NREL 5MW Reference Turbine - tilt and C_T

Hello,

we use the NREL 5 MW reference turbine data for our turbine parameterizations in an LES code.

We have complex parameterizations like actuator line model and enhanced actuator disk model (similiar calculations as in actuator line model, however not for discrete blade lines but for each point across the rotor disk). In these models we consider the 5° shaft tilt. But we also have a standard actuator disk model. Here, we specify a thrust curve (see attachment) which is based on Fig. 9-1 in the report or rather the thrust values given in this topic: NREL 5-MW reference turbine. C_T is calculated as: C_T = 2*thrust / ( rho * pi * r^2 * u^2 ) with rho = 1.225 kg/m3, r = 63 m, thrust and u the corresponding values from Fig. 9-1.

In the current code, the C_T values > 1 are set to 0.9999 to enable a calculation of the axial induction factor for determining the reference velocity. In the future we plan to include a Glauert correction.

I have some questions:

  1. Can you confirm these C_T values, are they reasonable and do you have similar values? And could you provide a more detailed table - for example I added values determined by eye from the graphic curve in the region where C_T drops very fast (11.3 and 11.7 m/s), with a linear interpolation between 11 and 12 m/s we would make a significant error.

  2. Do we have to account for the shaft tilt of 5° when we want the thrust to act on the mean flow? In other words: Do we need a tilt correction (which we apply in the actuator line model) - or is this already included in the thrust curve of Fig. 9-1?

  3. When we include the Glauert correction, this is only valid for C_T < 2 (although even with values near 2, the axial induction factor approaches 1 and the thrust would go to infinity) - so what to do with the C_T > 2 for u < 3.7 m/s?

Best regards,
Björn
NREL_5MW_C_T_new.txt (276 Bytes)

Dear Bjorn,

The basic problem I see is that the rotor thrust reported in Figure 9-1 of the NREL 5-MW specifications report is not the pure applied aerodynamic thrust, but it also includes contributions from gravity and inertia forces. See the following forum topic for a similar question and my detailed response: http://forums.nrel.gov/t/fast-output-rotct-is-not-constant-for-the-same-tsr/167/1.

The complete Cp/Cq/Ct versus tip-speed ratio (TSR) and pitch angle surfaces of the NREL 5-MW turbine based purely on aerodynamics are documented on our online forum – see the topic found here: http://forums.nrel.gov/t/nrel-5-mw-reference-turbine-cp-cq-ct-coefficients/456/1.

Both the thrust and thrust coefficients discussed in these forum topics are measured along the shaft, which is titled at 5 degrees; I would think you would want to account for this 5 degree tilt in your LES code.

Best regards,