Benjamin Hohermuth

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Hohermuth

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Benjamin

ORCID

0000-0001-8218-0444

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Publications 1 - 10 of 40

Repercussions of microplastics on the sea surface
Bergfreund, Jotam; Wobill, Ciatta; Evers, Frank; et al. (2022)
Preliminary Analysis on the Effect of Tunnel Profile Transitions on Air-demand and Flow Patterns of Low-level Outlets
Pagliara, Simone; Felder, Stefan; Hohermuth, Benjamin; et al. (2024)
Proceedings of the 10th International Symposium on Hydraulic Structures (ISHS 2024)
Reservoir dams are decisive for energy supply and water resources management, thus playing a crucial role in modern society and economy. Low-level outlets (LLOs) are important safety structures of dams, aiming at regulating the water level in the reservoir, and at allowing a fast drawdown in case of maintenance or emergency situations. A typical LLO consists of a pressurized inflow controlled by a gate and a free-surface outlet tunnel. The transition from pressurized to supercritical free-surface flow is responsible for the generation of a high-speed water jet, resulting in significant turbulence levels, air entrainment, and air transport along the tunnel. Sub-atmospheric pressures typically develop downstream of the gate, potentially triggering significant problems such as gate vibration and cavitation. A sufficient flow aeration is crucial to mitigate these issues, and an appropriate air supply system should be designed downstream of the gate. Several empirical design equations have been developed in literature to predict the relative air demand (i.e., the ratio of air discharge through the vent and water discharge in the outlet tunnel) of LLOs, accounting for the effects of flow patterns, air vent loss coefficient, tunnel slope, length, and roughness. Nevertheless, the comparison against prototype data showed a significantly larger air demand compared to the model-based equations mainly due to geometrical differences, resulting in non-negligible effects of tunnel roughness, profile transitions, and scale. In this regard, there has been no systematic study investigating the effect of tunnel profile transition on air demand and flow patterns. To narrow this gap, large-scale physical model tests were carried out to investigate the effects of an abrupt and a gradual linear tunnel profile transition on the LLO performance, for various combinations of gate opening, energy head at the gate and air vent properties. The two tunnel profile transitions resemble those commonly found in real-world prototypes. Preliminary observations showed that the profile transitions significantly affect the flow patterns compared to tunnels featuring no transitions, resulting in more complex and larger shockwave formation downstream of the gate. This also leads to an overall increase in the air demand for similar inflow conditions. This study provides preliminary recommendations for a safe design of LLO tunnels featuring gradual and abrupt profile transitions, contributing to a safer design of such structures.
Photovoltaik und Wasserkraftspeicher in der Schweiz-Synergien und Potenzial
Maddalena, Gioele; Hohermuth, Benjamin; Evers, Frederic M.; et al. (2022)
Wasser Energie Luft
In dieser Arbeit wurde das Potenzial von schwimmenden Photovoltaikanlagen (floating PV, FPV) und von an Talsperren montierten Photovoltaikanlagen (dam mounted PV, DMPV) an 23 Schweizer Speicherseen respektive Talsperren untersucht. Jeder Standort wurde mit einer Bewertungsmatrix anhand der Hauptkriterien «Akzeptanz», «Energie und Potenzial» sowie «Wirtschaftlichkeit» bewertet. Für den Bereich «Energie und Potenzial» wurde eine GIS-Analyse zur Bestimmung der geeigneten Fläche unter Berücksichtigung von Beschattung durchgeführt und die erwartete Energieproduktion wurde mithilfe des Tools SUNWELL berechnet. Die Anwendung der Bewertungsmatrizen führte zu separaten Ranglisten der Standorte für FPV und DMPV. Nach Ausschluss der am wenigsten geeigneten Standorte beträgt die geschätzte mögliche Gesamtstromproduktion 350 bis 450GWh/a für FPV und 11,5 bis 14,5GWh/a für DMPV. Ein Vergleich mit bereits realisierten oder geplanten Anlagen in der Schweiz zeigt eine gute Übereinstimmung mit den Ergebnissen gemäss der entwickelten Bewertungsmethodik. Die Ermittlung der möglichen Flächen ist zuverlässig, die Stromproduktion wird jedoch tendenziell leicht unterschätzt. Das erwartete Produktionspotenzial für die untersuchten 23 Standorte ist verglichen mit der gemäss Energieperspektiven 2050+ angestrebten Photovoltaikproduktion von 34TWh/a eher gering, im Vergleich zur aktuellen Produktion von 2TWh/a jedoch erheblich. Zudem liefern PV-Anlagen im Gebirge bis zu 50 Prozent der Jahresproduktion im Winter und die Installation an bestehender Infrastruktur schont natürliche Ressourcen. Eine weitere Abklärung der Machbarkeit für die in dieser Studie identifizierten Standorte sowie die Abschätzung des gesamtschweizerischen Potenzials scheint darum lohnenswert.
