34 research outputs found
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Investigation of the three-dimensional flow past a flatback wind turbine airfoil at high angles of attack
Flatback airfoils are airfoils with a blunt trailing edge. They are currently commonly used in the inboard part of large wind turbine blades, as they offer a number of aerodynamic, structural, and aeroelastic benefits. However, the flow past them at high angles of attack (AoA) has received relatively little attention until now. This is important because they usually operate at high AoA at the inboard part of Wind Turbine blades. The present investigation uses Reynolds averaged Navier–Stokes (RANS) and hybrid RANS + large eddy simulation predictions to analyze the flow in question. The numerical results are validated against previously published wind tunnel experiments. The analysis reveals that to successfully simulate this flow, the spanwise extent of the computational domain is crucial, more so than the selection of the modeling approach. Additionally, a low-drag regime observed at angles of attack before stall is identified and analyzed in detail. Finally, the complex interaction between the three-dimensional separated flow beyond maximum lift (stall cells) with the vortex shedding from the blunt trailing edge is revealed
The flow past a flatback airfoil with flow control devices: benchmarking numerical simulations against wind tunnel data
As wind turbines grow larger, the use of flatback airfoils has become standard practice for the root region of the blades. Flatback profiles provide higher lift and reduced sensitivity to soiling at significantly higher drag values. A number of flow control devices have been proposed to improve the performance of flatback profiles. In the present study, the flow past a flatback airfoil at a chord Reynolds number of 1:5_106 with and without trailing edge flow control devices is considered. Two different numerical approaches are applied, unsteady Reynolds-Averaged Navier Stokes (RANS) simulations and detached eddy simulations (DES). The computational predictions are compared against wind tunnel measurements to assess the suitability of each method. The effect of each flow control device on the flow is examined based on the DES results on the finer mesh. Results agree well with the experimental findings and show that a newly proposed flap device outperforms traditional solutions for flatback airfoils. In terms of numerical modelling, the more expensive DES approach is more suitable if the wake frequencies are of interest, but the simplest 2D RANS simulations can provide acceptable load predictions
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Vortex identification methods applied to wind turbine tip vortices
This study describes the impact of postprocessing methods on the calculated parameters of tip vortices of a wind turbine model when tested using particle image velocimetry (PIV). Several vortex identification methods and differentiation schemes are compared. The chosen methods are based on two components of the velocity field and their derivatives. They are applied to each instantaneous velocity field from the dataset and also to the calculated average velocity field. The methodologies are compared through the vortex center location, vortex core radius and jittering zone.
Results show that the tip vortex center locations and radius have good comparability and can vary only a few grid spacings between methods. Conversely, the convection velocity and the jittering surface, defined as the area where the instantaneous vortex centers are located, vary between identification methods.
Overall, the examined parameters depend significantly on the postprocessing method and selected vortex identification criteria. Therefore, this study proves that the selection of the most suitable postprocessing methods of PIV data is pivotal to ensure robust results
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Experimental study of drag-reduction devices on a flatback airfoil
Various trailing-edge drag-reduction devices, including a new flap device, were examined experimentally on a flatback airfoil in a wind tunnel. The tests concerned a 30% thick airfoil with 10.6% thick trailing edge. Pressure, hot wire, and stereo particle image velocimetry measurements were performed at a chord Reynolds number of 1.5e6. Results show that the best-performing devices decrease drag, increase the vortex shedding frequency, and reduce flow variation downstream of the wing trailing edge. The best-performing device was a combination of the flap with an offset cavity plate. Further investigation is required for the optimization of the new device to examine its effects on noise reduction, load mitigation, and control
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Aircraft turbulence and gust identification using simulated in-flight data
Gust and turbulence events are of primary importance for the analysis of flight incidents, for the design of gust load alleviation systems and for the calculation of loads in the airframe. Gust and turbulence events cannot be measured directly but they can be obtained through direct or optimisation-based methods. In the direct method the discretisation of the Fredholm Integral equation is associated with an ill conditioned matrix. In this work the effects of regularisation methods including Tikhonov regularisation, Truncated Single Value Decomposition (TSVD), Damped Single Value Decomposition (DSVD) and a recently proposed method using cubic B-spline functions are evaluated for aeroelastic gust identification using in flight measured data. The gust identification methods are tested in the detailed aeroelastic model of FFAST and an equivalent low-fidelity aeroelastic model developed by the authors. In addition, the accuracy required in the model for a reliable identification is discussed. Finally, the identification method based on B-spline functions is tested by simultaneously using both low-fidelity and FFAST aeroelastic models so that the response from the FFAST model is used as measurement data and the equivalent low-fidelity model is used in the identification process
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Experimental investigation of the atmospheric boundary layer flow past a building model with openings
