Turbine performance overprediction
 

Hi Folks,
I'm trying to carryout transient analysis on a 2D 3 bladed vertical axis wind turbine.
I want to then compare my CFD to experimental Cp results. Im determining my Cp by multiplying my Cm (based off the reference values) by the tip speed ratio. Cp = Cm X TSR I've found this is what alot of published authors have done. Im using the SST turbulence model with a Y+ =1 I reckon my mesh is pretty good with 1000 elements over each foil of 0.14 m chord and a total domain siz of ~300k elements with a max skewness of 0.76

Im using the Coupled Scheme and a Courant number of 100 aswell as the Presto model for pressure, this gives me a max Cp of 0.6 wheres it should be 0.3 according to experimental, Im aware that 2 D models will overpredict the performance and they are unable to capture 3D blade tip effect etc. but I was hoping to get a better solution that 100% over prediction.

Also when I run the second order model for pressure after about 3000 iteritions i get the warning
"turbulent viscosity limited to viscosity ratio of 1.000000e+05 in xxxcells" with x increasing until I get divergence and a floating point exception and failure.

has anyone any experience in this id appreciate any advice, or can explain why the Presto model works and the 2nd order doesnt? I also have this problem when I try to run Piso instead of Coupled. :confused:

Thanks any advice would be appreciated.

Thanks,:)
John