Elevated Turbulence Research Articles

Energy balance closure of eddy-covariance data: A multisite analysis for European FLUXNET stations

This paper presents a multi-site (>20) analysis of the relative and absolute energy balance (EB) closure at European FLUXNET sites, as a function of the stability parameter ξ, the friction velocity u *, thermally-induced turbulence, and the time of the day. A focus of the analysis is the magnitude of EB deficits for very unstable conditions. A univariate analysis of the relative EB deficit as function of ξ alone (both for individual sites and a synthesis for all sites), reveals that the relative EB deficit is larger for very unstable conditions ( ξ < −1.0) than for less unstable conditions (−0.02 > ξ ≥ −1.0). A bivariate analysis of the relative EB deficit as function of both ξ and u *, however, indicates that for situations with comparable u * the closure is better for very unstable conditions than for less unstable conditions. Our results suggest that the poorer closure for very unstable conditions identified from the univariate analysis is due to reduced u * under these conditions. In addition, we identify that the conditions characterized by smallest relative EB deficits (elevated overall turbulence, mostly during day time) correspond to cases with the largest absolute EB deficits. Thus, the total EB deficit at the sites is induced mostly under these conditions, which is particularly relevant for evapotranspiration estimates. Further, situations with the largest relative EB deficits are generally characterized by small absolute EB deficits. We also find that the relative EB deficit does generally not correspond to the regression line of absolute EB deficit with the net radiation because there is a (positive or negative) offset. This can be understood from theoretical considerations. Finally, we find that storage effects explain a considerable fraction of the large relative (but small absolute) nocturnal EB deficits, and only a limited fraction of the overall relative and absolute EB deficits.

On interaction of centrally-ignited, outwardly-propagating premixed flames with fully-developed isotropic turbulence at elevated pressure

A new apparatus for the study of turbulent premixed flames at atmospheric and elevated pressures is proposed. The apparatus includes a high-pressure fan-stirred cruciform burner, the inner chamber, which is resided in a relatively large pressure-absorbing safety chamber (the outer chamber). Both chambers are optically accessible, allowing direct visualization and measurement of flame and turbulence interactions. The inner burner applies the same turbulence generating mechanism as that previously reported by Shy et al. [10], capable of generating intense near-isotropic turbulence. The additional modification lies in its vertical vessel which has four sensitive pressure-releasing valves installed symmetrically, so that the pressure difference between the inner and outer chambers during explosion can be eliminated. Flame speed measurements for centrally-ignited, outwardly-propagating lean CH4–air flames at the equivalence ratio ϕ=0.8 under both quiescent and turbulent conditions are conducted over an initial pressure range of p=0.1–1MPa. It is found that, contrary to the popular scenario for laminar flames, the coupling influence of elevated pressure and turbulence significantly enhances turbulent flame speeds. Our experimental data show that the unstretched laminar burning velocities (SL) decrease with p−0.52, while turbulent burning velocities (ST) increase with p0.14 when a constant turbulent fluctuating velocity u′≈1.4m/s is applied. In terms of the power law relation proposed by Kobayashi and his co-workers, we found that ST/SL≈1.07[(u′/SL)(p/p0)]0.44 where p0 is the atmospheric pressure, showing a similar increasing trend but with much lower values of ST/SL to what they found in a Bunsen-type burner. It is suggested that ST∼p0.14 is attributed to further flame surface area increment induced by the enhancement of hydrodynamic instability due to the decrease of kinematic viscosity at elevated pressure.

Rapid dispersal of a hydrothermal plume by turbulent mixing

The water column imprint of the hydrothermal plume observed at the Nibelungen field (8°18 ′S 13°30 ′W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities K ρ were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of K ρ around 10 −3 m 2 s −1. The mixing close to the vent site was even more elevated, with an average of K ρ = 4 × 10 − 2 m 2 s − 1 . The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection.

Significance of Face Velocity Fluctuation in Relation to Laboratory Fume Hood Performance

In order to recognize the problems associated with the transport mechanism of containment during the ventilation process of a laboratory fume hood, a transparent, full scale chemical fume hood is constructed for experimental studies. Distributions of mean velocity and velocity fluctuation in the sash plane are measured using a thermal anemometer. Flow patterns and tracer-gas concentration leakages are respectively diagnosed via the laser-assisted flow visualization method and the EN 14175-3 test protocol. The magnitudes of measured velocity fluctuations exhibit a sharp peak along the perimeter of the sash opening. The results of flow visualization verify that the elevated turbulence fluctuations are induced by the boundary-layer separation when the flow passes over the edges of sash perimeter. The tracer gas experiment shows that the regions where high degree containment leakages detected are located along the perimeter of hood aperture. Eleven commercial hoods which are claimed with fine aerodynamic design are further tested for confirmation of these observations. The results show similar correlations. Conclusions thus are made that large-scale vortex structures occurring around the perimeters of hood aperture due to the boundary-layer separation could induce strong turbulence, and therefore enhance dispersion of the hood containment.

