions Research Articles

The Effects of Unsteady On-Road Flow Conditions on Cabin Noise: Spectral and Geometric Dependence

The in-cabin sound pressure level response of a vehicle in yawed wind conditions can differ significantly between the smooth flow conditions of the aeroacoustic wind tunnel and the higher turbulence, transient flow conditions experienced on the road. Previous research has shown that under low turbulence conditions there is close agreement between the variation with yaw of in-cabin sound pressure level on the road and in the wind tunnel. However, under transient conditions, sound pressure levels on the road were found to show a smaller increase due to yaw than predicted by the wind tunnel, specifically near the leeward sideglass region. The research presented here investigates the links between transient flow and aeroacoustics. The effect of small geometry changes upon the aeroacoustic response of the vehicle has been investigated. It was found that sideglass pressures showed close agreement at all turbulence levels while surface sound pressure levels also showed similar behaviour under a wide range of on-road flow conditions. While the overall sideglass sound pressure level changed under the various yaw conditions, the change in shape of the frequency spectrum was less significant. Geometry changes made to a base vehicle reduced the sensitivity of the in-cabin noise to on-road turbulence, showing that shape-change can modify sensitivity to on-road turbulence.

Performance of disturbance augmented control design in turbulent wind conditions

This paper investigates the use of disturbance models in the design of wind turbine individual pitch controllers. Previous work has used individual pitch control and disturbance models with the Multiblade Coordinate Transformation to design controllers that reduce the blade loads at the frequencies associated with the rotor speed. This paper takes a similar approach of using a disturbance model within the H ∞ design framework to account for periodic loading effects. The controller is compared with a baseline design that does not include the periodic disturbance model. In constant wind speeds, the disturbance model design is significantly better than the baseline design at canceling blade loads at the rotor frequencies. However, these load reduction improvements become negligible even under low turbulent wind conditions. The two controllers perform similarly in turbulent wind conditions because disturbance augmentation improves load reduction only at the multiples of the rotor frequency in the yaw and tilt moment channels whereas turbulence creates strong collective bending moments. In addition, turbulent wind contains energy across a broad frequency spectrum and improvements at multiples of the rotor frequency are less important in these conditions. Therefore inclusion of periodic disturbance models in the control design may not lead to the expected load reduction in fielded wind turbines.

The impact of data gaps and quality control filtering on the balances of energy and carbon for a Southwest Amazon forest

Fluxes of CO 2, water vapor, sensible heat and several heat storage terms were used to characterize the dependence of the energy balance closure and carbon balance on gaps introduced by screening for high quality or turbulent conditions. The current work is the first study where a sensitivity analysis was applied to the carbon balance and energy balance closure for an Amazonian forest site. The measurements were part of the RACCI/DRY-TO-WET Experiment, within the framework of the Large Scale Biosphere–Atmosphere Experiment in Amazonia (LBA). The energy balance closure varied from 88% to 98%, with an average intercept of −5 W m −2. The level of closure was dependent on the amount of data filtered out according to the quality control flag. A compromise between moderate data quality and good sampling of daytime periods was determined to be the best choice for this site. Different values of friction velocity threshold and data quality used in the screening of data resulted in a carbon balance ranging from −0.10 ± 0.15 to 0.31 ± 0.25 t C ha −1, where positive values indicate a net source of carbon in the period. Applying the screening based on friction velocity and quality control changed this ecosystem in the period analyzed from a sink into a source of carbon to the atmosphere. Year-round continuous measurements would be required to verify if the net release of carbon during this transition period is part of the natural cycle for this forest or if that was caused by disturbances in the ecosystem or local climatology. Our results show that the carbon sink strength associated with this forest site could have been overestimated in previous works due to underestimation of nocturnal respiration, caused by lack of filtering of low turbulence conditions.

