Mesh Cylinder Research Articles

Designing acorn protection for direct seeding of quercus species in high predation areas

Aim of the study: a) To present the trial-and-error approach followed in the design and patent of a manufactured seed and seedling protector effective against mice and voles, rabbits, wild boar and deer (http://bopiweb.com/elemento/829172/). b) To assess the viability of direct oak seeding with and without protection in the complex acorn predation reality of post fire restoration and underplanting in existing pine afforestations.Study area: Northern Plateau of Spain, in an area of extreme acorn predation.Material and methods: We followed a classical trial-and-error approach for problem solving. Different modifications to a wire mesh screen cylinder were tested in subsequent trials aiming to evaluate the effects on acorn predation and early emerging plant survival and growth. The final protector is based on a thin wire mesh cylinder with three innovations: a truncated cone, a circular crown and a sphere compartment. Further we assessed the viability of direct oak seeding with and without protection in the complex acorn predation reality of post fire restoration and underplanting in existing pine afforestations.Main results: The manufactured seed protector was found to be effective against synergic attacks of mice, rodents, wild boars and herbivores. Survival of protected oak was 77% under canopies and 32% in open light conditions two years after sowing.Research highlights: Our results confirm the viability of direct oak seeding for woodland restoration if seed predation is controlled. Keywords: sowing; underplanting; trial-and-error; seed protector; afforestation; oak.

Flexible conformable hydrophobized surfaces for turbulent flow drag reduction.

In recent years extensive work has been focused onto using superhydrophobic surfaces for drag reduction applications. Superhydrophobic surfaces retain a gas layer, called a plastron, when submerged underwater in the Cassie-Baxter state with water in contact with the tops of surface roughness features. In this state the plastron allows slip to occur across the surface which results in a drag reduction. In this work we report flexible and relatively large area superhydrophobic surfaces produced using two different methods: Large roughness features were created by electrodeposition on copper meshes; Small roughness features were created by embedding carbon nanoparticles (soot) into Polydimethylsiloxane (PDMS). Both samples were made into cylinders with a diameter under 12 mm. To characterize the samples, scanning electron microscope (SEM) images and confocal microscope images were taken. The confocal microscope images were taken with each sample submerged in water to show the extent of the plastron. The hydrophobized electrodeposited copper mesh cylinders showed drag reductions of up to 32% when comparing the superhydrophobic state with a wetted out state. The soot covered cylinders achieved a 30% drag reduction when comparing the superhydrophobic state to a plain cylinder. These results were obtained for turbulent flows with Reynolds numbers 10,000 to 32,500.

采用内插金属丝网管强化竖直蒸发管传热研究

基于蒸发管内流型与传热的内在关联，本文采用200目、孔隙尺寸为75微米的不锈钢金属丝网管作为内插物，通过金属丝网管对流型的调控来影响管内流动沸腾的换热特性；对比研究了普通光管及具有不锈钢金属丝网管作为内插物的蒸发管的传热特性，揭示了泡状流、弹状流、混块状流及环状流这四类典型流型条件下金属丝网管对管内流动沸腾传热的影响机理，为内插金属网管强化沸腾换热技术的应用奠定了科学依据。 Based on the relationship between the flow pattern and heat transfer in an evaporative tube, a stainless steel mesh cylinder of 200 ppi with a pore diameter of 75 μm was used as the insert to regulate the flow patter and enhance the heat transfer of flow boiling in the tube. A comparative study on the evaporative heat transfer was conducted for the bare tube and the tube with the mesh cylinder insert, respectively. In the flow regimes of bubbly flow, slug flow, churn flow and churn/annular flow, the influential mechanisms of the mesh tube on the flow boiling heat transfer were disclosed. The results of this paper will provide a scientific basis for the application of flow boiling heat transfer enhancement by the mesh cylinder insert.

