D.C. Lehmann

Bio: D.C. Lehmann is an academic researcher from Karlsruhe Institute of Technology. The author has contributed to research in topics: Pollinator & Microbiome. The author has an hindex of 2, co-authored 2 publications receiving 234 citations. Previous affiliations of D.C. Lehmann include Delft University of Technology.

Exchange Processes between a River and Its Groyne Fields: Model Experiments

Abstract: The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that is formed at the river-groyne-field interface. The large eddies formed in the mixing layer are the major cause of this exchange. Second, under certain conditions, even larger eddies are shed from the upstream groyne tip. Distortions in the flow field caused by such intermittent structures cause a much larger exchange than that by the mixing layer alone. The occurrence of large shed eddies depends on the presence of a sufficiently large, stationary, secondary gyre located at the upstream corner of the groyne field. The overall exchange of matter could be characterized as a first-order process, in accordance with the dead-zone-theory. The corresponding exchange coefficients agreed reasonably well with the results of earlier experiments and the effective coefficients as found in experiments in real river flows.

Protocol for Initiating and Monitoring Bumble Bee Microcolonies with Bombus impatiens (Hymenoptera: Apidae).

Abstract: Populations of some bumble bee species are in decline, prompting the need to better understand bumble bee biology and for assessing the effects of environmental stressors on these important pollinators. Microcolonies have been successfully used for investigating a range of endpoints, including behavior, gut microbiome, nutrition, development, pathogens, and the effects of pesticide exposure on bumble bee health. Here, we present a step-by-step protocol for initiating, maintaining, and monitoring microcolonies with Bombus impatiens . This protocol has been successfully used in two pesticide exposure-effects studies and can be easily expanded to investigate other aspects of bumble bee biology.

Effects of Groyne Layout on the Flow in Groyne Fields: Laboratory Experiments

Abstract: This research is aimed at finding efficient alternative designs, in the physical, economical, and ecological sense, for the standard groynes as they are found in the large rivers of Europe In order to test the effects of various groyne shapes on the flow in a groyne field, experiments were performed in a physical model of a schematized river reach, geometrically scaled 1:40 Four different types of schematized groynes were tested, all arranged in an array of five identical groyne fields, ie, standard reference groynes, groynes with a head having a gentle slope and extending into the main channel, permeable groynes consisting of pile rows, and hybrid groynes consisting of a lowered impermeable groyne with a pile row on top Flow velocities were measured using particle tracking velocimetry The design of the experiment was such that the cross-sectional area blocked by the groyne was the same in all cases Depending on the groyne head shape and the extent of submergence variations in the intensity of vortex shedding and recirculation in the groyne field were observed The experimental data are used to understand the physical processes like vortex formation and detachment near the groyne head It is demonstrated that the turbulence properties near and downstream of the groyne can be manipulated by changing the permeability and slope of the groyne head It is also observed that for submerged conditions the flow becomes complex and locally dominated by three-dimensional effects, which will make it difficult to predict by applying depth average numerical models or by three-dimensional models with a coarse resolution in the vertical direction

Numerical Investigation of Flow Hydrodynamics in a Channel with a Series of Groynes

Abstract: The flow hydrodynamics in a straight open channel containing a multiple-embayment groyne field on one of its sides is investigated numerically using large eddy simulation. The vertical groynes are fully emerged. The mean flow depth in the groyne region is about half that of the main channel and the length and width of the embayments are much larger than the mean depth in the embayment region. The model is validated using mean velocity and turbulent fluctuations data collected at the free surface in a previous experimental study. It is found that despite the fact that the flow inside the main recirculation eddy in the embayments can be characterized as being quasi-two dimensional, the flow inside the mixing layer region between the embayments and the channel is strongly nonuniform over the depth. As this region controls the mass exchange processes between the groyne field and the main channel, a three-dimensional description of the flow in this area is essential. The large-scale eddies that populate the mixing layer can penetrate the embayment region over lateral distances of the order of the channel depth. These eddies advect with them channel fluid inside the embayment. Eventually, the channel fluid is mixed with the embayment fluid by the small scales. The other main mixing mechanism is due to the injection of patches of high-vorticity mixing-layer fluid near the tip of the downstream groyne and their subsequent convection in the form of a wall-attached jet-like flow into the embayment, first parallel to the downstream groyne face and then to the sidewall. It is shown that on average, most of the fluid leaves the embayment region via the top layer of the embayment-channel interface (upstream half) and enters the embayment region at levels situated around the middepth (practically over the whole length of the embayment) of the interface surface. This explains why the mass exchange coefficients are overestimated when predicted using methods that employ floating particles as a tracer. The instantaneous bed shear stress inside the cavity is found to peak close to the downstream groyne face of each embayment and to show a high variability around the mean values due to the interaction of the mixing layer eddies with the tip of the groynes and the formation of the jet-like flow parallel to the droyne face.

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process

Abstract: [1] The present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start (flat bed) of the scouring process. Large eddy simulation (LES) is performed at a relatively low channel Reynolds number (Re = 18,000), in the range where most flume studies with clear water scour conditions are conducted. Similar to these studies, the incoming flow is fully turbulent and contains realistic turbulence fluctuations. Visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process. It is found that the structure of the horseshoe vortex (HV) system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex. However, its intensity is the largest at vertical sections situated around the tip of the spur dike. It is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes. In one of them (zero-flow mode), the necklace vortex is closer to the spur dike and more compact, and the near-bed jet flow beneath it is weak. In the other one (back-flow mode), a strong near-bed jet flow convects the primary necklace vortex away from the spur dike, and its core is more elongated and less compact. This explains the large amplification (by about 1 order of magnitude compared to the surrounding turbulent flow) of the turbulent kinetic energy and pressure fluctuations inside the HV system in the region situated around the tip of the spur dike and the double-peak distribution of the turbulent kinetic energy. Past the spur dike, in the legs of the necklace vortex, the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike. It is found that the legs of the horseshoe vortices can interact, at times, with the vortex tubes shed in the detached shear layer (DSL) and with the tip of the spur dike. These events typically result in a significant change in the coherence of the HV system. The largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike, but high bed shear stress values are also observed beneath the upstream part of the DSL. The bed shear stress fluctuations around the local mean values can be very high, especially in the region situated beneath the upstream part of the DSL. At random times, some of the vortices shed in the DSL merge or interact with eddies from the recirculation region. This leads to an increase in their strength and to a large increase of the bed shear stress along their path.