Using Experimentally Validated Navier-Stokes CFD to Minimize Tidal Stream Turbine Power Losses Due to Wake/Turbine Interactions

Tidal stream turbines fixed on the seabed can harness the power of tides at locations where the bathymetry and/or coastal geography result in high kinetic energy levels of the flood and/or neap currents. In large turbine arrays, however, avoiding interactions between upstream turbine wakes and downstream turbine rotors may be hard or impossible, and, therefore, tidal array layouts have to be designed to minimize the power losses caused by these interactions. For the first time, using Navier-Stokes computational fluid dynamics simulations which model the turbines with generalized actuator disks, two sets of flume tank experiments of an isolated turbine and arrays of up to four turbines are analyzed in a thorough and comprehensive fashion to investigate these interactions and the power losses they induce. Very good agreement of simulations and experiments is found in most cases. The key novel finding of this study is the evidence that the flow acceleration between the wakes of two adjacent turbines can be exploited not only to increase the kinetic energy available to a turbine working further downstream in the accelerated flow corridor, but also to reduce the power losses of said turbine due to its rotor interaction with the wake produced by a fourth turbine further upstream. By making use of periodic array simulations, it is also found that there exists an optimal lateral spacing of the two adjacent turbines, which maximizes the power of the downstream turbine with respect to when the two adjacent turbines are absent or further apart. This is accomplished by trading off the amount of flow acceleration between the wakes of the lateral turbines, and the losses due to shear and mixing of the front turbine wake and the wakes of the two lateral turbines

Virtual incidence effect on rotating airfoils in Darrieus wind turbines

Small Darrieus wind turbines are one of the most interesting emerging technologies in the renewable energies scenario, even if they still are characterized by lower efficiencies than those of conventional horizontal-axis wind turbines due to the more complex aerodynamics involved in their functioning. In case of small rotors, in which the chord-to-radius ratios are generally high not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be moreover influenced by the so-called "flow curvature effects". Recent works have indeed shown that the curved flowpath encountered by the blades makes them work like virtually cambered airfoils in a rectilinear flow. In the present study, focus is instead given to a further effect that is generated in reason of the curved streamline incoming on the blades, i.e. an extra-incidence seen by the airfoil, generally referred to as "virtual incidence". In detail, a novel computational method to define the incidence angle has been applied to unsteady CFD simulations of three airfoils in a Darrieus-like motion and their effective angles of attack have been compared to theoretical expectations. The analysis confirmed the presence of an additional virtual incidence on the airfoils and quantified it for different airfoils, chord-to-radius ratios and tip-speed ratios. A comparative discussion on BEM prediction capabilities is finally reported in the study

Critical Analysis of Dynamic Stall Models in Low-Order Simulation Models For Vertical-Axis Wind Turbines

Abstract The efficiency of vertical-axis wind turbines (VAWTs) still lacks from those of horizontal-axis rotors (HAWTs). To improve on efficiency, more accurate and robust aerodynamic simulation tools are needed for VAWTs, for which low-order methods have not reached yet a maturity comparable to that of HAWTs' applications. In the present study, the VARDAR research code, based on the BEM theory, is used to critically compare the predictiveness of some dynamic stall models for Darrieus wind turbines. Dynamic stall, connected to the continuous variation of the angle of attack on the airfoils, has indeed a major impact on the performance of Darrieus rotors. Predicted lift and drag coefficients of the airfoils in motion are reconstructed with the different dynamic stall models and compared to unsteady CFD simulations, previously validated by means of experimental data. The results show that low-order models are unfortunately not able to capture all the complex phenomena taking place during a VAWT functioning. It is however shown that the selection of the adequate dynamic stall model can definitely lead to a much better modelling of the real airfoils' behavior and then notably enhance the predictiveness of low-order simulation methods

Critical issues in the CFD simulation of Darrieus wind turbines

Computational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of vertical-axis wind turbines, helping this technology to rise towards a more mature industrial diffusion. The unsteady flow past rotating blades is, however, one of the most challenging applications for a numerical simulation and some critical issues have not been settled yet.In this work, an extended analysis is presented which has been carried out with the final aim of identifying the most effective simulation settings to ensure a reliable fully-unsteady, two-dimensional simulation of an H-type Darrieus turbine.Moving from an extended literature survey, the main analysis parameters have been selected and their influence has been analyzed together with the mutual influences between them; the benefits and drawbacks of the proposed approach are also discussed.The selected settings were applied to simulate the geometry of a real rotor which was tested in the wind tunnel, obtaining notable agreement between numerical estimations and experimental data. Moreover, the proposed approach was further validated by means of two other sets of simulations, based on literature study-cases

fine tuning of a two stoke engine in full power configuration provided with a low pressure direct injection system

