ECN Aero-module

The ECN Aero-module is currently being developed at ECN. It will be a module that is suitable for coupling to structural codes; multi-body or FEM, resulting in a complete simulation tool to simulate a wind turbine in the time domain. The Aero-module will be a module that can easily switch between basic BEM equations to calculate the aerodynamics and other more complex methods; AWSM and Rotorflow. These different options that can be used to determine the aerodynamic forces acting on the flexible structure are described below. For turbulent wind input it will be possible to couple the module to SWIFT. The module will also be integrated into FOCUS. 

Special attention has been paid to facilitate the coupling of the ECN Aero-module to a structural solver. The discretisation of structural and aerodynamic control points is flexible due to a built-in spline module for interpolation. A first try-out of the ECN Aero-module coupled to a structural code (the multi-body code SIMPACK) has yielded good results and holds a promise for the near future. The combination of a wind turbine model in SIMPACK with the Aero-module will, in future, be validated by comparing simulation results to both measurements in the field and wind tunnel tests. During this process the accuracy of the aerodynamic models will be looked at as well as the necessity of detail in the structural model of the wind turbine.  

Below short descriptions of the foreseen aerodynamic models to be included are given. More information on AWSM and ROTORFLOW can be found through the links.

BEM

The BEM options in the ECN Aero-module will include engineering models for turbulent wake, dynamic inflow, skewed flow, tower shadow, dynamic loops in the lift coefficient (dynamic stall), 3D corrections to the coefficients and Prandtl?s tip correction and root correction. This BEM option can be used for simulations of wind turbine models operating in conditions for which it is known that blade element momentum theory gives reasonable results. This option will be available in the first beta release which is expected in 2010.

AWSM

AWSM is based on non-linear lifting line vortex wake theory which is a more accurate model for rotor wake physics; an important aspect in wind turbine load simulations.  This aerodynamic module calculates the development of the shape and strength of the wake of the blades in time. In this approach the aerodynamic lift-, drag-, and pitching-moment characteristics of the blade cross-sections are assumed to be known and corrected for the effects of blade rotation. In comparison to the currently used blade-element-momentum BEM theory codes, more accurate predictions are expected in situations where local aerodynamic characteristics strongly vary with time and where dynamic wake effects play a significant role (e.g. yawed flow). AWSM, is expected to substantially reduce the number of uncertainties that accompany currently used blade-element-momentum methods. This option will be available in the first beta release which is expected in 2010.

ROTORFLOW

In the future the ROTORFLOW code, once finished, will also become a part of the ECN Aero-module. In the ROTORFLOW code the flow field is modelled by a combination of a potential flow and an integral boundary layer method. ROTORFLOW I uses the panel method for the unsteady inviscid flow around rotating bodies including their wakes. ROTORFLOW II uses a three dimensional unsteady boundary layer model/method, to be solved in strong interaction with the external pressure field from ROTORFLOW-I.

The three dimensional unsteady boundary layer model will be expressed and solved in integral form, instead of a field solution. In this way, both the external flow solver as the boundary layer solver are surface fitted and hence in essence two dimensional (over the surface). It is expected that this will lead to a method that is significantly faster than a full Navier-Stokes field solver. Still, through the strong interaction with the external flow, it is anticipated that this method will at least be able to obtain good results for mild separated flows.