Aerodynamics of a Letterbox Trailing Edge: Effects of Blowing Rate, Reynolds Number, and External Turbulence on Aerodynamic Losses and Pressure Distribution

A letterbox trailing edge configuration is formed by adding flow partitions to a gill slot or pressure side cutback. Letterbox partitions are a common trailing edge configuration for vanes and blades, and the aerodynamics of these configurations are consequently of interest. Exit surveys detailing total pressure loss, turning angle, and secondary velocities have been acquired for a vane with letterbox partitions in a large-scale low speed cascade facility. These measurements are compared with exit surveys of both the base (solid) and gill slot vane configurations. Exit surveys have been taken over a four to one range in chord Reynolds numbers (500,000, 1,000,000, and 2,000,000) based on exit conditions and for low (0.7%), grid (8.5%), and aerocombustor (13.5%) turbulence conditions with varying blowing rate (50%, 100%, 150%, and 200% design flow). Exit loss, angle, and secondary velocity measurements were acquired in the facility using a five-hole cone probe at a measuring station representing an axial chord spacing of 0.25 from the vane trailing edge plane. Differences between losses with the base vane, gill slot vane, and letterbox vane for a given turbulence condition and Reynolds number are compared providing evidence of coolant ejection losses, and losses due to the separation off the exit slot lip and partitions. Additionally, differences in the level of losses, distribution of losses, and secondary flow vectors are presented for the different turbulence conditions at the different Reynolds numbers. The letterbox configuration has been found to have slightly reduced losses at a given flow rate compared with the gill slot. However, the letterbox requires an increased pressure drop for the same ejection flow. The present paper together with a related paper (2008, “Letterbox Trailing Edge Heat Transfer—Effects of Blowing Rate, Reynolds Number, and External Turbulence on Heat Transfer and Film Cooling Effectiveness,” ASME, Paper No. GT2008-50474), which documents letterbox heat transfer, is intended to provide designers with aerodynamic loss and heat transfer information needed for design evaluation and comparison with competing trailing edge designs.

Pollination and seed set in tropical wetland grasses

This study concerns 39 different grass species occurring in the wetland regions of Godavari delta in the west Godavari district (16°15′–17°30′N and 80°50′–81°55′E) of Andhra Pradesh, southern India. In each species, the sexual status of lemmas, the period of stigma receptivity, pollen/ovule ratio, pollen longevity and daily pollen release were examined. The functional sexual systems were determined after performing controlled pollinations and testing seed quality by weight and germination potential. The florets of 35 species are morphologically hermaphroditic. Among the other four species, Chionachne koenigii and Iseilema laxum are monoecious; Chrysopogon aciculatus and Pennisetum pedicellatum are andromonoecious. Examination of the sequential opening of florets and controlled pollinations revealed that C. koenigii and I. laxum are obligate outcrossers with incompatibility to geitonogamous pollen; C. aciculatus and P.pedicellatum have a high level of compatibility to xenopollen. These and another nine species were treated as predominantly outcrossing species. Yet another five species were predominantly self-fertilized, while the remaining 21 species displayed a mixed breeding system. In a majority of the species the P/O ratios are not high and thus do not conform to the expected large P/O ratios in wind-pollinated plants and postulated breeding systems except in a few species. The majority of these grasses shed pollen over a short period spreading from 2 am to 8 am, when usually low turbulent conditions exist, thus allowing short distance transport and restricted pollen shadows with higher concentration of pollen near the source. This appears to be the appropriate strategy for maximization of pollination in view of the short lifespan of pollen that extends mostly over 1–3 h period, and the patchy distribution of the grasses under study. The restricted pollen shadows facilitate the low levels of pollen production in the grass species under study to fertilize most or all the available ovules.

Letterbox Trailing Edge Heat Transfer: Effects of Blowing Rate, Reynolds Number, and External Turbulence on Heat Transfer and Film Cooling Effectiveness

Heat transfer and film cooling distributions have been acquired for a vane trailing edge with letterbox partitions. Additionally, pressure drop data have been experimentally determined across a pin fin array and a trailing edge slot with letterbox partitions. The pressure drop across the array and letterbox trailing edge arrangement was measurably higher than for the gill slot geometry. Experimental data for the partitions and the inner suction surface region downstream from the slot have been acquired over a four-to-one range in vane exit condition Reynolds number (500,000, 1,000,000, and 2,000,000), with low (0.7%), grid (8.5%), and aerocombustor (13.5%) turbulence conditions. At these conditions, both heat transfer and adiabatic film cooling distributions have been documented over a range of blowing ratios (0.47≤M≤1.9). Heat transfer distributions on the inner suction surface downstream from the slot ejection were found to be dependent on both ejection flow rate and external conditions. Heat transfer on the partition side surfaces correlated with both exit Reynolds number and blowing ratio. Heat transfer on partition top surfaces largely correlated with exit Reynolds number but blowing ratio had a small effect at higher values. Generally, adiabatic film cooling levels on the inner suction surface are high but decrease near the trailing edge and provide some protection for the trailing edge. Adiabatic effectiveness levels on the partitions correlate with blowing ratio. On the partition sides adiabatic effectiveness is highest at low blowing ratios and decreases with increasing flow rate. On the partition tops adiabatic effectiveness increases with increasing blowing ratio but never exceeds the level on the sides. The present paper, together with a companion paper that documents letterbox trailing edge aerodynamics, is intended to provide engineers with the heat transfer and aerodynamic loss information needed to develop and compare competing trailing edge designs.

