scholarly journals Surrogate models for the blade element momentum aerodynamic model using non-intrusive Polynomial Chaos Expansions

2021 ◽  
Author(s):  
Rad Haghi ◽  
Curran Crawford
Keyword(s):  
Wind Turbine ◽  
Polynomial Chaos ◽  
Computational Cost ◽  
Turbulence Models ◽  
Accurate Estimation ◽  
Extreme Statistics ◽  
Time Step ◽  
Minimum Requirement ◽  
Thrust And Torque ◽  
Blade Element

Abstract. In typical industrial practice based on IEC standards, wind turbine simulations are computed in the time domain for each mean wind speed bin using a few number of unsteady wind seeds. Software such as FAST, BLADED or HAWC2 can be used to capture the unsteadiness and uncertainties of the wind in the simulations. The statistics of these aeroelastic simulations output are extracted and used to calculate fatigue and extreme loads on the wind turbine components. The minimum requirement of having six seeds does not guarantee an accurate estimation of the overall statistics. One solution might be running more seeds; however, this will increase the computation cost. Moreover, to move beyond Blade Element Momentum (BEM) based tools toward vortex/potential flow formulations, a reduction in the computational cost associated with the unsteady flow and uncertainty handling is required. This study illustrates the unsteady wind aerodynamic statistics' stationary character based on the standard turbulence models. This character is shown based on the output of NREL 5MW simulations. Afterwards, we propose a non-intrusive Polynomial Chaos Expansion approach to build a surrogate model of the loads' statistics, NREL 5MW rotor thrust and torque, at each time step, to estimate the extreme statistics more accurately and efficiently.

scholarly journals Surrogate models for unsteady aerodynamics using non-intrusive Polynomial Chaos Expansions

2020 ◽  
Author(s):  
Rad Haghi ◽  
Curran Crawford
Keyword(s):  
Wind Turbine ◽  
Polynomial Chaos ◽  
Computational Cost ◽  
Unsteady Aerodynamics ◽  
Turbulence Models ◽  
Accurate Estimation ◽  
Time Step ◽  
Extreme Loads ◽  
The Time Domain ◽  
Polynomial Chaos Expansions

Abstract. In common industrial practice based on IEC standards, wind turbine simulations are computed in the time domain for each mean wind speed bin using six unsteady wind seeds. Different software such as FAST, Balded or HAWC2 can be used to this purpose, to capture the unsteadiness and uncertainties of the wind in the simulations. The statistics of these simulations are extracted and used to calculate fatigue and extreme loads on the wind turbine components. Having only six seeds does not guarantee an accurate estimation of the overall statistics. One solution might be running more seeds; however, this will increase the computation cost. Moreover, to move beyond Blade Element Momentum based tools toward vortex/potential flow formulations, a reduction in the computational cost associated with the unsteady flow and uncertainty handling is required. This study illustrates the stationary character of the unsteady aerodynamic statistics based on the standard turbulence models. Afterward, we propose a non-intrusive Polynomial Chaos Expansion to build a surrogate model of the loads' statistics at each time step, to estimate the statistics more accurately and efficiently.

Strongly Coupled Fluid-Structure Interactions via a New Navier-Stokes Method for Prediction of Vortex-Induced Vibrations

2005 ◽  
Author(s):  
Yannis Kallinderis ◽  
Hyung Taek Ahn
Keyword(s):  
Stokes Equations ◽  
Computational Cost ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Design Tool ◽  
Navier Stokes ◽  
Time Step ◽  
Navier Stokes Equations ◽  
Vortex Induced Vibrations ◽  
Offshore Industry

Numerical prediction of vortex-induced vibrations requires employment of the unsteady Navier-Stokes equations. Current Navier-Stokes solvers are quite expensive for three-dimensional flow-structure applications. Acceptance of Computational Fluid Dynamics as a design tool for the offshore industry requires improvements to current CFD methods in order to address the following important issues: (i) stability and computation cost of the numerical simulation process, (ii) restriction on the size of the allowable time-step due to the coupling of the flow and structure solution processes, (iii) excessive number of computational elements for 3-D applications, and (iv) accuracy and computational cost of turbulence models used for high Reynolds number flow. The above four problems are addressed via a new numerical method which employs strong coupling between the flow and the structure solutions. Special coupling is also employed between the Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model. An element-type independent spatial discretization scheme is also presented which can handle general hybrid meshes consisting of hexahedra, prisms, pyramids, and tetrahedral.

