Alliau, Damien

Fretaud, Thierry

Genin, Margaux.

Lang, Pierre

English

Hydraulic Engineering Repository

Conference Paper, Published VersionAlliau, Damien; Fretaud, Thierry; Genin, Margaux.; Lang, PierreNumerical and Experimental Study of Favre Waves afterHydropowerplant Trigger with TELEMAC-3DZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/100428Vorgeschlagene Zitierweise/Suggested citation:Alliau, Damien; Fretaud, Thierry; Genin, Margaux.; Lang, Pierre (2013): Numerical andExperimental Study of Favre Waves after Hydropowerplant Trigger with TELEMAC-3D. In:Kopmann, Rebekka; Goll, Annalena (Hg.): XXth TELEMAC-MASCARET. User Conference2013. Karlsruhe: Bundesanstalt für Wasserbau. S. 53-58.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Numerical and experimental study of Favre waves after hydropowerplant trigger with TELEMAC-3D  D. Alliau, T. Fretaud, M. Genin Engineering department Compagnie Nationale du Rhône Lyon, France d.alliau@cnr.tm.fr   P. Lang Expertise et structure INGEROP Grenoble, France pierre.lang@ingerop.com    Abstract—CNR is the first producer of exclusively renewable energy in France, operating and managing 18 hydroelectric power plants on the Rhône River. When turbines shut down because of electric incidents, the stop is very sudden and generates a wave that raises the upper channel water level. This step is called trigger or disjunction.  To improve the knowledge of these wave phenomena is essential for CNR in order to implement suitable actions both at the barrage and power plant with respect to each specific development constraint (warning of sudden water release in the downstream reach, automatic backup, intrinsic security).  Previous modelling studies with 1D and 2D models of the actual trigger test carried out with a 500 m3/s discharge at Chautagne scheme in April 2010 showed some limitations including excessive dampening of reflected waves and underestimation of secondary waves amplitude. The aim of this study is to carry out the modelling of this test with the TELEMAC-3D software in order to get better results. Tasks consist of 3D model meshing, model calibration for trigger conditions, comparison of results with real test measurements, discussion on methods, analysis of the main parameters and extrapolation to a trigger test with a 700 m3/s discharge.  I. INTRODUCTION The of hydro-electric development scheme of the Rhone River is based on a regulation barrage and a power plant equipped with turbines. Electrical incidents (mechanical failure in the turbine or in the electrical network, etc…), or unfavourable hydraulic conditions can lead to a quick stopping of one or more turbines and thus stop the plant and its production. This phenomenon is also called trigger or disjunction. To avoid destruction of the machines due to overspeed, the flow that supplies hydraulic turbines is automatically cut off by valves. The discharge is then suddenly reduced to zero, creating a positive wave propagating in the upstream channel and a negative one in the downstream channel of the plant. The wave is called wave disjunction or swelling. As a consequence, the positive wave will temporarily increase the standard water level of the upper channel (i.e. the usual operating water level) and consequently affects security issues (e.g. flooding of banks equipments and structures, spillage over levees). The numerical study of this phenomenon is crucial for CNR in order to improve the understanding of the wave propagation and to prevent occurrence of any incidents. The paper is divided in four parts. First, the Chautagne development scheme features are explained. Second, the experimental trigger is described. Third, the TELEMAC modelling is presented. Finally, 2D and 3D results are analysed, limits of optimization are listed and improvements are proposed. II. OBJECTIVES The simulation of disjunction waves using TELEMAC-2D model (shallow water or Boussinesq equations) gives a good representation of the first passage of the primary wave amplitude and frequency. This model also has its limitations: • The dampening of reflected waves is too strong; • The amplitude of the wave from the second pass at the plant is underestimated compared to the expected results. In addition, the secondary waves are not modelled by the Saint Venant equations, because these assume a hydrostatic pressure distribution. That’s why 3D modelling seems necessary to reproduce this phenomenon. Study objectives are: • to realize a 3D exploratory approach from the existing 2D model and evaluate it by comparing with the experimental test data recorded on 29 April 2010 (500 m3/s); • To extrapolate the trigger test to the maximum discharge of the power plant. III. CHAUTAGNE SCHEME Chautagne hydropower plant is located in the Rhône River valley (in the eastern part of France), between Geneva (Switzerland) and Lyon. It was built in 1980. Chautagne follows the typical CNR development scheme. Indeed, it is composed of the following structures: • A hydropower plant (Usine d’Anglefort: US) with a total installed capacity of around 90 MW. It comprises two bulb-upstream units with a maximum power station discharge of 700 m3/s.  