Blade Wake Interaction Research Articles

Unsteady aerodynamics of the floating offshore wind turbine due to the trailing vortex induction and airfoil dynamic stall

The trailing vortex induction effect and the airfoil dynamic stall are two main sources of the wind turbine unsteady aerodynamics. The unsteady aerodynamics of the floating offshore wind turbine (FOWT) is more complicated than its monopile counterpart due to the existence of the additional six degrees of freedom motion (6-DOF) of the floating platform. This paper focuses on the unsteady aerodynamics of the FOWT under surge condition results from the trailing vortex induction and airfoil dynamic stall, especially the resulting hysteresis effect, by adopting the free vortex wake (FVW) method coupled with the dynamic stall model. The reasons and manifestations of the hysteresis effect are discussed, and the effect of the operation conditions on the hysteresis effect of the FOWT are analyzed. Besides, other unsteady aerodynamic phenomena associated with the hysteresis effect are also included. Results show that, when the FOWT operates under surge condition, there exists a phase lag between the loads response and the variation of the rotor inflow velocity, which is a manifestation of the hysteresis effect. With the variation of the operation conditions of the FOWT, the phenomena such as dynamic stall, blade-wake interaction and the transition of the operation state may occur, and the manifestation of the hysteresis effect will be different.

Acoustics and Forces from a Subscale Electric Vertical-Takeoff-and-Landing Rotor in Edgewise Flight

The far-field noise and forces of a subscale two-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The mean thrust and torque coefficients increased with the edgewise advance ratio in relation to the hover coefficients. The tonal and broadband noise contributions were separated and analyzed in both the time and frequency domains. The sound pressure level (SPL) at the blade pass frequency (BPF) had the greatest contribution to the overall SPL, and the dominant broadband noise source changed from high-frequency trailing edge noise to midfrequency blade wake interaction noise with an increasing edgewise advance ratio. The tip Mach number scaling of the noise sources was examined and found to be highly dependent on the oncoming freestream velocity. The BPF SPL directivity showed a dependency on the advance ratio, with a peak magnitude below and upstream of the rotor’s retreating side. The broadband noise had a dipole directivity with a minimum in the rotor plane. The midfrequency broadband noise had signatures indicative of blade wake interaction noise, and the high-frequency noise had signatures indicative of amplitude modulated trailing edge noise. The results demonstrate the importance of unsteady loading noise and broadband noise for subscale urban air mobility rotors.

Aerodynamic and acoustic study of a small scale lightly loaded hovering rotor using large eddy simulation

The aerodynamics and acoustics of a small-scale lightly loaded hovering rotor operating at low Reynolds number is investigated using high-fidelity numerical simulation. A canonical geometry is considered, which allows for different flow topologies typical of transitional flows to develop along the span. Specifically, regions with laminar attached flow, trailing edge laminar separation with prominently 2D structures, and successive laminar separation, transition and reattachment with prominently 3D structures are identified. Furthermore, the lightly loaded configuration is conducive to significant blade-wake interaction, that in turn affects the dynamics of the boundary layer. The acoustic footprint of these phenomena is analyzed by correlating pressure fluctuations in the flow to those at the blade surface. It is found that the radiated noise is characterized by tonal peaks at the blade passing frequency and harmonics, and by a broadband hump at higher frequencies, consistent with experimental observations. The broadband hump exhibits distinct peaks that are found to originate from the separated shear layers on both pressure and suction sides. In particular, two noise generation mechanisms arising at two different frequencies in the broadband hump are identified. One is due to the interaction between an instability transported in the tip vortex of one blade and the leading edge of the following blade, and another one is found to rely on the interaction between an instability transported in the tip vortex and in the wake of one blade scattered by the trailing edge of the following blade.

