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The HAWC2 code is a code intended for calculating wind turbine response in time domain. It has been developed within the years 2003-2006 at the Aeroelastic Design research program at Risø, National Laboratory Denmark.
The structural part of the code is based on a multibody formulation. In this formulation the wind turbine main structures is subdivided into a number of bodies where each body is an assembly of Timoshenko beam elements. Each body includes its own coordinate system with calculation of internal inertia loads when this coordinate system is moved in space, hence large rotation and translation of the body motion is accounted for. Inside a body the formulation is linear, assuming small deflections and rotations. This means that a blade modeled as a single body will not include the same nonlinear geometric effects related to large deflections as a blade divided into several bodies. The bodies representing the mechanical part of the turbine are connected by joints also referred to as constraints.
The constraints are formulated as algebraic equations that impose limitations of the bodies’ motion. This could in principal be a trajectory the body needs to follow, but related to the wind turbine implementation there are so far the possibility of a fixed connection to a global point (e.g. tower bottom clamping), a fixed coupling of the relative motion (e.g. fixed pitch, yaw), a frictionless bearing and a bearing where the rotation angle is controlled by the user. It may be worth to notice, that also for the last constraint where the rotation is specified, inertial forces related to this movement is accounted for in the response.
External forces are in general placed on the structure in the deformed state, which is especially important for pitch loads and twist of the blades and since large rotations are handled by a proper subdivision of bodies, the code is also especially suited for calculations on very flexible turbines subjected to e.g. large blade deflections. The aerodynamic part of the code is based on the blade element momentum theory, but extended from the classic approach to handle dynamic inflow, dynamic stall, skew inflow, shear effects on the induction and effects from large deflections. One example is the effect of large flapwise blade deflections causing a change in the effective rotor diameter and that the blade forces are no longer perpendicular to the rotor plane. This reduces the thrust on the rotor thereby changing the induced velocities and vice versa.
Two dynamic stall models have been implemented where the first is generally known as the Øye model, which includes the effect of stall separation lag. The second model is a modified Beddoes-Leishmann model that includes the effects of shed vorticity from the trailing edge (Theodorsen Theory), as well as the effects of stall separation lag caused by an instationary trailing edge separation point. These effects are especially important related to flutter analysis, but also generally to calculate loads and stability of blades with very low torsion stiffness. The hydrodynamic loads are based on the, within offshore technology well-known, Morison’s equation. The wave kinematics are not calculated within the HAWC2 code but provided externally through a defined DLL (Dynamic Link Library) interface.
The wind conditions are divided into deterministic and stochastic wind. The deterministic wind is mean wind velocity, wind steps, ramps, special gust events, special shears including the possibility for fully user defined shears. The stochastic wind normally referred to as turbulence is generated outside the HAWC2 code. In the HAWC2 code two formats for reading turbulence data are possible. One is in Cartesian coordinates (e.g. Mann turbulence generated by the code WAsP engineering) and the other is in polar coordinates (the Veers model used by FLEX5). Tower shadow effects are also a part of the wind module as it changes the wind conditions locally near the tower. For upwind turbines a potential flow method is used whereas a jet-model produces much better results for downwind turbines.
Control of the turbine is performed through one or more DLL’s. The format for these DLL’s is very general, which means that any possible output sensor normally used for data file output can also be used as a sensor to the DLL. This allows the same DLL format to be used whether a control of a bearing angle, an external force or moment is submitted to the structure.
Outputs from the code can be written to result files in either ASCII format or a more compressed binary format. It is also possible to write a special binary output file which can afterwards be used to animate the turbine behavior.
The code has internally at Risø been tested against the older validated code HAWC. Further on a detailed verification is at the moment performed in the IEA Annex 23 research project and the European UPWIND project. So far these simulations cover a 5MW pitch regulated turbine at land and on a monopile subjected to hydrodynamic loading. The calculation time is approximately a factor of 2 slower than real time on a 3GHz CPU, which is obtained using a Newmark beta solution scheme together with Newton-Raphson iterations within each time-step. The code is limited to Windows 32 primarily caused by its use of DLL’s. The source code is not public, but through the general DLL interface a lot of external coding can be performed by the user. These possibilities will be further expanded in the future. |