TUC 2013, 16-18/10/2013, Karlsruhe - Kopmann, Goll (eds.) - © 2013 Bundesanstalt für Wasserbau ISBN 978-3-939230-07-653XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013     VI. SCHEME MODELLING 1. Mesh The domain is 15 km long and is meshed with node located every 15 m (∆x). This resolution was calculated according to the wave speed (C) and the time for stopping the hydropower plant. The secondary waves are very quick and have a runtime of 15 s (T). It was assumed that between 5 and 10 observations (n) during plant stopping were enough to measure waves. Consequently the mesh resolution was calculated with:  ∆𝑥 =  𝐶.𝑇𝑛 𝑛=10���� ∆𝑥 = 15.6 𝑚 (1) The 2D grid comprises about 26 000 nodes and 50 000 elements. This mesh has been generated by Matisse. With regard to the 3D model, 6 horizontal levels are generally used in this study. Consequently, the 3D grid comprises about 150 000 nodes and 750 000 elements. 4 liquid boundary segments (SY, FI, BGE, and CE) are applied. A. Calibration The model calibration is very important because it determines numerical model reliability. At first, the calibration is done with the 2D model. Then 2D calibration coefficients are reused in the 3D model after a validation step (dependent to the water depth discretization, i.e. horizontal level number). For the calibration process five steady state discharges ranging from 436 m3/s to 1300 m3/s and two flood events with peak flows of 2400 m3/s and 2070 m3/s were selected. Water levels were imposed at the downstream hydropower plant (CE) and at the other three liquid boundaries (SY, FI and BGE).  The modelling of the turbulence was realised with a constant viscosity of 5.10-3m2/s. Calibration was focused on the bottom coefficient, which was computed following Strickler’s law. The model was divided into different Strickler zones as specified in Fig. 3.   Figure 3.  Strickler coefficients The comparison of computed and measured water levels shows a maximal error of 15 cm for the steady state conditions, however for the lowest discharge the error is less than 5 cm. For the flood events the upstream power plant water level during the peak flow is overrated with 15 cm,. Nevertheless the results match the measurements fairly well allowing to validate the numerical model. For the next step of the study, an average error between 5 and 15 cm has to be taken into account.  The Strickler coefficients from the 2D calibration were used in the 3D model, and the calibration procedure was applied again in order to check the model behaviour in 3D. A similar calibration quality could be reached.   Figure 4.  Calibration of steady state in 3D model VII. DISJUNCTION TEST MODELLING The disjunction is studied in this part. Water levels throughout the model were calculated to improve the knowledge about these phenomena. During the disjunction, the model was controlled by flow boundary condition.  Thus, 2 different kinds of boundary condition setups were used. On the one hand, the initialization (normal boundary conditions), on the other hand the disjunction (only flows).   A. Initialization A hydraulic model in subcritical flow is controlled by a downstream water level, consequently a water level was imposed on the power plant. For the initialization the unsteady state before the disjunction took place was taken from measurements. The model simulated a whole day prior to the disjunction until 9:30 a.m. The simulation of these 30 hours took only 8 minutes on 32 computational cores.  At first the initialization was run with TELEMAC-2D. The difference between the models will be explained later. The comparison of computed and measured water levels (∆Z) for the initialization run is presented in Table II for different locations of the model (Fig. 1).  At 9:30 a.m the water level. is close to the reality (average ∆Z about 3 cm). Moreover, the headrace channel discharge calculated at 9:30 am is about 500 m3/s, similar to the ADCP measurements. Therefore the last time step of this initialization was used as the first of the trigger modelling.       55XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013     TABLE II.  INITIALIZATION SUMMARY  Z [m NGFO] Init 5 [m NGFO] Measurement Point/Station 08:30 09:30 08:30 ∆Z 09:30 ∆Z P1 Seyssel Bridge 252.33 252.53 252.47 0.14 252.56 0.02 P2 leisure center 251.96 252.06 252.13 0.17 252.09 0.03 P5 BGE  upstream  251.96 252.03 252.09 0.13 252.04 0.01 P7 CA entrance 251.94 252.02 252.04 0.10 252.01 0.00 P8 mi- CA 251.84 251.94 251.82 -0.02 251.87 -0.06 P9 lock  upstream  251.57 251.73 251.64 0.07 251.78 0.05 P13 100m CE upstream  251.54 251.70 251.59 0.05 251.76 0.06 P14 CE upstream 251.59 251.74 251.67 0.08 251.74 0.00  B. Disjunction To start disjunction modelling, simulation was run with TELEMAC-2D to get an idea of the results. Then a simple 3D model was run with default setup parameters in TELEMAC-3D. The optimization of this case has been studied in a second phase. Again only flow boundary conditions were used (Fig. 5). Default setup parameters were chosen for TELEMAC-2D run.  • Strickler’s friction law with different zones as shown in Fig. 3.  • Constant turbulence model with a constant viscosity of 5.10-3 m2/s.  • The solver was chosen by default with “solving normal equation”.  • Shallow water equations were used in this model. A quick sensitivity test between Saint Venant and Boussinesq did not show any significant difference.  Figure 5.  Boundary conditions during trigger test For the TELEMAC-3D run, parameters were chosen similarly. Additional key-words like 2D continuation, 6 horizontal levels and no friction on lateral boundaries were added to the 3D model.  The time step (∆t) has been calculated according to Courant number Cr. This number must be less than 1 for rapidly changing flows.   𝐶𝑟 = 𝐶 . ∆𝑡∆𝑥 < 1 ∆𝑥=15𝑚������∆𝑡 < 1.4 𝑠 (2) Consequently, ∆t = 1s.  Both simulations (2D and 3D) were run with continuation on 2D or 3D initialization. Results were extracted at different locations; the study mainly focused on the upstream US sensor (6) and on the inlet of CA sensor (7).  Figure 6.  Comparison 2D vs. 3D models at upstream powerplant. The 2D model shows attenuation of the reflected waves and underestimated amplitude. The first wave peak is perfectly reached by the 2D model; the 3D model is also close even if there is a ∆z of 5 cm. Generally the 3D model reproduces the waves better than 2D model. Amplitude and frequency of the model results are almost in line with the measured signal. The amplitude of the reflected wave is quite acceptable even if the model results show a small attenuation; this could be improved with an optimization program.   Figure 7.  Comparison 2D vs. 3D models at inlet of headrace channel. The 2D model shows instabilities on the secondary waves (see the zoomed detail in Fig. 7). Some oscillations appear overlaying the general curve, but they might be linked to numerical instability. The primary wave is well 56XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013     simulated by the 3D model. Afterwards a specific study is performed on the representation of secondary waves. VIII. OPTIMISATION PROGRAM (1)  A. Key-words effect A large optimization program is set up to improve the representation of the disjunction wave and to mitigate the difference between simulation and measurements. This program is based on different key words available with TELEMAC-3D: • Number of horizontal levels: 6 to 10;  • Horizontal levels position: equidistant or close to free surface; • Modelling continuation: 2D or 3D; • Non-hydrostatic or hydrostatic version; • Horizontal and vertical turbulence model: constant viscosity, mixing length, Smagorinsky, K-Epsilon, K-Omega; • Solvers. Most of the tests are displayed on Fig. 8 and Fig. 9. Not all the tests performed with the different solvers are shown, because the results are almost the same.  Figure 8.  CE upstream models comparison  Figure 9.  Inlet of headrace channel models comparison. From the analysis shown in Fig. 8 some configurations could be dismissed: • Particular level distribution is not appropriate to this test; • Friction on lateral boundaries has no influence on the free surface level. Further investigations: • 3D continuation with restart file (equations solved at last time step);  • Non-hydrostatic version (starting hypothesis); • Horizontal turbulence model: constant viscosity (5.10-3 m2/s); • Vertical turbulence model: mixing length; • Steering word “Velocity profiles” at the discharge boundaries were calculated considering the water height. At the inlet of the headrace channel, only the primary waves could be calculated by TELEMAC-3D: it should be necessary to find an alternative set of parameters to investigate this phenomenon. B. Key-words coupling These selected key words are gathered in one simulation. A quick test showed that considering only 6 horizontal levels was enough for this trigger test. Besides boundary conditions were modified and shifted to +10 minutes, because records gave average on last 10 minutes. The values of coefficient diffusion for velocity and depth were fixed to 1 in accordance with the calibration done on v5p9 version. Moreover, free surface gradient compatibility was reduced to 0.9 to avoid instabilities. A new simulation was run with TELEMAC-3D considering the mentioned key-words concerning coupling and boundary conditions. The 2D model was also modified with the same key-words matching on TELEMAC-2D. Results were matching very well with the measurements as shown on Fig. 10 and Fig. 11.  Figure 10.  Comparison 2D vs. 3D optimized models at upstream of  power plant 57

Numerical and Experimental Study of Favre Waves after Hydropowerplant Trigger with TELEMAC-3D

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Numerical and Experimental Study of Favre Waves after Hydropowerplant Trigger with TELEMAC-3D

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