CFD simulations of a vertical axis wind turbine in dynamic stall: URANS vs. Scale-Adaptive Simulation (SAS)

Vertical axis wind turbines (VAWTs) are promising candidates for wind energy harvesting in the urban environment. However, their aerodynamic performance still falls behind of their horizontal axis counterparts. This could be associated to the comparatively small research they have received in the past decades as well as their complex unsteady aerodynamics. Computational Fluid Dynamics (CFD) has been widely used to evaluate and improve the aerodynamic performance of VAWTs. An extensive literature study reveals that the 2D unsteady Reynolds-Averaged Navier-Stokes (URANS) approach has been used in the majority of the CFD studies on VAWTs. The current study intends to evaluate the aerodynamic performance of a VAWT, calculated using 2D URANS, and compare it with that of 2.5D URANS and 2.5D scale-adaptive simulation (SAS). SAS is a hybrid RANS-LES model developed by Menter and Egorov [1]. The four-equation transition SST turbulence model is employed in the URANS simulations as well as in the RANS region of the hybrid RANS-LES simulation. The studied turbine is a one-bladed Darrieus H-type VAWT with a solidity of 0.125 operating at a low tip speed ratio of 2.0, which corresponds to the most complex case for VAWTs where dynamic stall is dominant. The reduced frequency is 0.125 representing the high unsteadiness in the flow. Significant benefits of the one-bladed turbine are: (i) less blade-wake interactions while the essential flow features, such as dynamic stall, are still present, (ii) reduced computational costs due to the smaller number of cells. The turbine characteristics is based on the experiment by Simão Fereira et al. [2]. Validation studies for the one-bladed turbine as well as the other turbines have been performed [3-5]. A comparative analysis of the instantaneous tangential and normal loads on the turbine (see Fig. 1), spatiotemporal distribution of pressure coefficient (see Figs. 2a-c) and skin friction coefficient (see Fig. d-f) on the blade suction side, the evolution of the shed vorticity by the blade, dynamic loads on the blade and the turbine wake are employed to evaluate the performance of URANS modeling in comparison to the SAS model. The instantaneous turbine loads calculated using the 2D and the 2.5D URANS, shown in Fig. 1, are in line with minor differences in the downwind side. Despite the 180 times higher number of cells and 10 times finer time step of the SAS modeling, an overall good agreement exists between the 2D URANS and the SAS results. The predicted thrust coefficients for 2D and 2.5D URANS and SAS are 0.422, 0.424 and 0.430, respectively. Nevertheless, there exist noticeable differences between the URANS and SAS results in the bursting location of the laminar separation bubble (LSB), the evolution of the dynamic stall vortex (DSV), the leading-edge secondary and tertiary vortices and the trailing-edge separation. The findings of the present study help to highlight the deficiencies of URANS modeling of VAWTs in dynamic stall

CFD simulation of two tandem floating offshore wind turbines in surge motion

High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulation is employed to investigate the variations in the power performance of two tandem in-line floating offshore horizontal axis wind turbines for the scenario in which the upstream rotor is oscillating in surge motion and the downstream rotor is positioned in a distance of 3D (D: turbine diameter) and is stationary. The rotors are the NREL-5MW reference turbine. The platform surge period and wave amplitude are 9 s and 1.02 m, respectively. The results are presented for 100 full surge periods. It is found that the surge motion of the upstream rotor results in: (i) sinusoidal fluctuations in the power and thrust coefficients (CP and CT) of the upstream rotor with a standard deviation (std) of 9.7% and 5.5%, respectively; (ii) such fluctuations in CP and CT are less regular with a std of 4.2% and 2.8% for the downstream rotor, respectively. A low-frequency oscillating mode with a period nearly 10 times the surge period is also observed for the downstream rotor. The mean Cp and Ct of the downstream rotor are 28.9% and 38.5% of the upstream one.peer-reviewe

Wake interactions of two tandem floating offshore wind turbines : CFD analysis using actuator disc model

Floating offshore wind turbines (FOWTs) have received great attention for deep water wind energy harvesting. So far, research has been focused on a single floating rotor. However, for final deployment of FOWT farms, interactions of multiple FOWTs and potential impacts of the floating motion on power performance and wake of the rotors need to be investigated. In this study, we employ CFD coupled with an Actuator Disc model to analyze interactions of two tandem FOWTs for the scenario, where the upstream rotor is floating with a prescribed surge motion and the downstream rotor is fixed and influenced by the variations in the incoming flow created by the oscillating motion of the surging rotor. We will investigate three different surge amplitudes and analyze the fluctuations in power performance of the two rotors as well as their wake interactions. The results show a light increase in the mean power coefficient of both rotors for the surging case, compared against the case with no surge motion. The standard deviation of the transient CP of the surging rotor linearly scales with the surge amplitude, while such impact for the downstream rotor is very limited. Surging motion of the upstream rotor is found to enhance flow mixing in the wake, which therefore, accelerates the wake recovery of the downstream rotor. This finding suggests prospects for research into redesigning wind farm layout for FOWTs, aiming for more compact arrangements.peer-reviewe

Towards smart blades for vertical axis wind turbines : different airfoil shapes and tip speed ratios

