Application of Turbulence Models to Separated Flow Over Rough Surfaces

1995 ◽  
Vol 117 (2) ◽  
pp. 234-241 ◽  
Author(s):  
V. C. Patel ◽  
J. Y. Yoon
Keyword(s):  
Sand Dune ◽  
Numerical Solutions ◽  
Turbulence Models ◽  
Separated Flow ◽  
Complex Flows ◽  
Flow Equations ◽  
Wide Range ◽  
Wall Flow ◽  
Extensive Experimental Data ◽  
Moody Diagram

Principal results of classical experiments on the effects of sandgrain roughness are briefly reviewed, along with various models that have been proposed to account for these effects in numerical solutions of the fluid-flow equations. Two models that resolve the near-wall flow are applied to the flow in a two-dimensional, rough-wall channel. Comparisons with analytical results embodied in the well-known Moody diagram show that the k–ω model of Wilcox performs remarkably well over a wide range of roughness values, while a modified two-layer k–ε based model requires further refinement. The k–ω model is applied to water flow over a fixed sand dune for which extensive experimental data are available. The solutions are found to be in agreement with data, including the flow in the separation eddy and its recovery after reattachment. The results suggest that this modeling approach may be extended to other types of surface roughness, and to more complex flows.

scholarly journals Derivation of Global Parametric Performance of Mixed Flow Hydraulic Turbine Using CFD

1970 ◽  
Vol 7 ◽  
pp. 60-64 ◽  
Author(s):  
Ruchi Khare ◽  
Vishnu Prasad Prasad ◽  
Sushil Kumar
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Numerical Solutions ◽  
Guide Vane ◽  
Hydraulic Turbine ◽  
Francis Turbine ◽  
Mixed Flow ◽  
Flow Equations ◽  
Laboratory Setup ◽  
Wide Range

The testing of physical turbine models is costly, time consuming and subject to limitations of laboratory setup to meet International Electro technical Commission (IEC) standards. Computational fluid dynamics (CFD) has emerged as a powerful tool for funding numerical solutions of wide range of flow equations whose analytical solutions are not feasible. CFD also minimizes the requirement of model testing. The present work deals with simulation of 3D flow in mixed flow (Francis) turbine passage; i.e., stay vane, guide vane, runner and draft tube using ANSYS CFX 10 software for study of flow pattern within turbine space and computation of various losses and efficiency at different operating regimes. The computed values and variation of performance parameters are found to bear close comparison with experimental results.Key words: Hydraulic turbine; Performance; Computational fluid dynamics; Efficiency; LossesDOI: 10.3126/hn.v7i0.4239Hydro Nepal Journal of Water, Energy and EnvironmentVol. 7, July, 2010Page: 60-64Uploaded date: 31 January, 2011

CFD Simulation of Ducted Dual Rotor Wind Turbine for Small-Scale Applications

2019 ◽  
Author(s):  
Amr Mohamed ◽  
Ahmed El-Baz ◽  
Nabil Mahmoud ◽  
Ashraf Hamed ◽  
Ahmed El-kohly
Keyword(s):  
Wind Turbine ◽  
Output Power ◽  
Cfd Simulation ◽  
Turbulence Models ◽  
Small Scale ◽  
Continuous Increase ◽  
Flow Equations ◽  
Wind Speeds ◽  
Wide Range ◽  
Power Curves

Abstract Due to growing needs for energy in our life, research in the wind energy field has increased significantly. There has been global concern towards the development of smart techniques and devices that could optimize the energy conversion and maximize the output power from the wind. Investigating such alternative solutions are required in order to meet the continuous increase in the power demand. The Dual Rotor Wind Turbine system (DRWT) offers higher energy extraction rates from the wind. In the present study, it is proposed to utilize the dual rotor configuration in a ducted system using wind lens in order to enable its application in regions of low wind speeds. The aerodynamic performance of ducted dual rotor wind turbine is investigated using CFD to solve three dimensional, turbulent-steady incompressible flow equations, using the k-ε Realizable and k-ω shear stress transport (SST) turbulence models. Several difficulties due to complexity of geometry and meshing requirements have been encountered. Mesh independence study was conducted to ensure the accuracy and validate the results. Power curves were obtained, detailed investigation of the wind turbine performance in different configurations are highlighted in order to explore the benefit and effect of each configuration to the output power. The final results of combined configuration for dual rotor wind turbine (DRWT) with lens show a considerable improvement to the performance of wind turbine over wide range of wind speeds.

