SOFTWARE OVERVIEW

CARDAAV

CARDAAV is a computer code based on the Double-Multiple Streamtube model with variable upwind and downwind induced velocities in each streamtube (DMSV model). A unique feature of the CARDAAV code, apart from the two actuator disks model, is that additional parameters or input options can be included, for example: to analyze the influence of the blade geometry, the airfoil type and the secondary effects such as the rotating tower, the presence of struts and aerodynamic spoilers on the Darrieus turbine. Those capabilities make CARDAAV a very attractive and efficient design and analysis tool. Therefore, although based on a momentum model, the results obtained with the DMSV model, supplemented with the above mentioned corrections that are available in CARDAAV, are in good agreement with the experimental data.

The numerous parameters that are necessary to fully describe the analyzed VAWT provide a rather large freedom in specifying its geometry. Among the most important in this category are: the rotor geometry and dimensions, the helical twist of the blades, the number of blades and the type of airfoil defining their cross section, the diameter of the central column (or tower), the geometry, dimension and position of the struts, the size of the spoilers, etc. Virtually any blade shape can be analyzed, including, of course, the Troposkein (or eggbeater) and straight ones. Moreover, the blade can be made of sections having different chord lengths and cross-sections (airfoils).

If the user wants to perform analyses with an airfoil that is not among those already available, this can be done quite simply, by including the values of its experimentally or numerically determined lift and drag coefficients in the actual airfoil database. These data must be given for several Reynolds numbers that correspond to those attained on the revolving blades and cover (at each Re) the full range for the angle of incidence attained.

Among the principal operating parameters that are readily modifiable to meet the needs of a specific analysis, one can mention: the wind speed, the rotation speed of the rotor, the local gravitational acceleration and the working fluid properties (density and kinematic viscosity). Either constant rotation speed for different wind speeds or different rotation speeds for a constant wind speed can be considered when performing an analysis. By specifying the adequate value for the atmospheric wind shear exponent, a power law type variation of the wind speed (as a function of altitude) will be taken into account during the computations.

In what regards the control parameters, the code requires the number of half cycle (azimuthal) divisions and vertical divisions which define the total number of streamtubes that are going to be considered in the computations as well as the number of integration points over the width of each tube. In the same category, the user has to specify the maximum number of iterations in the computation of the upwind and downwind interference factors along with the convergence criteria (relative error levels that must be satisfied when computing the interference factors and the dynamic stall). The decision on whether to apply or not the aerodynamic corrections related to the blade-tip (or finite span) effects, as well as several other secondary effects, such as those due to the rotating central column (or tower), the struts and spoilers and those due to the occurrence of dynamic stall must be specified in the control parameters. Four dynamic stall models are available, three derived from Gormont's method (adaptation of Strickland and al., the adaptation of Paraschivoiu and al. and the modification of Berg) and one derived from the indicial method.

Dynamic stall has a significant influence on the aerodynamic loads and the rotor performances at low tip-speed ratios, whereas the secondary effects are important at moderate and high tip-speed ratios. The local induced velocities, Reynolds number and angle of incidence, the blade aerodynamic loads and the azimuthal torque and power are the output data.

Due to DMSV model and to a quite large number of additional models and options regarding the geometrical configuration, the operational conditions and the control of the simulation process, CARDAAV proves to be an efficient and flexible software package, appropriate for the needs of VAWT designers. It computes the aerodynamic loads and rotor performance for VAWTs of any geometry at given operational conditions. It is also possible to couple CARDAAV with an optimization or structural analysis code in order to perform the optimized design of a VAWT.

CARDAAX

The CARDAAX computer code is a modified version of the CARDAAV code in which the expanding lateral displacements of the streamlines are calculated with respect to an undistorted central streamline (streamtube expansion effects). The streamline position, defined by its coordinates, depends on the local tip-speed ratio of the rotor. In this new numerical procedure, it is assumed that the upwind half-cycle of the rotor is divided into nine angular tubes corresponding to six angular tubes on the downwind half-cycle zone.