Kesan Rekabentuk Parametrik dalam Pengoptimuman Prestasi Turbin Angin Berpaksi Menegak: Satu Ulasan

Wind energy is one of the renewable energy resources that is gaining attention from industry players and researchers. In the last few years, there is an increasing interest in small-scale wind turbines as a power generator in built environments as the urgency to reduce carbon footprint in urban areas increases as well as reducing adverse effects of fossil fuels on human health and the environment. Generally, wind turbines can be categorized into two categories which are horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). VAWTs have good potentials to be developed considering their suitability to be used in complex wind conditions associated with built environments. However, the number of research, publications, as well as basic understanding of flow phenomena associated to the performance of VAWTs such as dynamic stall, flow separation, flow curvature effect and blade-wake interaction are scarce. These flow phenomena are attributed by operational and geometrical parameters that significantly affect the overall performance of VAWTs that includes turbine power generation and aerodynamic characteristics. This paper provides a review and discussion of the effects of various design parameters such as blade profile, blade pitch angle and turbine solidity on the performance of VAWTs and serves as a basic guideline to the designer in designing an ideal VAWT

Blade-Wake Interaction Noise for Small Hovering Rotors, Part I: Characterization Study

This work illustrates the use of artificial neural network modeling to study and characterize broadband blade-wake interaction noise from hovering small unmanned aerial systems rotors subject to varying airfoil geometries, rotor geometries, and operating conditions. Design of experiments was used to create input feature spaces, and a high-fidelity strategy was implemented at the discrete data points defined by the input feature spaces to design airfoils and rotor blades, predict the unsteady rotor aerodynamics and aeroacoustics, and isolate the blade-wake interaction noise from the acoustic broadband noise. A metric for the blade-wake interaction noise was developed, and the ANOPP2 Artificial Neural Network Tool was used to identify an optimal prediction model for the nonlinear relationship between the input features and the metric for blade-wake interaction noise. This optimal artificial neural network was then validated over training/test data and exhibited prediction accuracy over 91% for data previously unseen by the model. A sensitivity analysis was conducted, which showed that input features that directly modify the thrust coefficient had a dominant effect over blade-wake interaction noise. The optimal prediction model along with aerodynamic simulations were used to further study the effect of varying input features on blade-wake interaction noise, and three types of blade-wake interaction noise were identified.

Numerical investigation of h-Darrieus wind turbine aerodynamics at different tip speed ratios

PurposeThe purpose of the paper is to predict the aerodynamic performance of a complete scale model H-Darrieus vertical axis wind turbine (VAWT) with end plates at different operating conditions. This paper aims at understanding the flow physics around a model VAWT for three different tip speed ratios corresponding to three different flow regimes.Design/methodology/approachThis study achieves a first three-dimensional hybrid lattice Boltzmann method/very large eddy simulation (LBM-VLES) model for a complete scaled model VAWT with end plates and mast using the solver PowerFLOW. The power curve predicted from the numerical simulations is compared with the experimental data collected at Erlangen University. This study highlights the complexity of the turbulent flow features that are seen at three different operational regimes of the turbine using instantaneous flow structures, mean velocity, pressure iso-contours, blade loading and skin friction plots.FindingsThe power curve predicted using the LBM-VLES approach and setup provides a good overall match with the experimental power curve, with the peak and drop after the operational point being captured. Variable turbulent flow structures are seen over the azimuthal revolution that depends on the tip speed ratio (TSR). Significant dynamic stall structures are seen in the upwind phase and at the end of the downwind phase of rotation in the deep stall regime. Strong blade wake interactions and turbulent flow structures are seen inside the rotor at higher TSRs.Research limitations/implicationsThe computational cost and time for such high-fidelity simulations using the LBM-VLES remains expensive. Each simulation requires around a week using supercomputing facilities. Further studies need to be performed to improve analytical VAWT models using inputs/calibration from high fidelity simulation databases. As a future work, the impact of turbulent and nonuniform inflow conditions that are more representative of a typical urban environment also needs to be investigated.Practical implicationsThe LBM methodology is shown to be a reliable approach for VAWT power prediction. Dynamic stall and blade wake interactions reduce the aerodynamic performance of a VAWT. An ideal operation close to the peak of the power curve should be favored based on the local wind resource, as this point exhibits a smoother variation of forces improving operational performance. The 3D flow features also exhibit a significant wake asymmetry that could impact the optimal layout of VAWT clusters to increase their power density. The present work also highlights the importance of 3D simulations of the complete model including the support structures such as end plates and mast.Social implicationsAccurate predictions of power performance for Darrieus VAWTs could help in better siting of wind turbines thus improving return of investment and reducing levelized cost of energy. It could promote the development of onsite electricity generation, especially for industrial sites/urban areas and renew interest for VAWT wind farms.Originality/valueA first high-fidelity simulation of a complete VAWT with end plates and supporting structures has been performed using the LBM approach and compared with experimental data. The 3D flow physics has been analyzed at different operating regimes of the turbine. These physical insights and prediction capabilities of this approach could be useful for commercial VAWT manufacturers.

