This study presents a novel high-resolution simulation of free-surface flow and tracer retention over a streambed with ripples based on varying ripple morphologies, surface hydraulics and the transport of a tracer pulse from surface water to surface dead zone. For the simulations, the computational fluid dynamics (CFD) model OpenFOAM was used to solve the threedimensional Navier-Stokes equations in combination with an implemented transport equation. Pressure gradients at the streambed were used to account for hyporheic exchange, assuming water flow from high pressure zones to low pressure zones. Flow velocities, ripple sizes and spacing showed to significantly affect these pressure gradients, but also the transport of a passive tracer at the streambed, which was not investigated so far. Due to the velocity field, large parts of the tracer mass were transported alongside the main stream above the ripples. Tracer mass reaching the space between the ripples was temporarily retained due to low velocities and recirculations. It was shown that the retention is depending on the ripple size and space between the ripples as well as on the flow velocity. Decreasing ripple sizes and higher flow velocities lead to a smaller tracer retention. Furthermore we showed that the ripple length to height ratio controls the generation of recirculation zones which affect the residence time of the tracer significantly. Ripple spacing leads to temporarily higher tracer concentration at the streambed, but smaller tracer retention. We conclude that the impact of the streambed morphology on the hydraulics in combination with tracer retention should be addressed for a comprehensive understanding of compound movement, exchange and transformation within the hyporheic zone.
Suction Buckets sind etablierte Gründungselemente für Offshore‐Öl‐ und ‐Gasplattformen, die bisher in der Offshore‐Windindustrie nur vereinzelt eingesetzt wurden. Diese Fundamente können zusammen mit der Unterstruktur installiert werden, wodurch zusätzliche Offshore‐Operationen wie das Grouting von Transition Pieces oder von Jacket‐Beinen vermieden werden. Außerdem werden bei der Installation von Suction Buckets, im Gegensatz zu Pfahlgründungen, minimale akustische Emissionen erzeugt, sodass zusätzliche schallreduzierende Maßnahmen überflüssig sind. Aus diesen Gründen gelten Suction Buckets als eine praktikable und potenziell kostengünstige Gründungslösung für Offshore‐Windenergieanlagen (OWEA). Das Forschungsvorhaben ProBucket wurde 2020 initiiert und wird vom Bundesministerium für Wirtschaft und Klimaschutz (BMWK) gefördert. Durch die geplanten vielfältigen physikalischen und numerischen Untersuchungen strebt das Vorhaben an, die Unsicherheiten der existierenden Bemessungsverfahren zu verringern. In diesem Beitrag werden die aktuellen Herausforderungen zum Design und zur wirtschaftlichen Umsetzung von Suction Buckets für OWEA erläutert. Darüber hinaus werden die Forschungsaktivitäten von ProBucket beschrieben.
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