Wind Turbine Model Validation Is Improved by High‐Resolution, Measurement‐Derived Inflows

Computational fluid dynamics (CFD) is a simulation tool, that employs applied mathematics and powerful computer to model fluid flow situations to anticipate the mass, momentum and heat transfer and most favorable design in industrial processes. Turbine is a rotary engine actuated by the impulse or reaction of a current of fluid subject to pressure which is made with a series of curved vanes on a central rotating spindle. CFD at present is the most vital tool to design and analyze different types of turbines. This paper is a review of work carried out in the analysis of flow in different turbines employing CFD software. Literature review of CFD analysis in turbines by various researchers is elucidated. In addition, the merits and demerits of CFD analysis of flow in turbines are mentioned. Future work germane to CFD investigation in turbines is also provided.

Modeling of the flow in a Darrieus water turbine: Wall grid refinement analysis and comparison with experiments

Variable geometry twin-entry turbine has advantages in fuel economy, low-end torque, emission and turbo-engine matching of internal combustion engine. The reliability of performance prediction method for nozzled twin-entry turbine is heavily dependent on the understanding of flow mechanism of the turbine which is always confronted by time-dependent unequal admissions. This paper establishes a new meanline model of a nozzled twin-entry turbine based on the internal flow features. Firstly, flow mechanism of the twin-entry turbine with a vaned nozzle under different admissions is investigated via an experimentally validated CFD method. Results clearly demonstrate that the flow distortion in spanwise direction caused by unequal admissions notably influences the turbine efficiency discrepancy between symmetric unequal admissions, especially the two partial admissions. The evolution of flow distortion in the nozzle passage is the key to the performance discrepancy, which is impossible to be considered by a conventional performance model of a nozzled twin-entry turbine. Inspired by this, a newly-designed meanline model, named ‘parallel nozzle’ is proposed. Specifically, a nozzle passage is divided into two parallel sub-passages in spanwise direction to consider the flow distortion in nozzle passage. The model is validated against the detailed CFD results in terms of both overall performance and detailed flow parameters over different admissions. Results prove that turbine performance and flow parameters are well predicted by the model. Specifically, the influence of admissions on nozzled twin-entry turbine performance and flow parameters can be predicted by the method in satisfying accuracy.

SYNOPSIS This paper describes a study in which nucleation theory has been applied to the flow in turbines. The treatment is one dimensional and use is made of friction coefficients to allow for the aerodynamic losses. The experimental results quoted have been obtained independently from tests on a model turbine. The results of a dry run have been used to obtain friction coefficients which have then been adopted in the prediction of the wet test. The theoretical results are extracts from the computer prints out, the comparisons are with the turbine wet efficiency and the agreement obtained is remarkably good.

Introduction Different Instrumentation systems has been developed for the surgical correction of Adolescent idiopathic Scoliosis (AIS) with different deformity correction techniques. Material and Method: The purpose of this retrospective study was to compare top loading to side-loading instrumentation systems used in the surgical treatment of AIS looking at the clinical, functional and radiological outcomes as well as the cost difference between different systems. Thirty matched surgically treated female patients were assigned to 2 groups, group one were treated using a top loading system and in group 2 a side-loading spinal instrumentation system was used. The groups were evaluated for curve magnitude, percent correction, functional outcome using the scoliosis research society questionnaire-30. Results both groups were similar in their demographic data, preoperative clinical and functional variables. The cost of instrumentation was significantly higher in group 2. At 2 years follow up: both groups have significant improvement in all domaines of Scoliosis Research Society score-30. Group 1 had a better (not significant) correction of the rib hump and both groups had similar percent correction of the main curve. Conclusion The surgical treatment of AIS with side loading systems was significantly more expensive but had statstically similar functional and radiographic results to the top loading system. The top loading system achieved better correction of the rib hump.

