CARDAAV

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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.