Analysis of the Aeroelastic Dynamics of Wind-Turbine Blades

2012 ◽  
Author(s):  
Lucas I. Lago ◽  
Fernando L. Ponta ◽  
Alejandro D. Otero
Keyword(s):  
Wind Turbine ◽  
Strain Energy ◽  
Structural Model ◽  
Numerical Models ◽  
Turbine Blades ◽  
Computational Cost ◽  
Full Scale ◽  
Wind Turbine Blades ◽  
Time Step ◽  
Aerodynamic Model

Reducing the uncertainties related to blade dynamics by the improvement of the quality of numerical simulations of the fluid structure interaction process is a key for a breakthrough in windturbine technology. A fundamental step in that direction is the implementation of aeroelastic models capable of capturing the complex features of innovative prototype blades, so they can be tested at realistic full-scale conditions with a reasonable computational cost. We make use of a code based on a combination of two advanced numerical models implemented in a parallel HPC supercomputer platform: First, a model of the structural response of heterogeneous composite blades, based on a variation of the dimensional reduction technique proposed by Hodges and Yu. This technique has the capacity of reducing the geometrical complexity of the blade section into a stiffness matrix for an equivalent beam. The reduced 1-D strain energy is equivalent to the actual 3-D strain energy in an asymptotic sense, allowing accurate modeling of the blade structure as a1-D finite-element problem. This substantially reduces the computational effort required to model the structural dynamics at each time step. Second, a novel aerodynamic model based on an advanced implementation of the BEM (Blade Element Momentum) Theory; where all velocities and forces are re-projected through orthogonal matrices into the instantaneous deformed configuration to fully include the effects of large displacements and rotation of the airfoil sections into the computation of aerodynamic forces. This allows the aerodynamic model to take into account the effects of the complex flexo-torsional deformation that can be captured by the more sophisticated structural model mentioned above. In this presentation, we report some recent results we have obtained applying our code to full-scale composite laminate wind-turbine blades, analyzing the fundamental vibrational modes and the stress load in normal operational conditions.

Unsteady RANS and scale adaptive simulations of a turbulent spray flame in a swirled-stabilized gas turbine model combustor using tabulated chemistry

2015 ◽  
Vol 25 (5) ◽  
pp. 1064-1088 ◽  
Author(s):  
Alain Fossi ◽  
Alain DeChamplain ◽  
Benjamin Akih-Kumgeh
Keyword(s):  
Gas Turbine ◽  
Gas Turbines ◽  
Fluid Dynamic ◽  
Computational Cost ◽  
Turbulence Models ◽  
Combustion Model ◽  
Time Step ◽  
Content Type ◽  
Spray Flame ◽  
Tabulated Chemistry

