Actuator Disk Approach Research Articles

Wind turbine rotor simulation using the actuator disk and actuator line methods

The present paper focuses on wind turbine rotor modeling for loads and wake flow prediction. Two steady-state models based on the actuator disk approach are considered, using either a uniform thrust or a blade element momentum calculation of the wind turbine loads. A third model is based on the unsteady-state actuator line approach. Predictions are compared with measurements in wind tunnel experiments and in atmospheric environment and the capabilities and weaknesses of the different models are addressed.

Validation of the actuator disc approach in PHOENICS using small scale model wind turbines

In this study two wind turbine setups are investigated numerically: (a) the flow around a single model wind turbine and (b) the wake interaction between two in-line model wind turbines. This is done by using Reynolds averaged Navier-Stokes (RANS) and an actuator disc (ACD) technique in the computational fluid dynamics code PHOENICS. The computations are conducted for the design condition of the rotors using four different turbulence closure models. The computed axial velocity field as well as the turbulent kinetic energy are compared with PIV measurements. For the two model wind turbine setup, the thrust and power coefficient are also computed and compared with measurements. The results show that this RANS ACD method is able to predict the overall behaviour of the flow with low computational effort and that the turbulence closure model has a direct effect on the predicted wake development.

The Actuator Disc Concept in Phoenics

This study presents two models to simulate a wind turbine. This is done by employing the 1D momentum actuator disc theory in PHOENICS, a general purpose computational fluid dynamics software. To test the general applicability of these models, single wind turbine simulations are conducted using eight different wind turbine models from two manufacturers. The simulations are performed by imposing sheared inflow with hub height wind speeds ranging from 3 m/s up to 25 m/s. A range of computational parameters are investigated, including the resolution of the domain, the thickness of the actuator disc and the iterative convergence criteria. To investigate the wake development produced by these methods, a comparison study is performed with the more complex large-eddy simulation software EllipSys3D using an actuator disc approach for validation purposes. The resulting wind turbine thrust and power outputs from PHOENICS are compared with the experimental power curves and thrust values provided by the manufacturers for each wind turbine. The results show that actuator disc methods are able to provide a reasonable estimation of the conventional wind turbine power and thrust output with low computational effort. Moreover, the results from the preliminary comparison of the wake produced from these two rotor models compare well with the wake produced by the actuator disc in EllipSys3D.

Numerical Investigation of Engine Effects on a Transport Aircraft with Circulation Control

The scope of this paper is to illustrate the installation effects of a turboprop engine on a high-lift configuration of a short takeoff and landing aircraft with circulation control. In addition to the influence on the wing performance, the impact on the longitudinal static stability of the aircraft is also investigated. Furthermore, critical failure cases, namely one engine inoperative, as well as an asymmetric circulation control failure, are assessed. Therefore, steady computational-fluid-dynamics calculations based on the Reynolds-averaged Navier–Stokes equations were performed. The propeller is modeled with an actuator-disk approach. The results show strong potential of increasing lift by synergy effects between circulation control and propeller slipstream. However, the longitudinal stability and controllability are adversely affected. Regarding the case of one engine inoperative, the resulting yawing moments are twice as high as the actual yawing moments from the asymmetric thrust and are therefore higher than expected.

Analysis of long distance wakes of Horns Rev I using actuator disc approach

The wake recovery behind the Horns Rev wind farm is analysed to investigate the applicability of Large Eddy Simulations (LES) in combination with an actuator disc method (ACD) for farm to farm interaction studies. Periodic boundary conditions on the lateral boundaries are used to model the wind farm (as infinitely wide), using only two columns of turbines. The meteorological conditions of the site are taken into account by introducing wind shear and pre-generated synthetic turbulence to the simulation domain using body forces. Simulations are carried out to study the power production and the velocity deficit in the farm wake. The results are compared to the actual power production as well as to wind measurements at 2 km and 6 km behind the wind farm. The simulated power production inside the farm shows an overall good correlation with the real production, but is slightly overpredicted in the most downstream rows. The simulations overpredict the wake recovery, namely the wind velocity, at long distances behind the farm. Further studies are needed before the presented method can be applied for the simulation of long distance wakes. Suggested parameters to be studied are the development of the turbulence downstream in the domain and the impact of the grid resolution.

Large-eddy simulation of multiple wakes in offshore wind farms

In this study, high-resolution large-eddy simulations of two German offshore wind farms are performed with the LES model PALM in which the turbines are parameterized with an actuator disk approach. The simulation of a single wind farm is realized by a stationary model domain with a turbulent inflow. Different atmospheric stratifications from slightly stable to unstable have been simulated and their effect on the wake characteristics is investigated. The results show a clear development of the flow within the wind farms with large differences between single, double and triple wakes. The wake deficit is increasing up to the second wake for the neutral and stable boundary layer and up to the third wake in the convective case before reaching a nearly constant value. This is explained with the turbulence intensity being much higher behind the second and subsequent turbines compared to the wake of the first turbine. With increasing atmospheric stability the wake deficits are shown to be stronger due to reduced turbulence.

