Blade Wake Interaction Research Articles

Computational evaluation of an optimum leading-edge slat deflection angle for dynamic stall control in a novel urban-scale vertical axis wind turbine for low wind speed operation

This paper explores passive flow control via leading-edge (LE) slats to reduce the dynamic stall (DS) phenomenon and related blade-wake interaction in an H-Darrieus type vertical axis wind turbine (VAWT) operating under low wind speed conditions. A comprehensive 2D unsteady computational fluid dynamics (CFD) assessment has been carried out for the non-slatted baseline rotor and the advance slatted rotor (ASR) configurations. The unsteady Reynolds-averaged Navier-Stokes (URANS) approach with k-ω shear stress transport (SST) turbulence model and sliding mesh technique have been applied in Ansys Fluent. Optimum slat deflection angle δ has been evaluated using the single-blade oscillatory case with and without the LE slats. Results indicate a reduction in optimum δ from 16° at rated wind speed of 10 ms−1 to 12° for low wind speed operation at 5 ms−1. A significant increase in the maximum coefficient of lift CL,max by approximately 32% and a delay in stall angle of attack αmax by 3° is obtained with ASR configuration compared to the baseline. Further assessment of the ASR configuration on the three-blade rotatory case demonstrates an increase in the power coefficient CP by approximately 15% at the rated tip-speed ratio λ compared to the baseline.

Aerodynamic Behavior of a Floating Offshore Wind Turbine

The flowfield around the rotor blades of a wind turbine is extremely complex due to the occurrence of several aerodynamic phenomena. It is all the more true for floating offshore wind turbines (FOWTs), for which the six rigid-body motions of the floating platform can induce blade–wake interactions. Therefore conventional numerical approaches for wind turbine applications, such as Blade Element Momentum Theory, may be questionable for an accurate prediction of floating wind turbine aerodynamic loads. Consequently, the current paper investigates the aerodynamic behavior of an FOWT subjected to several idealized prescribed motions, representative of a wave movement, based on computational fluid dynamics simulations. These results, obtained on the NREL 5-MW wind turbine, are compared with previous results found in the literature and analyzed to provide a better understanding of the involved aerodynamic phenomena.

Performance Evaluation of H-Type Darrieus Vertical Axis Wind Turbine with Different Turbine Solidity

Among renewable energy resources, wind energy is one of the best alternative for power generation. Recently, vertical axis wind turbine (VAWT) received renewed interest as small-scale wind energy converter due to its suitability for urban application, where the wind condition is known to be unsteady and turbulence. Amongst various type of VAWTs, H-type Darrieus rotor has become more popular, thanks to its simple construction features, resulting to low manufacturing and installation cost. The aim of this paper is to evaluate numerically the power performance of straight-bladed Darrieus VAWT with different turbine solidity using computational fluid dynamic (CFD) technology. A series of two-dimensional CFD simulations of a three-bladed H-type Darrieus rotor were performed with 3 different solidities, σ (0.3, 0.5 and 0.7) to evaluate their power performance. Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations were used to calculate the instantaneous moment coefficient, Cm and power coefficient, Cp over a range of tip speed ratio, λ (0.5–4.5) with a free stream velocity of 8.0 m/s. The simulation results show that high solidity turbine performed well at low values of λ while turbine with low solidity has a wider operating range of λ and performed better at λ > 3.0 due to less blade-wake interactions between upstream and downstream halves of the turbine and lower blockage effect. The findings lend substantially to our understanding of physics flow around blades and turbine in order to optimize the power performance of small scale straight-bladed Darrieus VAWT operating in unsteady and turbulence wind condition.

Power coefficient measurements of a novel vertical axis wind turbine

AbstractWind tunnel experiments were performed to evaluate the power coefficients of three vertical axis wind turbines (VAWTs), namely, (a) conventional VAWT (two‐bladed troposkien shape), (b) novel 50% shifted troposkien shape‐vertical axis wind turbine (50% STS‐VAWT), and (c) novel 100% STS‐VAWT. All turbines had the same height, swept area, and NACA0015 airfoil with the same chord length. During the power coefficient measurements for each one of the turbines, the wind tunnel velocity was varied while the turbine rotation was kept constant through rheostatic load adjustment. Two different turbine rotations were used (600 rpm and 700 rpm). The resulting tip speed ratios (TSRs) ranged from approximately 2 to 6 in the current investigation. Both the power coefficients and the tip speed ratios were corrected against wind tunnel blockage effects. The 50% STS‐VAWT model showed an overall better aerodynamic performance (both corrected and uncorrected power coefficients) when compared against the other two configurations, likely due to a combination of (a) power generation increased due to blade‐wake interaction (BWI) reductions and (b) power generation capacity related to the blade length of each turbine.

