Keynote lecture - Potential, limits and perspectives of physical and numerical modelling

In the 19th Century, hydraulic engineering was typically addressed in two parallel approaches. On one hand, classical theoretical fluid mechanics with the use of analytical approaches to study various flow occurrences. Due to the difficulty and complexity of the phenomena under investigation and the lack of rational basis, hydraulic experimentation as the second approach played more and more an important role. Therefore, since more than one hundred years, physical experiments were relevant in science and engineering. With the rapid development of computational fluid mechanics and affordable high performance computing power, CFD is getting competitive with physical modelling that allows overcoming scale effect. Based on many years of experience in physical modelling at the LCH, some present and future needs and on-going changes observed are considered. Increasingly complex models are under investigation where regularly entire systems with dynamic behaviour or transient and unsteady flow discharge scenarios have to be considered. There is a trend towards larger all-inclusive models. These studies often exceed the essential hydraulic phenomena, and several parameters are included in one single model. These demands frequently lead to some suboptimal scale requirement. In these cases, and as there is also an increasing demand for hybrid numerical and physical modelling, the previously mentioned aspects can be avoided by treating each hydraulic phenomena at its proper scale and with the appropriate tool, either CFD or physical modelling. This allows reducing the physical model perimeter defining the boundary conditions from numerical modelling. Three examples are presented and discussed to highlight the potential of physical and numerical modelling. First rapid hydraulic transients due to the creation and collapse of a single cavitation bubble will be shown. Here the challenge comes from the observation and measurement techniques and time-space analysis, with acquisition frequencies up to 500 kHz for acoustic signals and 100 kHz for imaging and propagation velocities around the speed of sound in water. The second case of a complex surge tank with varying geometry. The capacity to physically model transient phenomena including a free surface is illustrated. The case of a simulated first rise due to mass oscillation between the upstream reservoir and the surge tank of a high head hydropower plant is presented. To terminate the study of air entrainment on a spillway chute is presented, where the limits of physical modelling due to scale effects are visible. In principle, there is no possibility to scale down the physics of air transport. Here numerical modelling could play a significant role in the future. As closing remark, besides the importance of hydro-engineering experimentation for the design process, and for the safe and efficient operation of present and future hydraulic structures and scheme, the author also emphasize the importance for fundamental research. Currently, the majority of scientific papers in peer-reviewed journals in the field of hydraulic research and engineering are still based on data that was collected in physical models and experimental installations.

Characterization of hydraulic behavior of orifices in conduits

Nicolas Jean François Pascal Ghislain Adam

Surge tanks in high-head power plants ensure safe and flexible transient operation of the hydraulic machinery. Orifices or throttles in surge tanks are often critical structural elements in view of the good performance of surge tanks and the stability of the whole waterway system combined with hydraulic-mechanical equipment. The design and the dimensioning of orifices ore throttles placed at surge tanks has to be carried out with great care since a non-functioning of this critical structural elements can endanger the safe operation of the whole hydropower scheme. Orifices or throttles have to produce a distinct head loss for flow entering and leaving the surge tank. In the design, the best geometry has to be found which produces the desired head losses. The search of the most adapted geometry of the orifice or throttle is often challenging and has frequently to be complemented by systematic hydraulicmodel tests. In order to allow a preliminary design of orifices, Dr. Nicolas Adam studied for the first time systematically with laboratory experiments and numerical simulations a large number of different geometries of throttles, i.e. orifices. Based on the extensive catalogue of the orifice geometries tested and the developed empirical relationships, he could give efficient design guidelines based on empirical formulae and on a useful expert sheet in order to find efficiently the appropriate orifice geometry for a wished head loss. Dr. Adam studied also for the first time systematically the transient head losses through orifices. The systematical experiments and numerical simulations allowed also a better understanding of the hydraulic behavior of orifices in view of the influence length of the orifice i.e. the reattachment length of the jet leaving the orifice and associated risk of cavitation. Finally, Dr. Adam gives helpful practical recommendation for an efficient and safe design of orifices in surge tanks.

EPFL-LCH2017

Interaction of side weir overflow with bed-load transport and bed morphology in a channel