Purpose – The purpose of this paper is to numerically investigate the three-dimensional (3D) reacting turbulent two-phase flow field of a scaled swirl-stabilized gas turbine combustor using the commercial computational fluid dynamic (CFD) software ANSYS FLUENT. The first scope of the study aims to explicitly compare the predictive capabilities of two turbulence models namely Unsteady Reynolds Averaged Navier-Stokes and Scale Adaptive Simulation for a reasonable trade-off between accuracy of results and global computational cost when applied to simulate swirl-stabilized spray combustion. The second scope of the study is to couple chemical reactions to the turbulent flow using a realistic chemistry model and also to model the local chemical non-equilibrium(NEQ) effects caused by turbulent strain such as flame stretching. Design/methodology/approach – Standard Eulerian and Lagrangian formulations are used to describe both gaseous and liquid phases, respectively. The computing method includes a two-way coupling in which phase properties and spray source terms are interchanging between the two phases within each coupling time step. The fuel used is liquid jet-A1 which is injected in the form of a polydisperse spray and the droplet evaporation rate is calculated using the infinite conductivity model. One-component (n-decane) and two-component fuels (n-decane+toluene) are used as jet-A1 surrogates. The combustion model is based on the mean mixture fraction and its variance, and a presumed-probability density function is used to model turbulent-chemistry interactions. The instantaneous thermochemical state necessary for the chemistry tabulation is determined by using initially the equilibrium (EQ) assumption and thereafter, detailed NEQ calculations through the steady flamelets concept. The combustion chemistry of these surrogates is represented through a reduced chemical kinetic mechanism (CKM) comprising 1,045 reactions among 139 species, derived from the detailed jet-A1 surrogate model, JetSurf 2.0 using a sensitivity based method, Alternate Species Elimination. Findings – Numerical results of the gas velocity, the gas temperature and the species molar fractions are compared with their experimental counterparts obtained from a steady state flame available in the literature. It is observed that, SAS coupled to the tabulated flamelet-based chemistry, predicts reasonably the main flame trends, while URANS even provided with the same combustion model and computing resources, leads to a poor prediction of the global flame trends, emphasizing the asset of a proper resolution when simulating spray flames. Research limitations/implications – The steady flamelet model even coupled with a robust turbulence model does not reproduce accurately the trend of species with slow oxidation kinetics such as CO and H2, because of the restrictiveness of the solutions space of flamelet equations and the assumption of unity Lewis for all species. Practical implications – This work is adding a contribution for spray flame modeling and can be seen as an extension to the significant efforts for the modeling of gaseous flames using robust turbulence models coupled with the tabulated flamelet-based chemistry approach to considerably reduce computing cost. The exclusive use of a commercial CFD code widely used in the industry allows a direct application of this simulation approach to industrial configurations while keeping computing cost reasonable. Originality/value – This study is useful to engineers interested in designing combustors of gas turbines and others combustion systems fed with liquid fuels.

scholarly journals Computational fluid dynamic (CFD) of vertical-axis wind turbine: mesh and time-step sensitivity study

2019 ◽  
Vol 13 (3) ◽  
pp. 5604-5624
Author(s):  
S. Ashwindran ◽  
A. A. Azizuddin ◽  
A. N. Oumer
Keyword(s):  
Wind Turbine ◽  
Numerical Study ◽  
Computational Cost ◽  
Vertical Axis ◽  
Sensitivity Study ◽  
Dynamic Mode ◽  
Vertical Axis Wind Turbine ◽  
Time Step ◽  
Sliding Mesh ◽  
Mesh Resolution

This paper presents mesh and time-step dependence study of newly designed drag type vertical axis wind turbine. Ansys FLUENT a commercially available CFD solver was used to perform CFD numerical study on the drag type wind turbine. In computational analysis, 2D models was simulated under unsteady flow fields using SST k-ω to achieve stabilized numerical convergence. The model was analyzed at static and dynamic mode, where sliding mesh technique was used to analyze the turbine in dynamic mode. Three main parameters were taken under careful consideration: mesh resolution, turbulence model and time-step. Aerodynamic force was used in mesh sensitivity study for both static and sliding mesh. A small discrepancy in results of 2D sliding mesh result at different time-step and mesh resolution was observed. The generated results showed good agreement between fine and medium mesh with small difference in the initial initialization. In time-step dependency study for static mesh, dt=0.0002 time-step size was chosen for economical computational cost.

Numerical Validation of Empirical Formula for Estimating Airfoil Drag Coefficient at Extreme Angles of Attack in Wind Turbine Applications

2015 ◽  
Author(s):  
Jaikumar Loganathan ◽  
Srinath Narayanamurthy
Keyword(s):  
Wind Turbine ◽  
Empirical Formula ◽  
Turbine Blades ◽  
Turbulence Models ◽  
Leading Edge ◽  
Operating Conditions ◽  
Accurate Estimation ◽  
Efficient Design ◽  
Wind Turbine Blades ◽  
Empirical Formulas

For optimal structural design of wind turbine blades, aerodynamic loads need to be estimated at all wind and operating conditions. Under parked conditions or during an emergency stop or in the event of a sudden gust, the blades can experience very high angles of attack (AOA). Generally, loads at these high AOA are design driving and hence an accurate estimation of force coefficients at these conditions are critical for efficient design. Experimental investigation of airfoils at high AOA in a wind tunnel is always a challenge due to blockage effects, Reynolds number limitation and large unsteady wake motion. Numerical simulations have their own deficiencies mainly associated with the limitations of turbulence models. Empirical formulas which are derived from experimental data for a variety of airfoils have been used with some degree of success. One such empirical formula proposed by C. Lindenburg is based on a limited set of airfoil geometry parameters like leading edge radius and wedge angles. In this study a CFD based numerical investigation is carried out on simplified airfoil geometries to validate the assumptions made in Linderburg’s formula.