Acoustic theory of the many-bladed contra-rotating propeller: physics of the wake interaction noise critical sources

In the theory of interaction noise from contra-rotating propellers with many blades, the usual far-field radiation formulae can be re-cast as a double integral, over a source surface, which can be evaluated asymptotically solely in terms of the contributions from critical points. The paper shows that these critical points have a particularly interesting physical meaning. They relate to locations on an event line, running between hub and tip, that represent the locus of the wake–blade interactions at a fixed point in time. The event line rotates at the speed of the spinning interaction tone but does not coincide with the radial variation in either the wake location or the rear blade leading edge. At the precise critical locations on the event line, it is shown that the Mach number of the event line is unity in the direction of the observer (the sonic condition) and the tangent to the event line – at a fixed time – is normal to a line drawn between it and the observer (the normal-edge condition). The zero-mode case is also considered, for which we show that, even though the event line rotates at infinite speed, there can still exist locations that satisfy the sonic and normal-edge conditions. The paper also discusses the physical meaning of the lower-order boundary solutions from the hub and tip.

A spline ROM of blade aerodynamic force to upstream wake

All related research nowadays are devoted to build ROM of aerodynamic forces induced by vibration of blades or wings. There is no discussion of ROMs of the aerodynamic forces of the blades corresponding to upstream wakes, which is more important to turbine machines' safety. In this paper, a spline Volterra ROM method is proposed to predict the blades' aerodynamic forces induced by the upstream wakes to fasten the blade–wake interaction analysis in turbine machine design. Traveling wave method is applied to simplify the inputs representing the upstream wakes. The kernels of the Volterra ROM are expressed in spline to reduce the number of parameters to be identified. The coefficients of the spline kernels are identified by minimizing the difference between ROM results and CFD data. The ROM method is used to predict a blade's aerodynamic forces under an actual wake. The results show that the ROM can capture most information of the aerodynamic forces due to the wake.

The effect of blade pitch on the flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine

This study analyses the variations in flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine for different blade pitches. A water towing tank facility has been used with a turbine model based on symmetric NACA-0015 profiles with a chord-to-diameter ratio of 0.16. The set-up was especially designed to measure the flow field inside and around the rotor, using planar digital particle image velocimetry. The experiments were made at a constant turbine diameter Reynolds number, [Formula: see text], of 6.1 × 104. The forced rotation of the cross-flow turbine using a direct current motor allowed changes in the ratio of the blade tangential speed to the freestream velocity (tip speed ratio), covering the range of 0.7–2.3. Toe-in and excessive toe-out angles have been associated in the past to low performances in this type of turbines. Pitch angle cases with expected low performances have been chosen, namely, [Formula: see text] of 8° toe-in and 16° toe-out, as well as a case with a high expected efficiency of 8° toe-out and another with [Formula: see text] as the reference case. The the goal is to better understand the flow dynamics and the blade–wake interactions inside the rotor of this type of machines, by studying cases with very different expected performances.

A panel method for floating offshore wind turbine simulations with fully integrated aero- and hydrodynamic modelling in time domain

ABSTRACTThe further development of the first-order panel method panMARE for simulating floating offshore wind turbines (FOWTs) in time domain is presented in this work. Based on a surface discretisation of platform, tower and rotor with its wake, the three-dimensional aerodynamic and hydrodynamic flow fields are calculated. A free wake deformation model is integrated to capture blade–wake interaction. Hydrodynamic, aerodynamic and mooring loads are cumulated by a six-degrees-of-freedom solver to compute the motion of the FOWT. The presented method is able to simulate unsteady and aperiodic motions and to predict the wake deformations and their influence on the rotor loads due to the platform motion. In order to verify the method, simulation results of the DeepCWind floater with the NREL 5-MW baseline turbine are compared with those obtained in the OC4 project. The ability to capture highly unsteady situations is studied as well by simulating a gust with varying length.