Strong transport and mixing of deep water through the Southwest Indian Ridge

TheIndianOceanharboursanimportantbutpoorlyunderstood part of the global meridional ocean overturning circulation, which transports heat to high latitudes 1 . Understanding heat exchange in the Indian Ocean requires knowledge of the magnitudes and locations of both meridional deep-water transport and mixing, but in particular the latter is poorly constrainedatpresent 2,3 .Herewepresentdetailedmeasurements of transport and mixing in the Atlantis II fracture zone in the Southwest Indian Ridge, one of the main conduits for equatorward-flowing deep water 4,5 . We observe a northward jet of deep and bottom water extending 1,000m vertically with a transport rate of 3 ! 10 6 m 3 s " 1 . Turbulent di! usivity within the jet was up to two orders of magnitude above typical deep ocean levels in our measurements. Our results quantify the flow through this narrow fracture zone to 20 to 30% of the total meridional overturning circulation in the Indian Ocean, and provide an example of elevated turbulence in a deep shearedflow that is not hydraulically controlled, in contrast to many other fracture zones 6‐9 . The lower limb of the Indian Ocean meridional overturning circulation (MOC) consists of northward-flowing deep and bottom water from the North Atlantic and Weddell Sea that ultimately returns south as lighter deep and intermediate waters before continuing on a global trajectory 1 . Estimates of the total rate of overturning integrated across the basin vary widely, ranging from 4Sv (ref. 10) to 27Sv (ref. 5), with rates from most recent studies clustering around 10‐12Sv (refs 2,11,12).

Mixing across the Arctic Ocean: Microstructure observations during the Beringia 2005 Expedition

Turbulent‐scale temperature and conductivity were measured during the pan‐arctic Beringia 2005 Expedition. The rates of dissipation of thermal variance and diapycnal diffusivities are calculated along a section from Alaska to the North Pole, across deep flat basins (Canada and Makarov Basins) and steep ridges (Alpha‐Mendeleev and Lomonosov Ridges). The mixing rates are observed to be small relative to lower latitudes but also remarkably non‐uniform. Relatively elevated turbulence is found over deep topography, confirming the dominant role of bottom‐generated internal waves. Measured patterns of mixing in the Arctic are also associated with other mechanisms, such as double‐diffusive structures and deep overflows. A better knowledge of the distribution of mixing is essential to understand the dynamics of the changing Arctic environment.

Barite and barium in sediments and coral skeletons around the hydrocarbon exploration drilling site in the Træna Deep, Norwegian Sea

Barite and barium concentrations in bottom sediments and coral skeletons from the vicinity of the hydrocarbon exploration well drilled in 1992–1993 in the Traena Deep, Norwegian Sea have been studied to assess the spreading of the drilling mud and related ecological effects on Lophelia petrusa coral reefs. Sand size barite crystals derived from the drilling mud and elevated Ba concentrations in surface (0–2 cm) sediments were found up to 4 km from the exploration drilling site. 210Pb-dating results on sediment cores indicate that Ba-rich surface intervals (0–2 cm) record ca. 20 years of sedimentation history, and connect Ba enrichment with exploration drilling. The geographic distribution of Ba contents in sediments allowed the reconstruction of the drilling mud dispersal pattern showing transport eastward from the drilling site, consistent with the prevailing current directions. The presence of relatively coarse-grained sediments and barite crystals trapped in coral polyps, ca. 500 m down current from the drilling site, reflects the elevated turbulence and sediment supply during the drilling activity. This elevated sediment dispersion likely placed a stress upon the coral reefs, but due to strong currents that effectively dilute episodic drilling waste and sediment discharges, the damage does not appear significant.