Surrogate models for twin-VAWT performance based on Kriging and artificial neural networks

Though twin vertical axis wind turbines (VAWTs) have great potential in the application in oceanic energy harvest, their aerodynamic characteristics is quite complex, especially in the case of sufficient wake-blade interactions. At design stages, the prediction of their power performance usually relies on high-fidelity unsteady simulations based on computational fluid dynamics, whose time budget is high. In this paper, two surrogate models, i.e., Kriging and artificial neural networks (ANN), were adopted for the performance prediction of a twin-VAWT with a close staggered arrangement. Turbines’ pitch angles and their averaged torques at the best tip speed ratio were taken as the input and output, respectively. The numerical study shows that both Kriging and ANN models can provide satisfactory predictions using only 22.45% of CFD observations as training set, and the R2 values for both upstream and downstream turbine models reach more than 0.99 and 0.98, respectively. Among them, the Kriging-based models appear to be more time-efficient and stable than those based on ANN under the moderate dataset in hand. In addition, the current sampling strategy was tested to be modest and robust through sensitivity analysis.

The noise-generating mechanism of rod-airfoil configuration and the effect of spanwise length on noise prediction

The wake-blade interaction in turbo-engines is the primary source of broadband noise. In this work, the noise mechanism of a benchmark rod-airfoil configuration is exhaustively analyzed using compressible Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings (FW-H) methods. The baseline model with a spanwise length shortened by 7d (d is the rod diameter) is simulated under periodic boundary conditions, and the effect of spanwise lengths on noise prediction and correlation is investigated in combination with two other models (3d and 1d). In terms of the baseline model, the noise predicted by different FW-H surfaces is compared first, and the noise radiated from different chordwise and spanwise regions of the airfoil surface reveal the generating mechanism of the main dipolar noise and the effect of periodic boundary conditions on noise prediction, respectively. Additionally, the fluctuating surface pressure characterized at four frequency bands confirms that the wake-airfoil interaction dominates the leading-edge noise at low frequencies. The characteristic frequencies of the noise and transient velocity fields reveal that the primary broadband noise source is the interaction of large-scale vortices with the airfoil surface at the leading-edge region. Finally, the effect of spanwise length on noise prediction and correlation is studied for three models. The differences in the flow structures lead to different features of their coherence coefficients. The results presented in this work provide a comprehensive understanding of the noise-generating mechanism of wake-airfoil interaction in turbomachinery and the practical application of simplified models in the future numerical simulations.

Numerical investigation of 3D unsteady flow around a rotor of vertical axis wind turbine darrieus type H

This article presents an analysis of the complex and unsteady flow a ssociated with the functioning of the rotor of a vertical axis wind turbine Darrieus - H. In this study, the influence of different numerical aspects on the accuracy of the simulation of the flow around a rotor of three straight blades in rotation is performed, which are the effect of the turbulence modeling, and the effects of the mesh and the time step. The Delayed Detached Eddy Simulation (DDES) approach is used. The aim of this article is to describe and analyze the unsteady flow in 3D predicted numerically considering the effects of arms like blade-arms interference, blade-wake interactions around the Darrieus rotor and the effect of tip vortices. Two-dimensional simulations are used in a preliminary numerical configuration. Then, three-dimensional simulations a re performed t o p recisely determine the characteristics of the complex aerodynamic flow associated with the operation of the wind turbine rotor. The flow field around the rotor is studied for several values of the tip speed ratio, dynamic quantities, such as the torque and the power of the rotor that are presented and analyzed. From the results obtained, it is clear that the approach of the Detached Eddy Simulation with the SST K-ω model can be considered as a reliable prediction. A comparison of the performance of the results showed that the predicted coefficients of performance are very close to the experimental data from the bibliography.

