Giovanni De Cesare

Effect of inclined jet screen on turbidity current

The sustainable use of reservoirs for irrigation, flood protection, water supply and hydropower may be endangered due to unavoidable reservoir sedimentation. Turbidity currents are the main process for the transport and deposit of sediments in reservoirs, especially in the deepest part near the dam where vital structures such as power intakes and bottom outlets are located. Besides other measures such as solid or permeable obstacles, turbidity currents can be influenced by means of an inclined water jet screen. Physical experiments of a turbidity current flowing through a water jet screen were carried out. Velocity profiles, front velocities, and deposit evolutions were determined. The results indicate that in certain configurations, turbidity currents can be partially stopped by the jet screen. Furthermore, the deposits downstream of the screen may be reduced up to a factor of two as compared with deposits of a free-flowing turbidity current.

Anregungen zur Qualitätssicherung in der 3-D-numerischen Modellierung mit FLOW-3D@@@Contribution to Quality Standards for 3D-Numerical Simulations with FLOW-3D@@@Предложения по обеспечению качества в трехмерном численном моделировании с использованием FLOW-3D

FLOW-3D ist eine leistungsfahige 3-D-numerische Stromungssimulationssoftware fur Freispiegelabflusse. Das strukturierte und entlang der Koordinatenachsen ausgerichtete Berechnungsnetz stellt eine Besonderheit der Software dar. Dadurch ergeben sich zusatzliche Freiheiten, wie die Implementierung von bewegten Objekten oder die vereinfachte Variationsuntersuchung von Einbauten. Im Hinblick auf die Qualitatssicherung muss die Adaptierung der Geometrie an das Berechnungsnetz im Detail kontrolliert werden. Dies und weitere Modellierungsaspekte werden in diesem Beitrag behandelt

Herausforderungen des heutigen wasserbaulichen Versuchswesens mit drei Beispielen

Die meisten technischen Universitaten oder Hochschulen, welche Bauingenieure ausbilden, verfugen seit annahernd 100 Jahren uber wasserbauliche Laboratorien. Darin werden mittels physikalischer Modellierung Elemente von Talsperren, Wasserkraftwerken oder Hochwasserschutz-Masnahmen uberpruft und optimiert. Mehrmals wurde dieser klassischen Methode bereits das Ende vorausgesagt, trotzdem sind die Versuchsanstalten ausgebucht. Die Autorenumreisen den Wandel der Branche und geben drei Beispiele dazu.

Flow field in a reservoir subject to pumped-storage operation - in situ measurement and numerical modeling

In pumped-storage reservoirs, turbulent kinetic energy input from plant operation can be considerably high. Flow velocities were measured by Acoustic Doppler Current Profilers near the intake/outlet structure to describe flow patterns induced by pumped-storage operation in an Alpine reservoir. Recorded data allowed reproducing 1D and 2D velocity profiles along the water column. The comparison between the main frequencies of the velocity signal and the discharge series from the plant reveals correlation between recorded flow patterns and pumped-storage operation. Numerical modeling enhanced the understanding of flow patterns developing near the intake/outlet structure. Both results reveal that water withdrawal by pumping only marginally affects flow patterns in front of the intake, whereas water injected during turbine mode leads to backflow areas and large-scale recirculation cells. Numerical modeling further revealed that steady flow patterns are developing only after some 2.5 h of continuous turbine operation.

Continuous Long-Term Observation of Suspended Sediment Transport between Two Pumped-Storage Reservoirs

Transport of suspended sediment between the two reservoirs of a pumped-storage plant in Switzerland was monitored over an 8-month period. A turbidity probe was installed on the upstream end of the pressure shaft, in which real-time monitoring was ensured by automatic data acquisition and remote control. The suspended sediment concentration in the reservoirs varies considerably over the course of a year, with low particle loads in the winter, when the reservoirs are covered by ice and the catchment area is entirely covered by snow. In late spring, when snowmelt starts, the suspended sediment concentration increases and remains high until summer. The short-term evolution of the suspended sediment concentration was found to be correlated to the pumped-storage sequences, especially when the reservoir levels are low. Higher concentrations during pumping mode were observed for low levels in the lower reservoir, and low levels in the upper reservoir corresponded to higher sediment concentrations in turbine mode. Concentration ratios between pumping and turbine mode between 0.93 and 1.16 were observed. Periods with higher particle load were observed more frequently during pumping mode. Nevertheless, the suspended sediment balance due to pumped-storage operations is equilibrated, which indicates a high degree of correlation between the transported sediment volume and hydropower operation. Connecting the applied monitoring system in an existing plant control and management system would allow the plant operator to detect increased suspended sediment loads in the pressure shaft and to adapt the operation mode to actively manage reservoir sedimentation or to estimate and predict the long-term sediment balance.