Separated Flow Measurements on a Highly Loaded Low-Pressure Turbine Airfoil

Boundary layer separation, transition, and reattachment have been studied on a new, very high lift, low-pressure turbine airfoil. Experiments were done under low freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Velocity profiles were acquired in the suction side boundary layer at several streamwise locations using hot-wire anemometry. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) ranging from 25,000 to 330,000. In all cases, the boundary layer separated, but at high Reynolds number the separation bubble remained very thin and quickly reattached after transition to turbulence. In the low Reynolds number cases, the boundary layer separated and did not reattach, even when transition occurred. This behavior contrasts with previous research on other airfoils, in which transition, if it occurred, always induced reattachment, regardless of Reynolds number.

Relationships among team's organizational context, innovation speed, and technological uncertainty: An empirical analysis

This study examines the moderating effect of uncertainty on the relationships between three organizational context factors (i.e. top management support, clarity of goals and speed-based rewards) and innovation speed. It considers two aspects of uncertainty: technology novelty and technological turbulence. Findings from 183 new product projects indicate that top management support has a more positive effect on innovation speed under conditions of high technology novelty and high technological turbulence. Clarity of goals is more important to innovation speed under conditions of medium technology novelty and low technological turbulence. The results suggest a curvilinear, positive relationship between speed-based rewards and innovation speed. This is true for all the situations considered in this study, with the exception of environments characterized by high technological turbulence, where a negative curvilinear relationship was found between speed-based rewards and innovation speed.

Gill Slot Trailing Edge Aerodynamics: Effects of Blowing Rate, Reynolds Number, and External Turbulence on Aerodynamic Losses and Pressure Distribution

Gill slots (also called cutbacks) are a common method to cool the trailing edge of vanes and blades and to eject spent cooling air. Exit surveys detailing total pressure loss, turning angle, and secondary velocities have been acquired for a gill slot vane in a large-scale, low speed cascade facility. These measurements are compared with exit surveys of the base (solid) vane configuration. Exit surveys have been taken over a four to one range in chord Reynolds numbers (500,000, 1,000,000, and 2,000,000) based on exit conditions and for low (0.7%), grid (8.5%), and aerocombustor (13.5%) turbulence conditions with varying blowing rate (50%, 100%, 150%, and 200% design flows). Exit loss, angle, and secondary velocity measurements were acquired in the facility using a five-hole cone probe at two stations representing axial chord spacings of 0.25 and 0.50. Differences between losses with and without the gill slot for a given turbulence condition and Reynolds number are compared providing evidence of coolant ejection losses and losses due to the separation off the gill slot lip. Additionally, differences in the level of losses, distribution of losses, and secondary flow vectors are presented for the different turbulence conditions and at the different Reynolds numbers. The turbulence condition has been found to have only a small effect on the increase in losses due to the gill slot. However, decreasing Reynolds number has been found to produce an increasing increment in losses. The present paper, together with a companion paper (2007, “Gill Slot Trailing Edge Heat Transfer—Effects of Blowing Rate, Reynolds Number, and External Turbulence on Heat Transfer and Film Cooling Effectiveness,” ASME Paper No. GT2007-27397), which documents gill slot heat transfer, is intended to provide designers with the heat transfer and aerodynamic loss information needed to compare competing trailing edge designs.

Coccolithophores in the upwelling waters of Portugal: Four years of weekly distribution in Lisbon bay