Effect of gravity levels on the flow pattern modulation by the phase separation concept

Suspending a mesh cylinder in a tube modulates flow patterns with gas bubbles in the annular region to form thin liquid film on the wall. We investigate the effect of gravity levels on the modulated flow patterns. The volume of fluid (VOF) method simulates the slug bubble train flow in bare tube and modulated flow sections. The bubble population density along the flow direction (β) and averaged liquid film thickness (δα) synthesize a parameter β/δa to characterize the enhancement of phase change heat transfer. It is found that at the normal gravity on earth, counter-flow appears with fast upward flow in the annular region and downward liquid flow in the core region. A lower β due to sparsely populated bubbles and thin liquid film form a larger β/δa to enhance the phase change heat transfer. Convective heat transfer in liquid plugs is enhanced by the fast fluid movement in the annular region and liquid circulations at three length scales. At the miniature gravity, quasi-co-current flow happens with upward flows in both annular region and core region, except that a liquid layer inside mesh cylinder flows downward. Bubbles are more densely populated than those at the normal gravity. Liquid circulation occurs only at the bubble length scale. At the micro gravity, the two-phases are thoroughly separated with co-current flows in both annular region and core region. Gas flows slowly and the residence time of gas is increased to result in β=1. Τhe liquid film is ultra-thin. These two factors create a significantly large β/δa to enhance the phase change heat transfer. We demonstrate the effectiveness of the phase separation concept at miniature and micro gravity environment.

Modulated flow pattern in a condenser tube with two-phase flow interacting with mesh screen surface at micro-gravity

Abstract Two-phase loop systems are important to maintain low temperature in a spacecraft at micro-gravity. It is known that the condenser size at micro-gravity can be one order magnitude larger than that at the earth gravity. This paper explores the effectiveness of the modulated flow technique at micro-gravity for the condensation heat transfer enhancement. A mesh cylinder is suspended in a tube, dividing the tube cross section into annular region and core region. When an intermittent two-phase flow interacts with the mesh screen structure, gas bubbles are prevented from entering the mesh cylinder thus they flow in the annular region to form the thin liquid film on the wall. Liquid can be separated to flow in the core region. Here the bubble dynamics in the bare tube section and modulated flow section are investigated for three different slug flow cases. It is found that the mesh cylinder modulates the slug flows in the bare tube section to form the elongated-ring-slug bubble train in the modulated flow section. Liquid plugs can disappear due to the continuous liquid flow from the annular region to the core region. Thus the two-phases are thoroughly separated with gas flowing in the annular region, forming ultra-thin liquid film thicknesses on the wall at micro-gravity. The modulated flow pattern forms the liquid film thicknesses to be about 1/30–1/10 of those in the bare tube section for the three cases. Based on the modulated bubble dynamics and parameters, the condensation rates per unit wall area ( m ) can be about 10–100 times of those in the bare tube section for the three cases. The modulated flow pattern and enhanced heat transfer are more obvious when the gas volume flow rates are decreased. This study provides a new approach to enhance the condensation heat transfer for space applications under micro-gravity environment.

Modulated heat transfer tube with mesh cylinder inserted

The concept of flow field modulation was proposed to enhance convective heat transfer in tubes. A mesh cylinder is suspended in the tube, dividing the cross section into an annular region and a core region. When the fluid flows over the mesh screen, a small portion of flow penetrates into the core region through mesh pores, while most of the fluid is within the annular region, leading to larger velocity gradient near the wall than that for the bare tube without mesh cylinder, this large velocity gradient can result in significant heat transfer enhancement. In this study experiments were performed at constant heat flux boundary conditions with water as the working fluid. Reynolds numbers vary from 2109 to 20,175, covering the transition and turbulent flow regimes. Experimental results showed that the heat transfer was enhanced over the whole length of tube, with the enhancement ratios from 1.21 to 1.84, and the largest enhancement ratio occurred in the transition flow regime. The mechanism for heat transfer enhancement could be attributed to: (1) The multi-scale characteristics of the heat transfer tube; (2) The modulated flow field with larger velocity and velocity gradient near the wall; and (3) The enhanced flow turbulence intensity.

The bubble leakage mechanism for vertical upflows by the phase separation concept

Recently, our research group proposed the phase separation condenser tube, in which a mesh cylinder was inserted to form the flow structure of “gas near the tube wall and liquid in the tube core”, significantly enhance the condensation heat transfer. But the bubble leakage towards the core region may worsen the heat transfer enhancement. In order to prevent the bubble leakage, the critical criterion was proposed based on the Young–Laplace equation, considering the inertia force, viscous force and pulsating flow. It was found that the critical criterion depends on the dimensionless parameter G *, the We number and a coefficient C. The numerical model was developed in terms of the volume of fluid method to predict the two-phase laminar flow in the phase separation condenser tube. The results show that the bubble leakage takes place at the bubble tip, which is agreed with the experimental observations. The critical curve distinguishing the non-bubble-breaking and bubble-breaking was obtained by comparing the bubble dynamics at different G* and We. The coefficient C was determined. The critical criterion for the bubble leakage is given as \( G^{*} \times We = \left( {0.22We^{0.99349} - \frac{{G^{*} }}{{4.7 \times 10^{3} \times We^{0.00651} }} + \frac{196.39}{{We^{0.00651} }}} \right) \times \frac{{4{ \cos }\alpha }}{\Delta \varTheta W} \), providing the design and operation guidance for the phase separation condenser tube.