The flow across a street canyon of variable width—Part 1: Kinematic description

Measurements have been made of the scalar dispersion of smoke released from a two-dimensional slot in the wall perpendicular to a boundary layer flow and located parallel to and midway between two square obstacles placed on the wall. The Reynolds number of the boundary layer at the slot location without the obstacles in place is R θ ≈ 980 . Two optical systems with CCD cameras facing each other have been used to measure simultaneously the velocity and scalar concentration fields, respectively, with PIV and Mie scattering diffusion. In Part B of this paper the data will ultimately provide detailed information about the scalar fluxes for this environmentally relevant geometry. Here in Part A the results of the velocity field measurements in the streamwise plane will be reported for spacings between the obstacles of 1–10 obstacle heights. The mean flow measurements reveal the increasing complexity of the canyon flow with increasing obstacle spacing. A primary vortex, with negative spanwise vorticity, occurs within the canyon for all spacings and is driven by the flow above. The circulation region of this vortex extends above the level of the tops of the obstacles. For spacings of 2 h and greater, a secondary vortex with positive vorticity appears in the upstream corner of the canyon, and a tertiary vortex with negative spanwise vorticity first appears in the downstream corner of the canyon for an opening of 6 h . The spatial distribution of the level of turbulence within and around the canyon is indicated by the contours of two-dimensional turbulent kinetic energy, 1 2 ( u 2 ¯ + v 2 ¯ ) . The region of elevated turbulence in the shear layer created by the upstream obstacle penetrates deeper into the canyon with increasing canyon opening. For all openings, the Reynolds shear stress is negative above the canyon. The vertical extent of the high negative stress region increases as the canyon opening increases, and it also penetrates well within the canyon. This region of negative stress is due to the transport, by the primary canyon vortex, of low momentum fluid from deep within the canyon upward and high momentum fluid from above the canyon downward.

Experimental Evaluation of an Inlet Profile Generator for High-Pressure Turbine Tests

Improving the performance and durability of gas turbine aircraft engines depends highly on achieving a better understanding of the flow interactions between the combustor and turbine sections. The flow exiting the combustor is very complex and it is characterized primarily by elevated turbulence and large variations in temperature and pressure. The heat transfer and aerodynamic losses that occur in the turbine passages are driven primarily by these spatial variations. To better understand these effects, the goal of this work is to benchmark an adjustable turbine inlet profile generator for the Turbine Research Facility (TRF) at the Air Force Research Laboratory. The research objective was to experimentally evaluate the performance of the nonreacting simulator that was designed to provide representative combustor exit profiles to the inlet of the TRF turbine test section. This paper discusses the verification testing that was completed to benchmark the performance of the generator. Results are presented in the form of temperature and pressure profiles as well as turbulence intensity and length scale. This study shows how a single combustor geometry can produce significantly different flow and thermal field conditions entering the turbine. Engine designers should place emphasis on obtaining accurate knowledge of the flow distribution within the combustion chamber. Turbine inlet conditions with significantly different profile shapes can result in altered flow physics that can change local aerodynamics and heat transfer.

On the Mechanisms Affecting Fluidic Vectoring Using Suction

Suction was applied asymmetrically to the exhaust of a rectangular subsonic jet creating a pressure field capable of vectoring the primary flow at angles up to 15deg. The suction simultaneously creates low pressures near the jet exhaust and conditions capable of drawing a secondary flow along the jet shear layer in the direction opposite to the primary jet. This countercurrent shear layer is affected both by the magnitude of the suction source as well as the proximity of an adjacent surface onto which the pressure forces act to achieve vectoring. This confined countercurrent flow gives rise to elevated turbulence levels in the jet shear layer as well as considerable increases in the gradients of the turbulent stresses. The turbulent stresses are responsible for producing a pressure field conducive for vectoring the jet at considerably reduced levels of secondary mass flow than would be possible in their absence.

Turbulence and feeding behaviour affect the vertical distributions of Oithona similis and Microsetella norwegica

The small copepods Oithona similis and Microsetella norwegica are often numerically abundant and widely distributed, but the factors controlling their vertical distributions and role in carbon cycling are yet unknown. Here we examined the vertical distributions of copepods during spring and summer in the Skagerrak and during autumn in the North Sea with respect to different physiochemical factors including turbulent dissipation rate. The ambush feeder O. similis numeri- cally dominated the copepod community; they were located in the layers with high microzooplank- ton abundance. M. norwegica were dominant in the Skagerrak and were observed within or just below the pycnocline; they are assumed to feed on sinking detrital aggregates. Both copepods use remote detection of either hydromechanical (O. similis) or chemical signals (M. norwegica) generated by the prey and both species migrated to deeper depths in response to elevated surface turbulence. The potential effect of turbulence on both types of feeding is theoretically shown to be negative and we suggest a turbulent dissipation rate in the range 10 -7 to 10 -6 m 2 s -3 as a threshold triggering the observed avoidance responses.