Accelerating the High-Fidelity Simulation of Turbulence: Ensemble Averaging

2016 ◽  
Author(s):  
Vakhtang Makarashvili ◽  
Elia Merzari ◽  
Aleksandr Obabko ◽  
Paul Fischer ◽  
Andrew Siegel
Keyword(s):  
Time Scales ◽  
Time Scale ◽  
Computational Cost ◽  
Turbulence Models ◽  
Ensemble Averaging ◽  
Time Step ◽  
Scale Separation ◽  
Industrial Systems ◽  
Time Scale Separation ◽  
Turbulent Statistics

Computational fluid dynamics (CFD) is increasingly used to simulate complex industrial systems. Most CFD analysis relies on the Reynolds-averaged Navier-Stokes (RANS) approach and traditional two-equation turbulence models. Higher-fidelity approaches to the simulation of turbulence such as wall-resolved large eddy simulation (LES) and direct numerical simulation (DNS) remain limited to smaller applications or to large supercomputing platforms. Nonetheless, continued advances in supercomputing are enabling the simulation of physical systems of increasing size and complexity. These simulations can be used to gain unprecedented insight into the physics of turbulence in complex flows and will become more widespread as petascale architectures become more accessible. As the scale and size of LES and DNS simulations increase, however, the limitations of current algorithms become apparent. For larger systems, more temporal and spatial scale must be resolved, thus increasing the time-scale separation. While the smaller time scales dictate the size and the computational cost associated with each time step, the larger time scales dictate the length of the transient. An increased time-scale separation leads to smaller time steps and longer transients, eventually leading to simulations that are impractical or infeasible. In practice the presence of multiple and strongly separated time scales limits the effectiveness of CFD algorithms for LES and DNS applied to large industrial systems. Moreover, the situation is likely to become worse as even larger systems are simulated, thus increasing the size and length of transients. At the same time transients currently simulated on petascale architectures are unlikely to become any faster on exascale architectures. This paper presents an ensemble-averaging technique for transient simulations, aimed at collecting averaged turbulent statistics faster. The focus is on ergodic flows and simulations. Ensemble averaging involves creating multiple models and combining them to produce a desired output. This technique is commonplace in machine learning and artificial neural networks, and it is at the basis of RANS/URANS turbulence modeling. In the proposed approach, multiple instances of the same ergodic flows are simulated in parallel for a short time and summed to create an ensemble. Provided each instance is sufficiently statistically decorrelated, this allows considerable reduction in the time to solution. This paper focuses on the theory and implementation of the methodology in Nek5000, a massively parallel open-source spectral-element code. Also presented is the application of the method to the DNS and LES simulation of channel flow and pipe flow.

Performance enhancement of Savonius wind turbine by blade shape and twisted angle modifications

2021 ◽  
pp. 095765092098794
Author(s):  
Ahmed M Nagib Elmekawy ◽  
Hassan A Hassan Saeed ◽  
Sadek Z Kassab
Keyword(s):  
Wind Turbine ◽  
Performance Enhancement ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Vertical Axis ◽  
Cfd Simulations ◽  
Sliding Mesh ◽  
Blade Shape ◽  
Rotor Shape ◽  
Twisting Angle

