Q. 94 – R. 5 ---Kyoto, Juin 2012 ---COMPLEMENTARY USE of PHYSICAL and NUMERICAL MODELLING TECHNIQUES in SPILLWAY
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Q. 94 – R. 5 COMMISSION INTERNATIONALE DES GRANDS BARRAGES ------- VINGT-QUATRIÈME CONGRÈS DES GRANDS BARRAGES Kyoto, Juin 2012 ------- COMPLEMENTARY USE OF PHYSICAL AND NUMERICAL MODELLING TECHNIQUES IN SPILLWAY DESIGN REFINEMENT * James WILLEY GHD Pty Ltd, Brisbane, QLD, Thomas EWING GHD Pty Ltd, Melbourne, VIC, Bob WARK GHD Pty Ltd, Perth, WA, Eric LESLEIGHTER Lesleighter Consulting Pty Ltd, Cooma, NSW, AUSTRALIA 1. INTRODUCTION The design of recent projects involving new or upgraded spillways have benefited from the complementary use of both Computational Fluid Dynamics (CFD) techniques and physical scale modelling. As well as providing significant benefits to the design process for these projects, this parallel use has provided invaluable data for the comparison of the techniques which may lead to greater confidence in the future use of standalone CFD analyses in spillway design. The design process recently adopted for complex spillway designs involved the initial development of a concept or preliminary design using theoretical and empirical design methods, together with published data. These arrangements * Utilisation complémentaire des techniques de modélisation numérique et physique pour l’amélioration de la conception des déversoirs. 55 Q. 94 – R. 5 were then analysed and optimised through CFD modelling and, where required, physical scale modelling was used for final verification and refinement. A comparison of the results from CFD and physical models is presented where both have been used in the design process and conclusions are drawn on the applicability of standalone CFD analyses. Projects on which GHD was the designer and which are discussed herein include the spillway for the new Enlarged Cotter Dam and remedial works at Lake Manchester, Blue Rock and Wellington Dams. 2. BACKGROUND Physical scale modelling has been used in the design and investigation of hydraulic structures for over 100 years. The design process has typically involved the development of a preliminary design on the basis of theoretical and empirical methods. A physical scale model of this arrangement would then be constructed in two- or three-dimensions and various scenarios run to confirm whether the hydraulic performance was acceptable and to extract data for input to the design. The methods are tried and tested and the outputs from the model testing in terms of data and observations are invaluable in the design process. However, the construction, operation, and testing of physical models is often a time-consuming and expensive exercise. Furthermore, the modification of the model to trial alternative arrangements or to optimise features can add weeks to a testing programme. Through recent advances in computing power and modelling software capabilities, it is now feasible to undertake complex three-dimensional analyses using CFD techniques. To date, CFD modelling has generally been used as a valuable tool in the optimisation phase of the project prior to the commissioning of a physical model study. The major benefit of the CFD modelling in this capacity was that it allowed the early identification of problematic flow features and modifications to the layout could be trialled rapidly and cost-effectively. Although CFD modelling packages now have the capability to analyse complex hydraulic conditions common in spillways such as air entrainment, flow separation, turbulence and shock waves, there is however a significant lack of calibration and validation studies (between CFD and physical models and also between model and prototype) for these advanced applications and caution should be applied to their use in design. 56 Q. 94 – R. 5 3. CFD MODELLING CAPABILITIES AND APPLICATION TO ANALYSIS OF SPILLWAYS The most common, and reliable CFD analysis of spillways involves model- ling of flow in the spillway approach, over spillway crests, and around obstacles such as piers and flow training walls. Models of this type are used to generate rating curves, predict water surface elevations and identify zones of unfavourable flow behaviour. Examples of practical and accurate modelling of these flows are presented in this paper, and many more can be found in the published literature. In these relatively simple cases the dynamics are dominated by what is fundamentally a transfer of potential energy to kinetic energy and flow around abrupt obstacles. Modelling of these processes to a reasonable degree of accuracy does not require accurate simulation of turbulence or boundary layers, allowing the use of coarse model grids and simple turbulence models. By contrast, the dynamics of high speed flow down steep spillways or chutes are dominated by the production and dissipation of turbulence and require careful model and mesh configuration. Aeration of spillway flows is another process that is very dependent upon the accurate representation of turbulence and boundary layer dynamics. Several CFD codes do have the capability to model interpenetrating phases (in this instance air bubbles in water), but at present only a relatively small number of aerated flows have been modelled successfully using CFD. This was not considered in the examples presented herein as the CFD modelling in these cases was used as an initial optimisation tool using a coarser grid resolution for more rapid processing. Further work is required in this area. Further comments on the limitations of CFD are included in the conclusions. 