The phase separation concept condensation heat transfer in horizontal tubes for low-grade energy utilization

This paper reports condensation heat transfer in a horizontal tube with a 14.81 mm inside diameter and a 1200 mm heat transfer length, by using the phase separation concept. The hollow mesh cylinder was formed by packaging two layers of mesh screen surface. The outer layer had the pore size of 76 μm. This invention constructs a multiscale condenser tube. The modulation of stratified flow and annular flow was focused on. Liquid was held within the mesh cylinder to increase the vapor covered tube wall surface to enhance the heat transfer for stratified flows. For annular flows, liquid droplets in the vapor core were captured by the mesh screen surface to increase the vapor void fractions near the tube wall, enhancing the hat transfer. The measurements showed that the heat transfer coefficients could be more than two times of those in the bare tube. The total thermal resistance was decreased by 45.6%, maximally. The heat transfer enhancement ratios were increased with the vapor mass fluxes, Gxin, for the condensation flow along the whole tube length. A pressure analysis explained the increased heat transfer trend. The increment of heat transfer enhancement ratios was suppressed with a liquid flow part in the tube.

Modulated flow patterns for vertical upflow by the phase separation concept

The passive phase separation concept was proposed to create thin liquid film on the wall surface. The inserted mesh cylinder made of a single layer of mesh pore surface divides the tube cross section into an annular region near the tube wall and a core region. Gas bubbles are prevented from entering the core region and liquids can be sucked towards the core region. Thus, the two-phases are majorly flowing in two different regions. The concept is expected to be used for multiphase (boiling/evaporation and condensation) heat transfer enhancement. Air–water two-phase flow experiments were performed with vertical upflows. Miniature bubbles are modulated to flow in the annular region. Cap/slug/plug bubbles are modulated to form the elongated-ring-slug bubble in the annular region to generate thin liquid films on the wall. For all the cases, the core region is full of liquid. Due to the large density difference between liquid in the core region and gas in the annular region, pulsating flow is self-sustained in the core region. The mesh pore surface promotes the mass and momentum exchange between the annular region and core region. For ring-slug bubbles such as longer than 10cm, miniature bubbles are emitted from the slug bubble front. The proposed concept is being verified by the phase change heat transfer experiment in our laboratory.

Numerical simulation of modulated heat transfer tube in laminar flow regime

The flow field modulation concept was proposed. A mesh cylinder was suspended in a tube, dividing the tube cross section into an annular region and a core region. The invention is called the modulated heat transfer tube (MHTT). The numerical simulation was performed in laminar flow regime at constant heat flux boundary condition with water as the working fluid. An equal flow area criterion was proposed for the conversion of 3D to 2D mesh pores. The non-uniform grids link micron scale of mesh pores with meter scale of tube. The results show double-peak velocity distribution over tube cross section. The near wall region has larger velocity and velocity gradient, accounting for the heat transfer enhancement mechanism. For any specific Reynolds number, there is a critical length beyond which heat transfer is deteriorated. Therefore, a set of short mesh cylinders was suspended in the tube. The configuration is called the improved modulated heat transfer tube (IMHTT). It is shown that the IMHTT ensures significant heat transfer enhancement over the whole tube length. This study provides a new enhancement mechanism and tube configuration for heat transfer, having a wide engineering application potential.

Numerical simulation of the modulated flow pattern for vertical upflows by the phase separation concept

