Cp greater than Betz in Aerodyn

[James.Manwell]

I was just running AeroDyn (v15.04.02a, 10-May-2017) using AeroDyn_driver (v1.02.00a, 12-Apr-2016) in the free-standing mode on NREL 5 MW files as inputs under the following simple conditions: WndSpeed=8 m/s; ShearExp=0 ; RotSpeed=7.55 rpm; Pitch=0 deg; Yaw=0 deg; dT=0.1 s; Tmax=1 s and the output had Cp = 0.696,i.e. greater than Betz. The power given was consistent with that Cp and the area was consistent with the 63 m radius. The aero data was of the form used in DU21_A17_10Hz.dat, etc.
Typical output was:
Time B1N5Alpha B1N5Cl B1N5Cd B1N5Cm RtAeroPwr RtArea RtAeroCp
(s) (deg) (-) (-) (-) (W) (m^2) (-)
0 1.20E+01 1.28E+00 5.97E-02 -1.03E-01 2.72E+06 1.25E+04 6.96E-01
Any idea what is happening here?

[Jason.Jonkman]

Hi Jim,

I would guess you are seeing some sort of start-up transient based on how the states of AeroDyn are initialized. Is the output changing over the 11 time steps you are simulating? (If so, perhaps see what happens when you simulate longer.) So that I can understand what states your model has, what are your settings for WakeMod and AFAeroMod?

Best regards,

[James.Manwell]

Hi Jason,
The results are the same for all time steps. WakeMod =0, AFAeroMod =1, TwrPotent =0. I ran 3 cases at the same time (8, 9, 10 m/s). All resulted in Cp>0.6. I could upload or send you all the files if that would help.
Jim

[Jason.Jonkman]

Hi Jim,

Oh, I didn't realize you have WakeMod = 0, which explains why you are receiving Cp > Betz. When WakeMod = 0, momentum theory is disabled, the inductions are set to zero, and it is possible for Cp to exceed Betz limit. I would using WakeMod = 0 only for simulations involving a parked or idling rotor.

Best regards,

[James.Manwell]

Ah hah! Thanks.
Jim