4. COMPARISON OF CFD AND PHYSICAL MODEL DATA 4.1 ENLARGED COTTER DAM 4.1.1 Brief Description of the Project The Enlarged Cotter Dam (ECD) is a new 87 m high roller-compacted concrete dam under construction at the time of writing in 2011. The spillway includes a central primary stepped spillway tapering from 70 m at the crest to approximately 45 m at the entrance to the stilling basin. Stepped secondary spillways were provided over each abutment and operate for floods greater than the 1:1000 annual exceedance probability (AEP) discharge. The discharge from 57 Q. 94 – R. 5 the secondary spillways is conveyed to the river channel by stepped abutment return channels or cascades. The primary spillway has an ogee crest designed for the maximum head, while the secondary spillway crest is horizontal with a downstream transition curve designed for half the maximum head. The primary spillway curve transitions into steps approximately 9 m horizontally from the upstream face. The design for the project utilised both two- and three-dimensional CFD analyses using in-house capabilities and a 1:45 scale three-dimensional physical model by Manly Hydraulics Laboratory (MHL). 4.1.2 Spillway Discharge Rating The initial rating curve for the project was derived from published data in [1]. Discharge coefficients were derived from the two-dimensional CFD analyses for the primary and secondary spillway crests. From the physical model study, it was only possible to calculate discharge coefficients for the primary spill- way crest for discharges up to the 1:1000 AEP event or a unit discharge (q) of about 8 m³/s/m. Beyond this level, the secondary spillways began to operate and it was therefore not possible to calculate discharge coefficients for either crest without making assumptions. The discharge coefficients derived from the various methods for the primary and secondary spillway crests are presented in Fig. 1. Fig. 2 presents a comparison of the discharge rating curves derived from the discharge coefficients plotted in Fig. 1. 2.6 2.4 2.2 2 1.8 Primary - USACE WES data 1.6 Primary - CFD model data Coefficient of of discharge Coefficient 1.4 Primary - physical model data Secondary - USACE WES data 1.2 Secondary - CFD model data 1 0 10 20 30 40 50 60 70 80 Unit Discharge (m³/s/m) Fig. 1 Relationship between unit discharge and discharge coefficient for Cotter Dam Relation entre le coefficient de débit et le débit unitaire pour le barrage de Cotter 58 Q. 94 – R. 5 559 558 557 556 555 554 USACE data Reservoir level (RL m) (RL level Reservoir 553 CFD data Physcial model 552 551 550 0 1000 2000 3000 4000 5000 6000 7000 Discharge (m³/s) Fig. 2 Comparison of discharge rating curves for Cotter Dam Comparaison des courbes de tarage pour le barrage de Cotter For the primary spillway crest, the discharge coefficients calculated from the physical model are within 5% of the USACE values. This was also the case for the coefficients derived from the CFD analyses for q < 3 m³/s/m and q > 36 m³/s/m. However, over the intermediate range, the calculated coefficients were typically 8-13% less than the USACE values. For q > 6 m³/s/m over the secondary spillway, the CFD analyses yielded discharge coefficients within 5% of the USACE values. However, at low discharges, the CFD showed the crest was less efficient than indicated by the USACE data. Despite the differences in the discharge coefficients noted above, the difference in the total head over the spillway for the probable maximum flood (PMF) discharge of 5,710 m³/s was about 0.5%, with a marginally higher head measured in the physical model. Similarly for the same reservoir level, the difference in discharge estimates was about 1%. 4.1.3 Pressures This comparison is based on results from the two-dimensional CFD model- ling and the three-dimensional physical model, and published data from [1] and [2]. In the physical model, pressures were measured using 145 pressure tap- pings connected to manometers on eight sections across the the primary and secondary spillways. Fig. 3 presents the calculated CFD and measured physical model pressures over the primary spillway crest for q = 48 m³/s/m together with published data from [1]. This represents the scenario with H/Hd = 1 (H = actual head on the crest and Hd = design head. This is plotted using the dimensionless parameters hp/Hd and x/Hd, (hp = pressure head and x = offset from the ogee curve origin). Reasonable correlation is evident between the three data sets.