Czech Academy of Sciences, Institute of Hydrology

An analytic model of free-surface flow over an erodible bed is developed and used to investigate the stability of the fluid-bed interface and the characteristics of the bed features. The model is based on the potential flow over a two-dimensional, moving, wavy bed with a sinusoidal profile of varying amplitude, and a sediment transport relation in which the transport rate is proportional to a power of the fluid velocity at the level of the bed. By assuming that the dominant wavelength is that for which the rate of amplitude growth is the greatest, expressions are obtained for the wavelength and velocity of the bed features. In addition, conditions for the occurrence of the different configurations, dunes, flat bed, and antidunes, are found from the model. The predicted wavelengths of antidunes and ranges of wavelengths of dunes, and the predicted conditions for change of bed configuration are found to be in good agreement with experimental data. Finally, brief consideration is given to the factors involved in determining the maximum heights of the bed features and surface waves.

In this paper, the key pathways and mechanisms through which soil moisture conditions affect future rainfall over the U.S. Midwest are investigated using a regional climate model. A series of numerical experiments are performed to identify these pathways using the drought of 1988 and flood of 1993 as representative events. The results suggest that the soil moisture–rainfall feedback is an important mechanism for hydrologic persistence during the late spring and summer over the midwestern United States. They indicate that the feedback between soil moisture and subsequent rainfall played a significant role in enhancing the persistence of the drought of 1988 and the flood of 1993. It is found that there is a pronounced asymmetry in the sensitivity of simulated rainfall to specified initial soil moisture. The asymmetry acts to favor a stronger soil moisture–rainfall feedback during drought conditions as opposed to flood conditions. Detailed analyses of the simulations indicate that the impact of soil moisture on both the energy and water budgets is crucial in determining the strength of the soil moisture–rainfall feedback. Anomalously high soil moisture tends to 1) increase the flux of high moist static energy air into the planetary boundary layer from the surface via an increase in net surface radiation, 2) reduce the planetary boundary layer height thus increasing the moist static energy per unit mass of air, and 3) reduce the amount of entrained air of low moist static energy from above the planetary boundary layer. Each of these effects are additive and combine to increase the moist static energy per unit mass of air in the planetary boundary layer. This increase results in an increase in the frequency and magnitude of convective rainfall events and a positive feedback between soil moisture and subsequent rainfall.

Abstract Laboratory flow visualization experiments, using glass beads as the porous medium, were conducted to study air sparging, an innovative technology for subsurface contaminant remediation. The purpose of these experiments was to observe how air flows through saturated porous media and to obtain a basic understanding of air plume formation and medium heterogeneity effects. The experiments indicate that air flow occurring in discrete, stable channels is the most probable flow behavior in medium to fine grained water saturated porous media and that medium heterogeneity plays an important role in the development of air channels. Several simulated scales of heterogeneities, from pore to field, have been studied. The results suggest that air channel formation is sensitive to the various scales of heterogeneities. Site‐specific hydrogeologic settings have to be carefully reviewed before air sparging is applied to remediate sites contaminated by volatile organic compounds.

Abstract A new online streaming video and multi‐media application called eTEACH, http:eTEACH.engr.wisc.edu was used to reform a large, lecture‐based computer science course for engineering majors. In‐class lectures were replaced with videotaped lectures and other materials that students viewed on the Internet on their own schedule, making it possible to use the live class periods for small, team problem‐solving sessions facilitated by the professors and a teaching assistant. By using the eTEACH application to transform course lectures into “homework” and free up the face‐to‐face class time for working on problems that were similar to homework assignments, the professors effectively reversed the lecture and homework paradigm of a typical large lecture course. A thorough course evaluation over two semesters showed that students who took the online lecture version of the course gave significantly higher ratings to all aspects of the course, including lecture usefulness, professor responsiveness, the course overall, and the instructor. Although a few students missed having the opportunity to ask questions during lectures, about two‐thirds of the 531 students surveyed felt it was easier to take notes and understand the lectures presented via eTEACH than it would have been while attending the same lecture live, and 78% of students appreciated the ability to view and review course lectures on their own schedule.

