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Helmut Kobus

Bio: Helmut Kobus is an academic researcher from University of Stuttgart. The author has contributed to research in topics: Groundwater & Aeration. The author has an hindex of 13, co-authored 65 publications receiving 524 citations. Previous affiliations of Helmut Kobus include Karlsruhe Institute of Technology & University of Iowa.


Papers
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Journal ArticleDOI
TL;DR: In this paper, experimental and modeled wind-induced oscillations in Upper Lake Constance with an emphasis on a coherent understanding of the basin-scale internal dynamics in this example of a large and stratified lake.
Abstract: This paper describes experimental and modeled wind-induced oscillations in Upper Lake Constance with an emphasis on a coherent understanding of the basin-scale internal dynamics in this example of a large and stratified lake. Data were collected with eight Lake Diagnostic Systems (LDSs) consisting of thermistor chains and wind anemometers. The isotherm displacements as measured by the LDSs were interpreted using the three-dimensional hydrodynamic Estuary and Lake Computer Model (ELCOM). Three types of basin-scale waves were found to dominate the wave motion: the vertical mode-one Kelvin wave that had an observed period around 90 h, two vertical mode-one Poincare waves that had periods near 8 h and 12 h, and a vertical mode-two Poincare wave that had a period near 14 h. After strong westerly winds, upwelling of cold bottom water was observed east of the Sill of Mainau, where the lake's two subbasins connect. The width and length ratios of the subbasins, spatial variations of the wind field, and rotational effects over the lake are shown to play critical roles in the details of the upwelling structure. A sudden fall of the isotherms in Lake Uberlingen formed a surge. The reflection of the surge from the northwestern boundary induced a vertical mode-two response leading to an intrusion in the metalimnion that caused a three-layer velocity structure in the smaller subbasin.

103 citations

Journal ArticleDOI
TL;DR: In this paper, the flow field and erosion pattern of continuous and pulsating impinging jets has been studied experimentally and the results show how harmonic velocity pulsations at the nozzle affect both the mean and the turbulent flow field.
Abstract: The flow field and erosion pattern of continuous and pulsating impinging jets has been studied experimentally. The mean velocity field as well as turbulent and pulsation components, wall pressure and wall shear stress distributions were investigated in an axisymmetric air jet impinging upon a rough wall. The results show how harmonic velocity pulsations at the nozzle affect both the mean and the turbulent flow field. Corresponding erosion tests with a vertical water jet impinging upon a uniform sand bed exhibited under certain conditions a greatly enhanced erosion rate. The increase in erosion capacity due to pulsation seems to be due to both the changes in the mean flow field and to the greatly increased lateral correlation of the velocity field in the impinging region.

56 citations

DOI
01 Jan 1973
TL;DR: In this paper, an experimental investigation of erosion of a uniform sand bed by a vertical submerged jet showed that the momentum flux of the jet and the distance between nozzle and sediment determined the rate of scour.
Abstract: An experimental investigation of erosion of a uniform sand bed by a vertical submerged jet shows that the momentum flux of the jet and the distance between nozzle and sediment determines the rate of scour. Under fully turbulent conditions, the relative dimensions of the scour hole depend only upon the time parameter and the ratio between the fictitious axial jet velocity at the bed and the fall velocity of the sediment particles. There exist two distinct values of this ratio, at which the eroding action of the jet becomes particularly effective. At the same mean volume flux, the erosion rate can be more than doubled by pulsations of the jet velocity. For the relative distance between nozzle and bed investigated, the continuous jet has the least erosion capacity as compared to pulsating jets. L'etude experimentale de l'affouillement d'un lit de sable nature! par un jet submerge et vertical montre, que la quantite de mouvement et la distance entre la buse et le fond mobile determine la loi de croissance de l'affouillement. On peut constater que les dimensions relatives de l'affouillement dependent seulement du parametre de temps et de la vitesse fictive sur l'axe du jet au niveau du fond, par rapport a la vitesse de chute des grains. Pour deux differentes voleurs de la proportion de ces vitesses, il y a un affouillement extreme a cause du jet. Un jet pulsant possede plus que la double capacite d'erosion qu'un jet permanent avec le meme debit volume moyen. A la distance recherchee le jet permanent effectue le minimum de volume d'affouillement compare aux jets pulsants.

