# MRF Rotation

#21
1. You will have to learn how to calculate Cp for your design, lots of info about that is out there.
2. With SimScale, you MAY be able to obtain a very rough guess as to how much power you can extract from your design at different wind speeds and rpms… This power that you calculate will be used in Cp calculation.
3. Power is a function of torque (moment) and rpm. Formulas are out there. Rpm is what you input in MRF (I think you may have the incorrect direction for that in your simulation). Torque for each blade is in your ‘Forces and Moments’ results (both pressure and viscous moments need to be added together).
4. In order to have any faith at all in the values of forces and moments results, those values should be stable within a 1% range of the value, over the last 500 iterations… Your values are not considered stable.
5. Again, be careful that you do not spin the MRF faster than the no load rpm of that design in the wind speed of the sim run. If you do, then you will be INPUTing power to the windmill to get it to the rpm you have set the MRF to (you will see that by the sign of the moments once you are sure you have the correct rotation direction in the MRF parms).
6. You should be working with coarser meshes until you zero in on having a sim setup that works.

#22

thank you so much

#23

i have one more question
for example
pressure moment x : -0.255 Pm y: 0.555 Pm z; -0.424
Pmx; 0.455 Pmy: 0.623 Pmz; -0.546
how i get resultant?
because i have two opinions and i am confused

#24

y is rotation axis, so pressure momentY sum is 0.555+0.623 Nm (edit, sorry decimal location was incorrect)

#25

if i have Pmy 1: -0.256
Pmy 2: 0.4
the resultant is 0.144 not 0.656

#26

thank you sir
you helped me alot