The proliferation of algal and cyanobacterial blooms globally has led to renewed interest in understanding the impact of cell populations on water treatment plant (WTP) performance and the resultant water quality, particularly the role of algogenic organic matter (AOM). This review discusses current knowledge on the composition of AOM, how it interferes with water treatment process technologies applied for the treatment of algal blooms, and how it affects water quality. Specifically, it was shown that AOM can lead to increased coagulant demand, increased propensity of membrane fouling, and contribute to disinfection by-products (DBPs). Identified knowledge gaps included a need for further research on the impact of cellular organic matter (COM) after an algal bloom collapses and the impact of interactions between AOM and natural organic matter (NOM).

Abstract. Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The “sapfluxnetr” R package – designed to access, visualize, and process SAPFLUXNET data – is available from CRAN.

The rate dependence of unsaturated hydraulic characteristics was analyzed using both steady state and transient flow analysis. One‐step and multistep outflow experiments, as well as quasi‐static experiments were performed on identical, disturbed samples of a sandy and a loamy soil to evaluate the influence of flow rate on the calculated retention and unsaturated hydraulic conductivity curves. For the sandy soil, a significant influence of the flow rate on both the retention and unsaturated hydraulic conductivity characteristic was observed. At a given matric potential, more water was retained with greater applied pneumatic pressures. Matric potential differences of 10 to 15 cm (for given saturation) and water content differences of up to 7% (for given potential) could be observed between the slowest and the fastest outflow experiments, predominantly at the beginning of drainage. The hydraulic conductivity also increased with increasing flow rate for higher saturations, while a lower hydraulic conductivity was observed near residual saturation for the higher flow rates. We observed a continuously increasing total water potential gradient in the sandy soil as it drained, especially for high‐pressure transient one‐step experiments. This indicates a significant deviation from static equilibrium, as obtained under static or steady‐state conditions. For the finer textured soil, these flow‐rate dependent regimes were not apparent. A number of physical processes can explain the observed phenomena. Water entrapment and pore blockage play a significant role for the high flow rates, as well as lack of air continuity in the sample during the wettest stages of the experiment.

Abstract The upward movement of water due to transpiration stops when soil water potential (Ψ s ) drops below leaf water potential (Ψ L ). Under these circumstances, water can move in any direction in the plant‐soil continuum through the passive conduits of roots and stems towards the lowest Ψ s . This is generally termed as hydraulic redistribution (HR), but the positioning and orientation of the driving water potential gradient may vary. Any experimental method that can measure bi‐directional and low flows in the sapwood of roots and stems will be suitable to detect HR. Using one approach for measuring sap flow (the heat field deformation technique, HFD) in several forest species and sites across Europe, we were able to provide evidence on different types of HR: vertical hydraulic redistribution (VHR), horizontal hydraulic redistribution (HHR), foliar uptake (FU) and tissue dehydration (TD). VHR is the vertical water movement through roots in response to water potential differences between deep and topsoil, either hydraulic lift or hydraulic descent. HHR is the lateral water movement through roots in response to horizontal water potential gradients, namely under localised irrigation. FU is the water movement from crown to soil through stems when the crown is wetted by foggy weather. TD is the downward movement of water in stems or roots from above‐ground tree tissues to soil under prolonged drought or frost. Results from direct sap flow measurements indicated the vectoral and widespread nature of HR, a phenomenon of paramount importance for overall physiology and ecohydrology. Copyright © 2010 John Wiley & Sons, Ltd.

Abstract This paper describes water-tunnel investigations into the mechanics of “fixed”-type cavitation and into the probable mechanism through which this type causes material damage. High-speed motion pictures were used to study the cavity mechanics, and indications of the damage pattern were obtained by measuring the pitting rate on soft aluminum test specimens. Information was obtained on the frequency and intensity of the damaging blows, the distribution of damage in relation to the area covered by the cavitation, and the variation of the intensity of cavitation with velocity.

The paper concerns the hydrodynamic turbulent motion in the lubricant layer. Proceeding from the Reynolds equations and introducing the approximations currently used in lubrication problems, owing to the lubricant film thickness, the general motion equations for turbulent lubrication are written. Using the Prandtl mixing length hypothesis, exact and approximate solutions are obtained for the velocity distribution into the lubricant layer. The results are discussed by pointing out the pressure gradient and the Reynolds number influence on the velocity distributions, as well as the differences with respect to the laminar flow. In order to obtain simple formulae, the exact dependence of the rate of flow on the pressure gradient into a dimensionless form is replaced by a linear relation, the slope of which depends on the Reynolds number. This approximation allows the obtainment of the pressure differential equation under a simple form. The pressure equation is integrated in case of journal bearings, by assuming a constant or a variable viscosity of the lubricant. The results are compared to the experimental data obtained by M. I. Smith and D. D. Fuller and the good qualitative agreement is pointed out.