From July 2001 to May 2005, seawater samples were collected once a week at a fixed station in Lisbon bay (38°41′N, 09°24′W) in order to describe the ecological dynamics of the coccolithophore community. The seasonal and interannual distribution patterns of the different species and their relationships with environmental parameters are addressed. The present work aimed to identify potential proxies for different local water bodies and environmental conditions. Throughout the period of study, the upwelling events were weak and progressively more persistent. High sea surface temperatures (SST) were observed earlier in the year; summers and winters were gradually warmer and colder, respectively. Salinity variations reflected the different weather conditions as they are strongly influenced by rainfall and thus by the Tagus river flow. The extended periods of weak upwelling and the overall increase in SST resulted in the development of phytoplankton populations as measured by chlorophyll a. However, the persistence of the upwelling, and thus shorter convergence periods, favoured other phytoplankton groups than coccolithophore populations as these decreased towards the end of the sampling period. The annual structure of the coccolithophore assemblage showed a pronounced and recurrent seasonal variability, mainly related with the intensity and persistence of upwelling. The highest cell densities were recorded from spring to autumn. An overall preference by most species for mature upwelled waters and low turbulent conditions was observed associated with high temperatures and salinities, although the species develop in different windows with mismatching maxima. The coccolithophores observed were capable of withstanding coastal processes such as turbulence and were well adapted to an environment rich in nutrients provided by both continental runoff and upwelling. The consistency of the results enabled local oceanographic tracers to be defined. Emiliania huxleyi and Gephyrocapsa species can be used as proxies of surface productivity waters during spring and summer while Coccolithus pelagicus indicates the presence of upwelling fronts. Calcidiscus leptoporus is a tracer of the convergence of subtropical oceanic waters onto the shelf, during winter while Coronosphaera mediterranea, Syracosphaera pulchra, Helicosphaera carteri and Rhabdosphaera clavigera revealed the presence of those waters during the short period that characterized the transition from upwelling to downwelling seasons.

A Comparison of Cylindrical and Fan-Shaped Film-Cooling Holes on a Vane Endwall at Low and High Freestream Turbulence Levels

Fan-shaped film-cooling holes have been shown to provide superior cooling performance to cylindrical holes along flat plates and turbine airfoils over a large range of different conditions. Benefits of fan-shaped holes include less required cooling air for the same performance, increased part lifetime, and fewer required holes. The major drawback, however, is increased manufacturing cost and manufacturing difficulty, particularly for the vane platform region. To this point, there have only been extremely limited comparisons between cylindrical and shaped holes on a turbine endwall at either low or high freestream turbulence conditions. This study presents film-cooling effectiveness measurements on an endwall surface in a large-scale, low-speed, two-passage, linear vane cascade. Results showed that film-cooling effectiveness decreased with increasing blowing rate for the cylindrical holes, indicating jet liftoff. However, the fan-shaped passage showed increased film-cooling effectiveness with increasing blowing ratio. Overall, fan-shaped holes increased film-cooling effectiveness by an average of 75% over cylindrical holes for constant cooling flow.

Grazing effects of blue mussel Mytilus edulis on the pelagic food web under different turbulence conditions

Benthic filter feeders have the potential to control phytoplankton biomass and size composition in shallow estuaries and fjords, while the in situ impact on the zooplankton community is so far unknown. In a companion study, the blue mussel Mytilus edulis was shown to graze on both microand mesozooplankton causing depletion above the mussel bed (Nielsen & Maar, Mar Ecol Prog Ser 339:185–198). However, the vertical zooplankton distribution varied between sampling days, and we thus examined the influence of turbulence on the escape capability of different zooplankton organisms. Two sampling days were selected for further analysis representing relatively low (e ≈ 10–7 m2 s–3) and high (e ≈ 10–6 m2 s–3) near-bed turbulence (estimated by a physical water column model, General Ocean Turbulence Model, GOTM). The grazing impact on the zooplankton was estimated by a random walk model taking zooplankton motility and escape behaviour into account as well as turbulence conditions. Protozooplankton and bivalve larvae were always captured efficiently by mussels irrespective of turbulence conditions, while the grazing impact on nauplii and copepodites was greatly reduced due to increased escape success in low turbulence conditions. Thus, the zooplankton community will be more diverse during low turbulence conditions and will compete with mussels for phytoplankton food. Seasonal importance of the different components of the pelagic food web for mussel consumption was evaluated by comparing the model results with plankton data from the studied fjord system. The analysis stresses the need, when evaluating the ecological role of mussels, to also consider the heterotrophic components of the food web.

Flow and heat transfer behaviors of phase change material slurries in a horizontal circular tube

The flow and convective heat transfer behaviors of microencapsulated phase change material (MPCM) slurries in a horizontal circular tube have been experimentally investigated. The slurry consisted of microencapsulated 1-bromohexadecane (C 16H 33Br) and water, with the mass fractions of MPCM varying from 5% to 27.6%. The pressure drop and local heat transfer coefficients were measured, and the influences of capsule fractions, heating rates, and flow structures on heat transfer performance were also studied. Heat transfer coefficients measured for MPCM slurry are significantly higher than for those for single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Moreover, a new simple heat transfer correlation equation was proposed that accurately predicts the local heat transfer coefficients of laminar MPCM slurry flow in a horizontal circular tube.

