Using FLOW-3D as an Engineering Tool for Hybrid Modeling

This Application Note was contributed by Armen A. Girgidov of JSC The B.E.Vedeneev VNIIG in Saint-Petersburg, Russia, the first winner of the Flow Science 30th Anniversary Simulation Contest.

Recommendations for Type and Construction of the Nizhne-Zejskaya HPP Spillway

There are several ways of carrying out research for designing or operating various hydrotechnical constructions.

Physical Modeling

Physical modeling is the most well-founded research method. Most hydrotechnical objects—spillways, bridges, stilling pools, etc. were built after model experiment. Research results give fully faithful process presentation. However, physical modeling during all the design stages, building and operation is laborious and expensive.

Numerical Simulation

When conducting hydraulic research, numerical simulation can be divided into two basic directions:

• Simulating projected constructions at project stages (preliminary modeling, for example at investment substantiation stage);
• Simulating hydraulic processes for the constructed objects (for example, modeling of possible hydrodynamic failures on hydrotechnical facilities).

In both cases, the mathematical model is verified by empirical data. This simulation method costs much less than physical modeling and can easily be performed. However, numerical simulations need to be validated. It is necessary to consider that running only numerical simulations of hydrodynamic processes for hydrotechnical facilities is not as accurate as building a physical model, even if they are verified on a number of classical and/or a few specialized problems. This purely computational method may not consider all the possible parameters that are in a physical model.

Hybrid Modeling

Hybrid modeling combines both physical and numerical modeling. During an initial research stage, physical and mathematical models are constructed. Using physical models, researchers collect all the model parameters (roughness, for example) and water stream parameters (discharge, velocity, etc.) that can be used in the mathematical model. After that, the numerical simulations are conducted and all the output data are verified by the data from the physical models. If the data agree, then the mathematical model is calibrated.

When the mathematical model gives the hydraulic parameters of the physical models with sufficient accuracy, the further need for the physical models will have been removed and so they can be disassembled.

Nizhne-Zejskaya Hydroelectric Complex

Figure 1. Physical model of the Nizhne-Zejskaya HPP spillway
(the view from tail-water).

Figure 2. 3-D stereolithography model of the Nizhne-Zejskaya
HPP spillway with topography and river (the view from tail-water).

One of the first projects in Russia in which hybrid modeling was used was studying the Nizhne-Zejskaya hydroelectric complex at the hydraulic laboratory of the B.E.Vedeneev VNIIG (Saint-Petersburg, Russia). The spatial physical model of the hydroelectric complex with the scale factor 1:100 (Fig. 1) was constructed and tested. Hydraulic parameters such as discharge, water stream velocity, and pressure acting on a stilling pool bottom were investigated.

At the same time, a mathematical three-dimensional model of the hydroelectric complex including its dam, spillway, hydropower plant model, and topography around the Zeya River (Fig. 2) was created in natural scale. Physical model parameters such as roughness were included into the mathematical model.

FLOW-3D was then used for numerical simulation. FLOW-3D results were compared to physical modeling results and calculation results derived by Russian standards. The spillway discharge is shown in Fig. 3. From the figure, it can be seen that discharge estimation yielded by the physical model is only slightly more conservative in comparison to the numerical modeling and standard calculations. At the same time, comparisons of the pressure diagrams on the stilling pool bottom (Fig. 4) and the maximum velocities in stilling pool agree well (Fig. 5). The velocities in the tail-race also show good comparison between the results (Figs. 6-7).

 Figure 6b. The velocities in the tail-race for 0.1% discharge in numerical simulation. Figure 7. The velocities in the tail-race for 1% discharge in numerical simulation.

References

FLOW-3D User's Manual, Version 9.3, Flow Science, Inc., 2008.

Recommendations for hydraulic calculations of the spillways (Rus). Рекомендации по идравлическому расчёту водосливов. /Ч.1. Прямые водосливы. Л. Энергия.1974. 38 с.

Technical information "Hydraulic researches of the Nizhne-Zejskaya HPP spillway.  Recommendations for type and construction" Part 4. Compares physical   modeling   and   numerical  simulation(Rus.) ТЕХНИЧЕСКАЯ ИНФОРМАЦИЯ  по  договору №  1 – 286 – 061 “Гидравлические исследования водосбросных  сооружений  Нижне  –  Зейской  ГЭС.  Рекомендации типа и конструкций”   Этап   4   «Сопоставление  результатов  исследований  с результатами   математического  оделирования»,   ОАО   "ВНИИИГ   им. Б.Е.Веденеева", СПб, 2009.