Experimental investigation of the effect of inward and outward propeller rotation on single blade and propeller loads

This paper is a sequel of the experimental investigation presented in Ortolani and Dubbioso (2019a) that addressed the single blade and propeller loads during the steady phase of the turning circle. In particular, the effect of inward and outward propeller sense of revolution on the propulsive performance has been investigated for a twin screw ship model equipped with a novel set-up pursued for the measurements of the single blade loads. On the basis of previous numerical investigations (Muscari et al., 2017; Dubbioso et al., 2022), the propeller sense of revolution should be evaluated as a passive mean to mitigate the amplification of the mean or fluctuating component of the blade loads and associated side effects (i.e., noise) caused by blade–wake interactions that occur during maneuvering conditions. The understanding and the accurate quantification of propeller loads, in these realistic operative scenario, is pivotal to design low emission and comfortable ships, fulfilling the requirements of safety and continuity of operations at sea. The analysis is carried out revisiting the investigation in Ortolani and Dubbioso (2019a) for the approach speed at FN=0.26 and a large set of rudder angles that span from moderate to tight maneuvers.

Analysis of flow-induced performance change of cycloidal rotors: Influence of pitching kinematic and chord-to-radius ratio

As a new type of the propulsive devices, cycloidal rotor has been attracting more attention recently. The present work concentrates on the analysis of the performance and internal flow field of a two-bladed cycloidal rotor operating at low Reynolds numbers, with special emphasis on understanding how the flow structure affects the performance at different working conditions. The symmetrical/asymmetrical pitching motions are evaluated initially and the primary results show that the asymmetrical pitching with a mean pitching angle of 5° improves the efficiency of the rotating system. Then, the effect of the chord-to-radius ratio at three different conditions is discussed, which shows that the chord-to-radius ratio of 0.45 is the best value to maximise the performance. The flow pattern, involving the blade-wake interaction, wake-wake interaction, three vortex structures on the blade surface, roll-up vortices inside the laminar boundary layer, separation bubble, leading-edge vortex, trailing-edge vortex and flow separation vortex, are discussed in detail under various operating conditions. The study on the performance of a single blade, the forces (lift and drag) distributions, the blade loadings and the near-wall flows on two blades, provide a new understanding of the global performance change of the propeller.

Computational-fluid-dynamics analysis of a Darrieus vertical-axis wind turbine installation on the rooftop of buildings under turbulent-inflow conditions

Abstract. The behaviour of a rooftop-mounted generic H-rotor Darrieus vertical-axis wind turbine (H-VAWT) is investigated numerically in realistic urban terrain. The interaction of the atmospheric boundary layer with the different buildings, topography, and vegetation present in the urban environment leads to the highly turbulent-inflow conditions with continuously changing inclination and direction. Consequently, all these factors can influence the performance of a VAWT significantly. In order to simulate a small H-VAWT at rooftop locations in the urban terrain under turbulent-inflow conditions, a computational approach is developed. First, the flow field in the terrain is initialized and computed with inflow turbulence. Later, the mesh of wind turbine is superimposed on the mesh of the terrain at two distinct locations and different heights for further computation in the turbulent flow field. The behaviour of the H-VAWT is complex due to the 3D unsteady aerodynamics resulting from continuously changing the angle of attack, blade–wake interaction, and dynamic stall. To get more insight into the behaviour of a rooftop-mounted H-VAWT in turbulent flow, high-fidelity delayed detached-eddy simulations (DDESs) are performed at different rooftop positions and the results compared against the behaviour under uniform-inflow conditions in the absence of inflow turbulence. It is found that the performance of a wind turbine is significantly increased near the rooftop positions. The skewed flow at the rooftop location increases the complexity. However, this effect contributes positively to increasing the performance of H-VAWT wind turbines.