The modernization of water turbines bears a high potential of increasing the already installed hydropower capacity. In many projects the existing waterways allow a substantial increase of the available flow capacity and with it the energy output. But also the upgrading onto a state of the art hydraulic, mechanical and electrical design will increase the available power considerably after the rehabilitation. The two phase nature of the flow in Pelton turbines requires for the hydraulic refurbishment special care in the application of the available design methods. Where the flow in the high pressure section of the turbine is mainly of one phase nature, CFD has been used as a standard tool for many years. Also the jet quality, and with it the exploration of the source of flow disturbances that cause poor free surface quality can be investigated with CFD. The interaction of the jet with the buckets of the runner is also examined by means of CFD. However, its accuracy with respect to hydraulic efficiency is, because of the two phase flow and the transient flow process, in very few cases good enough for a reliable and accurate prediction of absolute numbers. The optimization of hydraulic bucket profiles is therefore always checked with measurements in homologous scaled model turbines. A similar situation exists for the housing flow after the water is discharged from the runner. Here also CFD techniques are available to explore the general mechanisms. However, due to the two phase flow nature, where only a very small space is filled with moving water, the experimental setup in a model turbine is always the final proof for optimizations of housing inserts and modifications. The hydraulic design of a modernization project for a power station equipped with vertical Pelton turbines of two different designs is described in the proposed paper. It will be shown, how CFD is applied to determine the losses in the high pressure section and how these results are combined with the model tests carried out in the hydraulic laboratory. Finally a comparison is made in between the achieved model turbine results with measurements carried out in the prototype.

A comprehensive study was performed to analyze turbine wake characteristics by using a Proper-Orthogonal-Decomposition (POD) method to identify the dominant flow features from a comprehensive experimental database. The wake flow characteristics behind a typical three-bladed horizontal-axis wind turbine (HAWT) were measured in a large-scale wind tunnel with a scaled turbine model placed in a typical offshore Atmospheric Boundary Layer (ABL) wind under a neutral stability condition. A high-resolution Particle Image Velocimetry (PIV) system was used to achieve detailed flow field measurements to characterize the turbulent flows and wake vortex structures behind the turbine model. Statistically averaged measurements revealed the presence of the characteristic helical-tip vortex filament along with a unique secondary vortex filament emanating from 60% of the blade span measured from the hub. Both filaments breakup in the near-wake region (~0.6 rotor diameter downstream) to form shear layers, contrary to previous computational and experimental observations in which vortex filaments break up in the far wake. A Proper-Orthogonal-Decomposition (POD) analysis, based on both velocity and vorticity-based formulations, was used to extract the coherent flow structures, predominantly comprised of tip and midspan vortex elements. The reconstructions showed coherence in the flow field prior to the vortex breakup which subsequently degraded in the turbulent shear layer. The accuracy of the POD reconstructions was validated qualitatively by comparing the prediction results between the velocity and vorticity-based formulations as well as the phase-averaged PIV measurement results. This early vortex breakup was attributed to the reduced pitch between consecutive helical turns, the proximity between midspan filaments and blade tips as well as the turbulence intensity of the incoming boundary layer wind.

Constructing a small-scale hydroelectric power station that can run efficiently at a head lower than 10 m is one possible method for reducing the impact of the electricity crisis in remote areas of Indonesia. The Archimedes Screw turbine is one type of turbine that is ideal for discharges below 10 m. In this study, the simulation results show that the value of Turbulences Kinetic Energy is directly proportional to the increase in flow rate but inversely proportional to the level of immersion. This type of turbine is unless well in Indonesia due to a lack of information regarding the application of low head power plants. The turbine model that is suitable for the low head is investigated in this study. Before being tested, the turbine is first designed theoretically and then numerically evaluated. With discharges of 1 l/s, 2 l/s, and 3 l/s and turbine immersion levels of 30%, 50%, and 70%, ANSYS CFD (Computational Fluid Dynamic) software was used to investigate flow rate and level of immersion in the turbine. According to the investigation findings, the discharge has a considerable impact on the turbine's movement; the higher the flow rate, the higher the power to the turbine, which produces a torque on the turbine. The simulation findings indicate that the value of Turbulence Kinetic Energy is proportional to the increase in flow rate but inversely proportional to the level of immersion.

Performance tests of tornado-type wind turbine models

Power coefficients (Cp) and pressure distributions in the vortex tower were measured for two Tornado-Type Wind Turbine (TTWT) models (one a circular-shaped tower; the other, a spiral) in a wind tunnel and in real environments. All the maximum Cp's measured were greater than one and ranged from 4.7 to 15.2 times the Betz limit of the conventional wind generators. These Cp's indicate that the TTWT machine is not only able to extract the total kinetic energy, but also part of the pressure (or internal) energy of the captured wind. The low pressure vortex core established in the wind tower can be maintained at a very low energy state for efficiently pumping the turbine's discharging flow. The static pressure drop from the ambient value measured at the vortex center of the tower ground floor ranged from 4.3 to 10.2 times the wind dynamic pressure when the turbine flows were plugged. Further improvement of Cp requires further vortex intensification and optimal geometric design.