Towards accurate CFD simulations of vertical axis wind turbines at different tip speed ratios and solidities: Guidelines for azimuthal increment, domain size and convergence

The accuracy of CFD simulations of vertical axis wind turbines (VAWTs) is known to be significantly associated with the computational parameters, such as azimuthal increment, domain size and number of turbine revolutions before reaching a statistically steady state condition (convergence). A detailed review of the literature, however, indicates that there is a lack of extensive parametric studies investigating the impact of the computational parameters. The current study, therefore, intends to systematically investigate the impact of these parameters, on the simulation results to guide the execution of accurate CFD simulations of VAWTs at different tip speed ratios (λ) and solidities (σ). The evaluation is based on 110 CFD simulations validated with wind-tunnel measurements for two VAWTs. Instantaneous moment coefficient, Cm, and power coefficient, CP, are studied for each case using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with the 4-equation transition SST turbulence model. The results show that the azimuthal increment dθ is largely dependent on tip speed ratio. For moderate to high λ, the minimum requirement for dθ is 0.5° while this decreases to 0.1° at low to moderate λ. The need for finer time steps is associated to the flow complexities related to dynamic stall on turbine blades and blade-wake interactions at low λ. In addition, the minimum distance from the turbine center to the domain inlet and outlet is 15 and 10 times the turbine diameter, respectively. It is also shown that 20–30 turbine revolutions are required to ensure statistically converged solutions. The current findings can serve as guidelines towards accurate and reliable CFD simulations of VAWTs at different tip speed ratios and solidities.

Flow dynamics inside the rotor of a three straight bladed cross-flow turbine

In this work we study experimentally the flow dynamics inside the rotor of a three straight-bladed Cross-Flow Turbine (CFT). The CFT model used in the experiments is based on symmetric NACA-0015 profiles, with a chord to rotor diameter ratio of 0.16. The turbine model was designed in order to quantify the flow inside and around the rotor using planar Digital Particle Image Velocimetry (DPIV). Tests were made by forcing the rotation of the turbine with a DC motor, which provided precise control of the Tip Speed Ratio (TSR), while being towed in a still-water tank at a constant turbine diameter Reynolds number of 6.1×104. The range of TSRs covered in the experiments went from 0.7 to 2.3.The focus is given to the analysis of the blade-wake interactions inside the rotor. The investigation has allowed us to relate the interactions with the performance differences in this type of turbines, as a function of the operational tip speed ratio.

Aerodynamic Analysis of a Helical Vertical Axis Wind Turbine

Vertical axis wind turbines (VAWTs) are gradually receiving more and more interest due to their lower sensitivity to the yawed wind direction. Compared with straight blades VAWT, blades with a certain helicity show a better aerodynamic performance and less noise emission. Nowadays computational fluid dynamics technology is frequently applied to VAWTs and gives results that can reflect real flow phenomena. In this paper, a 2D flow field simulation of a helical vertical axis wind turbine (HVAWT) with four blades has been carried out by means of a large eddy simulation (LES). The power output and fluctuation at each azimuthal position are studied with different tip speed ratio (TSR). The result shows that the variation of angle of attack (AOA) and blade-wake interaction under different TSR conditions are the two main reasons for the effects of TSR on power output. Furthermore, in order to understand the characteristics of the HVAWT along the spanwise direction, the 3D full size flow field has also been studied by the means of unsteady Reynold Averaged Navier-Stokes (U-RANS) and 3D effects on the turbine performance can be observed by the spanwise pressure distribution. It shows that tip vortex near blade tips and second flow in the spanwise direction also play a major role on the performance of VAWTs.

Wind Tunnel Validation of Double Multiple Streamtube Model for Vertical Axis Wind Turbine

The performance and annual energy output have to be predicted to maximize the economic benefits from a wind turbine. Mathematically predicting the performance of Darrieus type lift based turbines are challenging due to the inconsistent angle of attack, blade wake interaction and local induced velocities giving rise to complex flow physics. A reliable and validated mathematical model is therefore essential to optimize the various design parameters prior to manufacture. The objective of the current study is to evaluate widely employed aerodynamic models based on their prediction accuracy, limitations, and computational requirements. Double multiple stream tube models have been discussed in detail and the predictions are experimentally validated through the wind tunnel test of three-bladed H-Darrieus rotor in terms of torque and power coefficient. The possible sources for the deviation between the predicted and measured values have been discussed and concluded with potential solutions.

