Galih Bangga

CFD studies on rotational augmentation at the inboard sections of a 10 MW wind turbine rotor

In the analysis of the aerodynamic performance of wind turbines, the need to account for the effects of rotation is important as engineering models often failed to predict these phenomena. Investigations are carried out by employing an unsteady computational fluid dynamics (CFD) approach on a generic 10 MW AVATAR (Advanced Aerodynamic Tools for Large Rotors) blade. The focus of the studies is the evaluation of the 3D effect characteristics on thick airfoils in the root area. For preliminary studies, 2D simulations of the airfoils constructing the blade and 3D simulations of the turbine near the rated conditions are carried out. The 2D simulations are in good agreement with available measurements within the linear lift region, but the accuracy deteriorates in the post stall region. For the 3D wind turbine rotor results, the prediction is consistent with other CFD computations obtained from the literature. Further calculations of the rotor are conducted at 5 different wind speeds ranging from below to above...

Investigations of the inflow turbulence effect on rotational augmentation by means of CFD

The present studies are addressed to gain more insights into the inflow turbulence effect on rotational augmentation using computational fluid dynamics. Three different cases were simulated and analysed focusing on the three-dimensional effects in the inboard blade region of a 10 MW generic wind turbine rotor. The evidence of rotational augmentation was presented and compared to two-dimensional simulations of the blade sections at consistent inflow conditions. Inflow turbulence has a very strong impact on the instantaneous blade loads and standard deviations, but the effect on the mean values is small. The amplitudes of the blade load fluctuations are amplified under turbulent inflow conditions and these are related to the blade passing frequency and the specified turbulence length scale at the inlet. Detailed examinations of these phenomena were performed and are presented in the present manuscript.

Effects of ambient turbulence on the near wake of a wind turbine

Developments of the near wake behind the Avatar research turbine (radius of 102.88 m) in ambient turbulence are investigated using high fidelity numerical simulations. A moderate level of background turbulence with a wide range of scales, which has not been considered in the previous studies is applied. With ambient turbulence, a significant impact on the near wake development is observed. The mean velocity profile becomes Gaussian after 450 m distance downstream, which is a demarcation between the near and the far wake. From the spectral analysis of the wake, clear peaks in the spectra are observed at the blade passing frequency, but the distributions of the peak extend into a wide range of frequency domain. Such aspects provide useful information in classifying periodic and stochastic fluctuations, and their contributions to the momentum mixing in the wake.

CFD Studies of the Dynamic Stall Characteristics on a Rotating Airfoil

The periodic changes of the angles of attack in time occurring in the Darrieus turbine blade result in the significant increase of unsteady characteristics, commonly referred as dynamic stall. CFD simulations using SST k-ω turbulence model with Kato-Launder correction were used to examine the physical phenomena occurring on the sectional airfoil of the blade. The blade was modeled as an airfoil rotating about its center of rotation. The CFD calculations show remarkable differences of the forces acting on the airfoil compared with the static condition due to the traveling vortices phenomena. The generation of leading edge and trailing edge vortices as the characteristics of dynamic stall was captured accurately. This provided more detailed information on the development of dynamic stall on the Darrieus turbine blade. The analysis of the stall event was obtained by considering the insertion flow occurring near the leading edge. The interactions of the travelling vortices in the upwind as well as in the downwind phases are presented in the present manuscript, providing deeper knowledge in the dynamic stall database of vertical axis wind turbines.

Unsteady Navier-Stokes studies on loads, wake, and dynamic stall characteristics of a two-bladed vertical axis wind turbine

Computational fluid dynamics (CFD) studies are carried out on a two-bladed vertical axis wind turbine operating at a wind speed of 8 m/s for tip speed ratios (λ) of 0.50–3.0. The blade is the NACA0021 airfoil with chord length 0.265 m and rotor radius 1 m. Basic sensitivity studies for various time step sizes are carried out. The results are validated against available measurement data from the literature. Excellent agreement is obtained for small λ up to optimum condition. For the higher tip speed ratios, the two-dimensional CFD computations predict higher results than the wind tunnel experiment, but they are very similar to the field measurement data. Wake characteristics are presented, showing that the wake becomes Gaussian at 5 times radius downstream of the rotor. It is shown that complex flow phenomena occur owing to dynamic stall onset, especially for the smaller tip speed ratio.