Groundwater inverse problems are concerned with the estimation of uncertain model parameters, such as hydraulic conductivity, from field or laboratory measurements. In practice, model and measurement errors compromise the ability of inverse procedures to provide accurate results. It is important to account for such errors in order to determine the proper weight to give to each source of information. Probabilistic descriptions of model and measurement errors can be incorporated into classical variational inverse procedures, but the computational demands are excessive if the model errors vary over time. An alternative approach based on representer expansions is able to efficiently accommodate time‐dependent errors for large problems. In the representer approach, unknown variables are expanded in finite series which depend on unknown functions called representers. Each representer quantifies the influence of a given measurement on the estimate of a particular variable. This procedure replaces the original inverse problem by an equivalent problem where the number of independent unknowns is proportional to the number of measurements. The representer approach is especially advantageous in groundwater problems, where the total number of measurements is often small. This approach is illustrated with a synthetic flow and transport example that includes time‐dependent model errors. The representer algorithm is able to provide good estimates of a spatially variable hydraulic conductivity field and good predictions of solute concentration. Its computational demands are reasonable, and it is relatively easy to implement. The example reveals that it is beneficial to account for model errors even when they are difficult to estimate.

In the Institute of Thermal Turbomachinery (ITSM) at Stuttgart University, a three-stage model steam turbine was used to study the complex steam flow through the last stages of low pressure steam turbines. The conditions in this test rig were such that condensation occurred after the first stage, so that wet steam was prevalent in the last stage. In order to determine the inlet conditions for the last stage and for the validation of results of a numerical simulation of the condensing steam flow through the model turbine, both steam quality and the droplet size spectrum were measured in front of the stator of the last stage using a miniature combined optical/pneumatic probe. However, while the measured steam quality matched the numerical results quite well, there were considerable differences between the computed and the measured droplet size spectrum. The latter showed considerably larger droplets than predicted by computational fluid dynamics. In this paper, the measurement principle (light extinction) is presented and the inherent limitations for the determination of very small droplets with diameters of less than 0.2 µm are discussed. Issues related to this challenge are the availability of UV-wavelengths and the optical length used for the measurements. The reliability of the inversion algorithm used to analyse the light extinction data is shown by comparison with data from experiments using polystyrene suspensions with known particle size and concentration. Thereafter, the validity of the measurement results is reassessed based on numerical results and data from repeated measurements. It can be shown that the experimental results are reliable, implying that the numerical results suffer from inaccuracies. Apart from modelling issues, one reason for this could be that the inherent unsteadiness in turbine flows is neglected in steady computational fluid dynamics computations.

CFD modelling of tidal turbines in arrays is described and assessed against experimental studies of turbines operating either at constant speed or constant torque. Rotor blades are represented by rotating actuator lines, whilst supports are represented by partially-blocked-out cells. For a single turbine the model successfully reproduces towing-tank measurements of thrust and power coefficients across a range of tip-speed ratios. For two turbines staggered streamwise, it is demonstrated that loads may be reduced or augmented, according as the downstream turbine is in the wake or bypass flow of the upstream turbine. When the downstream turbine is partially in the wake, individual blades are subject to large cyclic load fluctuations. Array performance is evaluated by comparison with experimental data, modelling up to 12 turbines in up to three staggered rows. The speed of each turbine is continuously adjusted in response to flow-induced torque. Distribution of thrust coefficients within the array is well reproduced, but there is greater discrepancy in angular speed. With actuator representation of blades, the choice of turbulence model has little effect on load coefficients for an isolated turbine or row of turbines, but a significant effect on the wake, and hence on downstream turbines in an array.

A new turbine model validation technique that is based on adaptation of look-up tables is described in this article. Simulation results from the VIDYN turbine simulation program and measurements from Big Glenn wind turbine, located outside Gothenburg, Sweden, are used as an input to this new model validation technique. The models of the flapwise bending moment and power coefficient are validated for Big Glenn turbine. Measurement data are acquired during normal turbine operation. Verification results show good agreement between model outputs and measured data. The method allows prediction in a wide range of turbine operating variables, using only few measured points.

Wind Turbine Model Validation: Fusion of Simulation and Measurement Data