Two hundred and eighty-four adults from the metropolitan New York area reported on their history of childhood sexual abuse (CSA), childhood physical abuse (CPA), and on the nature of their exposure to the terrorist attack on the World Trade Center. The respondents also completed the Impact of Events Scale-Revised (IES-R), the Attachment Style Questionnaire (ASQ) and the Dissociative Experiences Scale (DES). Those reporting histories of CSA and/or CPA were found to endorse more serious symptoms of PTSD, as did those who witnessed the terrorist attack live. The presence of secure attachments and dissociative symptoms were related significantly to both CSA and CPA, and to scores on the IES-R. Adult attachment and dissociation were found to mediate the relationship between childhood abuse and severity of PTSD.

Abstract This paper is a summary of the results of investigations of the drop in potential and boundary resistance in unsteady motion for cases of surface resistance caused by boundary shear stresses and cases of form-type resistance associated with the high shear and generation and diffusion of turbulence accompanying jet formation. These cases were obtained using uniform diameter conduits and orifices in conduits. From tests in the MIT unsteady flow water tunnel, the effects of accelerated and decelerated flows were studied. In the case of flow through a uniform tube, it was found that the boundary resistance at any instant during accelerated motion was slightly greater than the equivalent steady-state case while, for decelerated motion, it was slightly less. In the case of flow through orifices, it was found that the combined resistance of the orifice and the conduit was less during accelerated motion and more during decelerated motion than for the equivalent steady-state cases.

The ensemble-averaged characteristics of the turbulent near-wake flow around two side-by-side identical square cylinders at a Reynolds number ≈23100 have been studied using a two-component laser-Doppler velocimeter system. The work focuses on a single case with a gap/diameter ratio of 2, for which the resulting individual vortex streets are coupled so as to yield a flow predominantly symmetric about the line midway between the two cylinders. Data sorting or conditioning according to phase was performed with the aid of pressure signals taken from taps on a sidewall of each cylinder. The two-cylinder results are compared in detail to results from a previous study of the one-cylinder case. Vortex structures shed on the side towards the flow centreline, termed inner structures, are distinguished from those shed on the free-stream side, termed outer structures, and the differences between the features associated with the two different structures are examined. The circulation associated with outer structures evolves downstream in a manner similar to that observed in the one-cylinder case, but the circulation of the inner structures is found to decrease dramatically downstream. This not only gives support to previous theoretical predictions but also reconciles these with previously apparently conflicting experimental observations. Information regarding vortex structure motion and the relevant length and time scales is obtained. Differences between momentum and vorticity transport, particularly across the flow centreline are pointed out, and effective turbulent vorticity fluxes are defined. Similarities in local flow topologies in one- and two-cylinder cases are discussed, and the role of local velocity-gradient invariants and their relationship to critical points and turbulence statistics are examined.

Many studies have overlooked the role of pH in optimizing coagulation. Herein, the authors emphasize the importance of pH value in coagulation during the production of drinking water. We investigate the influence of pH value on the surface charges and forms of coagulants and impurities intended for removal. A methodology is suggested for optimizing key parameters for efficient coagulation – coagulant dosage and pH value. The study points out that various optimal pH ranges are required for coagulation of specific impurities and their mixtures. For natural organic matter of both humic and algogenic origin, acidic pH values are favourable for their removal through charge neutralization mechanism. Algal cells are effectively coagulated at slightly acidic to neutral pH values due to interactions with coagulant hydroxide precipitates. Inorganic particles are eliminated preferably at around neutral pH values. When mixtures of impurities are coagulated, mutual interaction between the impurities may impact dose of coagulant and also optimal pH ranges.

The stabilizing and cell-shape changing influence of a basic rotation upon thermally produced cellular circulations are developed in a first order, perturbation theory. The theoretical possibility of a special “overstable” oscillating mode in certain fluids under the influence of rotation is recognized. In this preliminary report, the theory obtained for ordinary convection is compared to results obtained in laboratory experiments with rotating cylinders of water, and the existence of the overstable oscillating mode is demonstrated experimentally. Several possible applications of the theoretical results to atmospheric convection are developed by means of numerical examples, including models that are consistent with the observed association of deep convection with cyclonic relative vorticity and the distribution of clouds in regularly spaced clear and cloudy areas.