Effect of invert roughness on smooth spillway chute flow
Bürgler, Matthias; Vetsch, David F.; Boes, Robert; et al. (2023)
Role of Dams and Reservoirs in a Successful Energy Transition
Spillway chutes are appurtenant dam outlet structures with the purpose to safely convey large discharges during extreme flood events. During such events, hydraulics plays a major role in the safety of the structure. Along a spillway chute, water is accelerated by gravity and may reach flow velocities in the order of 10 to 50 m/s, implying a considerable cavitation risk. On the spillway invert, turbulence is generated by shear stresses and surface roughness, which results in self-aeration of the flow once the turbulent boundary layer interacts with the free surface. For reliable design guidelines of spillways, knowledge of air concentrations along the spillway chute is essential, as entrained air concentrations can mitigate the risk of cavitation at the expense of risking overtopping of the chute walls due to flow bulking, or further accelerating the flow due to drag reduction. While it is well known that the invert roughness is the controlling parameter for boundary layer development and the self-aeration process (for a given slope and discharge), the quantitative understanding of roughness effects on air-water flow properties is still limited by the availability of data sets that target this variable. In this research, the effects of invert roughness on smooth spillway chute flow are investigated in a large-scale physical model. The investigated flow properties include the clear water and air-water mixture flow depths, depth-averaged flow velocities, air concentrations, and friction factors. Based on the experimental data, we demonstrate that the streamwise development of depth-averaged air concentration is significantly affected by invert roughness, which in turn also affects the bottom air concentration downstream of the inception point. Further, we found that friction factors are significantly affected by the relative boundary layer thickness in the developing non-aerated flow region, but also by bottom air concentrations in the aerated flow region. Good agreement between experimentally determined friction factors and established theoretical relations was found. Overall, our findings contribute to a qualitative description of invert roughness effects on air-water flow properties for a robust design of spillways, thus contributing to safer dam infrastructure.
Effects of Wall Roughness on Air-water Flow Properties of Low-level Outlets
Pagliara, Simone; Hohermuth, Benjamin; Felder, Stefan; et al. (2023)
Proceedings of the 40th IAHR World Congress
Reservoir dams serve many purposes, including hydropower production, climate change mitigation, and flood control. Low-level outlets (LLOs) are key safety structures of high-head dams. Their main purpose is to control the water level in the reservoir during flood events or maintenance works, and to support a rapid drawdown of the reservoir in case of emergency situations. Alternative purposes include controlling the first impounding of the reservoir, sediment flushing, turbidity current venting, and releasing of environmental flow to preserve downstream river health. In the free-surface tunnel, flow velocities can reach values of several dozen meters per second, leading to significant air transport and sub-atmospheric pressures which may induce or aggravate problems with cavitation and gate vibration. Sufficient flow aeration is crucial to mitigate these issues and to achieve a safe and reliable operation of LLOs. Despite recent new insights on the influence of hydraulic and geometric tunnel parameters on air-water flow properties in LLOs, the effect of wall roughness on their performances is still unclear. Considering that wall roughness and Reynolds number are often the key differences between scale models and prototype structures, novel physical model tests were performed to investigate the influence of tunnel wall roughness on key air-water flow properties including void fraction and interfacial velocity. Different combinations of gate opening and energy head at the gate were tested, corresponding to Froude and Reynolds numbers at the vena contracta up to 46 and 8·105, respectively. Two wall roughness configurations representing unlined rock and finished concrete were tested, and measurements were taken by means of a dual-tip phase-detection conductivity probe. Results indicate that the tunnel roughness has a significant effect on the air-water flow properties, leading to larger flow depths associated with higher void fractions and smaller interfacial velocities, while the turbulence levels increased. This study contributes to a safer design of LLO tunnels.