The Dinoflagellate Genera Brachidinium, Asterodinium, Microceratium and Karenia in the Open SE Pacific Ocean

Most of the species of the genus Gymnodinium Stein were described during the late 1800’s and early 1900’s. Very little progress has been made since then and the taxonomic system has therefore remained almost unchanged since the 19th century. Biecheler (1934) using a silver-impregnation method observed for the first time the apical groove in Gymnodinium. Takayama (1985) based on scanning electron microscopy (SEM) showed several types of apical grooves in unarmoured dinoflagellates. Daugbjerg et al. (2000) based on light microscopy, SEM, pigment composition and LSU rDNA sequences split Gymnodinium into four genera: Gymnodinium sensu stricto , Akashiwo G. Hansen et Moestrup, Karlodinium J. Larsen and Karenia G. Hansen et Moestrup. Since then, the description of new species of Karenia characterized by a short straight apical groove, with fucoxanthin and lacking peridinin, has proliferated. In some cases, a single species has been described under different names such as K. bicuneiformis Botes, Sym et Pitcher and K. bidigitata Haywood et Steidinger and other taxa may be conspecific such as K. longicanalis Yang, Hodgkiss et G. Hansen and K. umbella de Salas, Bolch et Hallegraeff (Yang et al. 2001; Botes et al. 2003; de Salas et al. 2004; Haywood et al. 2004). Among the recently described species, Karenia papilionacea Haywood et Steidinger showed several peculiar characteristics in cultures with an unusual plasticity and high size variability. In culture, K. papilionacea is also able to move forward the prominent apical process (Haywood et al. 2004, p. 170). Other close species, K. bicuneiformis, showed pointed or bulbaceous antapical tips in natural waters that were rounded when the species is cultured (Haywood et al. 2004, p. 173). The species of Karenia were described based on abundant materials cultured under optimal conditions for growth that does not reproduce the low turbulence and oligotrophic conditions of the open ocean. Consequently little is known on the morphology and the life cycle of species such as K. papilionacea under natural conditions. The dinoflagellate Brachidinium capitatum F.J.R. Taylor is a flattened unarmoured taxon easily identifiable by the four radiating elongate antapical extensions, apical process, numerous yellow-green chloroplasts and promiAlgae Volume 21(4): 445-452, 2006

Wind loads on curved roofs

Curved roofed buildings are increasingly used in the modern built environment because they offer aerodynamically efficient shapes and provide architects and designers with an alternative to regular rectangular building forms. However, there is little information available on the wind loads on these roof forms. The Eurocode for wind actions (EN1991-1-4) includes pressure coefficients for a limited range of aspect ratio cylindrical roofs from measurements in low-turbulence conditions but only for wind blowing normal to the eaves. There is some concern regarding the reliability of these data, consequently EN1991-1-4 allows National Choice (National Determined Parameter) for wind loads on these roofs. This paper describes a series of parametric wind tunnel studies undertaken at BRE to measure wind pressures on a wide range of curved roof models in a properly scaled atmospheric boundary layer simulation and gives an alternative to the EN1991-1-4 recommended procedure.

Flow over a system of several ribs under conditions of high free-stream turbulence

Results of an experimental hydrodynamic and heat-transfer study of the turbulent separated flow developing over a system of several cross-flow ribs are reported. Conditions with low and high free-stream turbulence are considered. Visualization and heat-transfer data are presented, and a comparison for two turbulence levels is given. In the system of three or more ribs, a very unstable flow in the second inter-rib cell was observed under low-turbulence conditions. Under a high level of free-stream turbulence, the flow in the first inter-rib cell is unstable; this observation is supported by measured pressure distributions. Addition of each next rib makes the separation flow region behind the last rib and the pressure recovery region less extended, and decreases the coordinate at which the rate of heat transfer attains its maximum. In the high-turbulent flow, the heat-transfer intensification in the second inter-rib cell amounts to 30 %.

