4.2.5. Modeling Considerations

AeroDyn was designed as an extremely flexible tool for modeling a wide-range of aerodynamic conditions and turbine configurations. This section provides some general guidance to help you construct models that are compatible with AeroDyn.

Please refer to the theory of Section 7 for detailed information about the implementation approach we have followed in AeroDyn.

4.2.5.1. Environmental Conditions

For air, typical values for AirDens, KinVisc, SpdSound, and Patm are around 1.225 kg/m³, 1.460E-5 m²/s, 340.3 m/s, and 101,325 Pa, respectively. For seawater, typical values for FldDens, KinVisc, SpdSound, and Pvap are around 1025 kg/m³, 1.004E-6 m²/s, 1500 m/s, and 2000 Pa, respectively.

4.2.5.2. Temporal and Spatial Discretization

For accuracy and numerical stability, we recommend that DTAero be set such that there are at least 200 azimuth steps per rotor revolution. However, when AeroDyn is coupled to FAST, FAST may require time steps much smaller than this rule of thumb. If UA is enabled while using very small time steps, you may need to recompile AeroDyn in double precision to avoid numerical problems in the UA routines.

For the blade and tower spatial discretization, using higher number of analysis nodes will result in a more accurate solution at the expense of longer computational time. When AeroDyn is coupled to FAST, the blade and tower analysis node discretization may be independent from the discretization of the nodes in the structural modules.

We recommend that NumBlNds be between 10 and 20 to balance accuracy with computational expense for the rotor aerodynamic load calculation. It may be beneficial to use a finer resolution of nodes where large gradients are expected in the aerodynamic loads e.g. near the blade tip. Aerodynamic imbalances are possible through the use of geometrical differences between each blade.

When the tower potential-flow (TwrPotent > 0), tower shadow (TwrShadow > 0), and/or the tower aerodynamic load (TwrAero = TRUE) models are enabled, we also recommend that NumTwrNds be between 10 and 20 to balance accuracy with computational expense. Normally the local elevation of the tower node above ground (or relative to MSL for offshore wind and floating MHK turbines or relative to the seabed for fixed MHK turbines) (TwrElev), must be entered in monotonically increasing order from the lowest (tower-base) to the highest (tower-top) elevation (or monotonically decreasing order for floating MHK turbines). However, when AeroDyn is coupled to FAST, the tower-base node in AeroDyn cannot be set lower than the lowest point where wind is specified in the InflowWind module. To avoid truncating the lower section of the tower in AeroDyn, we recommend that the wind be specified in InflowWind as low to the ground (or MSL for offshore wind turbines or seabed for fixed and floating MHK turbines) as possible (this is a particular issue for full-field wind file formats).

4.2.5.3. Model Options Under Operational and Parked/Idling Conditions

To model an operational rotor, we recommend to include the dynamic BEM model (WakeMod = 2) and UA (AFAeroMod = 2). Normally, the Pitt and Peters skewed-wake (SkewMod = 2), Prandtl tip-loss (TipLoss = TRUE), Prandtl hub-loss (HubLoss = TRUE), and tangential induction (TanInd = TRUE) models should all be enabled, but SkewMod = 2 is invalid for very large yaw errors (much greater than 45 degrees). The nonlinear solve in the BEM solution is in terms of the inflow angle, but IndToler represents the tolerance of the nondimensional residual equation, with no physical association possible; we recommend setting IndToler to DEFAULT.

While all of the UA models are documented in this manual, the original B-L model is not yet functional. Testing has shown that the González and Minnema/Pierce models produce reasonable hysteresis of the normal force, tangential force, and pitching-moment coefficients if the UA model parameters are set appropriately for a given airfoil, Reynolds number, and/or Mach number. However, the results will differ a bit from earlier versions of AeroDyn, (which was based on the Minnema/Pierce extensions to B-L) even if the default UA model parameters are used, due to differences in the UA model logic between the versions. We recommend that users run test cases with uniform inflow and fixed yaw error (e.g., through the standalone AeroDyn driver) to examine the accuracy of the normal force, tangential force, and pitching-moment coefficient hysteresis and to adjust the UA model parameters appropriately.

To model a parked or idling rotor, we recommend to disable induction (WakeMod = 0) and UA (AFAeroMod = 1), in which case the inflow velocity and angle are determined purely geometrically and the airfoil data is determined statically.

The direct aerodynamic load on the tower often dominates the aerodynamic load on the rotor for parked or idling conditions above the cut-out wind speed, in which case we recommend that TwrAero = TRUE. Otherwise, TwrAero = FALSE may be satisfactory.

We recommend to include the influence of the tower on the fluid local to the blade for both operational and parked/idling rotors. We recommend that TwrPotent > 0 for upwind rotors and that TwrPotent = 2 or TwrShadow > 0 for downwind rotors.

4.2.5.4. Linearization

When coupled to FAST, AeroDyn can be linearized as part of the linearization of the full coupled solution. When induction is enabled (WakeMod = 1), we recommend to base the linearized solution on the frozen-wake assumption, by setting FrozenWake = TRUE. The UA models are not set up to support linearization, so, UA must be disabled during linearization by setting AFAeroMod = 1. Linearization is not currently possible when modeling an MHK turbine, but we will attempt to enable it in an upcoming release.