39 citations

BookDOI
01 Jan 1993
TL;DR: The VEGAS (Research facility for subsurface remediation) aims at optimization of existing and development of new, improved technologies for in-situ remediation and exploration techniques.
Abstract: Das Konzept VEGAS (Versuchseinrichtung zur Grundwasser- und Altlastensanierung) hat die Optimierung existierender und die Entwicklung neuer, verbesserter in-situ Sanierungsverfahren und Erkundungstechniken zum Ziel. In einer Versuchshalle mit mehreren grosdimensionalen Versuchsstanden werden Ausbreitungs- und Sanierungsexperimente mit umweltrelevanten Schadstoffen unter kontrollierten Bedingungen durchgefuhrt. Als Einrichtung der begleitenden Forschung zur Grundwasser- und Altlastensanierung ermoglicht es VEGAS, parallel zu Labor- bzw. Feldversuchen physikalische, chemische und mikrobiologische Sanierungsverfahren unter naturnahen Bedingungen (Verweilzeiten, heterogener Untergrund) wissenschaftlich zu untersuchen, Sanierungserfolge mit begrenztem Aufwand vorherzusagen und neue Behandlungsverfahren ohne Risiko einer Kontaminationsverlagerung praxisnah zu erproben. VEGAS liefert damit einen wesentlichen Beitrag zur Neu- und Weiterentwicklung qualifizierter Umwelttechnologien. Das vorliegende Mitteilungsheft enthalt im ersten Teil eine Beschreibung der Konzeption und der technischen Ausstattung von VEGAS sowie im zweiten den Programmrahmen fur Forschungsvorhaben in VEGAS. The concept VEGAS (research facility for subsurface remediation) aims at optimization of existing and development of new, improved technologies for in-situ remediation and exploration techniques. Spreading, transport and remediation experiments with environmental pollutants are conducted under controlled conditions in large-scale test flumes and artificial aquifers located in a hydraulic laboratory. Complementary to small-scale labaratory experiments and field studies, VEGAS allows scientific investigations on physical, chemical and microbiological technologies for subsurface remediation under experimental conditions close to nature (residence times, heterogeneity). The efficiency of remediation technologies can be checked with limited effort, and new technologies can be tested without risk of contamination migration. Therefore, VEGAS provides a significant contribution to the initiation and further development of qualified environmental technologies. This booklet includes a brief description of the concept and the technical equipment of the facility in its first part and the programmatic frame for research activities in VEGAS in its second part.

39 citations

Journal ArticleDOI
TL;DR: In this paper, the transformation of tetrachloroethene (PCE) to ethene (DCE) was attempted by inoculation with Dehalospirillum multivorans and that of DCE to eethene with a mixed culture.
Abstract: Microbial reductive dechlorination is a naturally occurring transformation process tetrachloroethene (PCE) undergoes in aquifers. It contributes significantly to natural attenuation of chlorinated solvents. Stimulation of this process has been considered as a method of enhanced bioremediation. Experiments on the stimulation of reductive dechlorination were carried out in a large-scale quasi-2D sandbox model. The transformation of PCE to cis-1,2-dichloroethene (DCE) was attempted by inoculation with Dehalospirillum multivorans and that of DCE to ethene with a mixed culture. Ethanol used as the electron donor was introduced into the inlet of the domain, whereas water loaded with PCE was injected into a well. Limitations due to insufficient mixing could not be observed as high-permeability lenses enhanced the transverse exchange of the compounds. Both reductive dechlorination and competitive microbial reactions led to the acidification of the domain. The artificial aquifer was buffered by the concurrent injection of sodium sulfide with the electron donor. Under these conditions bioaugmentation of Dehalospirillum multivorans was successful, whereas stable dechlorination of DCE could not be achieved.