Acoustic versus aerodynamic installation effects on a generic propeller-driven flying architecture

The present work addresses the combined aerodynamic and acoustic installation effects observed as a subsonic propeller is partly crossing the near-wake of a wing. Only the tonal noise at multiples of the blade passing frequency is considered. The aerodynamic effect is the onset of additional sound sources caused by blade-wake interaction, compared to the case of the isolated propeller. The acoustic effect is the scattering by the wing. The work is aimed at demonstrating the ability of analytical models to estimate separately these effects, which is of primary interest for the preliminary design steps of a system. A basic experiment carried out in an anechoic, open-jet facility, is described, for validation purposes. The far-field sound measurements are compared to the predictions and some key outcomes are presented. In particular, the model provides guidelines to avoid configurations of excessive noise.

Noise from a Model-Scale Vertical-Axis Wind Turbine

Vertical-axis wind turbines are an attractive option for small-scale wind-power installations. In this application, noise is crucially important. To investigate the problem, measurements taken from a model-scale turbine in a wind tunnel are considered. The sound radiated by the model is clearly evident in the cross spectra from a pair of flush-mounted microphones. Parameter studies show that, in the normal operating range, speed is the dominant factor. The main source appears to be blade self-noise, although there are also indications of blade–wake interaction in the downstream half of the rotation path. Boundary-layer trips have a significant impact, showing that laminar instability is an important contributor to the self-noise. It will remain a risk at full scale, and its absence should be ensured at the design stage. To minimize the remaining sound radiation (at a given wind speed), configurations with lower tip-speed ratio at maximum power output should be preferred to ones having higher values of this parameter.

Evaluation of Finite-State Dynamic Inflow for Rotors

In an effort to evaluate the accuracy of the finite-state dynamic inflow of Peters and He (“Finite State Induced Flow Models Part II: Three-Dimensional Rotor Disk,” Journal of Aircraft, Vol. 32, No. 2, 1995, pp. 323–333) for use in rotor design, comparisons between dynamic inflow-based calculations and measured test data are presented for a wide variety of single rotor configurations and operating conditions. The quantities compared include rotor performance parameters, blade flap bending moments, blade airloads, and blade pitch angles. The dynamic inflow calculations are performed using the U.S. Army’s Rotorcraft Comprehensive Analysis System. Rotor performance calculations are mostly accurate prior to stall, except for rotor torque calculations from an individual blade pitch control phase sweep. Blade airloads calculations do exhibit the correct dominant features, including those from blade tip vortices, but the calculated amplitudes of the wake-induced loading are significantly less than the test data; this leads to significantly lower amplitudes in the flap bending moment response and is problematic for calculating half peak-to-peak values in conditions with significant blade–wake interactions. Blade collective and cyclic pitch angles are reasonably accurate in edgewise flight.

The control of quadcopter propeller radiated noise

Simple scaling analysis is used to show that significant reductions in quadcopter propeller radiated noise can be achieved, at the hover condition, by reducing the quadcopter and/or payload weight to, in turn, reduce the propeller tip speed. The scaling analysis shows that the propeller radiated noise, at the hover condition, scales as the cube of the combined quadcopter and payload weight. The reductions in radiated noise predicted from the scaling analysis are verified experimentally with quadcopter propellers in an anechoic chamber. Experiments also show that reducing the propeller tip speed by increasing the propeller blade pitch or blade number, while maintaining a given static thrust, is ineffective at reducing the radiated noise. In fact, it is shown that too high a blade pitch and/or blade number can significantly increase propeller radiated noise even though the blade tip speed is significantly reduced. This is attributed to increased propeller blade and blade wake interactions for the increased blade pitch and number.