On the Turbulence in the Upper Part of the Low-Level Jet: An Experimental and Numerical Study

The characteristics of low-level jets (LLJ) observed at the “Centro de Investigacion de la Baja Atmosfera” (CIBA) site in Spain are analysed, focussing on the turbulence generated in the upper part of the jet, a feature that is still to be thoroughly understood. During the Stable Boundary Layer Experiment in Spain (SABLES) 1998, captive balloon soundings were taken intensively, and their analyses have highlighted the main characteristics of the jet’s wind and temperature structure, leading to a composite profile. There are indications that the turbulence has a minimum at the level of the wind maximum, with elevated turbulence in a layer at a height between two and three times that of the LLJ maximum, but no direct measurements of turbulence were available at these heights. In September 2001, a 100-m tower at the same site was re-instrumented to give turbulence measurements up to 96.6 m above ground level. All occurrences of LLJ below this height between September 2002 and June 2003 have been selected and significant turbulence above the LLJ has been found. Simulations with a single-column turbulence kinetic energy model have been made in order to further investigate the generation of elevated turbulence. The results correlate well with the measurements, showing that in the layer above the LLJ, where there is significant shear and weakly stable stratification, conditions are conducive to the development of turbulence.

Vegetative and reproductive phenology of Zonaria tournefortii (Dictyotales, Phaeophyceae) in sublittoral populations off the Canary Islands

Sublittoral populations of Zonaria tournefortii off the Canary Islands were studied to monitor vegetative and reproductive phenology. Sterile individuals, sporophytes and gametophytes coexisted throughout the year, but significant differences were detected between seasons and life-history stages for most parameters tested. Density was maximal in autumn (3513 individuals m-2). Twenty-five percent of all individuals were fertile, with sporophytes at much higher densities than gametophytes in all seasons. The highest average individual thallus length occurred in winter, but the highest frequencies of long specimens and the greatest size inequalities were in summer. Fertile thalli were significantly longer than sterile thalli, however, sporophytes and gametophytes had similar average lengths. Populations exceeded 100% overall cover in all seasons (up to 552% in autumn), with highest cover occupied by fertile individuals. Overall biomass, produced mainly by sporophytes, was maximal in summer-autumn (981 g DW m-2). Results suggest that this perennial species exhibits a somewhat exceptional phenological pattern amongst dictyotaleans. Its phenology was regulated in most years by elevated water turbulence in early winter, which causes breakage or loss of whole individuals. Blade regeneration and recruitment from the remaining damaged individuals began in winter-spring, and continued until autumn. Individual growth, hierarchical structuring and maturation of populations occurred mainly in spring-summer. Optimal reproductive potential per individual occurred during summer, although the reproductive potential per unit area was similar among seasons.

Stable Boundary-Layer Scaling Regimes: The Sheba Data

Turbulent and mean meteorological data collected at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are analyzed to examine different regimes of the stable boundary layer (SBL). Eleven months of measurements during SHEBA cover a wide range of stability conditions, from the weakly unstable regime to very stable stratification. Scaling arguments and our analysis show that the SBL can be classified into four major regimes: (i) surface-layer scaling regime (weakly stable case), (ii) transition regime, (iii) turbulent Ekman layer, and (iv) intermittently turbulent Ekman layer (supercritical stable regime). These four regimes may be considered as the basic states of the traditional SBL. Sometimes these regimes, especially the last two, can be markedly perturbed by gravity waves, detached elevated turbulence (‘upside down SBL’), and inertial oscillations. Traditional Monin–Obukhov similarity theory works well in the weakly stable regime. In the transition regime, Businger–Dyer formulations work if scaling variables are re-defined in terms of local fluxes, although stability function estimates expressed in these terms include more scatter compared to the surface-layer scaling. As stability increases, the near-surface turbulence is affected by the turning effects of the Coriolis force (the turbulent Ekman layer). In this regime, the surface layer, where the turbulence is continuous, may be very shallow (< 5 m). Turbulent transfer near the critical Richardson number is characterized by small but still significant heat flux and negligible stress. The supercritical stable regime, where the Richardson number exceeds a critical value, is associated with collapsed turbulence and the strong influence of the earth’s rotation even near the surface. In the limit of very strong stability, the stress is no longer a primary scaling parameter.

Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf

Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf break, reveal a complex mixing environment. Depth- and time-averaged dissipation rates ( ε ¯ = 7.4 – 55.8 × 10 - 9 W kg - 1 ) and diapycnal diffusivities ( K ρ ¯ = 6.1 – 37.8 × 10 - 5 m 2 s - 1 ) were elevated above observations made over other continental shelves with no significant topography, but were below those influenced by topographic features. The close proximity of the shelf break/canyon rim, locally generated internal tides, and nonlinear internal waves all contributed to the elevated turbulence. The complex bathymetry associated with Monterey Submarine Canyon allowed an internal tide to be generated at depths greater than 1500 m, as well as at the shelf break. The observed velocity field was normally dominated by upward energy propagation from the local shelf break generated internal tide, but near low tide downward energy propagation from a surface reflection of the internal tide generated below 1500 m was observed. Turbulent dissipation rates were not well parameterized by either the open-ocean Gregg–Henyey model or the recently developed MacKinnon–Gregg shelf model. Like its application on the New England shelf, the MacKinnon–Gregg model had the correct functional dependence on shear and stratification (dissipation increasing with increasing shear and increasing stratification), however, the magnitude and range of values were too small. The most surprising finding was the presence of what we believe to be large, high-aspect-ratio, downslope-propagating nonlinear internal solitary-like waves of elevation. Upon reaching the canyon rim, these waves propagated into deep water and transformed into waves of depression. On the shelf south of the canyon, the waves of elevation accounted for 20% of the observed turbulent kinetic energy dissipation. Off the shelf, where the solitary-like waves changed to downward displacement, their average dissipation increased 10-fold to ε ¯ = 2.6 × 10 - 6 W kg - 1 , and accounted for nearly half the dissipation in the upper 150 m.

Influence of Wake Passing Frequency and Elevated Turbulence Intensity on Transition.

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section simulates the effects of unsteady wakes resulting from rotor-stator interaction In turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire thermal anemometry techniques are used to measure time-resolved and phase-averaged wall-normal profiles of streamwise velocity, turbulence intensity, and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 5 X 10 4 . This low Reynolds number would apply to small engines flying at high altitude

Effect of Elevated Free-Stream Turbulence on Transitional Flow Heat Transfer Over Dual-Scaled Rough Surfaces

The surface roughness over a serviced turbine airfoil is usually multiscaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low free-stream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2% to 6.0%. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading-edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 to 40 Ra=37-119 μm. Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30–40%-over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low free-stream turbulence cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

The Influence of Turbulence on Wake Dispersion and Blade Row Interaction in an Axial Compressor

The influence of free-stream turbulence on wake dispersion and boundary layer transition processes has been studied in a 1.5-stage axial compressor. An inlet grid was used to produce turbulence characteristics typical of an embedded stage in a multistage machine. The grid turbulence strongly enhanced the dispersion of inlet guide vane (IGV) wakes. This modified the interaction of IGV and rotor wakes, leading to a significant decrease in periodic unsteadiness experienced by the downstream stator. These observations have important implications for the prediction of clocking effects in multistage machines. Boundary layer transition characteristics on the outlet stator were studied with a surface hot-film array. Observations with grid turbulence were compared with those for the natural low turbulence inflow to the machine. The transition behavior under low turbulence inflow conditions with the stator blade element immersed in the dispersed IGV wakes closely resembled the behavior with elevated grid turbulence. It is concluded that with appropriate alignment, the blade element behavior in a 1.5-stage axial machine can reliably indicate the blade element behavior of an embedded row in a multistage machine.

The Antarctic Planet Interferometer

AbstractThe Antarctic Planet Interferometer (API) is a concept for an infrared interferometer located at the best accessible site on Earth. Infrared interferometry is strongly effected by both the strength and vertical distribution of thermal and water vapor turbulence. The combination of low temperature, low wind speed, low elevation turbulence, and low precipitable water vapor make the Concordia base at Antarctic Dome C the best accessible site on Earth for infrared interferometry. The improvements in interferometer sensitivity with respect to other terrestrial sites are dramatic; an interferometer with two meter class telescopes could make unique infrared measurements of extra solar planets that might otherwise only be possible with a space-based interferometer.

Steady Streamwise Structures in the Boundary Layer on a Swept Wing with Elevated Turbulence of the Incoming Flow

Results for a turbulized flow past the windward side of a swept wing model are presented. Origination of steady disturbances in the form of streamwise structures is found. The greatest effect on the formation of these disturbances is exerted by the curvature of the external flow streamlines. The secondary flow in the boundary layer leads to an increase in the characteristic scale of disturbances in the transverse direction, as compared to the flow around the model at a zero yaw angle.