Abstract The main drawbacks of two stroke (2S) engines consist in poor engine efficiency and high level of pollutant emissions. The contemporary opening of transfer and exhaust ports during the scavenging process causes the short circuit of fresh air-fuel mixture in case of indirect injection or carbureted engines. Despite the intrinsic strengths such as high power density, simplicity, compactness, lightweight and low production costs, 2S engines have been substituted by four stroke (4S) engines in many applications. Direct injection represents an effective solution to reduce the short circuit of fuel in 2S engines. Usually it is carried out by adopting high-pressure systems but the related increase of complexity and costs is inevitable. In order to maintain the intrinsic simplicity of a 2S engine, the most suitable solution is represented by a Low Pressure Direct Injection (LPDI) system. 2S LPDI engines are characterized by the presence of one or two injectors, working at 5 bar, installed on the cylinder wall. Previous works of the authors have shown the effectiveness of an LPDI system applied to a 300cc single cylinder engine in underpowered version with different ports timing and exhaust system with respect to the full power configuration. In the present paper, the authors show the fine-tuning of a 2S engine in full power configuration provided with two injectors installed on the cylinder and directed towards the exhaust port; the injector nozzles were located above the scavenge ports in order to guarantee the maximum interaction between injected fuel and inlet air flow. The engine has been deeply tested and analyzed at the test bench. Particular attention was paid to definition of the optimal injection timing in order to guarantee the best compromise between performance, efficiency and emissions. The experimental setup and the calibration methodology are discussed in detail. The results show the advantages of the LPDI system in terms of increased engine efficiency and emissions reduction with respect to the original carbureted engine maintaining the same level of performance

potential of the virtual blade model in the analysis of wind turbine wakes using wind tunnel blind tests

Abstract The present research frontier on wind turbine wake analysis is leading to a massive use of large-eddy simulations to completely solve the flow field surrounding the rotors; on the other hand, there is still room for lower-fidelity models with a more affordable computational cost to be used in extended optimization analyses, e.g. for a park layout definition. In this study, a customized version of the Virtual Blade Model (VBM) for ANSYS ® FLUENT ® is presented. The model allows a hybrid solution of the flow, in which the surrounding environment is simulated through a conventional RANS approach, while blades are replaced by a body force, calculated by a simplified version of the Blade Element Theory. The potential of the newly-customized VBM was evaluated by applying it to the famous NOWITECH-NORCOWE blind tests for horizontal axis wind turbines. Several test cases were analyzed and discussed including: 1) a single turbine; 2) an array of two turbines with one rotor working in the wake of the other one; 3) an array of two staggered rotors; 4) several configurations of rotors working in yawed-flow. The study proves that the VBM model can represent a valuable tool for the analysis of wind turbines wakes and of their interaction with near rotors

implementation of the virtual camber transformation into the open source software qblade validation and assessment

Abstract Thanks to the renewed interest in vertical-axis wind turbines, research efforts are devoted at improving the accuracy of present simulation tools, many of which are underdeveloped if compared to those for horizontal-axis turbines. In particular, recent studies demonstrated that a correction for the "virtual camber" effect has a major impact on the simulation. In cycloidal motion indeed the blade aerodynamics are equivalent to those of a virtually-transformed airfoil with a camber line defined by its arc of rotation. In this study, the implementation of a specific module to account for the virtual camber effect in the Open-Source code QBlade is presented. The effectiveness of the model is then validated by four 1-blade and a full 3-blade H-Darrieus turbines, for which both experimental measurements and detailed CFD calculations were available. A sensitivity analysis on the impact of the virtual camber correction on the accuracy of a low-order simulation model has been carried out as a function of the chord-to-radius ratio and the airfoil thickness-to-chord ratio. Reference thresholds for the model applicability are presented for both variables

On the influence of virtual camber effect on airfoil polars for use in simulations of Darrieus wind turbines