Analysis of the effect of curtailment on power and fatigue loads of two aligned wind turbines using an actuator disc approach

To study the effects of curtailment on both power production and fatigue loading, actuator disc (ACD) simulations of two turbines aligned in the wind direction are performed with the EllipSys3D code developed at DTU/Risø. A simple non-aeroelastic fatigue load evaluation method for ACD simulations is developed. Blade loads, extracted along a line that rotates in the rotor plane with the rotational velocity of the respective turbine, are used to calculate flapwise bending moments. After applying a rainflow counting algorithm an equivalent moment is calculated. Power curtailment is introduced by increasing the blade pitch angle of the first turbine. Evaluation is made with regards to fatigue load reduction at the second turbine and the change in the total production. Further parameters investigated are the spacing between the two turbines and the level of imposed pre-generated turbulence. The aeroelastic code Vidyn, Ganander [1], is used for validation of the ACD load evaluation method. For this purpose, the EllipSys3D simulations are rerun without the second turbine. Time series of cross sectional velocity fields are extracted at positions corresponding to the former placement of the downstream turbine and used as input for aeroelastic turbine load calculations in Vidyn. The results from Vidyn and the results based on the ACD loads show similar trends. Fatigue loads at the downwind turbine are clearly decreasing as the blade pitch angle of the upstream turbine is increasing. The achievable amount of fatigue load reduction depends on the level of the imposed pre-generated turbulence as well as the spacing between the turbines. The presented method is intended for further development of wind park optimization strategies.

Umerical simulation of atmospheric boundary layer and wakes of horizontal-axis wind turbines

Simulations of wind turbine wakes are presented in this paper using the three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations employing the k-e turbulence model appropriately modified for the atmospheric flows. The actuator disk approach is used to model the action of the turbine rotor. Modified formulations of the inlet conditions and the wall functions are used to allow consistency between the fully developed atmospheric boundary layer (ABL) inlet profiles and the wall function formulation. Results are presented and compared with three wind turbines running under neutral atmospheric conditions. The results demonstrate that the accurate simulation of the atmospheric boundary layer applying enhanced inlet conditions and wall function formulation consistent with the k-e model can give very useful information about the wakes, directly contributing to the accurate estimation of the power of the downstream turbines.

Wake prediction of horizontal-axis wind turbine using full-rotor modeling

This work is devoted to the study of the wake characteristics in the near and far wake regions of a horizontal axis wind turbine, with an exact representation of the rotor blades. The computational numerical solution was carried out by solving the conservation equations for one outer stationary reference frame and one inner rotating reference frame, wherein the blades and grids were fixed in reference to the rotating frame. The results were obtained using the steady state Reynolds-Averaged Navier–Stokes equations and the turbulence was simulated via the k–ε turbulence model. The results of the full rotor approach using the standard k–ε turbulence model are compared with the results of the actuator disc approach using the standard k–ε turbulence model and two modified k–ε models used by the earlier researchers. The wake behavior was tested and validated with the experimental results of the three blade Danwin 180kW wind turbine available in the literature. The results obtained from the full rotor model showed good agreement with the available experimental data, in comparison with the improvement achieved by the actuator disc approach using modified versions of the k–ε model.

Simulation of turning circle by CFD: Analysis of different propeller models and their effect on manoeuvring prediction

Propeller modelling in CFD simulations is a key issue for the correct prediction of hull-propeller interactions, manoeuvring characteristics and the flow field in the stern region of a marine vehicle. From this point of view, actuator disk approaches have proved their reliability and computational efficiency; for these reasons, they are commonly used for the analysis of propulsive performance of a ship. Nevertheless, these models often neglect peculiar physical phenomena which characterise the operating propeller in off-design condition, namely the in-plane loads that are of paramount importance when considering non-standard or unusual propeller/rudder arrangements. In order to emphasize the importance of these components (in particular the propeller lateral force) and the need of a detailed propeller model for the correct prediction of the manoeuvring qualities of a ship, the turning circle manoeuvre of a self-propelled fully appended twin screw tanker-like ship model with a single rudder is simulated by the unsteady RANS solver χnavis developed at CNR-INSEAN; several propeller models able to include the effect of the strong oblique flow component encountered during a manoeuvre have been considered and compared. It is emphasized that, despite these models account for very complex and fundamental physical effects, which would be lost by a traditional actuator disk approach, the increase in computational resources is almost negligible. The accuracy of these models is assessed by comparison with experimental data from free running tests. The main features of the flow field, with particular attention to the vortical structures detached from the hull are presented as well.

