[Bollonga]

Hello everybody!

I'm working on a three blade horizontal axis wind turbine of 29m of radius. I have a 120º sector cylinder of 5R of raidus, 5R upstream and 15R downstream.
My mesh is a 4 mill cells hexa mesh with rather good quality.

The target is to measure the torque generated for 15m/s and 10rpm. To achieve this I'm increasing the windspeed from 5m/s to 10m/s and 15m/s, with 1rpm, 2rpm and 3rpm of angular speed.

I'm using single precision (I've also tested double seeing no differences), SIMPLE, least squares for gradients, 2nd order for pressure and 2nd order for momentum. Residuals started at 1e-7, then I've reduced them to 1e-5 and 1e-4 to speed up convergence.

I've first tried the steady laminar case, but with no success. For 5m/s I got reversed flow at pressure outlet, and for 10 and 15m/s I got very low torque coefficient, almost 10 times lower than the experimental data.
At least flow vectors seemed to be correct.

Then I've tried the transient laminar case. I've used 1st order time scheme to use the adpative timestepping: 1e-7 to 1e-3 s.
Torque coefficient looks better but reverse flow appears and vectors looks the opposite way they should be! I've even doubted if the sens of rotation is okay and have tried the opposite. Results are the same.

How can I avoid the reverse flow at pressure outlet?
This can be due to not large enough domain, but I've checked literature and mine is pretty okay...
It can also be due to bad boundary conditions, but I'm using the usual: velocity inelt, periodic, symmetry and pressure outlet.

Can anybody tell me how reverse flow can be avoid?
And if it is the main cause of such a poor predicition?
Is my rotating reference frame okay? or it should be the opposite? See pictures.

Any comment or suggestion is highly appreciated.

Thanks a lot!