The sedimentation caused by the high density of suspended particles used in magnetorheological fluids is a significant obstacle for their wider application. In the present paper, core–shell structured carbonyl iron–polyaniline particles in silicone oil were used as a magnetorheological suspension with enhanced dispersion stability. Bare carbonyl iron particles were suspended in silicone oil to create model magnetorheological suspensions of different loading. For a magnetorheological suspension of polyaniline-coated particles the results show a decrease in the base viscosity. Moreover, the polyaniline coating has a negligible influence on the MR properties under an external magnetic field B . The change in the viscoelastic properties of magnetorheological suspensions in the small-strain oscillatory shear flow as a function of the strain amplitude, the frequency and the magnetic flux density was also investigated.

Abstract The effect of pressure and temperature on the shear and elongational deformation rate–dependent viscosities has been experimentally investigated for several polymers (HDPE, LDPE, LLDPE, PP, PC, PMMA, and PS) on a capillary rheometer with a back‐pressure device. Pressure, β, and temperature, α, coefficients have been determined through simultaneous fitting of the shear and extensional viscosity data by the modified White‐Metzner model. The dependence of β and α on temperature and pressure, respectively, was investigated and it has been found that simple relationships exist between pressure and temperature sensitivity coefficients for individual polymers. Polym. Eng. Sci. 44:1328–1337, 2004. © 2004 Society of Plastics Engineers.

ABSTRACT The composition and metabolic activity of cysts of the marine dinoflagellate Scrippsiella trochoidea (Stein) Loeblich were examined during dormancy, quiescence, and germination. On a per cell basis, newly formed cysts contained an order of magnitude more carbohydrate but significantly less protein and chlorophyll a than did exponentially growing vegetative cells. Loss of lipid and carbohydrate from cysts during the initial dormancy period reflected a respiration rate estimated to be 10% of the respiratory activity in vegetative cells. Among older, quiescent cysts the calculated respiration rate decreased further to approximately 1.5% of the vegetative rate and appeared to proceed largely at the expense of carbohydrate reserves. These estimated rates of respiration were in good agreement with direct measurements of cyst oxygen consumption. The transfer of quiescent cysts to conditions permissive for germination resulted in a rapid increase in respiration rate, as evidenced by carbohydrate loss and O 2 consumption. The increased respiratory activity was followed by an increase in protein content and, later, by an increase in chlorophyll a content and photosynthetic capacity. Just prior to germination the P/R ratio became greater than 1, and the estimated chlorophyll‐specific photosynthetic activity reached 75% of the rate in vegetative cells. Complete restoration of photosynthetic and respiratory capacity apparently was not achieved until after excystment. These data confirm the common assumption that dinoflagellate cysts represent true “resting” cells, containing extensive energy reserves and displaying greatly reduced metabolic activity.

Water level fluctuations induced by tidal strains can be analyzed to estimate the elastic properties, porosity, and transmissivity of the surrounding aquifer material. We review underutilized methods for estimating aquifer properties from the confined response to earth tides. The earth tide analyses are applied to an open well penetrating a confined carbonate aquifer. The resulting range of elastic and hydraulic aquifer properties are in general agreement with that determined by other investigators for the area of the well. The analyses indicate that passive monitoring data from wells completed in sufficiently stiff, low porosity formations can provide useful information on the properties of the surrounding formation.

Slurry pipe transport has directed the efforts of researchers for decades, not only for the practical impact of this problem, but also for the challenges in understanding and modelling the complex phenomena involved. The increase in computer power and the diffusion of multipurpose codes based on Computational Fluid Dynamics (CFD) have opened up the opportunity to gather information on slurry pipe flows at the local level, in contrast with the traditional approaches of simplified theoretical modelling which are mainly based on a macroscopic description of the flow. This review paper discusses the potential of CFD for simulating slurry pipe flows. A comprehensive description of the modelling methods will be presented, followed by an overview of significant publications on the topic. However, the main focus will be the assessment of the potential and the challenges of the CFD approach, underlying the essential interplay between CFD simulations and experiments, discussing the main sources of uncertainty of CFD models, and evaluating existing models based on their interpretative or predictive capacity. This work aims at providing a solid ground for students, academics, and professional engineers dealing with slurry pipe transport, but it will also provide a methodological approach that goes beyond the specific application.