The multiphase heat transfer could be enhanced by creating thin liquid film on the wall. The phase separation concept is called due to the separated flow paths of liquid and gas over the tube cross section to yield thin liquid film. Our proposed heat transfer tube consists of an annular region close to the wall and a core region, interfaced by a suspending mesh cylinder in the tube. The heat transfer tube is a multiscale system with micron scale of mesh pores, miniature scale of annular region and macroscale of tube diameter and length. Great effort has been made to link from micron scale to macroscale. The Volume of Fluid (VOF) method simulates air/water two-phase flow for vertical upflow. The three-dimensional system was successfully converted to a two-dimensional one by using three equivalent criteria for mesh pores. The non-uniform base grid generation and dynamic grid adaption method capture the bubble interface. The numerical results successfully reproduce our experimental results. The numerical findings identify the following mechanisms for the enhanced heat transfer: (a) counter-current flow exists with upward flow in the annular region and downward flow in the core region; (b) void fractions are exact zero in the core region and higher in the annular region; (c) the liquid film thicknesses are decreased to 1/6–1/3 of those in the bare tube section; (d) the gas–liquid mixture travels much faster in the annular region than in the bare tube; (e) three-levels of liquid circulation exists: meter-scale bulk liquid circulation, moderate-scale liquid circulation around a single-elongated-ring-slug-bubble, and microliquid circulation following the ring-slug-bubble tails. These liquid circulations promote the fluid mixing over the whole tube length and within the radial direction. The modulated parameters of void fractions, velocities and liquid film thicknesses in the annular region and three-levels of liquid circulation are greatly beneficial for the multiphase heat transfer enhancement.

Stratified two-phase flow pattern modulation in a horizontal tube by the mesh pore cylinder surface

Condensation heat transfer has been studied in the past century due to its wide applications in energy and power systems. The key scientific issue is the thick liquid thickness near the tube wall along the condenser tube length. The fabricated microstructures on the inner wall are the conventional technique to improve the performance. Here a passive phase separation concept was proposed to create distinct phase distribution. An empty cylinder made of a single layer of mesh pore surface was suspended in a tube, dividing the tube into an annular region and an inner region. The mesh pore surface prevents gas phase entering the inner region but sucks liquid towards the inner region. Thus largest possibility for gas directly contacted with the inner wall surface is ensured. An air/water two-phase flow experiment was performed and the stratified flow pattern modulation was investigated. When the liquid level in the horizontal tube is relatively higher, the liquid can be thoroughly within the mesh cylinder to form the “gas-floating-liquid” mode. The whole inner tube wall surface is covered by the gas phase. If the liquid content is relatively smaller, partial liquid can be sucked into the mesh cylinder. The contact area between the inner tube wall and gas is increased. The stratified flow pattern modulation is expected to significantly enhance the condensation heat transfer under low mass fluxes which is being verified by our continuous experiment.

Modeling the Uptake of Semivolatile Organic Compounds by Passive Air Samplers: Importance of Mass Transfer Processes within the Porous Sampling Media

Air sampling based on diffusion of target molecules from the atmospheric gas phase to passive sampling media (PSMs) is currently modeled using the two-film approach. Originally developed to describe chemical exchange between air and water, it assumes a uniform chemical distribution in the bulk phases on either side of the interfacial films. Although such an assumption may be satisfied when modeling uptake in PSMs in which chemicals have high mobility, its validity is questionable for PSMs such as polyurethane foam disks and XAD-resin packed mesh cylinders. Mass transfer of chemicals through the PSMs may be subject to a large resistance because of the low mass fraction of gas-phase chemicals in the pores, where diffusion occurs. Here we present a model that does not assume that chemicals distribute uniformly in the PSMs. It describes the sequential diffusion of vapors through a stagnant air-side boundary layer and the PSM pores, and the reversible sorption onto the PSM. Sensitivity analyses reveal the potential influence of the latter two processes on passive sampling rates (PSRs) unless the air-side boundary layer is assumed to be extremely thick (i.e., representative of negligible wind speeds). The model also reveals that the temperature dependence of PSRs, differences in PSRs between different compounds, and a two-stage uptake, all observed in field calibrations, can be attributed to those mass transfer processes within the PSM. The kinetics of chemical sorption to the PSM from the gas phase in the macro-pores is a knowledge gap that needs to be addressed before the model can be applied to specific compounds.

A bio-economic analysis of experimental selective devices in the Norway lobster (Nephrops norvegicus) fishery in the Bay of Biscay