Modeling wake effects in large wind farms in complex terrain: the problem, the methods and the issues

ABSTRACTComputational fluid dynamic (CFD) methods are used in this paper to predict the power production from entire wind farms in complex terrain and to shed some light into the wake flow patterns. Two full three‐dimensional Navier–Stokes solvers for incompressible fluid flow, employing k − ϵ and k − ω turbulence closures, are used. The wind turbines are modeled as momentum absorbers by means of their thrust coefficient through the actuator disk approach. Alternative methods for estimating the reference wind speed in the calculation of the thrust are tested. The work presented in this paper is part of the work being undertaken within the UpWind Integrated Project that aims to develop the design tools for next generation of large wind turbines. In this part of UpWind, the performance of wind farm and wake models is being examined in complex terrain environment where there are few pre‐existing relevant measurements. The focus of the work being carried out is to evaluate the performance of CFD models in large wind farm applications in complex terrain and to examine the development of the wakes in a complex terrain environment. Copyright © 2011 John Wiley & Sons, Ltd.

Numerical Investigation of Inlet Distortion on a Wing-Embedded Lift Fan

The inlet distortion arising on a generic fan-in-wing wind-tunnel model is investigated by means of unsteady Reynolds-averaged Navier–Stokes simulations. Flow separation occurs on the inlet lip and generates a separation bubble above the rotor blades. A distortion of the total pressure distribution at the fan inlet significantly influences the blade loading. The strong flow-velocity gradient over the inlet lip induces an increase of the axial-velocity magnitude, resulting in a reduction of incidence angle on the rotor blade. In the bubble-core region, the low axialvelocity magnitude enhances the blade incidence angle and provokes blade tip stall. Steady-state calculations, using an actuator disk approach, are also conducted to identify the key parameters affecting lip boundary-layer separation. The ratio of freestream velocity to fan jet velocity and the inlet-lip radius affect the total pressure distribution bymodifying the location of the flow separation. The angle of attack on the wing has a negligible impact on the inlet distortion. An inlet lip with a large radius at its front part suppresses the lip separation and considerably reduces the total pressure distortion. Injecting a jet over the inlet lip can be a solution to actively control the lip flow.

Recent advances in modeling of wind turbine wake vortical structure using a differential actuator disk theory

This paper presents the recent developments of a new CFD-based method aimed at predicting wind turbine aerodynamics, where velocity and pressure discontinuities are used to model the vortical system that creates lift on the turbine blades. To illustrate the ability of the present model to predict induced wake effect, the case of the taper wing is thoroughly analyzed and effects of both domain discretization and convection scheme are presented. Results are mitigated regarding predicted performance of induced drag, but accurate induced and upstream flow angles values are obtained. The method is even shown to be a useful calculator for the relationship between inflow angle measured upstream and effective angle of attack of a wing section. Interesting results for the NREL phase VI rotor have been obtained showing improvement of the method upon actuator-disk approach in handling tip vortices effect on the blade aerodynamics.

Rarefaction Wave Effects Upon Compressor Performance

THE response of a duct-embedded multistage compressor to downstream generated rarefaction waves and their reflections is discussed. Partial differential equations of flow and work in their hyperbolic form representing the compressor-duct system in unsteady flow were solved using the method of characteristi cs. Agreement between the analyzed results and an associated experiment was found in terms of the measured frequency of the pulse reflections, amplitude of the first few passes of the waves, and the final operating point of the system. Contents The Model The type of event investigated was that of a rarefaction wave generated downstream of a compressor, simulating, for example, the effects of extinguishing the re-heat in an air-gas turbine engine or the sudden opening of a downstream placed valve in a process plant. Governing equations of continuity of mass, momentum, and energy were solved in their partial differential form by the method of characteristi cs. Setting the equations in terms of the independent variables, axial distance x and time t and the dependent variables, speed of sound a, convection velocity Cx, and pressure p, resulted in a hyperbolic form of the equations permitting use of this technique. The flow considered was then inviscid, compressible, unsteady, and homentropic. The solution, being one-dimensional in space, predicted the passage of planar waves in the axial direction and compressor simulation was confined to high hub-tip ratio machines. Work transfer within a compressor was modeled by using an actuator disk approach. U3 Since

Propagation of Inlet Flow Distortions through an Axial Compressor Stage

This contribution will present an original calculation method predicting the development of an inlet flow distortion through a compressor stage. A finite difference technique is used to treat the flow equations outside the blade rows. That flow is two-dimensional, compressible and nonviscous. The blade rows are modelized using a quasi actuator disk approach, but include the unsteady transfer terms due to the rotor relative motion in a non uniform inlet flow. A set of experimental data, measured on a one stage axial compressor, submitted to a rectangular inlet total pressure distortion will be discussed and will serve as basis for a comparison between theory and experiments.