Influence of Solidity and Wind Shear on the Performance of VAWT using a Free Vortex Model

Performance analysis of a VAWT and HAWT is highly complex due to fluid structure interactions and blade vortex interactions.However, there are simplified methods such as momentum theory to most expensive CFD modelsavailable forperformance prediction. CACTUS is neither as simple as the momentum theory nor as complex as the CFD models, it is an open source code that uses the free vortex method to predict the performance of a wind turbine.In this paper, the effect of solidity and wind shear on the performance of an H-type Darrius VAWT is studied using CACTUS. Variation of solidity was achieved by changing the chord length ( c/R 0.04-0.07) and number of blades ( N 2,3). It has been observed that at lower tip speed ratio (TSR 3) shorter c/R was more efficient due to relatively low wake blade interaction. Effect of wind shear due to the tower height from the ground using the power law equation with values of power law coefficient ranging from 0.1 to 0.3has also been studied.Itis observedthat the power coefficients of the VAWT under a turbulent boundary layer are in congruence with the experimental results.

Effects of periodic wakes on boundary layer development on an ultra-high-lift low pressure turbine airfoil

The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than “simple” wake-blade interaction.

Effects of tip-gap width on the flow field in an axial fan

The turbulent low Mach number flow through an axial fan at a Reynolds number of 9.36 × 105 is investigated by large-eddy simulation (LES). Computations are performed for a fixed flow rate coefficient Φ=0.165 and two tip-gap widths, i.e., s/Do=0.01 and s/Do=0.005. A finite-volume flow solver in an unstructured hierarchical Cartesian setup for the compressible Navier–Stokes equations is used. To account for sharp edges, a fully conservative cut-cell approach is applied. A periodic boundary condition is used in the azimuthal direction such that only one out of five blades is resolved. The focus of this numerical analysis is on the impact of the tip-gap size on the overall flow field. Special attention is paid to the vortical structures around the tip-gap region and the turbulent transition on the suction side of the blade. It is shown that the reduction of the tip-gap size totally changes the flow field. The blade-wake interaction, which was evident for s/Do=0.01, vanishes for s/Do=0.005. Furthermore, depending on the tip-gap width different mechanisms are responsible for the turbulent transition on the suction side of the blade, i.e., for s/Do=0.01 a cyclic transition is observed which is triggered by the blade-wake interaction, where for s/Do=0.005 a separation bubble leads to a permanent transition. A reduction of the tip-gap width decreases the amplitude of the tip-gap vortex wandering and as such the region of influence of the turbulent wake and increases the frequencies of the dominant modes. Since this is confirmed by the experimental results of the sound power level, it also shows the dominant role of the tip-gap vortex in the noise emission.

Modeling Vertical-Axis Wind-Turbine Performance: Blade-Element Method Versus Finite Volume Approach

Vertical-axis wind turbines offer an inherently simpler design than horizontal-axis machines, and their lower blade speed mitigates safety and noise concerns. Although vertical-axis turbines do offer significant operational advantages, development has been hampered by the difficulty of modeling the aerodynamics involved, along with their rotating geometry. This paper presents results from a simulation of a baseline vertical-axis wind turbine computed using Star-CCM+, a commercial finite volume code, and compares them with data obtained from a multiple-streamtube model. Emphasis was placed on the dynamic stall characteristics and wake production, which have the greatest influence on turbine performance. A model was developed to replicate the blade–wake interactions common at higher tip-speed ratios and was found to greatly improve the accuracy of the blade-element momentum model.

Blade–wake interactions in cross-flow turbines

This paper presents analytical bounds for blade–wake interaction phenomenona occurring in rotating cross-flow turbines for wind and tidal energy generation (e.g. H-rotors, Darrieus or vertical axis). Limiting cases are derived for one bladed turbines and extended to the more common three bladed configuration. Additionally, we present a classification of the blade–wake type of interactions in terms of limiting tip speed ratios.These bounds are validated using a high order h/p Discontinuous Galerkin solver with sliding meshes. This computational method enables highly accurate flow solutions and shows that the analytical bounds correspond to limiting blade–wake interactions in fully resolved flow simulations.