36 citations


Cited by
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Journal ArticleDOI
TL;DR: Experimental observations and theoretical studies over the last 10 years or so have demonstrated that flow channeling or preferred flow paths is a common phenomenon in fractured rocks as discussed by the authors, and theoretical efforts to explain or model these observations are summarized.
Abstract: Experimental observations and theoretical studies over the last 10 years or so have demonstrated that flow channeling or preferred flow paths is a common phenomenon in fractured rocks. The reason it has come to the forefront of scientific investigation is the recent interest in predicting solute transport in geological media as part of safety assessment of geologic isolation of nuclear or toxic wastes. Solute transport is much more sensitive to medium heterogeneity than is temperature or pressure. In this paper, experimental observations of tracer transport over distances ranging from centimeters to hundreds of meters are reviewed, and theoretical efforts to explain or model these observations are summarized. Processes that may explain some of the experimental observations without the use of flow-channeling models are discussed. The paper concludes with a discussion of the implications of flow channeling on the practical problems related to contaminant transport in geologic systems.

358 citations

Journal ArticleDOI
TL;DR: An extensive survey of the results of possibly all available experimental and theoretical studies on gas entrainment by plunging liquid jets has been presented in this paper, where the following aspects of the phenomenon are thoroughly discussed: mechanisms, conditions, conditions of the onset of entrained gas, characteristics of the resulting bubble dispersion, and mass transfer.

310 citations

Book ChapterDOI
TL;DR: In this article, the authors discuss the problems of field-measuring equipment and the Pt electrode that often lead to spurious results in the field and suggest some suggestions for improvement are discussed.
Abstract: Reduction and oxidation measurements create important data for analysis of wet soils. These measurements are actually recordings of voltage (EH) over time between a reference electrode and a sensor electrode inserted into a soil. The sensor electrodes are usually made of platinum wire (Pt electrode). Hydric soils require a period of reduction, and these measures can provide the length of time that the reduction process is occurring. The voltage results from an exchange of electrons between a redox couple such as ferrous and ferric iron during the process of reduction and oxidation. In soils that have fluctuating wet and dry conditions, wide fluctuations in Eh occur. Micro site differences complicate these measurements in that anaerobes may be active and at 1‐cm away they are completely inactive. The ferrous–ferric iron couple usually dominates these measurements but other couple often contributes complicating the measurements and interpretations of the data. Reference electrodes are often fine for laboratory work but are not rugged enough for the field. In this chapter, suggestions for improvement are discussed. Field‐measuring equipment and the Pt electrode are also sensitive and subject to problems that often lead to spurious results in the field. These problems are discussed at length. Iron‐coated tubes and other methods of establishing redox conditions are relatively recent and are discussed, also.

255 citations

Journal ArticleDOI
TL;DR: In this paper, a steady bubble plume model is developed to describe a weak air (or oxygen) bubble injection system used for the restoration of deep stratified lakes, where the model is designed for two modes of operation, i.e., oxygenation and artificial mixing.
Abstract: A steady bubble plume model is developed to describe a weak air (or oxygen) bubble injection system used for the restoration of deep stratified lakes. Since the model is designed for two modes of operation, i.e., oxygenation and artificial mixing, gas exchange between water and bubbles has to be included. The integral model is based on the entrainment hypothesis and a variable buoyancy flux determined by the local plume properties and the ambient water column. Fluxes of eight properties are described by nonlinear differential equations which can be numerically integrated. In addition, five equations of state are used. The model leaves open two initial conditions, plume radius and plume velocity. Model calculations with real lake water profiles demonstrate the range of applicability for both modes of operation. The model agrees reasonably well with field data and with laboratory experiments conducted by various investigators.

245 citations

Journal ArticleDOI
TL;DR: A theoretical investigation of local scour downstream of grade control structures based on twodimensional jet diffusion and particle stability is experimentally verified in this article, where the particle stability of two-dimensional jet diffusion is evaluated.
Abstract: A theoretical investigation of local scour downstream of gradecontrol structures based on twodimensional jet diffusion and particle stability is experimentally verified Turbulent jet diffusion red

242 citations