The influence of animal density and water turbulence on growth and survival of cultured spiny lobster ( Jasus edwardsii) larvae

The survival and growth of early stage phyllosoma larvae are dependent on establishing suitable larval stocking densities, and controlling the velocity and type of water turbulence within a culture vessel. This study examined the effects of larval density and flow-induced turbulence on growth and survival of Jasus edwardsii phyllosoma in culture. In the first experiment, newly-hatched phyllosoma were stocked at six densities (5–160 larvae l − 1 ) into 1.6 l plastic beakers with water flow introduced at the container floor (0.5 cm above the beaker bottom and parallel to it) facilitating 2.5 exchanges h − 1 and fed 2.5 mm long Artemia supplied at 1.5 ml − 1 . Culture to Stage IV demonstrated that growth and survival were compromised at phyllosoma densities > 40 l − 1 . The second experiment examined the influence of flow-induced turbulence, with a combination of two water exchange rates (2.5 or 5 times h − 1 ), two inlet positions (2 cm above the water surface or 0.5 cm above the beaker bottom and parallel to it), and at two phyllosoma densities (10 and 40 l − 1 ). Larvae were significantly larger at the low density but only at the slower flow rate, while survival was highest at the low density with water introduced 2 cm from the water surface. These conditions allowed larvae to capture sufficient Artemia prey without the stresses associated with uncontrolled swimming. We recommend that larvae be cultured up to Stage IV at densities no higher than 40 l − 1 , under conditions of low turbulence, thus preventing excessive clumping of larvae, while maximising predator–prey ratios.

Volatile organic emissions from the distillation and pyrolysis of vegetation

Abstract. Leaf and woody plant tissue (Pinus ponderosa, Eucalyptus saligna, Quercus gambelli, Saccharum officinarum and Oriza sativa) were heated from 30 to 300°C and volatile organic compound (VOC) emissions were identified and quantified. Major VOC emissions were mostly oxygenated and included acetic acid, furylaldehyde, acetol, pyrazine, terpenes, 2,3-butadione, phenol and methanol, as well as smaller emissions of furan, acetone, acetaldehyde, acetonitrile and benzaldehyde. Total VOC emissions from distillation and pyrolysis were on the order of 10 gC/kgC dry weight of vegetation, as much as 33% and 44% of CO2 emissions (gC(VOC)/gC(CO2)) measured during the same experiments, in air and nitrogen atmospheres, respectively. The emissions are similar in identity and quantity to those from smoldering combustion of woody tissue and of different character than those evolved during flaming combustion. VOC emissions from the distillation of pools and endothermic pyrolysis under low turbulence conditions may produce flammable concentrations near leaves and may facilitate the propagation of wildfires. VOC emissions from charcoal production are also related to distillation and pyrolysis; the emissions of the highly reactive VOCs from production are as large as the carbon monoxide emissions.

Long-term tall tower carbon dioxide flux monitoring over an area of mixed vegetation

A methodology is described to determine the carbon budget of an extended region with mixed vegetation. The eddy covariance measuring system is operated at 82 m elevation on a tall radio/television transmitter tower above an area covered by agricultural fields and forest patches. The methodology ensuring the high quality of the calculated fluxes is described. Net ecosystem exchange (NEE) of CO 2 is determined as the sum of the eddy flux at 82 m and the rate of change of CO 2 storage below the measurement level. The gap filling technique used to patch the missing measurements is also presented. In contrast to several long-term NEE measurements low turbulent conditions does not seem to bias the annual sums of NEE at the site. Maximum daytime NEE reached −1.4 to −1.5 mg CO 2 m −2 s −1 (negative value indicates uptake) during the most active phase of the vegetation, while mean night-time respiration was around 0.1–0.3 mg CO 2 m −2 s −1. During the dormant season typical net exchange was around 1 g C m −2 day −1, while the vegetation adsorbed around 2–4 g C m −2 day −1 in the growing season. The overall dependence of the landscape-wide NEE on the environmental factors is described with non-linear regression functions. It is demonstrated that tall tower NEE measurements may provide repeatable, consistent estimates of the landscape-wide carbon exchange. The results may contribute to a better constraint on the “bottom-up” flux estimates. During the period of 1997–2004 (year 2000 is missing) the region mostly behaved as a weak net CO 2 sink on annual scale. Year-round NEE was in the range of −107 ± 48 and 69 ± 37 g C m −2 year −1. Climate data are presented to explain the interannual variability of NEE, gross primary production (GPP) and total ecosystem respiration ( R eco).

Large-Eddy Simulation of Transition in a Separation Bubble

In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low freestream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is refined to the extent that the simulation qualifies as a “coarse” direct numerical simulation. The simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.