scholarly journals Flood Hydrograph Prediction Using Machine Learning Methods

Water ◽  
2018 ◽  
Vol 10 (8) ◽  
pp. 968 ◽  
Author(s):  
Gokmen Tayfur ◽  
Vijay Singh ◽  
Tommaso Moramarco ◽  
Silvia Barbetta
Keyword(s):  
Machine Learning ◽  
Numerical Solutions ◽  
Central Italy ◽  
Computing Methods ◽  
Flood Routing ◽  
Flow Equations ◽  
Time To Peak ◽  
Wide Range ◽  
Flood Hydrograph ◽  
Artificial Neural Network Ann

Machine learning (soft) methods have a wide range of applications in many disciplines, including hydrology. The first application of these methods in hydrology started in the 1990s and have since been extensively employed. Flood hydrograph prediction is important in hydrology and is generally done using linear or nonlinear Muskingum (NLM) methods or the numerical solutions of St. Venant (SV) flow equations or their simplified forms. However, soft computing methods are also utilized. This study discusses the application of the artificial neural network (ANN), the genetic algorithm (GA), the ant colony optimization (ACO), and the particle swarm optimization (PSO) methods for flood hydrograph predictions. Flow field data recorded on an equipped reach of Tiber River, central Italy, are used for training the ANN and to find the optimal values of the parameters of the rating curve method (RCM) by the GA, ACO, and PSO methods. Real hydrographs are satisfactorily predicted by the methods with an error in peak discharge and time to peak not exceeding, on average, 4% and 1%, respectively. In addition, the parameters of the Nonlinear Muskingum Model (NMM) are optimized by the same methods for flood routing in an artificial channel. Flood hydrographs generated by the NMM are compared against those obtained by the numerical solutions of the St. Venant equations. Results reveal that the machine learning models (ANN, GA, ACO, and PSO) are powerful tools and can be gainfully employed for flood hydrograph prediction. They use less and easily measurable data and have no significant parameter estimation problem.

Perspective: Flow at High Reynolds Number and Over Rough Surfaces—Achilles Heel of CFD

1998 ◽  
Vol 120 (3) ◽  
pp. 434-444 ◽  
Author(s):  
V. C. Patel
Keyword(s):  
Numerical Solutions ◽  
Practical Importance ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Law Of The Wall ◽  
High Reynolds Numbers ◽  
Wall Flow ◽  
High Reynolds ◽  
Near Wall

The law of the wall and related correlations underpin much of current computational fluid dynamics (CFD) software, either directly through use of so-called wall functions or indirectly in near-wall turbulence models. The correlations for near-wall flow become crucial in solution of two problems of great practical importance, namely, in prediction of flow at high Reynolds numbers and in modeling the effects of surface roughness. Although the two problems may appear vastly different from a physical point of view, they share common numerical features. Some results from the ’superpipe’ experiment at Princeton University are analyzed along with those of previous experiments on the boundary layer on an axisymmetric body to identify features of near-wall flow at high Reynolds numbers that are useful in modeling. The study is complemented by a review of some computations in simple and complex flows to reveal the strengths and weaknesses of turbulence models used in modern CFD methods. Similarly, principal results of classical experiments on the effects of sand-grain roughness are reviewed, along with various models proposed to account for these effects in numerical solutions. Models that claim to resolve the near-wall flow are applied to the flow in rough-wall pipes and channels to illustrate their power and limitations. The need for further laboratory and numerical experiments is clarified as a result of this study.

Turbulence Modeling and Computation of Viscous Transitional Flows for Low Pressure Turbines

1999 ◽  
Vol 121 (4) ◽  
pp. 824-833 ◽  
Author(s):  
A. Chernobrovkin ◽  
B. Lakshminarayana
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Separated Flow ◽  
Low Pressure ◽  
Reynolds Stress Model ◽  
Suction Surface ◽  
Transitional Flows ◽  
Wide Range

Variation of the flow Reynolds number between the take off and cruise conditions significantly affects the boundary layer development on low-pressure turbine blading. A decreased Reynolds number leads to the flow separation on the suction surface of the blading and increased losses. A numerical simulation has been carried out to assess the ability of a Navier-Stokes solver to predict transitional flows in a wide range of Reynolds numbers and inlet turbulence intensities. A number of turbulence models (including the Algebraic Reynolds Stress Model) and transition models have been employed to analyze the reliability and accuracy of the numerical simulation. A comparison between the prediction and the experimental data reveals good correlation. However, the analysis shows that the artificial dissipation in the numerical solver may have a profound effect on the prediction of the transition in a separated flow.