Several fleets with various fishing strategies operate as a mixed fishery in the Bay of Biscay. Among the main fleets, bottom trawlers target Norway lobster (Nephrops norvegicus) and, together with gillnetters, they also catch hake (Merluccius merluccius). Trawling leads to average-size catches that are below the minimum landing size (MLS); such catches are discarded since they cannot be sold. These discards result in negative impacts on stock renewal, as most of them do not survive. This also results in an economic loss for both bottom trawlers and gillnetters since these discards represent a future loss of rent. This study, based on the 2009 and 2010 selectivity experiments at sea, assesses the short- and long-term bio-economic impacts of four experimental selective devices aimed at reducing N. norvegicus and M. merluccius discards over a 20-year simulation period. Tests were conducted at sea on a research trawler. Using the impact assessment model for fisheries management (IAM model), selectivity scenarios for trawlers in the Bay of Biscay were compared to a theoretical selective scenario of adopting an optimal device that catches only N. norvegicus and M. merluccius above MLS (9 cm and 27 cm total length, respectively). Costs and benefits were analyzed with the objective of finding the best compromise between a reduction in discards of undersized fish and a loss of valuable catches among the experimental devices. Selectivity scenarios show positive impacts on stocks but different economic impacts between fleets. The combination of a square mesh cylinder with a grid and square mesh panels gives the closest results to the theoretical scenario tested in terms of stock recovery and economic benefits. This experimental device leads to low economic losses in the short term and eventually to higher N. norvegicus yields, which would be favourable for fleets that greatly contribute to N. norvegicus fishing efforts.

Flow pattern modulation in a horizontal tube by the passive phase separation concept

A passive phase separation concept was proposed to modulate flow pattern in a condenser tube. An empty mesh cylinder is suspended in the condenser tube. The miniature mesh pores prevent gas bubble entering the mesh cylinder but capture liquid into the mesh cylinder, ensuring largest possibility for cold tube wall directly contacted with gas to form the perfect thin liquid film condensation heat transfer. We performed the air–water two-phase flow experiment. It was found that for a relatively higher liquid height in the horizontal tube, all liquid can be captured by the mesh cylinder to form the “gas-floating-liquid” mode. If the liquid height is small in the horizontal tube, partial liquid can be sucked by the mesh cylinder, the contact area between tube wall and gas is increased. When plug flow reaches the mesh cylinder surface, elongated saddle bubbles are formed in the annular region to envelop the mesh cylinder surface. When bubbly flow in the horizontal tube approaches the mesh cylinder area, miniature bubbles can merge to form large bubbles in the annular region. For the later two cases, all the gas flow rate is flowing in the annular region and the inside mesh cylinder is the liquid.

The Polyvinyl Alcohol Sponge Model Implantation

Wound healing is a complicated, multistep process involving many cell types, growth factors and compounds1-3. Because of this complexity, wound healing studies are most comprehensive when carried out in vivo. There are many in vivo models available to study acute wound healing, including incisional, excisional, dead space, and burns. Dead space models are artificial, porous implants which are used to study tissue formation and the effects of substances on the wound. Some of the commonly used dead space models include polyvinyl alcohol (PVA) sponges, steel wire mesh cylinders, expanded polytetrafluoroethylene (ePTFE) material, and the Cellstick1,2.Each dead space model has its own limitations based on its material's composition and implantation methods. The steel wire mesh cylinder model has a lag phase of infiltration after implantation and requires a long amount of time before granulation tissue formation begins1. Later stages of wound healing are best analyzed using the ePTFE model1,4. The Cellstick is a cellulose sponge inside a silicon tube model which is typically used for studying human surgery wounds and wound fluid2. The PVA sponge is limited to acute studies because with time it begins to provoke a foreign body response which causes a giant cell reaction in the animal5. Unlike other materials, PVA sponges are easy to insert and remove, made of inert and non-biodegradable materials and yet are soft enough to be sectioned for histological analysis2,5.In wound healing the PVA sponge is very useful for analyzing granulation tissue formation, collagen deposition, wound fluid composition, and the effects of substances on the healing process1,2,5. In addition to its use in studying a wide array of attributes of wound healing, the PVA sponge has also been used in many other types of studies. It has been utilized to investigate tumor angiogenesis, drug delivery and stem cell survival and engraftment1,2,6,7. With its great alterability, prior extensive use, and reproducible results, the PVA sponge is an ideal model for many studies1,2.Here, we will describe the preparation, implantation and retrieval of PVA sponge disks (Figure 1) in a mouse model of wound healing.