CARDAAS-1D & -3D

The CARDAAS code was developed in order to predict the aerodynamic loads on the Darrieus rotor in turbulent wind. This code is based on the Double-Multiple Streamtube (DMS) model and incorporates a stochastic wind model. The method used to simulate turbulent velocity fluctuations is based on the power spectral density. The problem consists in generating a region of turbulent flow with a relevant spectrum and spatial correlation. The first CARDAAS code developed was based on a one-dimensional turbulent wind model (CARDAAS-1D) and ignored the structure of the turbulence in the cross flow plane. An extension to three dimensions was made an integrated in the CARDAAS-3D code. The CARDAAS-1D and 3D code have the capability to predict the ensemble averaged values, the standard deviation, the instantaneous values, as well as the mean values with constant ambient wind.

CANICE-2D & -3D

Currently, atmospheric icing is one of the major concerns for the certification authorities as well as the aircraft manufacturers since it presents a major hazard to aircraft operating under natural icing conditions. Atmospheric icing also represents a major concern when installing wind turbines in cold climate. Ice accretion on the rotor blades can result in reduced power production and increased rotor loads, which may require stopping the turbine for safety reasons and to prevent damage to the turbine structure.

CANICE-2D & -3D uses an aerodynamic panel method to solve for the potential flow field which is then corrected for compressibility effects. The code uses Lagrangian tracking to determine droplet trajectories and impingement locations. The modified Messenger model is used for ice accretion thermodynamics, in conjunction with an integral boundary-layer solution for heat and mass transfer rates. The code includes roughness, runback and water splash/ice shed models based on water-bead model.

The code has the capability of simulating super cooled large droplet conditions. The ice accretion is re-paneled and the airflow field is re-computed at each time step to determine the growth of the ice accretion as time proceeds. The code has also a module for the computation of the effects of a hot-air based anti-icing system.

Invited paper, as code developer at the NATO/RTO Workshop at CIRA, in Italy, on Assessment of Icing Code Prediction Capabilities. CANICE is one of the five major icing simulation codes in the world. It has been approved by Transport Canada to be used for the certification of Bombardier aircraft in icing conditions.

DYNAMIST

DYNAMIST is a dynamic analysis code developed for assessing structural response of Vertical Axis Wind Turbine under dynamic loads. DYNAMIST will do the dynamic structural response analysis based on a finite-element formulation. In this code, the VAWT is modeled using the lumped mass approach. The rotor blades, struts and the central shaft are divided into simple straight segments and treated as 3-D beam elements. Each 3-D beam element has two nodes with six degrees of freedom at each node. Using the finite element technique, each element can be modeled as a second order differential equation. The formulation includes a mass matrix, a gyroscopic matrix, a stiffness matrix for each element and a second stiffness matrix which results from the spinning of the structure. The gyroscopic matrix has a stabilizing effect while the second stiffness matrix has a destabilizing effect. The force vector contains the aerodynamic loading (load per unit length) acting on the beam elements calculated by the CARDAAV code.

The output is the natural frequency and modes for both spinning and non-spinning rotor, as well as the modal frequency response at required rotational speeds. Output from forced response includes both linear and angular displacements (maximum values at required nodes) at desired rotational speeds. The program can also provide power density spectra. The load per unit length on each element, cross-sectional structural properties of each load element and the rotor operating conditions are required as input. The program was developed last year. The validation is currently underway. The theoretical basis is based on the concept of spinning finite elements.

AEROBRAKE

AEROBRAKE is a computer code developed to determine VAWT aerodynamic brake size requirements, as well as study the dynamic response of a VAWT under aerodynamic brake application.

CARDAAV-ICE

The CARDAAV has also been coupled with the state-of-the-art icing simulation code, CANICE, to predict the aerodynamic loads on the Darrieus rotor under icing conditions. Furthermore, the results of this code have also been used to study dynamic structural response of a VAWT under icing conditions.