The Use of Von Karman’S Similarity Hypothesis in Turbulent Fluid Flow Computation

1992 ◽  
Author(s):  
David J. Foster
Keyword(s):  
Turbulent Mixing ◽  
Numerical Solutions ◽  
Separated Flow ◽  
Mixing Length ◽  
Two Dimensional ◽  
Turbulent Fluid Flow ◽  
Flow Equations ◽  
Similarity Hypothesis ◽  
Step Flow ◽  
Type Flow

Abstract Von Karman’s similarity hypothesis for the turbulent mixing length in boundary layer type flow is extrapolated to a plausable two dimensional expression. The corresponding incompressible turbulent flow equations are developed in terms of the transient vorticity transfer and stream function equations. Numerical solutions for the separated flow behind a step and in a rectangular cavity were obtained and the results are presented pictorally. The streamlines computed for the step flow solution compare favorably with those calculated from experimental measurements at a similar Reynolds Number.

A Numerical and Experimental Investigation of Turbulent Heat Transport Downstream From an Abrupt Pipe Expansion

1983 ◽  
Vol 105 (4) ◽  
pp. 862-869 ◽  
Author(s):  
R. S. Amano ◽  
M. K. Jensen ◽  
P. Goel
Keyword(s):  
Heat Transfer ◽  
Numerical Solutions ◽  
Numerical Study ◽  
Separated Flow ◽  
Flow Region ◽  
Reynolds Numbers ◽  
Turbulent Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Pipe Expansion

An experimental and numerical study is reported on heat transfer in the separated flow region created by an abrupt circular pipe expansion. Heat transfer coefficients were measured along the pipe wall downstream from an expansion for three different expansion ratios of d/D = 0.195, 0.391, and 0.586 for Reynolds numbers ranging from 104 to 1.5 × 105. The results are compared with the numerical solutions obtained with the k ∼ ε turbulence model. In this computation a new finite difference scheme is developed which shows several advantages over the ordinary hybrid scheme. The study also covers the derivation of a new wall function model. Generally good agreement between the measured and the computed results is shown.

Redesign of High-Lift LP-Turbine Airfoils for Low Speed Testing

2010 ◽  
Author(s):  
Michele Marconcini ◽  
Filippo Rubechini ◽  
Roberto Pacciani ◽  
Andrea Arnone ◽  
Francesco Bertini
Keyword(s):  
Separated Flow ◽  
Low Pressure ◽  
High Lift ◽  
Low Speed ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Flow Configurations ◽  
Artificial Neural Network Ann ◽  
Lp Turbine ◽  
Turbine Airfoils

Low pressure turbine airfoils of the present generation usually operate at subsonic conditions, with exit Mach numbers of about 0.6. To reduce the costs of experimental programs it can be convenient to carry out measurements in low speed tunnels in order to determine the cascades performance. Generally speaking, low speed tests are usually carried out on airfoils with modified shape, in order to compensate for the effects of compressibility. A scaling procedure for high-lift, low pressure turbine airfoils to be studied in low speed conditions is presented and discussed. The proposed procedure is based on the matching of a prescribed blade load distribution between the low speed airfoil and the actual one. Such a requirement is fulfilled via an Artificial Neural Network (ANN) methodology and a detailed parameterization of the airfoil. A RANS solver is used to guide the redesign process. The comparison between high and low speed profiles is carried out, over a wide range of Reynolds numbers, by using a novel three-equation, transition-sensitive, turbulence model. Such a model is based on the coupling of an additional transport equation for the so-called laminar kinetic energy (LKE) with the Wilcox k–ω model and it has proven to be effective for transitional, separated-flow configurations of high-lift cascade flows.

scholarly journals A Multiple-Grid Lattice Boltzmann Method for Natural Convection under Low and High Prandtl Numbers

Fluids ◽  
2021 ◽  
Vol 6 (4) ◽  
pp. 148
Author(s):  
Seyed Amin Nabavizadeh ◽  
Himel Barua ◽  
Mohsen Eshraghi ◽  
Sergio D. Felicelli
Keyword(s):  
Natural Convection ◽  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Large Scale ◽  
Bgk Model ◽  
Flow Equations ◽  
Wide Range ◽  
Prandtl Numbers ◽  
Boltzmann Method ◽  
Benchmark Solutions

A multi-distribution lattice Boltzmann Bhatnagar–Gross–Krook (BGK) model with a multiple-grid lattice Boltzmann (MGLB) model is proposed to efficiently simulate natural convection over a wide range of Prandtl numbers. In this method, different grid sizes and time steps for heat transfer and fluid flow equations are chosen. The model is validated against natural convection in a square cavity, since extensive benchmark solutions are available for that problem. The proposed method can resolve the computational difficulty in simulating problems with very different time scales, in particular, when using extremely low or high Prandtl numbers. The technique can also enhance computational speed and stability while keeping the simplicity of the BGK method. Compared with the conventional lattice Boltzmann method, the simulation time can be reduced up to one-tenth of the time while maintaining the accuracy in an acceptable range. The proposed model can be extended to other lattice Boltzmann collision models and three-dimensional cases, making it a great candidate for large-scale simulations.

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