Three-dimensional CFD simulations are carried out to study the increase of power generated from Savonius vertical axis wind turbines by modifying the blade shape and blade angel of twist. Twisting angle of the classical blade are varied and several proposed novel blade shapes are introduced to enhance the performance of the wind turbine. CFD simulations have been performed using sliding mesh technique of ANSYS software. Four turbulence models; realizable k -[Formula: see text], standard k - [Formula: see text], SST transition and SST k -[Formula: see text] are utilized in the simulations. The blade twisting angle has been modified for the proposed dimensions and wind speed. The introduced novel blade increased the power generated compared to the classical shapes. The two proposed novel blades achieved better power coefficients. One of the proposed models achieved an increase of 31% and the other one achieved 32.2% when compared to the classical rotor shape. The optimum twist angel for the two proposed models achieved 5.66% and 5.69% when compared with zero angle of twist.

scholarly journals COMPUTATIONAL FLUID DYNAMICS STUDY OF DUSTY AIR FLOW OVER NACA 63415 AIRFOIL FOR WIND TURBINE APPLICATIONS

2017 ◽  
Vol 79 (7-3) ◽  
Author(s):  
Iham F. Zidane ◽  
Khalid M. Saqr ◽  
Greg Swadener ◽  
Xianghong Ma ◽  
Mohamed F. Shehadeh
Keyword(s):  
Numerical Simulation ◽  
Wind Turbine ◽  
South African ◽  
Stokes Equations ◽  
Lift Coefficient ◽  
Turbine Blades ◽  
Aerodynamic Performance ◽  
Accurate Estimation ◽  
African Countries ◽  
Sand Particles

Gulf and South African countries have enormous potential for wind energy. However, the emergence of sand storms in this region postulates performance and reliability challenges on wind turbines. This study investigates the effects of debris flow on wind turbine blade performance. In this paper, two-dimensional incompressible Navier-Stokes equations and the transition SST turbulence model are used to analyze the aerodynamic performance of NACA 63415 airfoil under clean and sandy conditions. The numerical simulation of the airfoil under clean surface condition is performed at Reynolds number 460×103, and the numerical results have a good consistency with the experimental data. The Discrete Phase Model has been used to investigate the role sand particles play in the aerodynamic performance degradation. The pressure and lift coefficients of the airfoil have been computed under different sand particles flow rates. The performance of the airfoil under different angle of attacks has been studied. Results showed that the blade lift coefficient can deteriorate by 28% in conditions relevant to the Gulf and South African countries sand storms. As a result, the numerical simulation method has been verified to be economically available for accurate estimation of the sand particles effect on the wind turbine blades.

Fast Conjugate Heat Transfer Simulation of Long Transient Flexible Operations Using Adaptive Time Stepping

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
R. Maffulli ◽  
L. He ◽  
P. Stein ◽  
G. Marinescu
Keyword(s):  
Heat Transfer ◽  
Conjugate Heat Transfer ◽  
Time Derivative ◽  
Computational Cost ◽  
Strong Impact ◽  
Time Step ◽  
Time Stepping ◽  
Adaptive Time ◽  
Fully Coupled ◽  
Adaptive Time Stepping

The emerging renewable energy market calls for more advanced prediction tools for turbine transient operations in fast startup/shutdown cycles. Reliable numerical analysis of such transient cycles is complicated by the disparity in time scales of the thermal responses in fluid and solid domains. Obtaining fully coupled time-accurate unsteady conjugate heat transfer (CHT) results under these conditions would require to march in both domains using the time-step dictated by the fluid domain: typically, several orders of magnitude smaller than the one required by the solid. This requirement has strong impact on the computational cost of the simulation as well as being potentially detrimental to the accuracy of the solution due to accumulation of round-off errors in the solid. A novel loosely coupled CHT methodology has been recently proposed, and successfully applied to both natural and forced convection cases that remove these requirements through a source-term based modeling (STM) approach of the physical time derivative terms in the relevant equations. The method has been shown to be numerically stable for very large time steps with adequate accuracy. The present effort is aimed at further exploiting the potential of the methodology through a new adaptive time stepping approach. The proposed method allows for automatic time-step adjustment based on estimating the magnitude of the truncation error of the time discretization. The developed automatic time stepping strategy is applied to natural convection cases under long (2000 s) transients: relevant to the prediction of turbine thermal loads during fast startups/shutdowns. The results of the method are compared with fully coupled unsteady simulations showing comparable accuracy with a significant reduction of the computational costs.

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