Darrieus vertical-axis wind turbines are experiencing renewed interest from researchers and manufacturers, though their efficiencies still lag those of horizontal-axis wind turbines. A better understanding of their aerodynamics is required to improve on designs, for example through the development of more accurate low-order (e.g. blade element momentum) models. Many of these models neglect the impact of the curved paths that are followed by blades on their performance. It has been theorized that the curved streamlines of the flow impart a virtual camber and incidence on them, giving a performance analogous to a cambered blade in a rectilinear flow. To test the extent of this effect, wind tunnel experiments have been conducted in a rectilinear flow to obtain lift and drag for three airfoils: a NACA 0018 and two conformal transforms of the profile. The transformed airfoils exhibit the virtual camber that the theory predicts is imparted to a NACA 0018 when used in a Darrieus turbine with blade chord-to-turbine radius ratios, c/R, of 0.114 and 0.25. A parallel computational fluid dynamics campaign has been conducted to study the aerodynamic behavior of the same blades in curvilinear flow in Darrieus-like motion with c/R = 0.114 and 0.25, at tip-speed ratios of 2.1 and 3.1, using novel techniques to obtain blade effective angles of attack. The analysis confirms that the theory holds, with the wind tunnel results for the NACA 0018 being analogous to numerical results for the relevant cambered airfoils. In addition, turbine performance is calculated using computational fluid dynamics and a blade element momentum code, for each of the blades in turn. The computational fluid dynamics results for the NACA 0018 agree closely to blade element momentum results for the equivalent cambered airfoil where c/R = 0.25, for both turbine power and blade tangential forces. Agreement between the two methods using geometrically identical blades is poor at both the blade and turbine level for c/R = 0.25. It is concluded that when modeling a Darrieus rotor using blade element momentum methods, applying experimental data for the profile used in the turbine will yield inaccurate results if the c/R ratio is high, in such cases it is necessary to select a profile based on the virtual shape of the blades

Darrieus wind turbine blade unsteady aerodynamics:a three-dimensional Navier-Stokes CFD assessment

Energized by the recent rapid progress in high-performance computing and the growing availability of large computational resources, computational fluid dynamics (CFD) is offering a cost-effective, versatile and accurate means to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines, increase their efficiency and delivering more cost-effective and structurally sound designs. In this study, a Navier-Stokes CFD research code featuring a very high parallel efficiency was used to thoroughly investigate the three-dimensional unsteady aerodynamics of a Darrieus rotor blade. Highly spatially and temporally resolved unsteady simulations were carried out using more than 16,000 processor cores on an IBM BG/Q cluster. The study aims at providing a detailed description and quantification of the main three-dimensional effects associated with the periodic motion of this turbine type, including tip losses, dynamic stall, vortex propagation and blade/wake interaction. Presented results reveal that the three-dimensional flow effects affecting Darrieus rotor blades are significantly more complex than assumed by the lower-fidelity models often used for design applications, and strongly vary during the rotor revolution. A comparison of the CFD integral estimates and the results of a blade-element momentum code is also presented to highlight strengths and weaknesses of low-fidelity codes for Darrieus turbine design. The reported CFD results may provide a valuable and reliable benchmark for the calibration of lower-fidelity models, which are still key to industrial design due to their very high execution speed

increased mean corpuscular volume of red blood cells predicts response to metronomic capecitabine and cyclophosphamide in combination with bevacizumab

Abstract Background There is an urgent need for the identification of commonly assessable predictive factors in the treatment of patients with metastatic breast cancer. Methods During the course of a treatment including low dose metronomic oral cyclophosphamide and capecitabine plus i.v. bevacizumab (plus erlotinib in one third of the patients) for metastatic breast cancer, we observed that a relevant number of patients developed repeatedly elevated levels of mean corpuscular volume (MCV) of red blood cells without a significant fall in hemoglobin levels. We conducted a retrospective analysis on these 69 patients to evaluate if the increase in MCV could be associated to tumor response. Results During the course of treatment 42 out of 69 patients (61%) developed macrocytosis. Using Cox proportional hazards modeling that incorporated macrocytosis (MCV≥100 fl) as a time-dependent covariate, macrocytosis resulted in a halved risk of disease progression (HR 0.45; 95% CI, 0.22–0.92, p-value 0.028). In a landmark analysis limited to patients with no sign of progression after 24 weeks of treatment, median time to progression was 72 weeks (48 weeks after landmark) in patients who had developed macrocytosis, and 43 weeks (19 weeks after landmark) in patients who had not (p = 0.023). Conclusion Macrocytosis inversely related to risk of disease progression in patients treated with metronomic capecitabine plus cyclophosphamide and bevacizumab for metastatic breast cancer. This finding may be explained through thymidylate synthase inhibition by capecitabine. Whether bevacizumab has a role in determining macrocytosis, similarly to what happens with sunitinib, has to be further investigated. If other studies will confirm our findings, macrocytosis might be used as an early marker of response during metronomic treatment with capecitabine and cyclophosphamide with or without bevacizumab