Burkhard Rosier

Side weirs, also known as a lateral weirs, and overflow dams are free overflow regulation and diversion devices commonly encountered in hydraulic engineering. They are set into the side of a channel or river allowing to spill a part of the discharge over their crest when the surface of the flow in the main-channel exceeds a certain level. The lateral loss of water is reducing the sediment transport capacity in the main-channel yielding to aggradation and the formation of a local sediment deposit in the downstream weir alignment. The reduced cross section generates backwater effects and additional contraction and expansion losses. As a consequence, the head over the side weir rises and the side overflow discharge as well. The design discharge to be diverted over the weir is increased by this flow-sediment transport interaction. Since the interaction of side overflow with bed-load and bed morphology in a channel has not been studied so far, systematic tests have been performed. Three test series and one reference experiment without side weir have been carried out in a 20.00 m long, 1.50 m wide and 1.20 m high rectangular flume. The first test series consisted of a 3.00 m long side weir, the second one had a 6.00 m long weir and the third series was characterised by two weirs of 2.50 m length each. The approach discharge varied between 0.098 m3/s ≤ Q1 ≤ 0.222 m3/s. The overall flow regime has been subcritical. The average initial bottom slope was 0.21 %. The mobile bed was characterized by a median particle size of d50 = 0.72 mm. During the experiments the water surface, the 2D-velocity field, the side overflow discharge and sediment supply were measured. The final bed morphology has been recorded by means of digital photogrammetry. Based on the systematic experimental flume study a one- and a two-dimensional empirical model for the prediction of the mobile bed evolution near the side weir have been developed. The models allow a simple and straightforward estimation of the interaction of a side overflow with bed-load transport and bed morphology in engineering practice. The 1D-model represents the overall mobile bed evolution in the weir reach. The model takes into account a deposit being uniform over the channel width. The height of the deposit only varies in longitudinal direction. In addition to the 1D-approach the 2D-model incorporates the variation of the deposit over the channel width. For the parameterisation of the two models a Maxwell-type distribution function is applied. Input parameters for both models such as the location and height of the maximum bed elevation and a shape factor are expressed in terms of non-dimensional geometric channel and side weir variables as well as hydraulic parameters and bed load transport relations. For the 2D-approach an additional relationship considering the spanwise variation is developed. To implement the models in numerical flow simulations expressions for the location of the empirical deposit relative to the side weir are established. Regarding the impact of the deposit on the intensity of side overflow it has been found out that the spilled discharge might increase by a factor of up to ≈ 3 compared to fixed plane bed conditions. In this context about 25 % of the total increase are attributed to effects of form roughness and about 75 % to bed aggradation phenomena. In this regard the height of the deposit represents the most important parameter. The location of the deposit with respect to the position of the side weir has a smaller influence, whereas the downstream shape of the deposit is of minor importance. To test the prediction accuracy of the models they have been implemented into 1D-flow calculations. The predicted side overflow was about 85 % for the 1D-model and about 91 % for the 2D-model. The difference is mainly caused due to the implementation of the spanwise variation in the 2D-model. Besides the two models a simple relationship for direct estimation of side overflow discharge in presence of bed-load transport has been established. The empirical models have been applied in a case study on the Rhone river upstream of Lake Geneva in Switzerland. In the case of a flat bed without deposit a protection for a flood event with a hundred year return period persists. Taking into account the mobile bed evolution a protection even for an extreme flood might be obtained. This presumes a sufficiently large storage volume of the retention bassin. The skewed deposit induces the formation of an oscillatory erosion gutter downstream of the weir. For the description of the sine-generated evolution of the thalweg indicative expressions are proposed. In order to test the capability of a numerical tool to reproduce the bed aggradation phenomena observed in the experiments, 1D-numerical simulations with bed-load transport have been performed (DUPIRO). From these computations it can be concluded that the most experimental phenomena are captured with reasonable accuracy.

EPFL2007

Experimental campaign on thin RC columns prone to out-of-plane instability: numerical simulation using shell element models

Katrin Beyer, Lisandro Arturo Jimenez Roa, Angelica Rosso, João Saraiva Esteves Pacheco de Almeida

Construction of multi-storey reinforced concrete (RC) wall buildings in areas of moderate to high seismicity has become a common practice in several Latin-American countries such as Colombia, Peru, Ecuador and Mexico. The large material cost has led to the common practice of designing walls with only one layer of reinforcement and very low thickness. The earthquakes in Chile (2010) and New Zealand (2011) have shown that such design approach can induce out-of-plane failure of the walls under seismic action. In order to study the influence of different parameters on the out-of-plane response of thin members, an experimental campaign on RC columns was recently carried out at École Polytechnique Fédérale de Lausanne. These columns, axially loaded in cyclic tension-compression, represent the boundary element regions of RC walls, which correspond to the parts of the wall mainly involved in the instability mechanism. This paper illustrates the application of a shell model with truss elements to simulate the response of the tested specimens. The accuracy of the numerical model is assessed by comparison against the experimental results. It is shown that good estimates of the critical tensile strain that leads to out-of-plane instability as well as the out-of-plane displacement in cycles prior to failure are obtained. The failure mode is adequately simulated and also local behaviour is consistently reproduced. Therefore, the application of similar models to assess the vulnerability of thin RC walls to out-of-plane instability appears to be a relatively simple and robust numerical tool for engineering practice.