Schwall-Sunk-Sanierung mit hybriden Ausgleichsbecken-Batterie-Systemen
Höfkes, Gereon; Evers, Frederic M.; Hohermuth, Benjamin; et al. (2022)
Wasser Energie Luft
Speicherwasserkraftwerke sind in der Lage, flexibel Strom zu erzeugen, um einen schwankenden Strombedarf zu decken. Dies führt jedoch zu stark intermittierenden Produktionsmustern mit einem schnellen An- und Herunterfahren der hydraulischen Maschinen. Bereits heute spielen Speicherkraftwerke eine zentrale Rolle für die Versorgungssicherheit und die Anforderungen an ihre Flexibilität werden durch Zubau von neuen erneuerbaren Energien tendenziell zunehmen. Der Flexibilität sind einerseits jedoch technische Grenzen gesetzt, da häufige, schnelle Lastwechsel die Turbinen stärker beanspruchen können. Andererseits erfordert das Gewässerschutzgesetz eine Dämpfung künstlicher Abflussschwankungen (Schwall-Sunk), um bei der Rückgabe in den Vorfluter nachteilige Auswirkungen auf aquatische Ökosysteme zu limitieren. Eine mögliche Alternative ist, die Lastwechsel durch stationäre Grossbatterien zu glätten und so Speicherkraftwerke weiter zu flexibilisieren. Die vorgestellte Abschätzungs methode ermöglicht eine Vordimensionierung von Gross batterien als Ergänzung zu einer Schwall-Sunk-Sanierung mittels Ausgleichsbecken. Neben technischen werden dabei auch wirtschaftliche Aspekte berücksichtigt. Anhand dreier Fallbeispiele wird die Abschätzungsmethodik auf bestehende Kraftwerkanlagen angewendet.
Effect of discharge variations and sediment supply on the stability of artificial step-pool sequences
Maager, Fiona; Hohermuth, Benjamin; Weitbrecht, Volker; et al. (2022)
Proceedings of the 39th IAHR World Congress
Built step-pool systems are promising to stabilize the bed in steep streams. Flume experiments have already been conducted to assess scour development and stability of such step-pool sequences. However, mainly stationary conditions were tested so far, which hardly represent the processes in steep streams. In this study, three distinct flow loading conditions (LC) were applied to the same step-pool sequence to determine potential differences in scour development and step stability. The 1:20 scaled physical experiments were conducted in a flume with a bed slope of 8%. The base material was selected according to typical Swiss mountain streams with dm = 0.005 m and d₉₀ = 0.0126 m (model scale). Six equally spaced steps were artificially placed into the channel bed. The steps consisted of two rows of blocks with a weight between 1.1 and 1.4 kg and a height of ~0.065 m each. First, a stationary hydraulic load was applied to the step-pool sequence for 80 min (LC A). The unit discharge was stepwise increased by Δq = 0.014 m²/s until the steps collapsed. Second, a series of hydrographs was applied to represent the processes occurring in steep streams more realistically (LC B). Similarly to LC A, the peak discharge of each hydrograph was increased by Δq = 0.014 m²/s until the steps collapsed. The total duration of the hydrographs was set to 54 min, the peak arrived after 13.5 min and lasted for 4.5 min. Due to the shorter peak duration and the descending limb of the hydrographs in LC B, differences in scour depth and stability were expected. LC C was identical to LC B, but a small stationary discharge was applied in between each hydrograph to simulate low flow periods occurring between major flood events. An increase in scour depth was expected in LC C, because the jet of low flows impinges more vertically on the channel bed. The step-pool system sustained a discharge of q = 0.110 m²/s for LC A and C, and q = 0.124 m²/s for LC B. Considering the uncertainties, the step-pool system failed at similar hydraulic loads independently of the LC applied. Major changes in channel bed appeared to occur within 5 to 10 min after the peak discharge was reached. Furthermore, all LC led to a similar scour depth. Even for periods of low flow in LC C, scour depths did not further increase at the toe of the steps. Consequently, applying either one of the LC appears to be reasonable to test step stability. However, all three LC were conducted under clear water conditions and the effect of sediment feed on the stability was neglected. Scour development of step-pool systems with sediment feed will be investigated in future experiments.
Air-Water Flow Patterns and Shockwave Formation in Low-Level Outlets
Pagliara, Simone; Hohermuth, Benjamin; Boes, Robert (2023)
Journal of Hydraulic Engineering