Future wind turbines will benefit from state-of-the-art technologies that allow them to not only operate efficiently in any environmental condition but also maximise the power output and cut the cost of energy production. Smart technology, based on morphing blades, is one of the promising tools that could make this possible. The present study serves as a first step towards designing morphing blades as functions of azimuthal angle and tip speed ratio for vertical axis wind turbines. The focus of this work is on individual and combined quasi-static analysis of three airfoil shape-defining parameters, namely the maximum thickness and its chordwise position as well as the leading-edge radius index I. A total of 126 airfoils are generated for a single-blade H-type Darrieus turbine with a fixed blade and spoke connection point at . The analysis is based on 630 high-fidelity transient 2D computational fluid dynamics (CFD) simulations previously validated with experiments. The results show that with increasing tip speed ratio the optimal maximum thickness decreases from 24 %c (percent of the airfoil chord length in metres) to 10 %c, its chordwise position shifts from 35 %c to 22.5 %c, while the corresponding leading-edge radius index remains at 4.5. The results show an average relative improvement of 0.46 and an average increase of nearly 0.06 in CP for all the values of tip speed ratio.peer-reviewe

Active flow control for power enhancement of vertical axis wind turbines: leading-edge slot suction

Vertical axis wind turbines (VAWTs) suffer from a poor power performance at low tip speed ratios, where their blade aerodynamics are dominated by unsteady separation and dynamic stall. Therefore, to enhance their aerodynamic performance, separation control is highly desired. The present study intends to suppress the flow separation on VAWTs using boundary layer suction through a slot located near the blade leading edge. High-fidelity computational fluid dynamics simulations extensively validated with experiments are employed. A characterization of the impact of the suction amplitude, 0.5% ≤ AS ≤ 10%, and the suction location, 8.5 ≤ XS/c ≤ 28.5, is performed. The dependency of the obtained power gain on operating conditions, i.e. tip speed ratio, 2.5 ≤ λ ≤ 3.5, Reynolds number, 0.51 × 105 ≤ Rec ≤ 2.78 × 105, and turbulence intensity, 1% ≤ TI ≤ 25%, is studied. The results show that applying suction along the chordwise extent of the laminar separation bubble (LSB) can prevent its bursting, eliminate/postpone its formation, avoid the formation of the dynamic stall vortex and trailing-edge roll-up vortex, and delay the incipient trailing-edge separation. This will significantly increase the blade lift force, decrease the drag force, delay the stall angle and suppress the aerodynamic load fluctuations. For the reference turbine and for AS = 0.5% and XS/c = 8.5%, the power coefficient at λ of 2.5, 3.0 and 3.5 is enhanced by 247%, 83% and 24%, respectively. The suction location is critical while a minimum amplitude, e.g. AS = 0.5%, suffices. The optimal suction location is insensitive to TI, weakly sensitive to λ while comparatively more sensitive to Rec

A framework for preliminary large-scale urban wind energy potential assessment: Roof-mounted wind turbines

status: publishe

Towards optimal aerodynamic design of vertical axis wind turbines: Impact of solidity and number of blades

© 2018 The Authors The current study systematically analyzes the impact of solidity (σ) and number of blades (n) on the aerodynamic performance of 2-, 3- and 4-bladed Darrieus H-type vertical axis wind turbines (VAWTs). Solidity varies within the wide range of 0.09–0.36. A large number of operational parameters, i.e., tip speed ratio (λ), Reynolds number (Re), turbulence intensity and reduced frequency (K) are investigated to provide a deeper insight into the impact of σ and n on the dynamic loads on blades, the turbine performance and the wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, extensively validated with experiments, are employed. The results show that the turbine optimal tip speed ratio (λopt) is invariant to a newly-introduced parameter ‘σλ3’ regardless of the turbine geometrical and operational characteristics. In addition, a new correlation is derived to estimate λopt as a function of σ which can also be employed to predict the optimal σ for a turbine with a given λ. It is also found that: (i) for constant-speed urban VAWTs, which due to the low mean wind speed in the urban environment, frequently operate at moderate to high λ a relatively-low σ is optimal; (ii) an optimal VAWT is a moderately-high-solidity variable-speed rotor maintaining a relatively-low λ where due to the large blade chord length the resulting Re and K are favorably high; (iii) within the turbine optimal operational range, turbine power coefficient (CP) is almost independent of n. The present findings support the optimal aerodynamic design of small-to large-scale VAWTs.status: publishe

On the accuracy of turbulence models for CFD simulations of vertical axis wind turbines

A comparative analysis of seven commonly-used eddy-viscosity turbulence models for CFD simulations of VAWTs is presented. The models include one- to four-equations, namely the Spalart-Allmaras (SA), RNG k-ε, realizable k-ε, SST k-ω, SST k-ω with an additional intermittency transition model (SSTI), k-kl-ω and transition SST (TSST) k-ω models. In addition, the inviscid modeling is included in the comparison. The evaluation is based on validation with three sets of experiments for three VAWTs with different geometrical characteristics operating in a wide range of operational conditions, from dynamic stall to optimal regime and to highly-rotational flow regime. The focus is on the turbine wake, the turbine power performance, and the blade aerodynamics. High-fidelity incompressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are employed. The extensive analysis reveals high sensitivity of the simulation results to the turbulence model. This is especially the case for the turbine power coefficient CP. The results show that the inviscid, SA, RNG k-ε, realizable k-ε and k-kl-ω models clearly fail in reproducing the aerodynamic performance of VAWTs. Only the SST model variants (SST k-ω, SSTI and TSST) are capable of exhibiting reasonable agreement with all the experimental data sets, where the transitional SST k-ω versions (SSTI and TSST) are recommended as the models of choice especially in the transitional flow regime

On the accuracy of turbulence models for CFD simulations of vertical axis wind turbines

status: publishe