As modern building design moves towards more sustainable solutions the use of natural ventilation is one of the options considered to improve indoor air quality and to minimize the energy cost of the buildings. The present cross-ventilation study is an experimental investigation of the atmospheric boundary layer flow past a cubic building model with vertical openings. Wind tunnel experiments were performed for two different simulated upstream boundary layer conditions and for two different cube options (with and without openings). Pressure measurements on the building model surface are in very good agreement with benchmark measurements. Stereo Particle Image Velocimetry measurements were performed to examine the effect of both the upstream condition and the openings. It is found that both conditions significantly alter the pressure and flow structure around the building model. Ventilation rate is estimated using two methods, the orifice equation and the measured velocity profile in the vicinity of the apertures. The comparison shows that the orifice equation overpredicts the ventilation rate and the effect of the upstream boundary layer. All data in the present report are freely available for validation purposes
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Improving Wind Tunnel "1-cos" Gust Profiles
A vane-type gust generator has been designed and characterized in the Swansea University wind tunnel to enable the validation of the response of aircraft models to gust loads. The experimental results reveal the complexity of the flow between the gust vanes and the aircraft model location. Previous studies have shown that generating a predetermined gust profile at the desired location in the wind tunnel is a challenging problem. In this work, two techniques to improve the “1-cos” gust have been considered. In the first case, the transfer functions between the vane rotation and the gust produced at the aircraft model location have been identified, and its inverse has been used to calculate the vane rotation. The strong aerodynamics nonlinearity limits the improvements of this method. A parametric study on vane rotation has shown that a more complicated vane rotation function made it possible to obtain “1-cos” gusts at the aircraft model location with a mean square error two orders of magnitude smaller than the initial case. Creating “1-cos” gusts with similar frequency content as the regulations require will help design more efficient gust load alleviation systems
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DES vs RANS: The flatback airfoil case
Using flatback airfoils at the root of wind turbine (WT) blades is becoming more popular as the WTs increase in size. The reason is that they provide significant aerodynamic, aeroelastic and structural benefits. However, due to the blunt trailing edge (TE), the wake of such airfoils is highly unsteady and rich in three-dimensional vortical structures. This poses significant challenges on the numerical simulation of the flow around them, given the highly unsteady, three-dimensional turbulent character of their wake. In this work, computational predictions for a flatback airfoil employing both RANS and DES approaches on three successively refined grids up to 25 million cells are compared with available experimental data. Results suggest that even though URANS and DDES are in good agreement in terms of lift and drag, RANS simulations fail to accurately capture the turbulent wake unsteady characteristics
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Revisiting the assumptions and implementation details of the BAY model for vortex generator flows
Today, Vortex Generators (VGs) are becoming an integral part of a Wind Turbine blade design. However, the challenges involved in the computation of the flow around VGs are yet to be dealt with in a satisfactory manner. A large number of VG models for Reynolds Averaged Navier Stokes (RANS) solvers has been proposed and, among them, the Bender–Anderson–Yagle (BAY) model (ASME Pap. FEDSM99-6919) is one of the most popular, due to its ease of use and relatively low requirements for user input. In the present paper a thorough investigation on the performance and application of the BAY model for aerodynamic VG flows is presented. A fully resolved RANS simulation is validated against experiments and then used as a benchmark for the BAY model simulations. A case relevant to wind turbines is examined, which deals with the flow past a wind turbine airfoil at Reynolds number 0.87e6. When the grid related errors are excluded, it is found that the generated vortices are weaker in the BAY model simulations than in the fully resolved computation. The latter finding is linked to an inherent deficiency of the model, which is first found in this study and which is explained in detail
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On the use of vortex generators to improve tidal turbine performance
Vortex Generators (VGs) are a passive flow control device with multiple applications, including horizontal axis wind turbines (HAWT). Their most popular version is that of thin vanes protruding normal to the blade surface, at an angle to the oncoming flow. Despite their popularity and success in the more established wind turbine industry there has been very little application of VGs on tidal turbines. The present investigation builds on the success of VGs on HAWTs and attempts to examine their effect on a tidal turbine. A numerical VG parametric study is performed on two hydrofoil profiles of different thickness, 30% and 20%. An in-house Reynolds Averaged Navier Stokes solver (MaPFlow) is used for this part of the study. The best performing VG configurations are selected and their effect on the profiles’ lift and drag polars are used to predict the effect on the tidal turbine performance. This is evaluated using an in-house Blade Element Momentum code. Results are very promising and indicate that the use of VGs could significantly improve the performance of tidal turbines over a range of tip speed ratios. Future work includes wind tunnel tests to further validate the present simulations, blade resolved RANS analysis of the turbine blade and high-fidelity simulations to analyse the VG effect on the boundary layer flow