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Galvan, Sergio,Reggio, Marcelo,Guibault, Francois,Solorio, Gildardo Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.4
The effect of the inlet swirling flow in a hydraulic turbine draft tube is a very complex phenomenon, which has been extensively investigated both theoretically and experimentally. In fact, the finding of the optimal flow distribution at the draft tube inlet in order to get the best performance has remained a challenge. Thus, attempting to answer this question, it was assumed that through an automatic optimization process a Genetic Algorithm would be able to manage a parameterized inlet velocity profile in order to achieve the best flow field for a particular draft tube. As a result of the optimization process, it was possible to obtain different draft-tube flow structures generated by the automatic manipulation of parameterized inlet velocity profiles. Thus, this work develops a qualitative and quantitative analysis of these new draft tube flow field structures provoked by the redesigned inlet velocity profiles. The comparisons among the different flow fields obtained clearly illustrate the importance of the flow uniformity at the end of the conduit. Another important aspect has been the elimination of the re-circulating flow area which used to promote an adverse pressure gradient in the cone, deteriorating the pressure recovery effect. Thanks to the evolutionary optimization strategy, it has been possible to demonstrate that the optimized inlet velocity profile can suppress or mitigate, at least numerically, the undesirable draft tube flow characteristics. Finally, since there is only a single swirl number for which the objective function has been minimized, the energy loss factor might be slightly affected by the flow rate if the same relation of the axial-tangential velocity components is maintained, which makes it possible to scale the inlet velocity field to different operating points.
Zahra Mortazavinia,Ricardo Camarero,Francois Guibault 한국유체기계학회 2018 International journal of fluid machinery and syste Vol.11 No.3
Two-way Fluid-Structure Interaction of a three dimensional hydrofoil subjected to viscous flow at Re=750000 is investigated in this paper. A rectangular, cantilevered NACA66 hydrofoil is simulated using a finite element based CSD code, while the fluid flow is modeled with the incompressible, unsteady Reynolds Averaged Navier-Stokes equations using a finite volume based CFD code. The numerical computations are carried out through a strong coupling between these two separate solvers. The strongly coupled FSI solver is validated by comparing the present numerical results with experimental measurements in a water tunnel at the French Naval Academy. To quantify the FSI effects, rigid (stainless steel) and flexible (POM Poly-acetate) hydrofoils with the same undeformed geometry are simulated and compared.
Validation of a CFD model for hydraulic seals
Roy, Vincent Le,Guibault, Francois,Vu, Thi C. Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4
Optimization of seal geometries can reduce significantly the energetic losses in a hydraulic seal [1], especially for high head runner turbine. In the optimization process, a reliable prediction of the losses is needed and CFD is often used. This paper presents numerical experiments to determine an adequate CFD model for straight, labyrinth and stepped hydraulic seals used in Francis runners. The computation is performed with a finite volume commercial CFD code with a RANS low Reynolds turbulence model. As numerical computations in small radial clearances of hydraulic seals are not often encountered in the literature, the numerical results are validated with experimental data on straight seals and labyrinth seals. As the validation is satisfactory enough, geometrical optimization of hydraulic seals using CFD will be studied in future works.
Unstructured Grid Smoothing for Turbomachinery Applications
Falsafioon, Mehdi,Arabi, Sina,Camarero, Ricardo,Guibault, Francois Korean Society for Fluid machinery 2017 International journal of fluid machinery and syste Vol.10 No.4
In the present study, two mesh smoothing techniques, Laplace and Winslow smoothing techniques, for unstructured grids on turbomachinery application are investigated. These operators are based on the solution of elliptic equations. In the first case, Laplace's equations are solved using a barycentric averaging procedure. Solution of Winslow's equations has been a challenging work for unstructured grids because of existence of cross derivative terms in the equations. This issue is addressed devising a local control volume. Both methods are compared using different grid quality criteria. Finally, these operators have been applied to turbomachinery configurations and the advantages and disadvantages are discussed.
Unstructured Grid Smoothing for Turbomachinery Applications
Mehdi Falsafioon,Sina Arabi,Ricardo Camarero,Francois Guibault 한국유체기계학회 2017 International journal of fluid machinery and syste Vol.10 No.4
In the present study, two mesh smoothing techniques, Laplace and Winslow smoothing techniques, for unstructured grids on turbomachinery application are investigated. These operators are based on the solution of elliptic equations. In the first case, Laplace`s equations are solved using a barycentric averaging procedure. Solution of Winslow`s equations has been a challenging work for unstructured grids because of existence of cross derivative terms in the equations. This issue is addressed devising a local control volume. Both methods are compared using different grid quality criteria. Finally, these operators have been applied to turbomachinery configurations and the advantages and disadvantages are discussed.