Venting of turbidity currents approaching a rectangular opening on a horizontal bed

ABSTRACT Reservoir sedimentation is a worldwide problem hampering the sustainable use of reservoirs and the sediment balance of impacted rivers. Various techniques are applied for sediment mitigation. However, as turbidity currents are a major source of sediments in reservoirs, venting them through outlets reduces sedimentation. The sediment release efficiency associated with venting turbidity currents on a horizontal bed is experimentally investigated in this paper. The outflow discharge and duration of venting are the main parameters assessed. Venting efficiency is studied based on two different concepts: (1) a global venting efficiency comparing inflow and outflow sediment fluxes during the total venting operation and (2) a local venting efficiency comparing masses starting at the arrival of the turbidity current to the outlet and taking into account deposited sediment masses. An efficiency indicator accounting for water losses is also introduced. Results can be used to improve the efficiency of venting by employing adequate outlet discharges.

Experimental investigation on turbidity current venting under restrained outflow discharges

Reservoir sedimentation is a worldwide problem affecting sustainable storage use as well as sediment transport downstream of dams. Various techniques are applied for sediment mitigation, among which, venting of turbidity currents. This paper reviews, discusses and evaluates venting of turbidity currents based on an experimental approach. Data acquisition during the tests is accomplished using five different measuring instruments. The efficiency of venting is analyzed by varying the relative outflow discharge. The study reveals the potential of optimizing venting operations in terms of water losses by employing adequate outlet discharges during venting.

Understanding turbulent free-surface vortex flows using a Taylor-Couette flow analogy

Free-surface vortices have long been studied to develop an understanding of similar rotating flow phenomena observed in nature and technology. However, a complete description of its turbulent three-dimensional flow field still remains elusive. In contrast, the related Taylor-Couette flow system has been well explicated which classically exhibits successive instability phases manifested in so-called Taylor vortices. In this study, observations made on the turbulent free-surface vortex revealed distinguishable, time-dependent “Taylor-like” vortices in the secondary flow field similar to the Taylor-Couette flow system. The observations were enabled by an original application of 2D ultrasonic Doppler velocity profiling complemented with laser induced fluorescence dye observations. Additional confirmation was provided by three-dimensional numerical simulations. Using Rayleigh’s stability criterion, we analytically show that a wall bounded free-surface vortex can indeed become unstable due to a centrifugal driving force in a similar manner to the Taylor-Couette flow. Consequently, it is proposed that the free-surface vortex can be treated analogously to the Taylor-Couette flow permitting advanced conclusions to be drawn on its flow structure and the various states of free-surface vortex flow stability.

Optimization of a Shared Tailrace Channel of Two Pumped-Storage Plants by Physical and Numerical Modeling

Pump and turbine operations lead to head losses in the tailrace channel. Pumping discharge may be limited due to potential air entrainment into the pump shaft when the downstream reservoir is at its lowest level and the head losses are too high. Regarding turbine operations, the limit is given by the maximum level in the Pelton turbine chamber due to high water level in the downstream reservoir and head losses. The rapid starting and stopping of turbines and pumps lead to highly unsteady flow in the tailrace channel system of the two connected hydropower plants. Negative and positive surges may lead to similar consequences as for stationary operations such as sudden air entrainment into the pump shaft and submerging of the Pelton runner under operation. Therefore, flow and head losses test were conducted on a physical model at 1:30 scale together with numerical simulations using FLOW-3D. On-site measurement of the existing power plant allows validating the results. Representative and extreme operational scenarios have been simulated; the main results are discussed and presented.

Design of a Throttled Surge Tank for Refurbishment by Increase of Installed Capacity at a High-Head Power Plant

AbstractThe Swiss confederation aims to phase out nuclear power production with its Energy Strategy 2050 program by increasing the renewable energy contribution to its overall energy generation. Hy...