The effect of the wake on the separated boundary layer in a two-stage compressor

This experimental study provides striking examples of the separated boundary layer development resulting from blade–wake interaction in a multistage turbomachine. Particle image velocimetry measurements are performed within the second-stage rotors of a two-stage compressor. Phase-lock results confirm that wake impingement greatly changes the passage flow, as well as affecting the boundary layer flow. The high turbulence level and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface. By correlating the flow state of the boundary layer with the spatial position of the wake, the influence of the wake on the blade boundary layer flow is revealed, and the mechanism restraining boundary layer separation on the suction surface is studied. It is found that the wake itself does not inhibit separation, and instead, the boundary layer of the region swept by the wake is thickened and separation is strengthened. However, the wake impingement produces a turbulent spot, and the calmed region behind this spot inhibits separation, as well as making the boundary layer thinner. As a consequence, the periodic sweeping of the wake makes the boundary layer exhibit a clear periodicity.

Loss Assessment of the Axial-Gap Size Effect in a Low-Pressure Turbine

The aim of this work is the decomposition, quantification, and analysis of losses related to the axial-gap size effect. Both experimental data and unsteady RANS calculations are investigated for axial gaps equal to 20%, 50% and 80% of the stator axial chord. A framework for identifying sources of loss typical in turbomachinery is derived and utilized for the low-pressure turbine presented. The analysis focuses on the dependency of these losses on the axial-gap variation. It is found that two-dimensional profile losses increase for smaller gaps due to higher wake-mixing losses and unsteady wake-blade interaction. Losses in the end-wall regions, however, decrease for smaller gaps. The total system efficiency can be described by a superposition of individual loss contributions, the optimum of which is found for the smallest gap investigated. It is concluded that these loss contributions are characteristic for the medium aspect-ratio airfoils and operating conditions investigated. This establishes a deeper physical understanding for future investigations into the axial-gap size effect and its interdependency with other design parameters.

Unsteady simulation of marine controllable pitch propeller using boundary element method

In this paper, flow around a submerged controllable pitch propeller is analyzed by developing a general 3D unsteady numerical model, based on boundary element method (BEM) in an infinite extended flow. BEM is combined with time stepping method to model the freely moving/unsteady trailing vortex sheet emanating from each blade trailing edge. In this way, the vortex blade–wake interaction can correctly be taken into consideration. As boundary integral equations are inherently unstable, a smoothing mollifier is applied to damp the singularity effect. Through this effort, attempt has been made to investigate the effect of blade rotation about its spindle on hydrodynamic performance of the controllable pitch propeller. For this reason, relative motion is applied on equation for each blade. This is the main novelty of the paper since little or no effort has been devoted to this subject, thus far. On the other hand, by using an unsteady time stepping method, the rollup wake pattern can be captured throughout simulation by changing the propeller pitching angle. It is demonstrated that the time stepping scheme combined by filtering method can successfully capture the rollup wake pattern during simulation time.

Reduced-order analysis of the acoustic near field of a ducted axial fan

Spatio-temporal and modal analyses of the tip-vortex system of a ducted axial fan are performed based on highly resolved large-eddy simulations by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The Reynolds number based on the tip speed of the blade and the diameter of the fan Do is Re=9.36×105, the Mach number is M=0.136, and the rotational speed Ω=3000 rpm. The tip clearances s/Do=0.001,0.005, and 0.01 are investigated for the design and off-design operating conditions. Only at the off-design condition, a blade-tip-vortex interaction is observed for s/Do=0.01 and s/Do=0.005 while further reduction of the tip clearance to s/Do=0.001 prevents the cyclic interaction of the neighboring blades tip vortex with the current blade and a fully transitional region develops on the suction side of the blade. The POD results capture the narrowband cyclic interaction of the blade wake at the off-design condition. The first two energetic mode pairs can be linked to the interaction of the blades with the incoming wakes of the preceding blades. Compared to the POD results, the DMD modes corroborate the findings of blade-wake interactions. Furthermore, they highlight the connection to boundary layer transition on the suction side of the blades. The spatio-temporal DMD analyses at the characteristic frequencies of the blade-wake interaction reveal the generation of pressure waves which evolve in the azimuthal direction like isolated bubbles and propagate upstream of the blade. It causes two peaks in the spectrum of the acoustic pressure in the near field acoustics in agreement with the experimental sound spectra for s/Do=0.01 and s/Do=0.005 at the off-design condition.