The Polyvinyl Alcohol Sponge Model Implantation

Wound healing is a complicated, multistep process involving many cell types, growth factors and compounds1-3. Because of this complexity, wound healing studies are most comprehensive when carried out in vivo. There are many in vivo models available to study acute wound healing, including incisional, excisional, dead space, and burns. Dead space models are artificial, porous implants which are used to study tissue formation and the effects of substances on the wound. Some of the commonly used dead space models include polyvinyl alcohol (PVA) sponges, steel wire mesh cylinders, expanded polytetrafluoroethylene (ePTFE) material, and the Cellstick1,2. Each dead space model has its own limitations based on its material's composition and implantation methods. The steel wire mesh cylinder model has a lag phase of infiltration after implantation and requires a long amount of time before granulation tissue formation begins1. Later stages of wound healing are best analyzed using the ePTFE model1,4. The Cellstick is a cellulose sponge inside a silicon tube model which is typically used for studying human surgery wounds and wound fluid2. The PVA sponge is limited to acute studies because with time it begins to provoke a foreign body response which causes a giant cell reaction in the animal5. Unlike other materials, PVA sponges are easy to insert and remove, made of inert and non-biodegradable materials and yet are soft enough to be sectioned for histological analysis2,5. In wound healing the PVA sponge is very useful for analyzing granulation tissue formation, collagen deposition, wound fluid composition, and the effects of substances on the healing process1,2,5. In addition to its use in studying a wide array of attributes of wound healing, the PVA sponge has also been used in many other types of studies. It has been utilized to investigate tumor angiogenesis, drug delivery and stem cell survival and engraftment1,2,6,7. With its great alterability, prior extensive use, and reproducible results, the PVA sponge is an ideal model for many studies1,2. Here, we will describe the preparation, implantation and retrieval of PVA sponge disks (Figure 1) in a mouse model of wound healing.

Comparison of Insect Vacuums for Sampling Asian Citrus Psyllid (Homoptera: Psyllidae) on Citrus Trees

Vacuum devices were compared for sampling the abundance of psyllids. One vacuum device was an AC rechargeable, handheld, cordless model, while a second was a handheld, DC model powered through a cord connected to a 12-volt vehicle battery. Each of the devices had a mesh cylinder (substituted for a dust bag) in which the insects were captured. The third device was a reversed leaf blower with a two-cycle gasoline engine, with the insects captured in a standard-sized aerial insect net. Each device had advantages and disadvantages over the others depending on the collecting situation. However, the gas-powered device captured the most psyllids. The handheld models provided ease in handling compared to the bulkier (and noisier) leaf blower. The DC-powered sampler was tethered to its power source, in this case, a vehicle that could access trees in a commercial grove, whereas, the AC cordless device may be more suitable for dooryard situations. The disadvantage of the cordless device was that its operating time of approximately 10 minutes was sufficient for sampling psyllids on only two trees before it had to be recharged for 16 hours. Because of greater air flow, the gas device captured greater numbers than did either of the smaller electrical devices. The sampling procedure consisted of vacuuming a target tree for 5 minutes. The mean numbers of psyllids captured per tree with the AC, DC, and leaf blower devices were respectively: 17.4 (n = 44), 33.0 (n = 72), and 96.8 (n = 47). All mean differences were statistically significant. To test the efficiency of the leaf blower, some trees were immediately re-sampled. The proportion of psyllids in the first sample compared to the resample was approximately 3:1.

Time and depth scales of fine sediment delivery into gravel stream beds: Constraints from fallout radionuclides on fine sediment residence time and delivery

Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t1/2=53days) and 210Pbex (t1/2=22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function.

Where’s the ‘reef’? A five year study of serpulid tube bioerosion in a Scottish sea loch

In the British Isles, aggregations of the tubicolous polychaete Serpula vermicularis L. occur only in a few localities in western Scotland and Ireland. In Loch Creran, Argyll, where written observations extend back to the 1880s, build-ups of skeletal debris constituting a true reef framework are notably absent. To investigate the taphonomic processes affecting the residence time of relict tube material, cleaned and weighed skeletal fragments were deployed on panels amongst living worm aggregations. Fragments were either open to the environment or enclosed in mesh cylinders to exclude grazing urchins. Panels were recovered and fragments re-weighed after 1, 2, 3 and 5 yr in situ. Caged fragments typically increased in dry weight over time, while most open fragments remained at steady-state or showed small weight decreases. Change in weight was correlated with the number of living tubeworms which had settled onto the skeletal fragments. Open fragments showed no consistent temporal trend in weight change, suggesting that urchin grazing was not a major bioerosive process over the 5 yr experimental timescale. Data from another locality where ser- pulid aggregations suffered mass mortality between 1984 and 1994 show that tube debris can persist for at least 15 yr in sea loch environments. S. vermicularis aggregations in sea lochs may be relatively transient features, appearing and disappearing over decadal timescales.