[blackmask]

You realize that your rotor speed is around 30 rpm, do you?

[Bollonga]

Originally Posted by blackmask

You realize that your rotor speed is around 30 rpm, do you?

Yes, I do. Is it too much? I've been reading that this is the rotor speed for some comercial wind turbines.

[Bollonga]

I've run the k-omega SST case with 1% inlet turbulence and 0.05m as length scale. Now the moment coefficient is negative! I'm measuring moment along (0,0,1) while rotating reference frame is about (0,0,-1).
The issue is that the laminar case results in a positive coefficient for the same axis definition!

Which is the correct coefficient sign? Is it acounting for the work the fluid is doing on the blade (so it should be positive with my setup) or is it the for the blade working on the flow?

Watching the coefficient history it appears to oscillate over a wide range.

Flow looks the right direction, but I see some separation and I guess it shouldn't be any.

I attach pictures of residuals, moment coefficient history, relative velocity vectors, static pressure contours and pathlines.

[Far]

is magnitude ok?

[Bollonga]

Originally Posted by Far

is magnitude ok?

Which magnitude? Everything is full scale.

[Bollonga]

I've reduced the angle of attack from 12º to 10º by reducing wind speed, Cm has improved but there's still some oscillations around its value (picture 1). I've also compared it for TI=1% and TI=5%. The first being logically a little bit higher.

There still some separation (pictures 2, 3 and 4), is it supposed not to have any? Reynolds number is pretty high at this section (Re=6 e6) so even for a 10º angle of attack I guess dettachment is possible.

Also, I'm not pretty sure about which is the optimun angular speed for a given windspeed if I can also choose the blade alignment.

[abrahamgx]

Originally Posted by 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!

To avoid the reverse flow at pressure outlet, you should use Alternate Rotation Model in the Domain setting.
The attachment picture shows the streamline without/with alternate rotation model.

[Far]

What is the difference between both models?