Hybrid RANS/LES Capabilities of the Flow Solver FLOWer—Application to Flow Around Wind Turbines

The compressible block-structured flow solver FLOWer of the German Aerospace Center (DLR) has been extended towards state of the art detached-eddy simulation (DES) methods in order to conduct hybrid RANS/LES simulations of flow around rotary wings. The large-eddy simulation (LES) capabilities of the code are demonstrated for decaying isotropic turbulence. Excessive numerical dissipation is avoided by using the fifth-order WENO scheme and an appropriate low-Mach number correction. The DES implementations are validated first for the well documented test cases backward facing step and NACA0021 airfoil at \(60^\circ \) incidence, before increasing the complexity by simulating the flow around the MEXICO model wind turbine operating in stalled conditions and comparing with experimental data. From the latter, recommendations on the numerical settings are derived to successfully set up eddy resolving simulations for wind turbine or helicopter applications.

Three-dimensional flow in the vicinity of a circular cylinder mounted to a flat plate at high Reynolds number

A numerical investigation on the flow behavior past a circular cylinder at Reynolds number of Re = 1.0e6 − 8.4e6 is performed using Computational Fluid Dynamics (CFD) approach. The study is carried out using an overset grid method employing the two-equation eddy viscosity Menter SST turbulence model. For preliminary studies, simulations of two-dimensional as well as infinite length circular cylinders are conducted and compared with available measurement data. The predicted drag coefficient and Strouhal number are in the good agreement compared with the measurements. With the presence of a flat plate, CFD simulations demonstrate that the three-dimensional effects strongly influence the flow behavior near the junction. The horseshoe vortex and the characterized 3D flow phenomena are observed and investigated in the present study.

Aerodynamic performance of a small vertical axis wind turbine using an overset grid method

The present paper aims to asses the aerodynamic performance of a small vertical axis wind turbine operating at a small wind speed of 5 m/s for 6 different tip speed ratios (λ=2-7). The turbine consists of two blades constructed using the NACA 0015 airfoil. The study is carried out using computational fluid dynamics (CFD) methods employing an overset grid approach. The (URANS) SST k − ω is used as the turbulence model. For the preliminary study, simulations of the NACA 0015 under static conditions for a broad range of angle of attack and a rotating two-bladed VAWT are carried out. The results are compared with available measurement data and a good agreement is obtained. The simulations demonstrate that the maximum power coefficient attained is 0.45 for λ=4. The aerodynamic loads hysteresis are presented showing that the dynamic stall effect decreases with λ.

Hybrid RANS/LES Simulations of the Three-Dimensional Flow at Root Region of a 10 MW Wind Turbine Rotor

Numerical computations using the Unsteady Reynolds Averaged Navier-Stokes (URANS) and Delayed Detached-Eddy Simulations (DDES) approaches are carried out to investigate the complex three-dimensional flow in the root region of a generic 10 MW wind turbine rotor. Preliminary studies regarding the time step size and the number of rotor revolution required for the time averaging procedure are conducted. In the blade outer region, URANS is sufficient to predict the general flow characteristics, but small discrepancies are observed in the blade root area where the flow is massively separated.

Energy assessment of two vertical axis wind turbines in side-by-side arrangement

The present study is intended to investigate the energy production of two vertical axis wind turbines (VAWTs) arranged in a side-by-side configuration for 5 different rotor distances and three combinations of rotational directions. The studied two-bladed rotor is constructed by the NACA 0021 airfoil with a radius of 1 m and operates at a wind speed of 8 m/s. Computational Fluid Dynamics approaches are employed in these analyses using the Menter SST turbulence model, and the simulations agree well with the available measurement data for a single rotor. The investigations reveal that the attained power of the two VAWT rotors depends strongly on the distance of the turbines and the direction of the rotor rotation. Load fluctuations are further examined in the present study. The performance improvement and vortex shedding of the dual-rotor system depend strongly on the tip speed ratio. An improved performance of the rotor can be achieved by considering the mentioned parameters, and layouts of the turbine array are designed based on these studies.The present study is intended to investigate the energy production of two vertical axis wind turbines (VAWTs) arranged in a side-by-side configuration for 5 different rotor distances and three combinations of rotational directions. The studied two-bladed rotor is constructed by the NACA 0021 airfoil with a radius of 1 m and operates at a wind speed of 8 m/s. Computational Fluid Dynamics approaches are employed in these analyses using the Menter SST turbulence model, and the simulations agree well with the available measurement data for a single rotor. The investigations reveal that the attained power of the two VAWT rotors depends strongly on the distance of the turbines and the direction of the rotor rotation. Load fluctuations are further examined in the present study. The performance improvement and vortex shedding of the dual-rotor system depend strongly on the tip speed ratio. An improved performance of the rotor can be achieved by considering the mentioned parameters, and layouts of the turbine arra...