Reservoir dams play a significant role in society and the economy. Low-level outlets (LLOs) are key safety devices providing reservoir drawdown for maintenance and emergency purposes, sediment flushing, and release of environmental flow. High velocities and turbulence levels of the free-surface flow lead to air entrainment and air transport along the LLO tunnel, resulting in subatmospheric air pressures. In addition, shockwaves formed downstream of the gate may lead to a complete filling of the tunnel cross section, possibly resulting in slug flow and flow pulsations, which should be avoided for operational safety. In this study, physical model tests were performed to investigate the effects of gate opening and hydraulic head on shockwave patterns. Especially for large contraction Froude numbers, the shockwaves were strongly aerated, resulting in a complex air-water flow pattern. The results provide insights into the formation and propagation of shockwaves, contributing to an improved design of LLO tunnels.
Air Demand and Flow Patterns of Low-Level Outlets: Accounting for Wall Roughness
Pagliara, Simone; Felder, Stefan; Hohermuth, Benjamin; et al. (2025)
Journal of Hydraulic Engineering
Low-level outlets (LLOs) are key safety elements of high-head dams, typically consisting of a pressurized inflow controlled by a vertical sluice gate that discharges into a free-surface flow tunnel. The transition from pressurized to free-surface flow generates a high-velocity water jet with considerable air entrainment and transport along the tunnel, resulting in subatmospheric pressures downstream of the gate. These conditions potentially aggravate serious safety issues such as cavitation, gate vibration, and, in combination with sediment transport, hydroabrasion. Sufficient air supply can mitigate these problems. Several empirical equations have been developed to predict the air demand of LLOs, incorporating the effects of flow patterns, air vent loss coefficient, and tunnel geometry. However, reported model and prototype air demand data scatter over one order of magnitude, with tunnel roughness identified as a potential reason for these differences. To date, the influence of wall roughness on the performance of LLOs has not been systematically investigated. In this study, physical model tests were conducted with varying wall roughness, representing finished concrete, abraded concrete, and unlined rock at prototype scale. The results showed a significant increase in the air-water mixture flow depth with increasing wall roughness, where excessive filling of the tunnel may trigger foamy flow, flow choking, and the formation of hydraulic jumps, resulting in severe degradation of LLO performance. Increased wall roughness also led to a higher air demand, suggesting a predominant effect of the invert roughness over the wall and soffit roughness. A novel empirical equation was derived for air demand, incorporating the effects of tunnel roughness. The equation showed good agreement with previous laboratory and prototype data, indicating that other design parameters were not affected by the tunnel roughness. Finally, design recommendations were updated to account for roughness effects in LLO design, thereby contributing toward a safer design of these structures.
Auswirkung der Sohlenrauheit von Schussrinnen auf die Entwicklung der Wasser-Luft-Strömung
Friz, Carolin; Bürgler, Matthias; Hohermuth, Benjamin; et al. (2025)
WasserWirtschaft
Schussrinnen an Talsperren sichern die Hochwasserableitung. Ihre steilen Neigungen führen zu einer schwerkraftbedingten Beschleunigung des Abflusses, die an der Gerinnesohle Turbulenzen erzeugt und zur Selbstbelüftung des Abflusses führt. Ein angemessenes Prozessverständnis ist für die präzise Schussrinnenbemessung entscheidend. An einem großskaligen Versuchsmodell wurden mit modernster Messtechnik umfassende Experimente durchgeführt, deren Ergebnisse zur Kenntnis über hochenergetische Wasser-Luft-Strömungen und die Dimensionierung von Hochwasserentlastungsanlagen beitragen. Reservoir dams are critical for hydropower, irrigation, and flood protection, with spillways serving as essential safety outlets for flood waters. Their steep slopes produce high flow velocities, while turbulence at the spillway invert interacts with the free surface to induce self-aeration, resulting in “white waters”. A thorough understanding of these phenomena is vital for the robust design of spillways. However, progress in quantitatively describing these processes is limited by scarce, comprehensive air-water flow data downstream of the aeration inception point. Experiments on a large-scale physical model, with systematic variations in discharge and invert roughness, employed advanced instrumentation such as laser Doppler anemometry and dual-tip conductivity probes to measure flow velocities and air concentrations. Video analysis was used to determine a reference inception point. This study deepens our insight into high energy air-water flows and supports improved design of spillways.

Publications 1 - 10 of 40

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