Devals, Christophe,Vu, Thi C.,Guibault, Francois Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.3
This paper presents a CFD-based methodology for the prediction of guide vane torque in hydraulic turbine distributor for aligned and misaligned configurations. A misaligned or desynchronized configuration occurs when the opening angle of one guide vane differs from the opening angle of all other guide vanes, which may lead to a torque increase on neighbouring guide vanes. A fully automated numerical procedure is presented, that automates computations for a complete range of operation of a 2D or 3D distributor. Results are validated against laboratory measurements.
Hosseinimanesh, Hossein,Devals, Christophe,Nennemann, Bernd,Guibault, Francois Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.3
No-load speed is an important performance factor for the safe operation of hydropower systems. In turbine design, the manufacturers must conduct several model tests to calculate the accurate value of no-load speed for the complete range of operating conditions, which are expensive and time-consuming. The present study presents steady and unsteady methods for calculating no-load speed of a Francis turbine. The steady simulations are implemented using a commercial flow solver and an iterative algorithm that relies on a smooth relation between turbine torque and speed factor. The unsteady method uses unsteady RANS simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step and friction torque. The main goal of this research is to evaluate and compare the two methods by calculating turbine dynamic parameters for three test cases consisting of high and medium head Francis turbines. Overall, the numerical results agreed well with experimental data. The unsteady method provided more accurate results in the opening angle range from 20 to 26 degrees. Nevertheless, the steady results showed more consistency than unsteady results for the three different test cases at different operating conditions.
Bahrami, Salman,Tribes, Christophe,von Fellenberg, Sven,Vu, Thi C.,Guibault, Francois Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.3
A robust multi-fidelity optimization methodology has been developed, focusing on efficiently handling industrial runner design of hydraulic Francis turbines. The computational task is split between low- and high-fidelity phases in order to properly balance the CFD cost and required accuracy in different design stages. In the low-fidelity phase, a physics-based surrogate optimization loop manages a large number of iterative optimization evaluations. Two derivative-free optimization methods use an inviscid flow solver as a physics-based surrogate to obtain the main characteristics of a good design in a relatively fast iterative process. The case study of a runner design for a low-head Francis turbine indicates advantages of integrating two derivative-free optimization algorithms with different local- and global search capabilities.
Steady and unsteady flow computation in an elbow draft tube with experimental validation
Vu, Thi C.,Devals, Christophe,Zhang, Ying,Nennemann, Bernd,Guibault, Francois Korean Society for Fluid machinery 2011 International journal of fluid machinery and syste Vol.4 No.1
Steady state computations are routinely used by design engineers to evaluate and compare losses in hydraulic components. In the case of the draft tube diffuser, however, experiments have shown that while a significant number of operating conditions can adequately be evaluated using steady state computations, a few operating conditions require unsteady simulations to accurately evaluate losses. This paper presents a study that assesses the predictive capacity of a combination of steady and unsteady RANS numerical computations to predict draft tube losses over the complete range of operation of a Francis turbine. For the prediction of the draft tube performance using k-${\varepsilon}$ turbulence model, a methodology has been proposed to average global performance indicators of steady flow computations such as the pressure recovery factor over an adequate number of periods to obtain correct results. The methodology will be validated using two distinct flow solvers, CFX and OpenFOAM, and through a systematic comparison with experimental results obtained on the FLINDT model draft tube.
Investigation of Cavitation Models for Steady and Unsteady Cavitating Flow Simulation
Tran, Tan Dung,Nennemann, Bernd,Vu, Thi Cong,Guibault, Francois Korean Society for Fluid machinery 2015 International journal of fluid machinery and syste Vol.8 No.4
The objective of this paper is to evaluate the applicability of mass transfer cavitation models and determine appropriate numerical parameters for cavitating flow simulations. CFD simulations were performed for a NACA66 hydrofoil at cavitation numbers of 1.49 and 1.00, corresponding to steady sheet and unsteady sheet/cloud cavitating regimes using the Kubota and Merkle cavitation models. The Merkle model was implemented into CFX by User Fortran code. The Merkle cavitation model is found to give some improvements for cavitating flow simulation results for these cases. Turbulence modeling is also found to have an important contribution to the prediction quality of the simulations. The relationship between the turbulence viscosity modification, in order to take into account the local compressibility at the vapor/liquid interfaces, and the predicted numerical results is clarified. The limitations of current cavitating flow simulation techniques are discussed throughout the paper.