The platform pitching motion of floating offshore wind turbine: A preliminary unsteady aerodynamic analysis

The flow-field around the rotor blades of an FOWT may be significantly influenced by the six rigid-body motions of the floating platform via the blade–wake interaction. Therefore, the accurate prediction of unsteady aerodynamic load which is calculated by many conventional numerical approaches is still questionable for an FOWT. In this study, the periodic pitching motion of the rotating turbine blades due to the floating platform motion is considered to investigate the effects of vortex–wake–blade interaction for the aerodynamic performance of an FOWT. The unsteady computational fluid dynamics (CFD) simulations based on the dynamic mesh technique were applied for analyzing the pitching motion of wind turbine due to the platform motion. The in-house unsteady blade element momentum code using the direct local relative velocity approach was also applied to simulate the unsteady aerodynamic performance. The equivalent average velocity approach which simplifies the relative velocity contribution due to the platform motion was proposed and incorporated to the in-house code. It is shown that the unsteady aerodynamic loads of the floating offshore wind turbine become sensitively changed due to the variation of frequency and amplitude of the platform motion. Additionally, there are strong flow interaction phenomena between the rotating blades with oscillating motions and generated blade-tip vortices.

Acoustic Kinematics of Helicopter BVI Noise

Helicopter blade-vortex interaction (BVI) noise is known as a critical source of noise annoyance from helicopters. A key feature of the noise is that the trace velocities of the sources generated by the BVI process are supersonic, and thus have high radiation efficiency. This further means that noise is generated selectively by only a limited part of the overall interaction. The consequences of this have been studied using a simple cycloidal shed wake model. This allows the whole process to be explicitly described mathematically. The trace velocity can be infinite for both forward and retreating blades. This will generate strong noise radiation in directions which can be simply defined as a function of advance ratio. Analysis shows that many features are functions of rotor disc position only, independent of the number of rotor blades. This offers a helpful simplification. Radiation generates a series of caustics which can be defined by the analysis. Simplified models of the blade-wake interaction show that both the advancing and retreating blade interactions can generate cusps in the noise. Under some conditions these cusps are predicted to continue to the far field in the rotor disc plane, due to a kinematic balance of the change in propagation direction of the basic source vector and the change in the Mach angle of the principal radiation. This gives a two-dimensional decay in sound levels and will give rise to locally intense BVI field. The model also shows areas of the stronger noise fields will be generated in the direction of motion of the interacting blade. A detailed analysis has been undertaken for a four blade rotor at an advance ratio of 0.2. Multiple caustics from successive blade interactions can be identified, some of which have cusps. The analysis shows that the BVI radiation field can have large levels locally and helps to explain why helicopters are often perceived to be noisier than a simple dBA level might suggest. Each part of the rotor disc noise radiation to the far field can be related back to specific positions on the blade and specific parts of the shed wake. This offers the prospect of providing local adjustments to minimize BVI sources under particular conditions. All these results are based on the two dimensional cycloidal model. However it is believed that these effects must be reflected in more complex computer models and in real helicopters.

MESH CONVERGENCE STUDY FOR 2-D STRAIGHT-BLADE VERTICAL AXIS WIND TURBINE SIMULATIONS AND ESTIMATION FOR 3-D SIMULATIONS

Mesh resolution requirements are investigated for 2-D and 3-D simulations of the complex flow around a straight-blade vertical axis wind turbine (VAWT). The resulting flow, which may include large separation flows over the blades, dynamic stall, and wake-blade interaction, is simulated by an Unsteady Reynolds-Averaged Navier–Stokes analysis, based on the Spalart-Allmaras (S–A) turbulence model. A grid resolution study is conducted on 2-D grids to examine the convergence of the CFD model. Hence, an averaged-grid residual of y+ > 30 is employed, along with a wall treatment, to capture the near-wall region’s flow structures. Furthermore a 3-D simulation on a coarse grid of the VAWT model is performed in order to explore the influence of the 3-D effects on the aerodynamic performance of the turbine. Finally, based on the 2-D grid convergence study and the 3-D results, the required computational time and mesh to simulate 3-D VAWT accurately is proposed.